DESIGN OF SPUR· TYPE Turner-Fairbank Highway

STREAMBANK STABILIZATION Research Center

6300 Georgetown Pike
McLean, Virglnl8 22101
FDREWDRD
Based on a thorough review of literature, analysis of several hundred field
STRUCTURES sites, end a recent laboratory study conducted by the Federal Highway
Administration, recommendations for ~e general application and design of
Report No. spur-type flow control and streambank stabilization structures are given. An
u.s. Department FHWAlRD-B4/101 example outlining the recommended procedure for establishing the geometric
of Tronsportation layout of spurs within a spur scheme is included.
Final Report
FederqI Hlghway July 1985 Research and development in streambank stabilization is included in the
AdmInistratlon Federally Coordinated Progran of Highway Research, Development, and Technology
Project SH "Highway Drainage and Flood Protection." Dr. Roy E. Trent is the
Project Manager and the Contracting Officer's Technical Representative for
this study.
FOR DISPLAY ONLY Sufficient copies of this report are being distributed to provide a mlnlmum of
two copies to each FHWA regionsl office, one copy to each division office, and
DO NOT REMOVE two capies to each State highway agency. Direct distributian is being made to
the division offices,

Wf!~
Richard E. Hay,' rector
î

Office of Engi ering

and Highway perations
Research and Development
Federal Highway Administration

NDTICE
This document is disseminated under the aponaorship of the Departrnentof
Transportation in the interest of information exchange. The United States
Government assumes no liability for its contents or use thereof.
The contents of this report reflect the views of the cantractor who is
responsible for the accuracy of the data presented herein. The contente do
not neeessarily reflect the official policy of the Department of
lransportation.
This report does not constitute a standard, specification, or regulation.
The United States Government does not endorse products or manufacturers.
Trade or manufacturers' names appear herein only beeause they are considered
essential to the object of this document•

. Ihro-.b m, ... tio,.. Techniullnformltion SeNiel, Springfilld. Virginil22161

 Technical Report Oocumentation Page
1. Repo" No, 12. Go,.. nm.n' A,,, .. ion No. 3. Rec:ipi.n"s Cqtolog No.

FlIWA/RD-84/101
4. TIIi. Ol"ld Subli'!e S. R.porT 001.
Ju1y 1985
DESIGN OF SPUR-TYPE STREAMBANK
6. P.,fo,,,,ing OrlJoni101ionCod.
STABILIZATION STRUCTURES
8. P.,'o,ming Or,oni:totion Report No.
7. AUlho,fs)
Scott A. Brown SCR-371-83-039
9. P.do,mlng Orgal'liJ;otlonNome end Addr.s. 10. Wo,kUnit No, (TR,A,IS)

Sutron vorporation
11. Controct or Gronl No.
2190 Fox Mil! Road
DTFH6l-80-C-OOI47
Herndon, VA 22071 METRIC COIfVERSIOI' FACTORS
13. Type of Repo,t ond Period Covered
12. Sponsoring Agenel' Nome ond Addren Final Report
Federal Highway Administration October 1980 to
Office of Engineering & Highway Operations R&D SeDtember 1983
Multiple By To Obtain
Structures oivision 14. Sponsoring Alleney Code
McLean, Virginia 22101
1 S. Suppl.menlorl' No'u
inches 2.5 centimeters
Contracting Officerls Technical Representative: Dr. Roy Trent (HNR-IO) feet 30 centimeters
yards 0.9 meters
16. A.bstrac:t mlles 1.6 kilometers
A study of the applicability and design of .pur-type flow-con trol and square inches 6.5 square centimeters
streambank stabilization structures has been conducted to establish design square feet 0.09 square meters
guidelines and other criteria for the use of spurs. The recommendations and square yards 0.8 square meters
findings are based on a thorough review of pertinent literature, analY8i8 of square mlles 2.6 square kilometers
several hundred field sites, and on a recent laboratory study conducted by the acres 0.4 hectares
Federal Highway Administration. Recommendations for the general application
of spur-type structures are given in relation to function of the spur, the ounces 28 grams
erosion mechanisms that are countered by spurs, the environmental conditions pounds 0.45 kilograms
best suited for the use of spurs, and potential negative impacts produced by short t ons 0.9 tonnes
spurs. An in troduc tion to the most common types of spurs is given, along with (2000 Ibs)
discussions of the factors most important to the design of specific 'spu r
types. Design guidelines for establishing spur permeability, the required
extent of protection, spur length, spur spacing, spur'orientation, spur height,
spur crest profile, and the shape of the spur tip or head are presented. An
example outlining a recommended procedure for establishing the geometric layout
of spurs within a spur scheme is recomaended.

17. K• .,. Wanh 18. 0, .t,ibulion Statement

River training No restrictions. This document is
Streambank stabilization available to the public through the
Spurs National Technical Information Service,
Groins Springfield, Virginia 22161
Jetties
19. S.c:urily Clo"lI. (a' tki. repo,t)
I ~.Unclassified
Sec:u,itl' CJouif. (of 'hi. pa;e) 121. No. o. P•••• 122. Pric:.

Unclassified 106

Fo,m DOT F 1700.1 (8_721 R"production of c=ompleted page authoria.ed

11
 TABLI OF CONTEIITS

TABLE OF COIITEIITS(Continueel)

LIST OF FIGURES v
011IERCOIISIDEUUOIIS 41
LIST OF TABLES viii
Costs 41
CHAPTER 1. IlITRODUCTIOIi
Oumnel Si:r:e 44
CHAPTER 2. CO.SIDER.lTHIiS lil TUE SELECTIO. .llID DESIG. OF Cbannelbed Fluctuations 44
SPOK-TYPE STRUC11JRES 3 Yegetation 45
Vandalisia and "aintenanee 45
GEIERlL APPLICABILITY OF SPURS 3
CHlPTER 3. DESIGN OF SPUI SYSTEIIS 46
Function 3
Erosiom Kecbanisas 3 PEIIl4E.IBILITY 46
River Envirou.ent 4
Channel Size 4 GEOHETRY 52
Bend Radius 4
Channelbank Characteristics 5 E:r:tentof Bank Protection 52
Syste. r.pacts 5 Spur Lengtb 56
Environmental Impacts 6 Spur Spaclng 60
Esthetic Impacts 8 Spur Orientat1on 64
Construction-Related Considerations 8 Geoaet ..ie Design Ezaaple 72
Costs 9
STBucrulE HEIGBT 81
SPUR TYPES 9
CREST PROFILE 84
Retardance Spurs 10
Fence Type 10 BED JJID BAH COWT"CT 85
Jack/Tetrahedron Type 19 Cbannelbed Contact 85
Retardanee/Diverter Spurs 19 Cbannelbank Contact 90
Light Fence Type 21
Heavy Diverter Spurs 24 SPUR HElD FORM OR DESIGN 91
Diverter Spu"s 27
Hardpoints 28 SIMWIY OF SPUR DESIGN RECOIIIIElIDlUONS 91
Transverse-Dike Spurs 28 Peraeabil1t,. 91
Dtent of Oumnelbank Protection 93
PRlKARY FACTORS IWFLUEIICDG THE DESIGN .llID SELECTIOII Spur Lengtb 93
OF ..SPUI TYPE 31 Spur Spacing 93
Spur lngle/Orientation 94
Spur FuDctionlPurpose 32 Spur Syst.. Geomet ..y 95
Eros1on "ecbanisa 34 Spu ..Heigbt 95
Sediment Environment 35 Spu.. C..est Profile 95
Flow Enviro~ent 37 Cbannelbed and Cbannelbank Contact 96
ehanael Yelocity Environment 38 Spur Bead Fora 96
Flow Stage 38
Bend Radius 39 REFERENCES 97
Debris and Iee-Load Enviro .. ent 40

iii
 LIST OF FIGURES LIST OF FIGURES (Contlnued)

TIMBER PlLE SPUR SHOWING THE IMPACT 17 STEEL JACK AND TETRAHEDRON DETAILS 20
7
OF EXCESSIVE FLOW DEFLECTION
16 LAYOUT DETAILS OF TETRAHEDRON SPURS 21
2 HENSON TYPE SPUR JETTY; BARZOS RIVER 11
NEAR ROSHARON, TEXAS 19 TYPICAL DESIGN SKETCH OF WOOD-FENCE SPUR 22

11 20 DETAILS OF LIGHT-FENCE-TYPE SPUR 23

3 TETRAHEDRON SPURS; SAN BENITO RIVER,
CALIFORNIA TIMBER-PILE AND WIRE-MESH SPUR
21 23
4 WOOD-FENCE SPUR; BATUPAN BOGUE, 12
GRENADA, MISSISSIPPI 22 TYPICAL DESIGN SKETCHES FOR STEEL-PILE AND
WI RE-MESH SPURS 25
5 WIRE FENCE SPURS 12
23 TYPICAL DESIGN OF TIMBER-PILE DIVERTER SPURS 26
6 DOUBLE-ROW TIMBER PILE AND WIRE-FENCE 13
SPUR 24 TYPICAL DESIGN SKETCH FOR TIMBER-PILE AND 27
WOOD PLANK RETARDANCE/DIVERTER SPUR
7 WELDED-WIRE AND STEEL H-PILE PERMEABLE 13
SPUR; ELKHORN RIVER AT SR-32 25 TYPlCAL ROCK HARDPOINT DESIGNS 29
AT WEST POINT, NEBRASKA 26 TYPICAL DESIGN SKETCH FOR DUMPED RIPRAP 29
STEEL PILE/WELDED WIRE MESH SPUR; 14
TRANSVERSE DlKE SPUR
6
LOGAN CREEK NEAR PENDER, NEBRASKA 27 TYPICAL DESIGN DRAWING FOR GABION TRANSVERSE 30
DIKE SPUR
9 TIMBER PILE SPURS; BIG BLACK RIVER 14
AT DURANT, MISSISSIPPI TYPICAL DESIGN SKETCH FOR WIRE CRIB DESIGN
26 30
10 TIMBER PILE/SUSPENDED LOG SPURS; '15
ELKHORN RIVER WEST OF ARLINGTON, 29 SKETCH OF RECTANGULAR TIMBER ROCK-FILL CRIB SPUR 31
NEBRASKA SPUR CaST COMPARISON 42
30
11 TIMBER PILE AND HORIZONTAL WOOD 15
PLANK DIVERTER STRUCTURE 31 PLOT OF SPUR PERMEABILITY VS. SCOUR DEPTH 49

ROCK RlPRAP SPUR; LOYALSOCK CREEK 16 32 SPUR ANGLE VS. V' 49

12
NEAR MONTOURSVILLE, PENNSYLVANIA
33 SPUR PERMEABILITY VS. EXPANSION ANGLE 51
13 GABION SPURS; LOYALSOCK CREEK NEAR 16
LOYALSOCKVlLLE, PENNSYLVANIA 34 FLOW OVER IMPERMEABLE SPURS 51

14 CRIB SPURS 17 35 EXTENT OF PROTECTION REQUIRED AROUND A CHANNEL BEND 53

FENCE-TYPE RETARDANCE SPURS 16 36 SHIFT IN MAXIMUM CURRENT THREAD WITH 54

15 CHANGING STAGE
16 HENSON SPUR JETTY LAYOUT ON RED 20
RIVER AT PEROT, LOUISIANA

v vi
 LIST OF FIGUBES (Continueel) LIST OF FIGUBES (Contlnued)

54 FINAL SPUR SCHEME GEOMETRY 82

37 RELATIONSHIPS BETWEEN SPUR LENGTH AND 57
Al DESIGN SKETCH FOR PILE CLUSTER SPUR 55 COMPARISON OF SCOUR PATTERNS GENERATED BY 84
Bl DESIGN SKETCH FOR DOUBLE-ROW, SINGLE-PILE (Al SUBMERGEO, AND (Bl NONSUBHERGED
SPUR IMPERHEAABLE SPURS
Cl DESIGN SKETCH FOR TIMBER PILE SPUR WITH
SLASHED TREES 56 ROCK RIPRAP SPUR ILLUSTRATING LAUNCHING OF 86
STONE TOE PROTECTION
38 DEFINITION SKETCH OF FLOW EXPANSION ANGLE 61
57 GABION SPUR ILLUSTRATING FLEXIBLE MAT TIP 87
RELATIONSHIP BETWEEN SPUR LENGTH AND 61 PROTECTION
39
EXPANSION ANGLE FOR SEVERAL PERMEABILITIES
58 PERHEABLE WOOD-SLAT, FENCE SPUR SHOWING 88
40 DEFINITION SKETCH FOR SPUR SPACING CRITERIA 63 LAUNCHING OF STONE TOE MATER lAL

COMPARISON OF FLOW THALWEGS FOR TWO SPUR SPACINGS 64 59 WIRE MESH SPUR WITH THE MESH SCREEN 88
41
EXTENDED BELOW THE MAXIMUM ANTICIPATED
42 DEFINITIONS SKETCH FOR SPUR ANGLE 65 SCOUR DEPTH

43 FLOW PATTERNS OBSERVED AROUND SPURS OF DIFFERENT 67 60 HENSON SPURS (Al RESTING ON ORIGINAL CHANNELBED, 89
ORIENTATIONS AND (Bl AFTER DROP IN CHANNELBED LEVEL

LOCAL SCOUR PATTERNS AT THE TIP OF IMPERHEABLE 68 61 HENSON SPUR SHOWING OUTFLANKING 90
44
SPURS WIRE-MESH PERMEABLE SPUR ILLUSTRATING SPUR 91
62
FLOW COHPONENTS IN THE VICINITY OF SPURS WHEN 70 ROOT EXTENDING INTO CHANNELBANK
45
THE CREST IS SUBHERGED
LIST OF TABLES
46 COMPARlSON OF THALWEG POSITION~ PRODUCED 71
BY SPURS ANGLED AT 1200 AND 150

47 CHANNELBEND SHOWING ERODED AREA, DESIRED 73 Table 1 SPUR TYPE SELECTION TABLE 33
FLOW ALIGNMENT, AND DEPOSITED SANDBAR

48 SETTING THE LIHITS OF PROTECTION 74

49 SETTING MAXIMUM FLOW CONSTRICTION 75

50 ESTIHATES OF THALWEG LOCATIONS FOR VARIOUS 76

FLOW CONDITIONS
51 LOCATION AND ORIENTATION OF FIRST SPUR 78

52 LOCATION OF SECOND SPUR 79

53 ORIENTATION OF SPUR NUMBER 2 80

vii
 This report i s based on a thorough llterature review, extensive review
and evaluation of spur fleld installations, numerous personal contacts with
design engineers actively involved in designing flow-control structures, and
a laboratory study designed to evaluate critical spur design parameters.

Chapter 1

INTRODUCTIOIf

The purpose of this report is to provide guidelines for the application

and design of spur or jetty type flow control structures. Spurs (or jetties,
as they are of ten called) are defined as linear structures, permeable or
impermeable, projecting into a channel from the bank for the purpose of
altering flow direction, ohannelbank protection, lnducing deposition, or
reduoing flow velocity al ong the bank. This report is intended to alert
engineers to the utility of spur s , including economie and other advantages,
as well as to provide a treatment of the effectiveness and limitations of
spur-type structures as flow control and streambank-stabilization
structures.

In the past, little guidance has been available for the design of
spur-type structures. Few design guidelines have been available; those that
are avallable are limited in scope and generally inaccessible to highway
design engineers. The design of these structures has been primarily based on
the designer' e experience and numerous rules-of-thumb. While actual field
design experience is indispensable when designing flow-control structures,
many highway design engineers have only limited experience, indicating a need
for some design guidance. There is also a need for more definite criteria
relating to the behavior of spurs under various river-flow conditions. This
would remove some of the uncertainty in their design and permit greater
economy in the design of spur schemes by minimizing over-design as well as
under-design. This design document addresses these needs by presenting
guidel1nes for the design of spur-type flow control and bank-stabilization
structures.

In this report the first consideration is the overall applicability of

spur-type structures. This includes the function of the spur , the erosion
mechanisms that are countered by spur s , the environmental conditions best
suited for the use of spur s , an introduction to the most commontypes of
spurs, and discussions of the factors most important to the design of
specific spur types.

The actual design of spur systems is considered next. Guidelines for

establishing spur permeability, the required extent or upstream and
dowostream limits of protection, spur length, spur spacing, spur orientation,
spur height, spur crest profile. the shape of the spur tip or head, and
maintaining channelbed and bank contact are included. An example outlining
the procedure for establishing the geometrie layout of spurs within a spur
seheme is al.o included.

2
 counter these particle displacement erosion mechanisms by diverting the
high-energy streamflow away from the bank. The immediate consequence is that
the flow dynamics and forces responsible for bank eros ion are moved away from
the bank, greatly reducing or eliminating the potential for erosion . Spurs
are particularly well-suited for protecting lower portions of the bank from
erosion at the bank toe. Toe scour and the resulting undermining of
Cbapter 2 channelbanks are discussed in FHWA(1984). Toe scour has been identifi ed as
a primary cause of bank failure. By moving the flow forces responsible for
COISIDERATIONS IN TOE SELECTlOl AID DESIGI OF SPUR-TYPE toe scour away from the bank, this erosion mechanism is effectively
STRUC'ruRES countered .

Bank-erosion processes also require a transporting mechanism to oarry

away the eroded material. BY shifting the main flow stream away from the
Criteria for the select ion of a specific spur type are presented in this bank, the transporting mechanism is removed , Therefore, a channelbank th at
chapter. Th i s includes a discussion of the gener al applicabili ty of spurs , has been weakened by subsurface flowerosion , wave erosion , surface erosion ,
the applicability of each of the major spur types, and a closer look at the chemical act ion , or some other bank-deterioration mechani sm (see F'f1WA,1984)
attributes of individual spur types. will be made less susceptible to total failure.
GEIlERAL APPUc&BILITY OF SPURS River Environaent
Spurs are defined as permeable or impermeable linear structures th at Spur-type structures have been used successfully in a wide variety of
project into the channel for the purpose of altering flow direction, inducing channel environments. The channel environment plays more of a role in the
deposition, and/or reducing flow velocities along a channelbank. Spurs can design and selection of a specific type of spur or ot her countercneasure than
be classified as permeable or impermeable. They can be further classified by it does in dictating the use of a gener al countermeasure type or group; this
functional type as retardance-type structures, retardance/di ver ter will be illustrated in later sections. Some general comments, however. can
structures, and diverter structures. Retardance and retardance/diverter be made concerning channel st ze , bend radius, and bank characteristlcs as
structures are permeable structures; diverter structures are impermeable. they relate to the use of spurs.
Retardance spur s are designed to reduce the flow velocity in the vicinity of
the bank as a means of protecting the channelbank. Retardance/diverter Channel Size
structures produce a flow retardance along the channelbank, but they al so
produce a deflection of flow currents away from the bank. Diverter spurs, on Spur-type structures are not well-suited for use on small-width (less
the other hand, function by diverting the primary flow currents away from the than 150 feet) channels. On these narrow-width channels, spur design orten
channelbank. will create excessive flow constriction at high streamflows and cause current
deflections towards the opposite bank. Also, the excess channel constriction
F... ction can cause greater channelbed scour than other countermeasure types that do
not cause flow constriction. Deeper, more expensi ve foundations would be
The functions or purposes for which spur-type structures are best suited required to protect the flow structure from undermining caused by the exces.
include protecting an existing bank-Hne, reestabl1shing some previous flow bed scour , Spurs can be used effectively, however, on small channels where
path or alignment, and controlling or constricting channel flows. These their funct 10n is to shift the location of the channel. In these cases,
functions or pur poses are discussed in detail in FHWA(198Q), The primary there usually is sufficient area available sa that excessive flow
advantage of spurs over other countermeasure types is their ability to constriction is not a problem.
provide flow control and constriction as well as the reestablishment of a
previous or new flowpath. While spur s also are effective at streambank Bend Radius
stabilization and protection in general, other countermeasure types can
provide equivalent or perhaps better protection against gener al bank erosion The use of spur-type structures for flow control and bank stabilization
CFHWA, 1984). on short-radius bends (less than 350 feet) is usually not cost effective when
compared to other countermeasure types. This is due to the short interspur
Erosion Hechanis.s spacing that would be required. Also, short-radius bends are typically found
on channels having small w1dths; the consequences of using spurs on smaU
Erosion mechani sms that can cause streambank fallures are discussed in channels has already been discussed.
FHWA(1984). The erosion mechanism countered best by spurs is bank-particle
displacement caused by abrasion and streamflow-induced shear stresses. Spurs

3
Channelbank Characteristics Environmental Impacts

Channelbank characteristics related to the use of spur s include bank Environmental impacts include impacts on channel geometry, water
height, bank configuration, and bank vegetation. Spurs are best suited for quality. and biology.
the protection of 10101- (Le ss than 10 ft) to medium-height (from 10 to 20 ft)
banks from the erosion mechanisms discussed above. Protecting high banks Changes in channel geometry caused by channelbank stabilization are
with spurs of ten requires special design considerations and/or excess discussed in detail in FHWA(1984); discussions of the channel deepening that
structural material. However, spur s that have successfully protected high occurs in stabilized channelbends also are included. In channelbend.
channelbanks have been designed (see Figure 22a for example). stabilized with spur=type st.ruct.ur es , this channel deepening can be
magnified, particularly at the spur head. There are two reasons for this.
Bank configuration refers to the geometry of the bank. Because, in most First, spur schemes naturally constrict river flows in channelbends. In an
cases, spur s do notrequire extensive bank reshaping or grading prior to attempt to maintain its previous level of discharge or flow conveyance,
construction, they are well-suited for use along steep-cut banks where further scour Lng of the channelbed occur s , In addi ti on, flow concentration
significant site preparation would be required for other cauntermeasure types at the spur head results in severe scour holes at and just downstream of the
(see FHWA, 1984). Also, the use of spurs is not adversely affected by spurs. This channel reshaping has been documented both at field sites (Brice
irregular bank lines. Again, spur use is recommended along irregularly et al., 1978; Littlejohn, 1969; Fenwiek, 1966) and in laboratory studies
shaped banks because excessive bank preparation and reshaping is not required (FHWA,1983; Ahmad, 1951a and b, and 1953; Franco, 1966).
to produce a smooth alignment around the bend.
The location of the scour trough discussed above provides another point
One advantage in the use of spur-type structures is th at they have been of comparison between spurs and other countermeasure types. Because spur s
observed to provide an enhancing influence on bank vegetation. The erosive shift the flow current away from the bank, they a1so shift the scour trough
action of currents impinging directly on the bank wi11 of ten prevent or away from the bank, thus removing the immediate danger from undermining away
hinder the natural volunteering of plant materials down the bank. Since from the bank. Streambank-stabilization schemes that have their primary
spurs shift these main flow currents away from the bank. a greater component parallel to the channelbank (i.e., revetments, retardance
opportunity exists far the natural volunteering of vegetation down the bank structures, longitudinal dikes, and bulkheads) must be designed to proteet
and into the .. spur zone," helping to stabilize both the upper and lower against undermining along the entire length of the bank, adding significantly
sections of the channelbank. In environments characterized by high sediment to the cost of the stabilization schemes. Because only the riverward ends of
loads, the vegetation will usually volunteer to the berm deposited between spur-type structures are impacted by the scour trough, only localized
the spurs, enhancing the stabilizing characteristics of the spur scheme. In protection at the spur heads is required. Also, the risk of a catastrophic
low sediment-yield environments, the reduced flow veloeities within the spur failure of the entire stabilizatian scheme as a result of toe erosion and
zone create a more acceptable environment for vegetative growth, therefore undermining is lower with spurs than with other structure types because on1y
allowing the advance of vegetative materials down the bank and into this zone the ends of the spur are impacted at any gi ven time. Fai I ure of the spur
during low-flow periods. Again, the additional vegetative growth thus head still leaves additional spur length to provide partial protection for
created will enhance bank stabilization and help counter the lack of a the bank until repairs can be made.
deposited sediment berm in 10101 sediment-yield environments. It al so helps
minimize the bank-scalloping characteristic of impermeable' spur Several factors will affect the magnitude of the channel reshaping just
installations . The development of thick vegetation on the banks and between discussed. First, the more severe the channel constriction, the more
spurs also prov ides a mechanism for flow retardance and energy dissipation pronounced the resulting channel scour patterns will beo The channelbed
for spur-topping flow conditions, further enhancing bank stabilization. Bank composition also plays a role in the magnitude of these erosion patterns;
vegetation also enhances the appearance of the bank by presenting a more channels cut in silt- and sand-size materials will exhibit greater depths and
natural-looking bankline. extents of erosion than channels in gravel- and cobble-size materiais. Since
impermeable spur s have a greater constricting effect on channel flows than
System Impacts permeable spur s , the erosion patterns produced by impermeable spurs can be
expected to be more severe (assuming similar channel environments).
The general impaots of stabilizing a channelbend are discussed in FHWA
(1974) in terms of channel morpho10gy. The impact produced by Impacts on channel geometry can a1so result from incorrect design and/or
bank-stabilization schemes was also mentioned as a countermeasure selection construct ion of the spur scheme. The geometrie layout of the scheme is of
criterion in FHWA(1984). The system impacts produced by spur-type flow primary importance. Misalignment of spurs can cause severe flow deflection
control and bank-stabilization structures can be classified as environmental and could initiate an erosion problem on the opposite bank. Figure 1
and esthetic. illustrates a case in point. The timber-pile spur shown was designed with a
projected length (length perpendicular to the flow line) of 50 percent of the
channel width. The resulting flow deflection has severely eroded the

5 6
 than many other countermeasures, these impacts will be minimized if spurs are
used,

Biological impacts can be broadly categorized as either terrestrial or

aquatic, The major terrestrial impact is related to the alteration or
elimination of riparian zone vegetation due to construction of project
features, The riparian zone can provide support to a wide variety of plant
and animal life and of ten provides a critical habitat for certain species.
Riparian vegetation also supports aquatic species by providing a habi tat and
food-chain input for these species. Again, since these activitles are
primarily associated with construction activities, they' are temporary in
nature and are minimized through the use of spurs. In fact, spur schemes
have been found to enhance the aquatic environment along the bank because of
the flow retardance they produce near the bank.

Esthetic Impacts

Esthetic impacts relate to the appearance of the project area. These

impacts are discussed in detail in FHWA(1984). Esthetic considerations
relate more to the selection of a specific spur type than to the general
applicabili ty of spur-type structures. In this regard, comments relati ng to
esthetics will be made when discussing individual spur types. Several
general comments, however, can be made relating to the potential hazards
FIGURE1. TIMBER-PILESPURSHOWINGTHE
associated with the use of spur schemes.
IMPACTOF EXCESSIVE
FLOWDEFLECTION.
The hazards associated with spur schemes are related to recreational use
of the r iver , The potential hazard spur-type structures can pose to boaters
is of primary concern. Besides obstructing flow, spur s can also obstruct
opposite channelbank as shown. Also, if the spur s produce too much flow boats. Smal! boats can be pinned broadside along these structures,
constriction. excessive channel deepening may occur, which can undermine and particularly the permeable spur types, if flows are below the spur crest.
cause the eventual failure of the spur structures. Time delays between Also, when the spurs are just submerged, they can be hidden obstacles to
initial design surveys and construction can also result in a final spur power boats. To avoid these hazards, adequate warning signs should be posted
configuration whose geometrie layout does not coincide with existing flow to alert boaters and other recreational users to the potential hazard.
conditions. The U.S. Army Corps of EngIneer s (1981) has dccumenteö several
cases where changes in stream pattern occurred between the time the initial Spurs can also pose hazards in other recreational us es of a river, such
design survey was conducted and construct ion was started. The shifting as swimming and fishing. The hazards discussed above for boats also apply to
stream pattern resulted 1n a final spur configuration that was not compatible people if they are swimming or fishing in the water around the structures.
with flow conditions af ter the scheme was constructed. The potential impacts In urban areas, there is also a potential hazard to children who might find
resulting from inappropriate spur-scheme layouts are the most significant spur s attractive structures to play on or around. In gener al , permeable
drawbacks to the use of spur-type flow-control and bank-stabilization spurs and spur structures with sharp or pointed edges create a greater hazard
structures. The geometrie layout of spur schemes is a more critical design than impermeable spurs. It is recommended that spur s not be used in areas
consideration for spur-type structures than for other countermeasure types. that are heavily used for recreational activities.
This points out the need for careful and efficient planning, design, and
construct ion of spur schemes. Construction-Related Consideratlons
Water-quality impacts result from changes in turbidity together with
Construction-related factors influencing the choice of a countermeasure
alteration of the local riverine habitat. The primary impacts are the
type include:
increased turbidity and stripping of bank vegetation dur Lng construction.
These activities can affect stream temperature and photosynthetic activities
• required acceas and right of way,
that in turn may affect algae or aquatic plant populations, dissolved oxygen,
and other water-quali ty parameters. These are usually temporary impacts.
Also, since the construct ion of spur schemes produces less bank disturbance • extent of bank disturbance,
 • required construction methods, and
• DIVERTER SPURS
• local availability of construction materials. -handpoints
-transverse dike spurs
Spurs provide an advantage in two of these areas. First, spurs
generally require less construction right-of-way than revetments and other Common spur types from within these functional groups were i11ustrated in
countermeasures because they do not necess1tate bank grading or extensive Figures 2 through 14. Additional descriptions of the more common spur types
bank reshaping/rebuilding. Also, construction of spur s produces less bank within each of these groups wi11 be given below. The spur designs listed
disturbance during construction than other flow-control and bank- below are based on typical designs that have been used in the past. lIany
stabilization countermeasures, thus producing less of an environmental impact design variations of these spurs are possible using different materials and
on the channel during construction. The minimum bank disturbance created by configurations.
the construction of spurs will also minimize the susceptibility of bank
material to loss caused by exposure of the bank surface during high-flow Retardance Spurs
periods.
As mentioned previously. retardance spur s are designed to reduce the
eosts flow velocity in the vicinity of the channelbank or over the region of
influence of the spur scheme. Retardance spur s are very similar in design
A cost analysis and comparison of the most commontypes of flow control and function to the gener al countermeasure classification of retardance
and streambank-stabilization structures is presented in FHWA(1984). This structures as described in FHWA(1984). The primary difference 1s that
comparison indicates that spur-type structures wil 1 of ten provide a retardance spurs are designed with their primary structural component
significant economie advantage over other countermeasure types for flow perpendicular instead of parallel to the channelbank. Retardance 'pur, are
control and bank-stabilization purposes • This has been found to be further classified as fen ce-type and jack/tetrahedron spurs.
particularly true where long reaches of gently curving meanders need to be
stabilized. Spurs have also been found to prov1de a significant economie Fence Type
advantage where flow-control and/or flow realignment are the primary
purpose(s) of the bank-stabilization scheme. The significant economie The most common fence-type retardance spur is the Henson 'pur jetty,
advantage that can be realized through the use of spurs is of ten the deciding which is illustrated in Figure 2. A typical design sketch of a Henson spur
factor in the selection of a spur scheme over some other countermeasure. jetty is illustrated in Figure 15(a). Henson spurs are constructed of
individual wood-fence panelS mounted on steel-pipe piles or posts. The fence
The data presented in FHWA(1984) indicate spur costs ranging fr om sections are typically constructed of 2-inch by 8-inch treated wood slats
$13/ft to $445/ft, with an average of $56.2/ft (1982 dollars). This cost mounted vertically to a frame on 18-inch centers. Individual fence units can
var iance reflects the di versi ty of the spur designs available, as well as vary in size depending on the specific application. but they are typically 20
site-specific costs such as channel environment, required si te preparation, to 30 feet in length. The fence units, consisting of two pipe piles and one
etc. Cost data for individual spur types will be presented in later fence panel, are then used in multiples to make up the spur structure. One
sections. Note th at all cost data reported herein have been adjusted to 1982 jetty can con,1st of any number of fen ce panels. The fence panelS are
dollars. mounted to be movable in the vertical direction and r i g i d in the lateral
direction. The purpose of the free-floating design is to allow the struéture
SPUB TYPES to flex or shift with the channel bottom to maintain contact with the
channelbed during flow events that would otherwise scour under the fence
A wide variety of spur types are available. Spurs are classified by uni ts. This is parti cularly important in channels hav ing regime/low
functional type as retardance spurs, retardance/diverter spurs, and diverter threshold sediment environments. The design and function (vertical
spur.. Retardance and retardance/diverter structures fall into the flexibility) of these structures are patented by Hold That River Inc. under
permeable-spur category; diverter structures are impermeable. Spurs within U.S. Patent No. 3,333,320. A similar wood-fence retardance spur design was
each of these categories can be further categorized by material and reported by the eOE (1978). The primary difference is that this design is
construction type as follows: fixed rigidly in the vertical direction. This design alternative is
illustrated in Figure 15(b). Another spur type similar in function to the
• RETARDANCE SPURS Henson spur (vertical flexibility) is marketed by the Ercon Corporation;
-fen ce type (wood or wire) patents are pending for this design. This structure is referred to as a
• RETARDANCE/DIVERTER SPURS
-light fen ce (wood or wire)
-heavy di ver ter

9 10
 FIGURE 4. WOOD-FENCE SPUR; BATUPAN BOGUE, GRENADA, MISSI3SIPPI.
FIGURE 2. HENSON TYPE SPUR JETTY; BARZOS RIVER
NEAR ROSHARON, TEXAS.

FrGURE 5. WIRE FENCE SPURS.

FrGURE 3. TETRAHEDRON SPURS; SAN BENITO RIVER, CALIFORNIA. (AFTER CALIFORNIA DEPT. OF PUBLIC WORKS, 1970)
(AFTER CALIFORNIA DEPT. OF PUBLrC WORKS, 1970)

1?

11
FIGURE 6. DOUBLE-ROW TIMBER PILE AND WIRE-FENCE SPUR. FIGURE 8. STEEL PILE/WELDED WIRE MESH SPUR;
(AFTER CALIFORNIA DEPT. OF PUBLIC WORKS, 1970) LOGAN CREEK NEAR PENDER, NEBRASKA.
(AFTER BRICE ET AL., 1978)

FIGURE 7. WELDED-WIRE AND STEEL H-PILE PERMEABLE SPUR;

ELKHORN RIVER AT SR-32 At WEST POINT, NEBRASKA. FrGURE 9. TIMBER PILE SPURS; BIG BLACK RIVER AT DURANT,
(AFTER BRICE ET AL., 1978) MISSISSIPPI.

13 14
FIGURE 10. TIMBER PILE/SUSPENDED LOG SPURS; ELKHORN RIVER WEST FIGURE 12. ROCK RIPRAP SPUR; LOYALSOCK CREEK
OF ARLINGTON, NEBRASKA. NEAR MONTOURSVILLE. PA. (COURTESY, PENNSYLVANIA DEPT.
OF TRANSPORTATION. DISTRICT 3-0)

FIGURE 11. TIMBER PILE AND HORIZONTAL WOOD PLANK DIVERTER.

STRUCTURE (AFTER BRICE ET AL., 1978) FIGURE 13. GABION SPUR; LOYALSOCK CREEK NEAR
LOYALSOCKVILLE. PA.

15
 WOOD ..... U
,TR"'UDI

,.1

FIGURE 1~. CRIB SPURS. (AFTER CALIFORNIA DEPARTMENT

Of PUBLIC WORKS. 1970)

ELEVATION
ELEYATION
,., Ccl

FIGURE 15. FENCE-TYPE RETARDANCE SPURS. (A) HENSON TYPE.

(B) RIGID-WOOD FENCE TYPE (C) CHAINLINK FENCE TYPE.

17 18
pa.l i s ade and hasa net section made of strapping material that is supported
by steel-pipe piles instead of the wood-fence unit. Additional variations on
the fenee-type retardance spurs are also possible; for example. using
ehainlink panels or other materiais. A rigid chainlink design is shown in
Figure 15c. Chainlink panels that are vertically flexible could also be
used ,

Fence-type retardance spurs are typically placed perpendicular to the

channelbank to be protected, forming a flow retardance zone along the toe of
the channelbank. A typical layout for a Henson-type retardance spur scheme
is illustrated in Figure 16.

JacklTetrahedron Type

Jack and tetrahedron units have also been used to form retardanee
spur s , The basic structural units of these spur s , the jacks and
tetrahedrons, are illustrated in Figure 17; part (a) illustr~tes a jaek; part
(b) i11 ustrates a tetrahedron. These structural units are skeletal frames
adaptable to permeable spur s by tying a number of similar units together in
longitudinal alignments. Cables are used to tie the units together and
ancnor key units to deadmen. Struts and wires are added to the basic frames ICAU
as needed to increase impedance to flow (either directly by their own 1000 o 1000 1000 FT
resistance or indirectly by the debris they collect). Flgure 3 illustrates a
typical tetrahedron spur unit. The basic frame of the Jack [see Figure 17
(a) ] is a triaxial assembly of three mutually perpendicular bars acting as .IGURE 16. HEKSOK
SPURJETTYLAYOUT
ONREDRIVERAT PEROT,LA.
six canti lever legs from their central connection. Besides the
steel-membered jack illustrated, concrete jacks have also been used. The
tetrahedron frame [see Figure 17 (b)] is assembied from six equal members,
three forming the triangular base and the others the three faces sloping
upward from the base to an apex. Like other permeable spurs, jacks and
tetrahedrons rely primarily on flow retardance and sediment deposition as
their primary bank-protection mechanism. Various jack and tetrahedron
designs have been patented in the past; the current status of these patents
is unknown.

As mentioned above, jack and tetrahedron

retardance spurs by stringi ng them together
units are used to form
with cables to form the spur
~ ......
system. Figure 18 illustrates a typical layout detail for tetrahedron
spurs. A similar configuration would be used for j ack spurs. ITIIUII liDI

PLAN VIEW
BetardancelDlverter Spurs

As mentioned previously, retardance/diverter spurs are permeable

structures that are designed to function by retarding flow currents along the
ELI!VATION
channelbank and providing flow deflection. This combination of functions
makes them the most versatile of all spur types. Retardance/diverter spurs
have been further classified as light fence structures and heavy di ver ter (., (bI
structures. These classifications generally separate the retardance/diverter
structures by size and degree of permeability. In general, the light fence FIGURE17. STEELJACKANDTETRAHEDRON
DETAILS.
structures are smaller and more permeable than the heavy di verter (A) STEELJACKDETAILS,(B) STEELTETRAHEDRON
DETAILS.
structures. Retardance/diverter spurs are generally oriented with a
downstream angle to enhance their flow-diversion qualities.

19
 ~ CIIOWN._W'DTN '_T?N!_FOU_NDATION 1
tYP,CAL LAYOUT ~ ".
WI1I8 O •••••••• 14,
T,.• .cAI,

_:_·&lhA::::i;·""-......
aT"A. IItOI

P'AIitT PLA"

FIGURE la. LAYOUT DETAILS OF TETRAHEDRON SPURS.

Light Fence Type r- 'TIIUC. AZ. LINE

TI ... t:1I PILt:

A variety of both wood and wire or chainlink structures have been used

-
as light-fence type retardanoe/diverter structures. Figures through 6
illustrate the three most typical designs: a wood-fence spur. a light-link or
wire-fence structure, and a double-row timber pile and wire-fence structure. FLOW

Figure 19 illustrates a typical design sketch of a wood fence type

structure. In this particular design the vertical supports are timber piles,
and the horizontal members are 3-inch by a-inch planks. Note how the
structure is braeed to provide additional strength against flow currents and
that a stone foundation is used to reslst undermining and to provide a key to
tie the structure to the channelbank.

Figures 20 and 21 illustrate design sketches for two wire-fence

I
retardance/diverter spurs. In Figüre 20, a l1ght-duty wire fence structure
is shown. This design conslsts of a wire mesh supported by vertical pipe
posts, with pipes used as horizontal and diagonal bracing. Figure 21 shows a
.
I
I
I
II :I
timber-pile wire fence structure. Timber piles are used as the vertical
l......
)
support members in this design with 8-inch by 8-inch timbers used as
horizontal bracing. Again, a wire-mesh screen is attached to this structural
frame. Although both figures show double-row structures, both single and END VIEW
double-row configurations have been used. The double-row configuration has
been much more successful than the single-row design because of the FIGURE 19. TYPICAL DESIGN SKETCH OF WOOD-fEN CE SPUR.
additional structural rigidity and flow retardance provided by the second
row. To provide protection against undermining, the entire fence screening
is usuaH y extended below the channe Lbed . Also, the structure is usually
designed to extend into the channelbank to prevent outflanking.

22
21
 Heavy Diverter Spurs

Heavy diverter spurs are illustrated in Figures 7 through 11; steel pile
and welded wire-mesh spurs and numerous timber-pile designs are detailed.

Two steel-pile and welded-wire mesh spur s are illustrated in Ffgur es

and 8. Typical design sketches for these structures are given in Figure 22.
These structures are the most permeable of the permeable di verter
structures. They are constructed by su.spending a wire-mesh or welded-wire
fabric on a support frame of steel "I" or "H" beams. Other materials such as
IIIUIM flLL OJITIONAL timber piles could. be used for the support frame. Part (a) of Figure 22
UJllnlAIl illustrates a structural design that has been used for the protection of high
WIIII IIIIM liDI channelbanks; part (b) illustrates a design for lower channelbanks. In both
design configurations a triple-pile header is used to provlde sufficient
structural rigidity to the spur head to resist damage from large nosting
debris. Here again, the welded-wire mesh is extended to below the channelbed
~ a to minimize underscouring, and the strueture is extended into the channelbank
to prevent outflanking.
ELEVATION IECTION
Figures 9 and 10 illustrate two timber-pile spurs. Timber piles are the
FIGURE 20. DETAILS OF LIGHT-FE~CE-TYPE SPUR, basic component of most permeable diverter structures designed. Single piles
or pile clumps (three or more piles to a clump) constitute the basic
construct ion unit for these structures. Timber-pile spurs of various designs
have been used including single piles in line, single piles .taggered, .ingle
piles in multiple rows, single and multiple rows of pile clumps, and
staggered rows of pile cl umps. Both single piles and pi Ie cl umps have been
spaced at various distances to provide various degrees of permeability. Rows
of piles or pile clumps are then usually braeed with planks or additional
piles,

--tte'7f
~81r:_ Figure 23 [(a) through (c») illustrates design sketches for three
timber-pile spur designs. The design illustrated in Figure 23 (a) consists
of three pile clusters joined by horizontal timber-pile stringers lashed to
the vertical pile clusters. As mentioned above, single or multiple rovs of
U w ~j LJ w••• 'iJ"N Ol ., •• .., W.I.
piIe clusters and stringers can be used, depending on the needs of individual
sites; up to three rows have been used in the past. An al ternate design is
PART ILIYATIOII
illustrated in Figure 23 (b). This design consists of alternate single
vertical piles straddling a single horizontal-pile stringer. This design is
commonly used by the eOE on large rivers to provide flow constriction for
n~vigational purposes. The design is also applicable for bank-stabilization
application". Figure 23 (c) illustrates another timber-pile structure. Th1s
design uses wldely-spaced vertical piles with trees slashed to the hor1zontal
~ .' ~ stringers to reduce the structure's permeability.
~ W'I ..... Ol •••• D Willi
U... T.... II 1101 Another retardance/diverter spur using timber piles for the vert10al
support structure are horizontal wood-plank struetures. Figure 11
PART PLAN l11ustrates one aueh structure. As is the case w1th other spur types, many
design variations are possible for pile and horizontal-plank .truetures.
FIGURE 21. TIHBER-PILE AND WIRE-MESH SPUR. Figure 24 shows a typieal design sketch for the spur illustrated in Figure
11. This design uses a double row of timber piles as vertieal supports.

23
 )~9:9.·:

(.1

(b'

--~ WILDID WIIII IIIIH

~Ar~
(bI (0'

rIGURE 22. TYPICAL DESIGN SKETCHES FOR STEEL-PILE AND WIRE-MESH FIGURE 23. TYPICAL DESIGN Or TIMBER PILE DIVERTER SPURS
SPURS (A) HIGH BANK DESIGN, (B) LOW BANK DESIGN. (A) DESIGN SKETCH rOR PILE CLUSTER SPUR
(B) DESIGN SKETCH FOR DOUBLE-ROW, SINGLE PILE SPUR
(C) DESIGN SKETCH rOR TIMBER PILE SPUR WITH SLASHED TREES.

25 26
 retardance/diverter structures discussed abovel. The two primary
subclassifications of diverter structures are hardpoints and transverse-dike
spur s , The primary difference between these two types of di vert.e r "pur. is
the structure's length.

Hardpoints

Hardpoints are short structures that extend only a limited distance

outward from the channelbank, and have a slight downstream orientation.
Their primary function is to protect an existing bankline; by definition,
they are not long enough to be used for flow oontrol or realignment, or to
provide flow constriction. Figure 25 illustrates a typical hardpoint
design. The designs shown are constructed of d unped riprap; however, gabion
designs could also be used. Hardpoints are made up of two parts; a spur
section and a root section. The spur section functions as the hard point and
ii deflects flow currents away from the channelbank. The root section extends
into the channelbank to help anchor the structure to the bank and prevent
outflanking dur i ng high flows. Rock hardpoints are particularly well-suited
I 11 for use on narrow channels because they do not create any signi fi cant flow

~ I
obstruction •

Transverse-Dike Spurs

Transverse-dike spurs are the most widely used impermeable spurs. These
structures are most commonly constructed of dumped rock riprap. Where rock
FIGURE24. TYPICALDESIGNSKETCHFORTIHBER-PILEANDWOOD of sufficient ai ze is not available, however, gabion and er tb designs have
PLANKRETARDANCE/DIVERTER
SPUR. also been used. Sheet-pile. asphalt, and ooncrete spurs have also been
designed. The cost of these structures will be prohibitive in most cases.

Transverse-dike spurs are similar to the rock hardpoints described above

Four by eight diagonal and horizontal bracing is used between the two except that the spur section is longer in length. In general , transverse
rows. Horizontal four by eight timbers are also used as horizontal sheathing dikes will extend into the stream past the point where the highest velooities
on the upstream face of the upstream row of pil es • In this particular occur , Their function is to move the thalweg from i ts posi tion along an
design, pole screening is used on the upstream face of the downstream row of eroding bank to a more favorable alignment. Transverse-dike spurs are
piles. Other designs use the downstream row of piles for bracing and do not illustrated in Figures 26 through 29
include a facing material.
Figure 26 shows a riprap-dike design. These structures can be
As is the case for other retardance/deflector spurs, the structural constructed using a uniform stone gradation, or with a smal! rock or earth
members of these structures should be well anchored to the channelbank to core surrounded with a larger rock facing. The stone used on the exterior of
pre vent outflanking and should be extended below the channelbed for a the structure must be of sufficient size to resist the erosive action of
sufficient distance so that they will not be undermined by looal scour. river floW5. Where stone of a size large enough to resist the erosive foroes
in a river is not available, a gabion or crib design can be used.
Diverter Spurs
A typlcal gabion 5pur structure is illustrated in Figure 27. Gabions are
Diver ter spurs are impermeable struotures that are designed to function compartmented rectangular containers made of galvanized steel hexagonal wire
by di verting the primary flow currents away from the channelbank. Several mesh and filled with stone. A typical gabion detail is illustrated in Figure
diverter spur s were illustrated in Figures 12 through 14 Diverter spurs 27. Individual gabion baskets are then stacked, wired together, and filled to
are most commonly construoted of dumped riprap "inoe it is almost universally form the spur structure. Note the base mat used in the design to support the
available and economieal. Furthermore. constructing spurs with this material spur structure; this mat helps to proteet the structure from fallure caused
is relati vely easy. Diverter spurs have a130 been constructed using gabion by undermining from local scour.
and cr ib designs. To enhance their flow-diversion qual1 ties, di ver ter spur s
are usually constructed with a downstream orientation (as are the

27
 (.1

-=:-~ -AI
TON. PLL

TYPICAL lECTION

HARD POINT .YITE ..

FIGURE 25. TYPICAL ROCK HARDPOINT DESIGNS. PERIPECTIVE OF GAllON OllOIN TYPICAL SECTION
(bi (ol

FIGURE 27. TYPICAL DESIGN DRAWING FOR GABION

TRANSVERSE DIKE SPUR.

~
ELEVATION
~
ITitl •• "DI WuI. M'IM 0 .. '''''110 WIIII

PART PLAN

-___ .1, Y LI 'ECTION

u U
AOJUIT"'LIOII
",.ID I"IKIT
WITM ROCIt PILt.

PLAN .EeTION

FIGURE 28. TYPICAL DESIGN SKETCH FOR WIRE CRIB DESIGN.

FIGURE 26. TYPICAL DESIGN SKETCH FOR DUMPED RIPRAP
TRANSVERSE DIKE SPUR.

29 30
 • bend radius/flow alignment, and

• ice and debris conditions.

Consideration of these factors provides guidance for the select ion of an

appropriate functional spur type. It is important to remember that the
factors listed are often interrelated, and it is their combined effect or the
total environment that must be considered when designlng a bank-protection
scheme.

Table 1 has been constructed to aid in the selection of en appropriate

spur type for a given situation. In Table 1, the primary factors influencing
the selection of a specific spur type are listed across the top, and the
primary spur types are evaluated in terms of those selection criteria. A
scale from 1 to 5 is used in the table to indicate a specifio spur type's
applicability for the given condition. A value of 1 indicates a disadvantege
in using thet spur type for the given condition. and a value of 5 indicates a
definite advantage in using that spur type. Table 1 is designed to be a
FIGURE 29. SKETCH OF RECTANGULAR TIMSER ROCK-FILL CRIS SPUR. design aid for selecting a spur type. The table can be used by summing the
values for the specific site conditions along horizontal lines. The spur
type having the highest sum would ideally be the best for the given
situation. It is not advisable, however, to seleot only one spur type from
A typical crib design is illustrated in Figure 28. This design is identical this table. Several of the better spurs should be selected for more detailed
to the double-row timber pile and wire-fence retardance/deflector spur consideration based on other factors such as cost, availability of materials,
illustrated in Figure 21 except that the space between the fences is filled maintenance requirements, structure impacts, etc.
with stone. Other double-row fence designs could be converted to impermeable
diverter spurs by adding rock fill as weIl. Other cr1b designs could also be The following discussions provide general guidance regarding the manner
used, such as the tïmber crib 11lustrated in Figure 29. Of significant in which the primary spur selection criteria affect the selection and design
importance to crib-spur design is the security of the base of the crib from of various spur types.
loss of the fill material upon scour along the base of the structure. The
structure should be extended to a sufficient depth below the channelbed, and Spur FunctiOlllPurpose
a sufficient volume of rock should be used in environments where local scour
might threaten the stability of the structure. Flow-control and/or bank-stabilization schemes are generally constructed
to function in one of the following capacitles:
As with the hardpoint designs discussed above, all the transverse-dike
spurs mentioned should be designed with a root section to anchor the • to proteet an existing bankline,
structure to the channelbank to prevent outflanking.
• to reestablish some previous flow alignment. and
PRIMlJlY FAC10IS IJtFLUElICIIIG TUE DESIGN AJU) SELECfION OF A SPUI TYPE
• to provide flow constriction.
There are numerous factors that influence the selection of a specific
spur type for a given streambank-stabilization situation. However, aix Combinations of the above functions are also possible.
primary factors have been identified. These include:
Retardance-type spurs are usually light structures designed to reduce
• spur function or purpose, the flow velocity in the vicinity of the channelbank. As suoh, they are best
suited for protecting an existing bank line. They are not as well-suited for
• erosion mechanism countered, either of the other functions mentioned, although wire-fence and
jack/tetrahedron-typespurs have been used to reestablish some prevfous flow
• sediment environment, alignment where only a minor shift in flow orientation ls neoessary.

• flow environment,

31
 TABLE 1. SPUR TYPE SELECTION TABLE. Erosion Heehanism

Erosion mechanisms countered by spur-type flow-con trol and

streambank-stabilization structures are:
EROSION SEDIMENT no" BENO lCE/DEBRIS

.~ ~ g
SPUR TYPE FUNCTION
!1ECHAIHSH ENVIRONMENT tNVIRONMENT RADIUS ENVIRONMENT
-e
• transport by streamflow,
~j =
." ·"·"
• "0
...~~ 0 ... VELOCln STA.GE

Ol ~
c 0
, ,~ displacement at the toe of the bank
,
.
~ ·
·. ~ , ~
~
. ~"
• particle

... a ~... ..• ...~

Jl E " • particle displacement along mlddie and upper bank by
. . H~ ;;

.'" ~ .
.0
C c
~
·
0 0 streamflow-induced shear stresses, and
" , c ! 3 ~ j M

.. 0

" .:: ~ ..." .c'" '" :i! .c .:! 11

'" ~
.c
S
-e
.c
;j !I'
"
)
.3
-e .":l" -e
:l!
M

'"
.c
.; ~ j s"" • partlcle displacement by abrasion.

RETARDANCE Combinations of these mechanisms are also possible. Detailed descriptions of

Pence Type ) 2 2 ) )' I I 4 ) 2 ) ) 2 ) 2 I ) 2 I ) ) 2 each of these mechanisms are presented in FHWA(198~).
Jack/Tetrahedron 3 3 1 33 1 1 4 3 I 3 2 I ) 2 1 ) 2 I 2 4 I
REt ARDANCE/DEFLECTOR

Light rence 3 3 3 3 ) 2 2 ) ) 2 3 ) 2 )
, 42
) 3 3 2 ) ,, 2 A sediaent-transporting lIleehani_ must be present for erosion to occur.
This mechanism is provided by the flowing water. All spur types will
H.avy utver eer 3 4 4 3 ) 4 3 2 3 3 3 3 2 3 3 J 2 ) 3
effectively counter this mechanism by retarding and/or deflecting the

,,
streamflow currents in the vicinity of the bank erosion. However, under some
nEFLECTOR

Hardpoirtt 3 ,, ,, 33 J 4 2 3 ,
4 3 3 ,, ) 3 2 ) 4 3
)
3
)
5 medium to high flow-velocity environments, some of the more permeable
Tn.n&ver.e Dike 3 ) 3 3 4 2 ) ) ) 1 J 2 ) J 5
retardance and retardance/diverter spurs will not provide sufficient flow
retardance to reduce flow velocities below the critical transport level.
Welded wire mesh (Figure 23), other wire-fence spurs (Figures 15, 20, and
'Henoon 'po, Je"' .. ilO nted a Î to' thio <on"juon 21), and jack and tetrahedron designs (Figure 17) are examples of structures
that might not provide sufficient flow retardance in some flow environments.

Part iele dlsplae_ent at tbe toe of the strealllbank caused by

1. Definite diaadvantast' to the use of this type sr ruc tue e . streamflow-induced shear stresses can also be countered by most of the spur
2. Somedhadvant8ge ec ene use of this type at ruc ture . types identified, as long as ot her conditions (to be discussed below) are
J. Adequate for condition.
4. seee advant.ge to the use cf th is type arrucrure . met. Again, the vehicles used are flow deflection and/or flow retardance.
5. Significant advantase to thl! uae of this type st.ruc tur e . As is the case with the transport mechan1sm, however, the more permeable
retardance and retardance/diverter structures m1ght not provide sufficient
flow retardance in some high-flow veloc1 ties to resist eros ion caused by
Retardance/deflector str.uctures have been used effectively for all three streamflow-induced shear stresses.
functions or pur poses l1sted. As is the case with retardance structures,
retardance/diverter structures function by producing a flow retardance along Part iele displac_ent on the middle or upper portions of the streambank
the channelbank. They are also designed to produce a diversion of flows. caused by streamflow-induced shear stresses can be best countered through the
The heavier diverter-type retardance/deflector spurs have been found to use of the larger retardance/deflector or deflector-type spurs.
prov1de an advantage over other types of permeable. structures .where flow Retardance-type structures will usually oply provide protection to the toe of
constriction and/or the reestablishment of some pr ev i ous flow allgnment are the streambank, and therefore, are not effective for upper-bank protection.
primary concerns. Some of the larger retardance/deflector structures provide some advantage in
this area, especially if moderate to high banks need to be protected. One
Impermeable deflector spurs function by deflecting the main flow current design particularly adaptable to protecting middle and upper portiens of the
away from the bank. Like retardance/deflector spurs, they have been found to channelbank is the steel-pile and wire-mesh spur illustrated in Figu·re 22(a).
provide an advantage where flow constriction and/or the reestabliShment. of
some new or previous flow alignment is desired. They are also as eff~ctlve lbrasion occurs when solid materiaIs, sueh as debris and iee, earried by
as other spur types when the primary function is to proteet an ex i s t i ng the flowing water collide with and dislodge surface soil particles.
bank-Hne. Countering streambank erosion caused by abrasion requires a spur that
provides flow deflection and will not be significantly damaged by the agent
causing the abrasion.. For these applications. the impermeable deflector

33 34
structures have two significant advantages over other spur types. First, adjacent to the bank. This is important in cases where erosion resulting
impermeable diverter spurs function by deflecting currents and any floating from bank-weakening mechanisms (wave erosion , subsurface flow and dral nage ,
debris away from the channelbank. Impermeable structures also have more etc.) is occurring. As discussed previously, Henson-type spur s provide a
structural mass than most permeable structures and, therefore, are subject to particular advantage in these highly dynamic environments because of thei r
less damage from floating debris. The light retardance structures have a vertical flexibility. Other fence-type structures will also function well.
history of being severely damaged by floating debris. This is because of Jack and tetrahedron structures have also been quite effective in these
their small at ze and the fact that permeable structures will become cIogged environments except where there are high-flow velocities. In high-velocity
with floating debris, increasing the hydraulic forces on the structure. environments the jacks and tetrahedrons do not provide sufficient flow
Therefore, these structures should not be used , Retardance/deflector spurs retardance and are of ten lost to scour.
are designed to deflect flow currents, as are the impermeable deflector
spurs. Their permeability, however, makes them debris skimmers like the Permeable retardance/deflector spur s have al so performed well in
retardance structures. The light fence retardance/diverters are prone to regime/low-threshold channels. Because of their flow deflection
damage from the floating debris and therefore, are not recommended. However, characteristics, however, they are bet ter sut ted for medium-threshold
some of the heav ier retardance/di ver ter structures have been found to be environments. This is particularly true of the larger heavy diverter
effective at resisting abrasion forces. structures. Local scour problems associated with these larger structures
have resulted in struatural undermining in some cases when they are used in
regime/low-threshold environments.

When discussing a spur's effectiveness in a given sediment environment, The above discussion is not meant to imply that permeable spur s should
it is appropriate to refer to spurs as either permeable or impermeable. not be used on channels that do not carry large sediment loads. Tn some
Referring to the classification scheme outlined above, retardance spur e and cases, the flow retardance produced by the spur scheme can be designed to
retardance/deflector spurs are permeable, and deflector spurs are provide the desired level of bank protection. This is particularly true of
impermeable. permeable retardance/deflector structures. These structures are designed to
function as flow deflectors as weIl as retardance structures. Permeable
Both permeable and impermeable spurs have been used in a wide range of retardance spur s and the light fence retardance/deflector structures are not
sediment environments. Sediment environments (or channelbed conditions) can suited as well for use on high threshold/rigid channels.
be defined as regime, threshold, or rigid. For purposes of identifying an
appropriate spur type, the sediment environment can be classified as Impermeable deflector spur s are best sui ted for use on high
regime/low threshold, medium threshold, or high threshold/rigid. A regime threshold/rigid channels. They have been used effecti vely, however, in some
channel is one whose bed is in motion under virtually all channel-flow regime and low-threshold environments. There are several drawbacks that make
conditions. Low threshold channels are those channels whose channelbeds are impermeable deflector spur s Less acceptable than permeable spur s in truly
in motion under all but some very low flow conditions. Therefore, regime/low alluvial channels (regime/low threshold and some medium-threshold
threshold environments are characterized by large suspended and bed-sediment environments). In tru1y alluvial environments, impermeable di ver ter spur s
loads under most flow conditions. These channels are typically cut through will cause sediment deposition along the channelbank in 8 similar fashion 8S
noncohesi ve sand- and silt-size materials . Medium threshold channels are permeable structures. However, this deposition will be to a much lesser
typically cut through sand- and gravel-s1ze materials whose channelbeds are degree than with permeable structures. The primary source of deposition
mobile for moderate and high channel-flow condit Icns , Channels cut through between impermeable spurs is from spur-topping flows. These flow conditions
cohesive materials can also be considered medium threshold. High have been observed to carry significant amounts of suspended material into
threshold/rigid channels are typically cut through larger gravel-, cObble-, zones between spur s , where it is then deposited as a result of the lower
and boulder-size materials. These materials will remain stable or rigid transport capacity between spur s , Another sour ce of sediment for deposition
under most flow conditions, but will become mobile during high flows. cornes from suspended materials carried into the interspaces by the expansion
of flow as it passes the spur tips. Again, this material deposits due to the
Permeable spur s are best suited for regime/low threshold and medium low transporting capacity of the currents between spurs. It 1s important to
threshold environments. Permeable retardance spurs have been found to be keep in mind that the amount of deposition that can be expected between
particularly effecti ve in regime/low threshold environments. In fact, they impermeable spurs is less than that induced by permeable structures.
generally provide an advantage over other spur types in these environments.
The flow retardance created by retardance spur schemes creates a depositional When using impermeable deflector structures in alluvial environments lt
environment within the retarded flow zone along the channelbank for the is important to recognize the potentially detrimental impacts they aan have.
suspended and bed-sediment Loads carried by these channels. This produces a Flow concentration and 10ca1 scour are primary smong these impacts. flow
sediment berm or bench that will stabilize the base of the channelbank. concentration is inherent in impermeable spur design. A oonsequence of the
Also, by lowering flow velocities in this zone, permeable retardance spur flow-constricting effect produced by spurs is a concentration of flow 11nes
schemes will reduce or el1minate the transporting ability of channel flows along the riverward tip of each spur. The flow concentration in this area

35
results in a magnified potential for erosion of the channelbed in the Channel Velocity Environment
vicinity and just downstream of the tip of the impermeable structures. This
condi tion is much more pronounced in high-veloci ty environments and around The applicabili ty of various spur types with respect to the channel' s
sharp bends than it is in low-velocity environments and around mild bends. velocity environment is in many ways related to the channel's sediment
The occurrence of significant erosion at and downstream from the spur tip has environment. It is the interaction of the flow environment and the channel's
been obser ved by the authors at numerous field si tes and is well documented bed-material constituency that determines the sediment-transport environment
in reported laboratory studies (FHWA, 1983; Ahmad, 1951a and 1951b). Local of a particular stream. The channel-flow velocity is also related to the
scour is a primary concern in alluvial environments because of the highly s i ze and structural integr i ty of a spur , Generall y, the larger and more
erosive nature of the gravel-, sand-, and silt-size material comprising the rigid the spur scheme, the better its adaptability to the more severe flow
channelbed. The potential for excessi ve erosion at the scour tip, combined environments.
with the high cost of providing protection against the erosion is a drawback
in the use of impermeable diverter spurs in alluvial environments. As discussed above , retardance spurs are best suited for regime and
low-threshold sediment environments. Within these environments. however ,
The flow concentration and local scour conditions just described are retardance spurs have not been successful in high-velocity environments. or
characteristic of impermeable installations in all river environments. In some of the higher medium-veloci ty environments. In these environments, the
high threshold/rigid channels (those cut through large gravel- and cobble- retardance spurs generally do not provide sufficient flow retardance and are
size materiaIs) ; however, these conditions pose less of a threat to the of ten undermined or outflanked due to the dynamic nature of the channelbed
stability of impermeable spur schemes. Flow concentration at the spur tip combined with the high flow veloeities. This has been found to be
will still cause eros ion in these environments. Because of the low particularly true for jack and tetrahedron structures. Jack and tetrahedron
transportability of the coarse materials making up the channeIbed , and the designs should not be used in the higher medium- or high-veloci ty
natural channelbed armoring that occurs in these environments, however, i t environments. Retardance spurs are also smaller and less structurally rigid
will be of a much smaller magnitude. In most cases, onlya limited amount of than other spur types and, therefore, are more susceptible to structural
erosion (in comparison with truly alluvial environments) will occur. This damage in high-velocity environments than other types of spurs.
can usually be anticipated and adeQuately designed at little addltional
cost. Because of their permeability to flow, retardance/deflector spurs are
also subject to undermining and outflanking in high- velocityenvironments.
Flow Envlrooaent
However. because they divert channel flows and provide flow retardance, they
have been effective in higher velocity environments than retardance spurs .
The channel-flow environment includes consideration of both channel-flow Retardance/deflector spurs are also more structurally r i g i d than retardance
velocities Bnd flow stage. Consideration of channel-flow velocities includes spurs, and therefore, can withstand higher flow forces. However, the
both the magnitude of the velocity, as weIl as the freQuency of occurrence of extremely permeable retardance/ diverter spurs (such as the welded wire mesh
a specified flow velocity. For classification purposes, channel flow structures illustrated in Figure 22) should not be used in the higher medium-
velocities will be clBssified as low. medium, and high. Low-velocity and high-veloci ty environments because they wi 11 not prov lde sufficient flow
environments are defined as those where the dominant or controlling flow retardance.
veloeities are less than four feet per second , Medium velocity environments
are defined as those where the dominant or controlling flow velocities are Deflector spurs have been found to be effective over the widest range of
greater than four feet per second but less than eight feet per setond. flow conditions. Because of their structural rigidity, impermeable deflector
High-velocity environments are defined as those where the dominant or spur s are the least susceptible to damage in high-veloci ty environments of
controlling flow velocities are greater than eight feet per second. The any of the spur types. For this reason they are generally considered to be
f'r equency of ocourrence is reflected in the terms "dominant or controlling." applicable for low-, medium-, and high-velocity environments. It must be
The dominant or controlling velocities are those primarily responsible for remembered, however, that they are subject to limitations in regime and
the erosion process. In one si tuation these veloci ties might be associated low-threshold sediment environments.
with normal low-flow conditions. In Bnother situation the dominant or
controlling velocities might be associated only with extreme flow events. Flow Stage
Flow stage can be classified in terms simllar to flow velocity. A low Flow stage must be considered in light of the height of bank to be
flow stage will be considered to be one where the dominant or controlling protected. For example, if the primary cause of eros ion to be protected
flow stage is less than 10 feet. A medium flow stage is one where the against occurs at low stages (as defined above ) , or affects only the lower
dominant or controlling flow stage is greater than 10 feet but less than 18 portions of the channelbank, then spurs suitable for low-stage conditions
feet. A high flow stage is one where the dominant or controlling flow stage
should be used. Conversely, if the primary cause of erosion occurs at high
is greater than eighteen 18 feet.
stages. or impacts upper portions of high banks , spurs suited for countering
high flow stages should be used.

37 38
 retardance/deflector spurs have been used effectively on both large- and
As indicated in Table 1, all of the major spur types are suited for use medium-radius channelbends. Because of their permeability, however, they
under low stage conditions. Under medium stage conditions, retardance spurs have not been as effecti ve as impermeable deflector spur s on smaU-radius
are at a sl1ght disadvantage because at this point some of the outflanking channelbends. As indicated in Table 1, impermeable deflector spur s provide
characteristics discussed above have been observed, However, this an advantage over other spur types on both medium and small channelbends.
disadvantage can be overcome in some cases by increasing the structure height Thi.s is primarlly due to their capacity as positive flow-oontrol structures.
and ensuring that the retardance-spur structures are adequately tied to the On extremely smaU radius bends (bend radii less than 350 feet) , the larger
channelbank to prevent or minimize the potential for outflanking. Although transverse-dike impermeable structures will cause eXcessive flow constriction
spur-type structures are generally not well-suited to protecting against high and scour problems th at will make them unacceptable. Impermeable hard point
stage conditions, some large retardance/deflector spurs have been found to be spurs have however, been used effectively on some channelbends less than 350
adaptable to these conditions. This is due to their structural design feet in radius because they do not cause a significant flow obstruction.
carrying up and into the channelbank. For example, see Figure 22(a).
Debris and Ice-Load EDyir~nt
Another stage consideration is the impact produced by spur-topping flow
stages. As the flow stage reaches and exceeds the spur crest, a zone of Debris and iee-load environments are defined in Table as minimal
magnified flow turbulence is created just downstream of the spur structure debris, light debris, and large debris and ice. Minimal debrls refers to
along the channelbank (FHWA, 1983). This zone of flow turbulence can cause flow environments that rarely carry ice or debris of any size. Light debris
accelerated streambank erosion between individual spur s , particularly 1f the rerers to the flow environments thet typically carry debrls loads of smallor
channelbank between spur s is not well vegetated or protected with a light lightweight materiel. Lerge debris and iee refer to large braneh- and tree-
layer of riprap or some other revetment. The laboratory studies conducted size material, as weIl as significant ice loadings.
for FHWA(1983) indicate that this is primarily a problem with impermeable
spur s . Because permeable spur s allow flow to pass through the structure, Debris and ice-load environments affect the function as well as the
there is very little additional disturbance as the spur crest is exceeded. stability of spurs. Retardance spurs function best when there is light
However, this is not the case with impermeable structures. As the stage debris present to reduee the permeability of the structures and enhance their
exceeds the crest elevation of impermeeble structures, a high level of flow-retardance qual1ties. However, large debris and ice will damage these
turbulence is generated on the downstream side of the structure as the flow light structures and render them ineffective. This is particularly true of
passes over the structure and into the zone between spurs. It has been the wire-fence and jaek/tetrahedron designs. The wire-fence and jack/
observed that the greater the spur angle in the downstream direction , the tetrahedron designs have also been found to be less effective than ot her spur
greater the generated turbulence. The implication is that spurs should be types in minimal debris environments. Without light debris to clog partially
designed with cr est elevations that should not be exceeded frequently, If or block the structural frames of some of these structures. they do not
th is is not practical, an impermeable structure should be used. provide sufficient flow retardance to protect the channelbank adequately.
Bend Radius Retardanee/deflector spur s have been used suceessfully in most debris
and ice environments. Like retardance spur s , the presence of light debrls
The radius of the channelbend to be protected is another factor that enhances the effectiveness of retardance/deflector spurs and makes them
must be considered when selecting a spur type. Channelbend radii csn be particularly adaptable 'to environments where light debris is present.
classified as small, medium, and large. These definitions correspond to Because of their flow-deflection qualities, these structures have al so been
channelbend radii greater than 350 feet but less than 800 feet, greater than moderately effecti ve in minimal debris environments. The large structural
800 feet but tess thsn 2000 feet, snd greater than 2000 feet, respectively. size of heavy diverter spurs makes this type of retardance/diverter
Spur-type structures are not well-sui ted for use on small channels having acceptable in large debris and ice environments as well. However, some of
channelbend radii less than 350 feet. Therefore, the small channelbend the lighter fence-type retardance/diverters are susceptible to extensive
category is limi ted to channels having radii greater than 350 feet but less damage in environments characterized by large debris and ice.
than 800 fe et .
Impermeable deflector spurs have been used effectively in all categories
The degree of bend curvature required or desired is directly of debris and ice environments. They provide a significant advantage,
proportional to the level or intensity of flow control needed to eliminate or however, over other spur types in large debris end ice environments.
minimize the streambank erosion. As is indicated in Table 1, the more Impermeable deflector spurs divert much of the floating debris instead of
passi ve, permeable retardance structures perform as well as other spur types skimming it from the surface as do permeable structures. AIso, their
on large-radius channelbends. This statement can be extended to include some structural mass makes them less susceptible to damage than the lighter
of the larger medium-radius bends as well. However, smaller radius bends permeable structures. This does not, however, imply that they will not be
require a more positive flow control , and retardance-type spur s become less damaged by floating debris, only th at the damage will be less severe.
acceptable. Because of their flow-deflection qualities, permeable

39
OTHEB CONSIDER1TIOIS eee r
...
(1'''001'1

The selection criteria discussed above are by no means the only factors
...
that should be considered when selecting a type of spur for a specific site.
Other considerations include:

• costs,

• channel size, .OAIIIID " .... Cl!

ITII&.
WIL ••••
PIU AIID
,,,.
.. ..
TI••• II PlLI
• channelbed fluctuations,

• vegetation, .","RAP
11........
MAIIDPO'M"
DI'4' •• '11I
_
_

OUIO.' DI""'III _
• vandalism and maintenance,

• construction-related impacts,

• channel geometry impacts, and COlT fI"NO~

"",IRAOe: COlT ."'10 0111LlMITID "1.,O,,TaD DATA

• aesthetics.
FIGURE30. SPURCOSTCOMPARISON.
Channel geometry impacts, aesthet1cs, and construction-related impacts were (ALLCOSTSREPORTEDIN 1982 DOLLARS)
discussed under "Environmental Impacts" earlier in this Chapter. Each of the
rema1ning items will be discussed briefly below. Of these, structure costs
have the most significant impact on the ultimate selection of an appropriate
spur type. Cost data for individual spur types are presented in Figure 30. cost
data for spur installations are not readily available; in many cases, no cost
Costs records are kept for spur installations. In other cases where cost data are
available, they are reported as a lump sum along with other items such as
The fina1 cost of a spur scheme wil! be dependent on many factors bridge-repair costs. For these reasons, co st data are not available for many
including, but not limited to: spur types. Also, the data that are available usually are biased by the
specific design requirements of the sites for which they were designed. The
• the spur type and specific design. following in format ion on spur costs should only be used as a rough guide in
any cost anal ys i s . The actual cost of a spur scheme should be based on the
• channel size and bank height, specific design being considered and the local cost of required construction
activities and materials. All cost data have been adjusted to a 1982 base
• hydraulic condition", using Engineering News Record's average annual construction eost index.
Also, all costs are reported as dollars per foot of bank protected.
• right-of-way cost",
The only retardance spur for which reported cost data were availab1e was
• site-preparation requirements, the Henson spur jetty. illustrated in Figures 2 and 15. The cos t e reported
ranged from $110/foot to $380/foot. All sites where eosts were reported were
• local labor and material costs, on medium-width channels with medium to high banks. Also, they all had
moderate ehannelbend radii. However, all Henson spur installations consist
• maintenance casts, etc. of the same components aod protect only lower portion" of the bank.
Therefore, bank height is not a significant cons1deration. The component
primarl1y responsible for the cost varianee reported was spur spacing.
Spacings reported ranged from ~O to 100 reet. Costs reported for sites
having "pur spac i ngs from ~O to 50 feet ranged from $3DO/foot to $380/foot:

42

41
at the other end of the scale, schemes having 100-foot spacings had reported high channelbank heights. Also, co st data were not reported for larger
costs in the neighborhood of $110/foot to $150/foot·. Although less structures. Cost data for large riprap diverter structures ranged from
expensive, the schemes designed with 100-foot spacings have not been as $50/foot to $226/foot. Here again, a major factor reflected in the cost
effective at stabilizing channelbanks as the 40- to 50-foot spacings. range is the spur length and spacing.
Cost data were found for four of the retardance/diverter spur s , Data Channel Size
for the board-fence structures (similar to those illustrated in Figures 4 and
19) were reported by the U.S. Corps of Engineers (1981). Five installations Channel st ze considerations were discussed earlier in this chapter in
were reported having an average cost of $51/foot. These structures were on relation to the applicability of spurs in general. It was stated that spurs
small- to medium-width channels with medium-height banks and mild are generally unacceptable for use on small or narrow-width channels (widths
channelbends. They were constructed at 100-foot spacings and had lengths of less than 150 feet). In general, this is true. Several spur types, however,
approximately 25 feet. have been used effectively on some of the larger narrow-width ohannels. The
spur types that have been used effect i vely on narrow-width channels include
The other retardance/diverter structures for which cost data were the smaller permeable fen ce structures and rock hard points . Aotually, any
available were all heavy diverter structures. Two steel-pi Ie and welded-wire spur that can be designed only to produce a minimal flow constriotion (less
fence structures were documented on the Soldier River by Brice et al. (1978) than 10 to 15 percent of the channel width) could be used. However, spurs
(see figures 22(a) and 8). The average reported cost for these structures was should not be used at sharp bends on narrow channels.
$230/foot. The Soldier River is a medium-width channel with medium to high
channelbanks. The structures were placed on meandering channelbends. Cbannelbed Fluctuatiou3
Structure length was about 110 feet with a interspur spacing of 110 feet.
These structures are designed to proteet the entire bank height. The streambed elevation of alluvial channels is known to fluctuate as a
result of local scour, general scour, dynamic scour, and aggradation and
Cost data were also available for several timber-pile degradation processes. In truly alluvial regime/low-threshold channels,
retardance/deflector spur a. The costs ranged from $295/foot to $445/foot. these processes occur continually and can cause extreme fluctuations in
These structures were all on medium-width channels with medium to high channelbed surface levels. Channelbed surface level fluctuations caused by
channelbanks and were on moderate channelbends. Spur spacing ranged from 130 one or more of the above-mentioned actions have been a primary cause of
reet to 450 feet; spur lengths ranged from 55 feet to 150 feet. The two structural undermining and fallure of spur-type structures as well as other
designs for which cost data were available were plle structures with timber structures constructed in ri ver environments. Spur structures designed for
piles as horizontal members (see Figures 23 end 9), and timber-pile use in alluvial environments must be designed to contend with these bed-level
structures with wood-plank sheathing as horizontal members (see Figures 24 fluctuations. Henson spur jetties (see Figures 2 and 15) are particularly
and 11). The cost of the timber-pile struoture with horizontal-pile stringers adaptable to these environments. This is due to the vertical flexi bi 11ty of
was $445/ft.; the average cost of the timber-pile struoture with wood-plank the fence panels. As discussed previously, these panels shift downward with
sheathing as horizontal members was $332.50/foot. the bed profile. This allows them to maintain contact with the channelbed at
all times so that the retardance structure is not undermined. Thus, the toe
Cost data were also available for diverter spur s , Costs for riprap of the channelbank remains stabie even under severe bed-scour condi tions.
hardpoints (see Figure 25) ranged from $13/foot to $110/foot. The primary Other permeable spurs are designed to counter undermining by extending the
factor affecting the reported costs is hardpoint spacing, which is dependent spur' s retardance structure (wire or wood facing) for a distance below the
on channelbend radius. Other factors influencing the cost of these channelbed. This is sufficient in many cases, except where the anticipated
structures are site preparation and bank height. The low end of the reported scour depth is underestimated. Extending the retardance structure to below
range was for hardpoints spaeed at 100 feet and having lengths of 68 feet. the channelbed is also costly in many cases because of the extra excavation
The $110/foot hardpoints were designed with 100-foot lengths, spaeed at 40 that is required. This is particularly true if the si te is underwater. To
feet on mild channelbends in channels having large widths and medium bank. A avoid the need to extend the permeable facing below the channelbed, many
comparison of these costs indioates that hard point spacing is one of the permeable structures, particularly the retardance di verter structures, are
important design parameters that must be defined. designed with a rock toe or blanket to proteet them against undermining fr om
local scour , Impermeable diverter spurs can be designed with extra
Casts for bath gabion and riprap diverter structures were reported. The structural mass (rock volume) to armor the channelbed in the vicinity of the
cost.s reported for gabion spur installations (see Figures 22 and 13) ranged spur to proteet it against undermining.
from $32/foot to $126/foot. The low end of the scale was for 10-foot long
spur s in a small channel with low channelbanks. The higher cost was reported
for 25-foot long spurs on a medium-width channel with low channelbanks. Both
ends of the cost range reported were documented on channels having sharp bend
radii. No cost data were reported on channels having mild bends or medium to

43
Yegetation

The existence or lack of channelbank vegetation is another environmental

characteristic that should be considered dur Lng the design of spur schemes.
The advantages of bank vegetation were discussed in general earlier in this
chapter. As mentioned, in areas where significant bank vegetation exists, Cbapter 3
this vegetation will usually volunteer to the bank and into the "spur zone"
helping to stabilize both the upper and lower sections of the channelbank. DESIGN OF SPUR SYSTEKS

In regard to the selection of a specific spur type, it should be noted

th at when impermeable diverter structures are used in environments lacking
channelbank vegetation, severe bank scalloping has been observed between the The previous chapter discussed at length considerations important to the
spur structures. This scalloping has been known to outflank spur s , leaving selection and design of spurs. In this chapter criteria for the design of
them unattached to the channelbank. Environments lacking bank vegetation are spur systems will be presented; criteria for spur permeability. geometry, and
usually located in arid regions of the country where most riverbeds are cut structure height will be presented first, followed by general comments on
through all uvial materials . In these environments, permeable retardance or spur-crest profile, bed and bank contact, and spur-head form.
retardance/diverter structures should be used.
The criteria presented here are based in part on a recent laboratory
Vandalls- and MaiDteDaDce investigation of spur-type structures conducted by FHWA (1983). The
laboratory report produced as a result of this study is available for
Vandalism, particularly in urban areas, is a problem that must be dealt interested researchers . However, i t contains li ttle information beyond what
with when designing spurs as well as other bank-protection schemes. Both the is presented here that would be useful to the design engineer.
U.S. Army Corps of Engineers (1981) and Keeley (1911) document cases of
vandal ism. Vandal1sm can render ineffecti ve a technically effecti ve PERMEABnITY
bank-protection scheme. Vandals' efforts include dismantling; burning;
cutting with knives hatchets, and axes; etc. Ir vandalism is determined to Considerations of spur permeability were discussed in relation to the
be an important co~sideration, steps can be taken to reduce the vandals' selection of an appropriate spur type (retardance structure,
chances of succeeding. For example, steel structural members could be used retardance/diverter structure, diverter structure) in the last chapter.
instead of wood, or the wood could be treated to eliminate or minimize the However, for both the retardance and retardance/diverter s t r uct ures , a
possibil1 ty of burning. Also, other structural types that are less variety of spur permeabilities can be and have been designed. Spur
susceptible to vandalism could be used, such as rock riprap structures. permeabili ty as referred to in this report is defined as the percentage of
the spur I 5 surface area that is open or unobstructed. In environments where
Maintenance requirements also must be considered. Virtually all the permeable structure can be reasonably assumed not to clog with floating
streambank protection schemes require some degree of maintenance. The need debris or other material, the determination of a particular spur's
to repair a bank stabilization structure can result from vandalism or damage permeability only requires computation of the unobstructed flow area within
from excessive hydraulic conditions and/or ice and debris conditions. In the structure. In most environments, however, the spur f s effecti ve
general, the greater the structural integrity of the spur, the less permeabili ty will be reduced as floating debris clogs the face of the epur ,
susceptible it is to adverse flow and debris condition.. However, the An estimate of the amount of spur clogging that will occur must be considered
dynamic nature of rivers makes it virtually impossible to predict all 1n the computation of a given spur's permeability. The amount of spur
possible combinations of forces to which a bank-stabilization scheme will be clogging that can be expected to occur is difficult to estimate and must in
subject. Also, it is not usually economically justifiable to build most cases be based on experience.
countermeasures that will resist all possible combinations of flow and debris
impingement foroes. Therefore, a regular program of inspection and The magnitude of spur permeabllity appropriate for a given flow control
maintenance is important to ensure economical, efficient, and reliable or channelbank stabilization application is inversely proportional to the
streambank protection. Of course, there will be an associated cost, which magnitude of flow retardance required, the level of flow control desired,
must be considered when evaluating alternative bank-stabilization schemes. and/or the channel bend radius. In all cases. the greater the magnitude of
the variable. the lesser the degree of spur permeability. It is recommended
that where it is necessary to provide a significant reduction in flow
velocity. a high level of flow control. or where the structure is being used
on a sharp bend, the spur's permeability should not exceed 35 percent. Where
each of the above variables is moderate, spur permeabilities up to 50 percent

45 46
are acceptable. In environments where only a mild reduction in velo city is One area of comparison between spurs of different permeabi1ities is the
required, where bank stabilization without a significant amount of flow scour pattern produced downstream of the spur tip. As might be expected, the
control is necessary, or on mildly curving channelbends, spurs having 1aboratory data indicated that the greater the spur permeabili ty, the less
effective permeabilities up to 80 percent have been used effectively. severe the scour pattern downstream of the spur tip. As spur permeabil1ty
However, these high degrees of permeability are not recommended unless increases, the magnitude of scour downstream of the spur decreases s1ightly
experience has shown them to be effective. in size, but more significantly in depth. rigure 31 illustrates the
relationship between spur permeability and scour depth for spurs having
Additional comments can be made regarding specific spur types identified lengths equal, to 20 percent of the channel' s width. As can be seen, the
in Chapter 2 based on their field performance. The permeabili ty of jack and scour depth decreases with increasing spur permeablli ty regardless of the
tetrahedron retardance spurs (see F1gure 11) 15 set by their design. The "pur angle to flow. rigure 31 also illustrates that impermeable spurs
permeability of these structures is generally greater than 80 percent. produce the greatest change in scour elevation over a given range of "pur
However. because of their high level of permeability, they do not provide angles, indicating a greater variabili ty of local scour at the spur tip for
sufficient flow retardance on their own to be effective as bank-stabilization the range of spur angles tested. Similar trends were also observed for other
or flow-control structures. Where they have been effect! ve, it has been spur lengths. Therefore, if an important design consideration is to minimize
because they have trapped a sufficient volume of light floating debr Ls to the size and depth of local scour just downstream of the spur, spur
reduce their effective permeability to an estimated value of approximately 50 permeability should be maximized.
to 60 percent. Thus , i t is recommended that jack and tetrahedron retardance
spurs be used only where it can be reasonably assumed that the structures The type of vertical structural member used in the permeable spur also
will trap a sufficient volume of floating debris to produce an effective has a bearing on the amount of scour produced downstream of the spur tip.
permeability of 60 percent or less. Round-membered verticals produced significantly less scour than square
vertical members, This impl1es that all vertioal structural members should
Henson-type retardance spurs (see rigure 15) are characteristically be round or streamlined to minimize local scour where possible. Here again,
built with a structural permeability of approximately 50 percent. This if minimizing 10ca1 scour depth is an important consideration for a
degree of spur permeabllity has been found to be adequate for most cases particular design, spurs having round or streamlined vert1cal support members
reported. However, in environments characterized by significant volumes of should be used,
large floating debris and high flow veloeities, the reduced permeability
caused by spur clogging often produces hydraulic forces that damage the rlow concentrationat the spur tip is another area of comparison between
structure. In these environments, a greater permeability of the spur spurs of various permeabilities. A dimension1ess velocity, V', defined as
structure should be considered. It is recommended that Henson-type spurs be the ratio of the velocity recorded in the vicinity of the spur tip to the
designed to have an effective permeability of approximately50 percent. average cross section velocity upstream of the spur was used to define flow
concentration at the tip of spurs in the rHWA laboratory investigation. The
A variety of retardance/diverter spurs were documented in Chapter 2 findings indicated that the greater the spur permeabllity, the lower the
(Figures 19 to 24). There was no standard spur permeabil1ty found for any of value of V'. Again, this finding held regardless of spur projected length or
these structures, although most of these structures feIl in the 25 percent to angle. However, the more significant finding was the magnitude of the
50 percent effective permeability range. Exceptions were found in the difference in flow concentration (as measured by V') between impermeable and
lightweight wire and welded wire mesh spurs illustrated in Figures 1 through permeable spurs, F1gure 32 illustrates this difference. Note how the V'
9. which typically had structural permeabilities of 80 percent or more and curve plotted for the impermeable spurs falls significantly higher than those
effective permeabilities of approximately 10 percent. These high- plotted for the permeable apurs, Also, note that the curves plotted for the
permeability structures were used in environments where only a mild reduction permeable spurs fall over a fairly narrow band width. indicating that V I is
in velocity is required, where bank stabilization without a significant Leas sens1tive to changes in spur permeabllity when the degree of
amount of flow control is necessary, or on mildly curving channelbends. In permeability is greater than 35 percent than it is when the degree of
general, the criteria for retardance spurs is as discussed above for permeability is less than 35 percent. Although different in magn1tude,
permeable spurs in general. similar relationshipswere found for other spur angles.

Recent laboratory investigations (FHWA,1983) provide additional insight Additional comments can be made regarding the magnitude of V I found
into how various spur permeabilities impact the behavior of spurs. The during the laboratory studies ror spurs having permeabil1tiesgreater than 35
following is a brief summary of the conclusions and findings from the rHWA percent. Note in rigure 32 that for spur angles greater than 120 degrees and
laboratory investigation relating to spur permeability. This information can permeabilities greater than 35 percent the corresponding vaIues of V' are
be used in conjunction with the information provided above. and the spur-type less than 1. This indicates that the maximum veloeity off the spur tip for
selection criteria presented in the previous chapter to select an appropriate these spurs is less than the average channel velocity upstream of the spur,
spur permeability for a given bank-stabilizationsituation. or that there is very little acceleration of flow around the spur tip for
these spur COnfigurations. Based on thi" information, if minimizing flow

47
 concentration off the spur tip is important to a particular spur design, a
'.0 spur with a permeability greater than 35 percent should be used.

It is important to note that the curves plotted in Figure 32 are based

.. on experimental data collected
lengths
in a straight
equal to 20 percent of the channel width.
flume, for spurs with projected
Similar trends were
~ 0.' observed for other spur lengths. The values of V' reported in the laboratory

·"
0
study are quali tati ve in nature, and are not recommended for field
0 application. Values of VI would be expected to be higher in real

·..·
U
channelbends due to centrifugal acceleration and the natural flow
0.' concentration at the outside of the channelbend in curved channels.
...~
•....
0 Spur permeabili ty was also found to impact the length of bank protected
downstream of the spur. An expansion angle downstream of the spur tip was
•
D 0.' used as a measure of the length of bank protected during the FHWAlaboratory
study. The expansion angle was defined as the angle between a flow tangent
at the spur tip, and a line between the spur tip and a point on the near bank
where the flow has reexpanded to impact the channelbank. This measure of
length of bank protected was used to avoid including the projected spur
O.II±.::.----::I';:O---~.'=O---~.O::----:'.O length parallel to the channelbank in the measure of leng th of bank
projected. Figure 33 illustrates the relationship between spur permeabili ty
and the leng th of bank protected as measured by the expansion angle for spurs
having projected lengths equal to 20 percent of the channel' s width. Figure
FIGURE31. PLOTOF SPURPERHEABILITY
VS. SCOURDEPTH. 33 indicates that the expansion angle 1ncreases with increasing spur
permeabili ty in all instances. This indicates that the more permeable the
spur, the shorter the length of channelbank protected downstream of the spurs
riverward tip. Figure 33 also illustrates that the expansion angle remains
almost constant until a permeability of almost 35 percent is reached. Beyond
this point the expansion angle increases much more rapidly. Similar trends
were found for other spur configurations during the FHWAlaboratory study.
The implication here is that spurs with permeabilities up to approximately 35
percent protect almost the same length of channelbank downstream of the spur
tip as do impermeable spurs; spurs having permeabilities greater than
approximately 35 percent protect shorter lengths of channelbank, and this
length decreases with increasing spur permeability. Relationships for the
length of bank protected for the various spur types will be discussed in the
next section with considerations of spur geometry.

One additional observation from the laboratory studies sponsored by FHWA

relating to spur permeability is the difference in the impact caused by
spur-topping flows. During the laboratory studies, it was found that as the
flow stage exceeds the crest of the spur there is an exces s I ve amount of
turbulence caused in the vicinity of the spur root and immediately downstream
TI-.-.--"""---,-"-,---,-,,,,,,--~--~
0.1
that results in erosion of portions of the upper channelbank in this area,
This bank disturbance was much more evident for the impermeable spurs
investigated than it was for the permeable spurs studied. However, there was
no significant difference observed in this re gard among the various degrees
FIGURE32. SPURANGLE
VS. V'. of permeabil i ty of the permeable spur s tested. The excess flow turbulence
and bank erosion evidenced in the case of the impermeable spur s is caus ed by
acceleration and deceleration of the channel flows as they pass over and down
the downstream face of the impermeable structures (see Figure 34). Because

49 50
 permeable spurs allow a flow equalization on both sides of the structure this
acceleration/deceleration turbulence is only minimal for permeable spurs.
4. Because of the increased
vicinity
potential for erosion of the channelbank
of the spur root and immediately downstream when the flow stage
in the

exceeds the crest of impermeable spur s , it is recomrnended that impermeable

spur s not be used along channelbanks composed of highly erodible material ,

I.
1'0·
unless measures are taken to proteet the channelbank 1n this area.
110·

Ol GEOMETlIY
ol 0.'
0
•c The geometry of a spur system is made up of several components that
•
0 10 produce the spur system' s geometrie form when combined. These components

..·
ii include the longitudinal extent of the spur system, the length of individual
c spur s , the spacing of individual spurs, and the orientation of individual
oe spurs. The longitudinal extent of the spur system describes the length of
Ol

,. channelbank that is to be protected;

individual spurs are self-explanatory.
the length,
In this
spacing,
section,
and orientation
each of these
of

components will be looked at individually and then as a whole to provide

criteria for delineating an appropriate spur geometry.

Extent of Bank Protection

O~O------~IO~------4TO------~'O------~'O
The extent of channelbank protection required on a typical eroding
"U. ' ••• IA.ILITY channelbend has been investigated by several researchers, including Parsons
(1960), Apmann (1972), and the U. S. Army Corps of Engineers (1981). These
FIGURE33. SPUR PERMEABILITY
VS. EXPANSION
ANGLE. investigators, as well as others, have found that a common misconception in
streambank protection is to provide protection too far upstream and not far
enough downstream. The following discussions will consider criteria for
establishing the longitudinal ex tent for bank-stabilization measures.

Criteria for establishing the extent of channelbank protection have been

developed by the U.S. Army Corps of Engineers (1981). These criteria are
based on a series of model studies to define more completely the limits of
bank protection as suggested by Parsons (1960). From these studies, it was
concluded that the minimum di stances for extension of protection are an
upstream di stance of 1.0 channel widths and a downstream di stance of 1.5
channel widths from corresponding reference lines as shown in Figure 35. A
similar criterion for establishing the upstream limit of protection was found
by FHWA(1983); however, a downstream 1imi t of 1.1 times the channel width
was found. The FHWAstudy was not, however, as extensive in this respect as
the COEstudy.

The above criteria are based on analysis of flow conditions in symmetrie

channelbends under ideal laboratory conditions. Real-world condi tlons are
rarely as simplistic. In actu81ity, many site-specific faetors have 8
bearing on the actual length of bank that should be protected. A des igner
will find the above criteria difflcult to apply on mlldly ourving bends or on
channels having irregular, nonsymrnetric bends. Also, other channel con trols
(such as bridge abutments) might already be producing a stabl1izing effect on
FIGURE34. FLOWOVERIMPERMEABLE
SPURS. the bend 50 that only a part of the channelbend needs to be stabilized. In
addition, the magnitude or nature of the flow event might only cause

51
 FIGURE 35. EXTENT OF PROTECTION REQUIRED AROUND
A CHANNELBEND. (AFTER U.S. ARMYCORPS OF ENGINEERS, 1981)

MAIUMUM CUftRENT
THIIUO LOW PLOW -- - - --

MAX'MUM CURftENT _
erosion problems in a very locali zed portion of the bend, again requiring THREAD HIOH FLOW

only that a short channel length be stabilized. Therefore, the above

criteria should only be used as a starting point. Additional analysis of FIGURE36 .. SHIFT IN MAXIMUM
CURRENT
THREAD
WITH
site-specific factors will define the actual ex tent of protection required. CHANGINGSTAGE.

In many cases, the longitudinal extent of the channelbank that should be

protected can be identified through field reconnaissance. If the channel is
actively eroding, the upstream limit of eros ion scars on the channelbank will indicates how these flow patterns change with flow magnitude, flow stage, and
identify the upstream limit of the channelbank that should be protected. It whether or not the flow event is occurring on the r i s Lng or falling limb of
is recommended that any bank-stabilization scheme extend approximately one the runoff hydrograph. Figure 36 illustrates a typical shift in the location
channel width upstream of the point where the bank scars first appear. The of the main flow thread or thaI weg between the low and high flow conditions.
downstream limit of protection is not as easy to de fine . Since the natural The cr i tical eros ion zones for these flow condi t i ons are also indicated.
progression of bank erosion is in the downstream direction, the present Consideration of these critical erosion zones dictates the length of
visual limit of erosion might not define the downstream limit of potential channelbank th at must be protected by a bank-stabilization scheme.
erosion. Additional analysis based on consideration of flow patterns in the
channelbend may be required. Additional analysis is also required if no When establishing the length of channelbank that will be impacted by
defini te erosion scars are present to define the upstream limit of channel flow forces severe enough to cause dislodging and/or transport of
protection. bank material, the first step is to establish the ri ver' s flow paths for
various flow conditions. As illustrated in Figure B-31, this is done by
An important factor In the consideration of. the length of bank to be delineating the main flow paths for several flow conditions. The general
protected is the channel bank length th at will be impacted by channel-flow discussion in FHWA(1984) of flow in channelbends can be used to help
forces severe enough to cause dislodging and/or transport of bank mater i al , determine the locations of the channel' 5 thalweg for various flow stages.
The dynamics of flow in channelbends are covered in detail In FHWA(1984). However, this will probably not provide sufficient information. More
This coverage includes discussions of flow patterns in channelbends and explicit information can be obtained for the low flow condition by conducting
channel surveys during low flow periods. Channel surveys are usually

53
54
impractical dur ing medium to high flow per Iods so that other means must be
used to establish flow conditions for these higher discharges. Some of the
best information available can come from aerial photographs taken of the • review of flow and eros ion forces for various flow-stage
sites under different flow conditions. Additional information can be condi tions.
obtained by flying over the site dur t ng periods of high flow. or observing
the channelbend in question from a vantage point such as a bridge or nearby Information from these approaches should then be combined with personal
hi Ll , Accurate prediction of the location of shifting flow patterns in a judgement end a knowledge and awareness of the flow conditions impacting at
channelbend requires a thorough knowledge of flow processes in channelbends the site to establish the appropriate limits of protectien.
and an understanding of the flow conditions characteristic of the bend in
question. Spur Length
The above analysis will indicate the bank regions impacted by channel Spur length as referred to here is the projected length of the spur
flows under various flow conditions. Not all of these flow conditions, perpendicular to the main flow direction ; it is reported as a percentage of
however, wi11 necessarlly cause bank erosion problems. As discussed the channel width at bank-full stage. Both the projected spur length and the
previously, evidence of the upstream limit of eros ion can usually be channel width used in these computations reflect lengths measured from the
identified by field observations. If no evidence of an initial point of desired channelbank line. On channels having smooth, regular bank lines
erosion can be discerned (ei ther from field investigation or observations these lengths are measured from the actual bank. When the spur s are being
from aerial photographs), other methods must be used , Dne such method is to used to shift the channel to a new location or provide a new smooth alignment
estimate the shear stress in the channelbend for var reus flow conditions. along channelbanks th at have been severely eroded, the actual spur projeoted
Methods for estimati ng shear stress in channe Ibends are presented in FHWA length and the channel width should be measured from the desired bank l1ne
(1984). Comparing the actual shear stresses computed with critical shear and not the actual bank line. In these later cases, the actual spur
stresses Cor the channelbank will define the flow condition for which erosion projected length will be longer than the projected lengths to be recommended
begins. The point where the flow pattern for this critical flow condition here. Actual spur lengths may vary within a spur scheme to ensure th at the
impacts the channelbank would define the upstream limit of bank protection . flow alignment provided lines up to an even curvature.
The downstream limit of channelbank protection would be defined as the
furthest downstream contact point for the design discharge being considered. A review of pertinent literature reveals that available criteria for
Normally, this downstream limit is extended to provide a factor or margin of establishing spur length are very site-specific. For example, Richardson and
safety in the design. Simons (1974) recommend that the minimum length be 50 fe et and the maximum
length be less than 10 to 15 percent of the bank-full channel width on
As indicated previously, the extent of bank protection can also be straight reaches, long radius bends, and braided channels. The 50-foot
influenced by existing channel controls. The most common situation minimum length is based on economie considerations, since the use of shorter
encountered is the existence of a bridge somewhere along the channelbend. If spurs might make it cheaper to riprap the bank. Also, Acheson (1968) reports
the bridge has an abutment immediately adjacent to the channelbank, it wi11 that gabion spurs should extend 20 to 30 feet out from the bank. However,
act as a control point with respect to channel stability. The location of these are rather broad-based statements that do not consider many of the
the br idge abutment (or other channel control such as a rock outerop) wi11 site-specific factors influencing spur length considerations.
usua11y define the downstream limit of the protection r equi r ed , It is rare
that significant erosion will occur downstream of the channel control; The appropriate length of spurs wUhin a bank-stabilization scheme is
however, if the analysis of flow patterns indicates th at excessive erosion dependent on the spur t s behavior in the particular environment, as well as
might occur downstream of the channel control , the protection should extend the desired flow alignment (as discussed abovel. The behavior of specific
beyond the control. spur types was investigated during laboratory studies sponsored by FHWA
(1983). During these studies it was shown that the length of both permeable
The above discussions provide techniques by which the extent of bank and impermeable spurs impaots the local soour depth at the spur tip, the
protection required can be estimated. Due to the uncertainties in the magnitude of flow concentration at the spur tip, the length of channelbank
analytical methods presented, no one of them should be used independently. protected by individual spurs, and the apparent current deflection angle
The recommendation is that the extent of bank protection be evaluated using a caused by the spur s , The relationships between each of these parameters and
variety of techniques including the following: spur length are illustrated in Figure 37. For each of the variables plotted
in Figure 37 (with the exception of the length of channelbank protected), as
• empirical methods, the spur length increases the dependent variabie moves in a direction
indicating a worsening condition with respect to the spur's performance.
• field reconnaissance. Figure 37 illustrates that the length of bank protected mcr eaees with spur
length. The relationships plotted are for spurs of various permeabilit1es
• evaluation of flow traces for var10us flow-stage conditions, and

55
 constructed perpendicular to the main flow current. Similar relationships
were found for spurs having orientations ranging to 150 degrees. The
following is a brief description of the trends illustrated in Figure 37.

In Figure 37(A), a dfmens t onl.e

as scour depth is used to illustrate the
trends between spur length and scour depth. The dimensionless scour
elevation is defined as the depth of scour divided by an arbitrary depth to
unitize the values. As indicated in the figure, as the spur length increases
the scour depth increases. AIso, the figure indicates that aS the spur
length increases, the rate of increase of the scour depth decreases. Thus,
to minimize scour depth, spur length should be minimized.

The dimensionless velocity plotted in Figure 37(B) demonstrates how flow

concentration at the spur tip varies with spur length. The dimensionless
'.a ~.'''''I ••'''att."'' •.r velocïty (V') is defined as the maximum measured velocity in the vicinity of

...
'.r
~,v",v"aL'
... / .. the spur tip divided by the average approach velocity upstream of the spur.

v---".' ,,,,, '.4

Figure 37 (Ilo) indicates that the greater the spur length, the greater the
value of VI (or the greater the magnitude of flow concentration at t ne spur
tip) • Figure 37(B) also indlcates that the greater the spur I 5 permeabill ty,
i
• •.t
'.1 /
~
~ ••••••
......, •••
M
III ••••
ILI'Y
,UT'
the less sensitive the value of V' is to spur length. Therefore, a unit
increase in length for a permeable spur will have less increase in spur-tip

. ..
! .'+--~-~-~-~
CO"".'t:no WIO'" ,.,
.~,+---~--~--~---.
•

cu.
••
....u
",.c,n
I.

Ol' O
, ..
I'

L ••• TM'
••
velocity than will a comparable increase in the leng th of a impermeable
spur.

Another important design parameter is the amount of flow deflection

caused by the spur. !'1gure 37(C) illustrates the impact of spur length on
the flow defleetlon angles produced by various spur types. The flow

i·,..
deflec·tion angle is defined as the angle between the directior. of flow

~_
deflection off the spur tip and the flow tangent at the spur tip measured in
the upstream direction from the former to the latter . As illustrated in the
figure, as the spur Length increases, the flow deflection angle decreases,

··
c .. indicating a steeper cross channel deflection of flow currents.
impermeable spurs are much more sensitive to this parameter than are
Also,

permeable spurs, meaning that a unit increase in the spur I s length has a
~ ...
~~, ••... " LIT...
~ •• " "".'''I'L'TT greater impact on flow deflection angles for lmpermeable spurs than it does
for permeable spurs.
~-'."'''.'.'L'
-----......... ~ ....• ,,, •• "ln.lT\'
•
~
--.....;;._, A.ILlT'
Another important design parameter is t he length of channelbank
....j_-...,...-~-~--.
. 1. ...• protected by individual spurs. To define this relationship, a term length of
....". Lt•• T.
e , 0' e '''V. LI •• '. channelbank protected div i ded by the spur I s projected length (LBP/PL) was
I'" IIT . e'...... C•• T 0' c." •• tL ."'"'
evaluated. The relationship between spur length and LLB/PL is illustrated in
Figure 37(0). The trend illustrated for impermeable spurs indicates that
LBP/PL increases slightly with spur projected length to a maximum of
approximately a 20 percent constricted width, and then decreases. This
FIGURE 37. RELATIONSHIPS BETWEEN SPUR LEMGTH ANO implies that an optimum spur leng th exists at the 20 percent constricted
(A) SCOUR OEPTH, (B) A OIMENSIONLESS SPUR TIP VELOCITY,
width length. The inerease in the value of LBP/PL up to the maximum at 20
(C)FLOW OEFLECTION ANGLE, AND (0) THE LENGTH OF
percent is only minor, however. and does not indicate a significant advantage
CHANNELBANK PROTECTEO 6Y INOIVIOUAL SPURS.
to the 20 percent length over shorter lengths. Oata colleeted from permeable
(RELATIONSHIPS FOR SPURS OF VARIOUS PERMEABILITIES ALL
spur experiments did not indicate a similar maximum. The pe rme ab Le spur
CONSTRUCTEO PERPENOICULAR TO THE MAlM FLOW CURRENT.J
trend indicated is that the greater the spur length. the smaller the relative
length of channelbank protected. Figure 37 (0) also indicates that the value
of LBP/PL remains fairly constant for both permeable and 1mpermeable spurs to

57 58
a .pur length of about 20 percent of the channel's width. Therefore, to thi. Spur Spacing
point there i. a ne ar !inear relationship between the spur leng th and the
length of bank protected by the spur . For spur lengths greater than 20 The spacing of spur s in a bank-protection scheme is a function of spur
percent of the channel's width, LBP/PL drop. off more rapidly indicating th at length, angle, and permeability, as well as the ohannelbend'. degree of
increasing the spur length beyond this point produces less of en increase in curvature (FIIWA, 1983).
length of bank protected. The significanee of this is that a spur having a
leng th not greater than 20 percent of the channel width should be used to Typically, spur spacing has been related to spur length by a spaoing
maximize the length of channelbank protected per unit projected length of the factor, which is the ratio of a spur ' s spacing to i ts projected length.
spur. Although not indicated in the figure, the laboratory data also Values of the spaoing factor reported in the 11terature range from less than
indicate that the greater the spur angle, the more rapid the drop in LBP/PL 1 for retardanoe spurs to 6 for impermeable d1verter spur s , Fenw1ck (1969)
with increasing spur leng th beyond 20 percent of the channel's width. reports spacing ratio values of 2 to 2.5 tor flow constriction applications
(comparable to retardance spur design) on large rivers and a value of 3 for
Evaluation of field sites also provides insight into the determination angled dikes used for bank protection (comparable to retardance diverter and
of an appropriate spur length. A review of field- site data ind1cates th at di ver ter structures). Richardson and Simons (1974) recommend val ues of 1.5
spur projected Lengt hs used at successful spur field installations ranges to 2.0 for retardance-type applications, and 3 to 6 for retardanoe-di verter
fr om 3 percent of the channel width to approximately 30 percent of the and diverter applications. On straight- or large-radius bends, Richardson
channel width. The most common range, however, is 10 to 20 percent. and Simons recommend values of 4 tO 6: values of 3 to 4 are recommended on
Impermeable .purs generally fell in the lower end of this range, with lengths small- to moderate-radius bends. Additionally, Acheson (1968) reoommends a
usually less than 15 percent of the channel width. Permeable spurs were spac1ng factor of 2 to 4, depending on the degree of bend curvature. While
commonly found with lengths up to 20 or 25 percent of the channel width. these recommendations hint at the relationship between spur spacing, the
However, the effective length of permeable spurs is a function of spur spur' s permeability, and the degree of channelbend curvature, they do not
permeability, and only the more permeable structures were effective at the provide defin1te criteria in these respects.
longer lengths.
The recent laboratory investigation sponsored by FHWA(1983) provides
The above discussions indicate that the appropriate length of spurs additional information th at is useful in establishing a criterion for spur
within a given bank-stabilization scheme are dependent on the spur's behavior spacing. In the FHWAstudy, two parameters were used to de fine the length of
in the given environment. This makes the selection of an appropriate spur channelbank protected by individual epur s in a straight flume: the length of
length site-specific. The proper approach is to identify the factors channelbank protected divided by the spur' s projected length (LBP/PL), and
important to the si te (e.g. , Is minimizing the magnitude of flow the flow expansion angle downstream of the spur tip. The results of the FHWA
concentration at the spur tip of greater importance than providing a greater study indicate that the length of channelbank protected by individual spurs
length of protected bank per individualspur?) and select a spur length that is best represented by the flow expansion angle.
appears to provide the best balance between the conflicting criteria. This
will require determining the magnitudes of flow concentration, local scour The flow expansion angle is defined as the angle between a flow tangent
depth, and the length of bank protected for various configurations to see how at the spur tip and a line between the spur tip and the point on the
each varies with spur length at the given site. ehannelbank where the flow reexpands to impact the channelbank. The
definition of expansion angle is illustrated in Figure 38. The results of the
The following general recommendations are given with regard to spur FHWAlaboratory study indicated that for a spur of given permeability, the
length: expansion angle downstream of the spur tip varled only with the spur's
length. Figure 39 illustrates the relationships found between spur length
• The projected length of impermeable spurs should be held to Iess and the expansion angle for various spur permeabilities. As indicated in
than 15 percent of the channel width at bank-full stage. Figure 38, the expansion angle for lmpermeable spurs is almost constant at a
value of 17 degrees. In contrast, the expansion angles for the permeable
• The projected length of impermeable spurs should be held to less spurs were found to increase exponentially with spur projected length.
than 25 percent of the channel width. However, this criterion Additionally, for spur lengths less than approximately 18 percent of the
depends on the magnitude of the spur's permeability. Spurs channel width, spurs having a permeabili ty of 35 peroent produoe
approximately the same expansion angles as impermeable spurs. This ind10ates
having permeabilities of less than 35 percent should be limited
that they proteet approximately the same length of channelbank. Also, as
to projected lengths not to exeeed 15 percent of the channel's
spur permeability lncreases, the length of channelbank protected by the spur
bank-full flow w1dth. Spurs having permeabilities of 80
decreases and is lndicated by an lncreasing flow expansion sngle.
percent should be limited to projected lengths of up to 25
percent of the channel's bank-full flow width. Between these
two limits, a linear relationship between the spur permeability
and spur length should be used ,

59
 The use of an expansion angle as a criterion for establishing spur
spacing (or the length of channelbank protected by an individual spur ) has
several advantages over other criteria, such as the ratio LBP/PL. As
illustrated above, the expansion ang l e is largely dependent only on
permeability and the spur's length perpendicular to the direction of the flow
field. In comparison, the LBP/PL parameter is also dependent on the spur' s
projected length parallel to the channelbank. Also, the value of LBP/PL will
vary with bend radius, whereas a single expansion angle can be applied
regardless of the bend curvature (as will be demonstrated below) . Also, it
was determined from the data coilected during the FHWA study that the
expansion angle is not significantly affected by spur angle as long as the
angle was held to a value of 120 degrees or less. For these reasons , i t is
recommended that an expansion angle be used to define the appropriate spur
spacing. .

Additional information relative to spur spacing was cocument ed during

experiments conducted during the FHWA studies on multiple spur schemes in
PLO. U.I-
meandering channelbends. It was found that the direction and or Lent.at i on of
the channel thaI weg plays a major role in determining an acceptable spacing
between individual spur s in a bank-stabilization scheme. It was found that
the maximum acceptabie spacing between spurs can be determined by projecting
a tangent to the flow thaI weg at and through the spur tip and defining the
location of the next downstream spur by the point where the projected flow
tangent intersects the channelbank on the bend. A simple example of the
cfl.. ICOUlt ".OLI
application of this principle is illustrated in Figure qO. The first step is
to locate the channel thalweg. As discussed previously, the location of the
main flow current or thaI weg in a channelbend shifts with flow stage. This
FIGURE38. DEFINITIONSKETCHOF FLOWEXPANSION
ANGLE. concept was illustrated in Figure 36. For simplicity, the flow thalweg
illustrated in Figure ijQ corresponds to a low-flow condition.

With the channel thalweg located, a tangent to the thalweg at the point
where the bend radius passes through the spur tip (line OR) is drawn (line
AB). This flow tangent is th en projected to the spur tip as illustrated by

.5
!
JO

:::Z= ~. ,., .
line A'B'. The point where this line intersects

the location

designer.
the channelbank (point
defines the location of the root of the next downstream spur .

As illustrated above, the spacing criteria

of the flow thalweg through the bend.
knowledge of flow conditions
Also, since the flow thalweg shifts
1)

are extremely dependent on

Therefore,
in the channelbend will be required
a thorough

with flow stage, consideration

of the

of multiple flow thalwegs is required to establish the appropriate spacing

within a channelbend. The channel thalweg that produces the steepest flow
tangent at the tip of each spur will dictate the spacing between that spur
and the next downstream spur , This implies that different flow thalwegs
(corresponding to different flow-stage conditions) will be critical for spurs
located at different points in the bend. Also, because of the sharp
.±.------~----~----~~----~ curvature of the flow thalweg near the downstream end of the channelbend
•• UII U••TM , .... A _."C •• " 0' CM"..... L WIDTM'
during high flow conditions, these spacing criteria indicate that it will be
necessary to space spur s in the downstream end of the bend closer together.
FIGURE39. RELATIONSHIP
BETWEENSPURLENGTHAND This, in fact, was found to be the case in the FHWAstudies. Also. review of
EXPANSIONANGLEFORSEVERALSPURPERMEABILITIES.

61 62
 ____ THALweo LOCATION

----'LOW TNALWIO

-_ - ......
'" , <,
" ,,
,

(bI

o
FIGURE 41. COMPARISON OF FLOW THALWEGS FOR TWO-SPUR SPACINGS.
FIGURE 40. DEFINITION SKETCH FOR SPUR SPACING CRITERIA.

further recommended that the spacing determined in this fashion (as

field sites where spur schemes have failed indicate that this failure usually illustrated in F"igure 40) be reduced by an amount equal to the expansion
occurs near the downstream end of the scheme, which indicates a need for more angle for that particular spur type, as indicated in Figure 39. Application
concentrated protection in this area. of this spacing concept will be illustrated in a later example.
Several additional comments can be made based on the results of the FHWA Spur OrieDtation
studies. It was found that reducing the spacing between individual spurs to
spacings closer than the maximum indicated by the spacing criteria presented Spur orientation refers to the spurts angle with respeot to the
above resulted in a reduction of locel scour at the spur tips. Reducing the orientation of the main flow ourrent withln the channelbank. Figure 42
spacing between spurs in this way reduces the magnitude of the illustrates the definition of spur angle as used within the context of this
expansion/contractionbetween spurs and as such, minimizes the magnitude of report. Historically, guldelines for spur orientation have been based
flow acceleration at the tip of the downstream spur in each of the two-spur primarily on the personal experience and judgement of design engineers. Spur
sets. Also, it was found that reducing the spacing between spurs caused the angles used at documented spur sites range from 30 to 150 degrees. They are,
stabilized thalweg to shift further away from the concave bank towards the however, typically greater than 90 degrees.
centerline of the channel. This finding is illustrated in Figure 41, which
provides a comparison of the flow thalweg resulting from wide and close Although both permeable and impermeable spurs have been construoted at
spacings of spurs oriented at 120 degrees. These findings indicate that some various angles to flow, permeable spurs should be placed normal to the flow
spacing closer than the maximum reoommended by the spaoing oriteria indloated line unless their purpose is flow diversion. Th1s is an economie
above should be used. consideration. Permeable retardance spurs are usually designed to provide
flow retardance within a given flow zone; therefore, they function equally as
In summary, a spac1ng criteria based on the projection of a tangent to weU in this respect whether they are eonstructed parallel or at an angle to
the flow thalweg and projected off the spur tip is recommended. It ls the flow line. Since spurs normal to the bank provide the shortest

63
 FLOW DI~.CTIOII The primary criterion for establishing an appropriate orientation for
the spurs within a given spur scheme is to provide a scheme that efficiently
and economically guides the flow through the channelbend, while at the same
time protects the channelbank and minimizes the adverse impacts on the
channel system. Meeting these criteria requires consideration of how various
spur angles impact flow patterns around individual spurs, flow concentration
at the spur tip, scour depths at and just downstream of the spur tip, the
length of channelbank proteoted by individual spurs, and flow deflection.

Figure 43 illustrates flow patterns around single impermeable spurs

having angles ranging from 30 to 150 degrees in a straight flurne. Note that
the most abrupt constriction ooours for the spur angled at 90 degrees; the
least abrupt constriotion ocours for the spur angled at 150 degrees,
signifying a milder impaot on channel flows. From the figure, it can also be
seen That spurs angled downstream produce a less severe constriction of flows
than those angled upstream or oriented norm al to flow. Similar findings were
found for permeable spurs during a recent study by FHWA (1983). During the
FHWA study, flow ooncentration at the spur tip was measured using the
parameter V' as described previously. The trend found was for V' to decrease
6- ,pu~ ""OL. with inoreasing spur angle beyond 90 degrees, implying a reduction in flow
concentration and relative flow velocity at the spur tip with increasing spur
angle,
FIGURE 42. DEFINITION SKETCH FOR SPUR ANGLE.
Figure 43 also documents the length of channelbank protected by spurs of
various angles. As indicated, the greater the spur angle, the greater the
length of bank protected. However, as indicated in the last section, the
connection between the bank and the spur head, they are oheaper and should be increase in the length of channelbank proteoted with increasing spur angle is
used where appropriate, Besides being cheaper to construct, spurs equal to the increased projected length of the spür parallel to the
perpendlcular to the bank are less susceptible to damage from wave action. channelbank. Ahmad's findings (illustrated in Figure 43) confirm that the
length of channelbank protected downstream of the spur tip does not vary with
In general, permeable retardance/diverter and impermeable diverter spurs spur angLe , and the flow expansion angle for impermeable spurs is
should be oriented so that they guide flows efficiently through the approximately 11 degrees as found during the FHWA study. The implication is
channelbend while proteoting the channelbank for all the flow conditions to
that spur orientation does not in itself result in a greater leng th of
which they will be subject. There is, however, a difference of opinion as to channelbank protected; it is the greater spur length associated with spur
how this should be accompl1shed. As ment ioned above, spurs typically have oriented at steeper angles that results in the greater leng th of channelbank
been set at angles of 30 to 150 degrees. However, at a symposium on the protected. Thus, the tradeoff between spur orientat ion and length of bank
design of spurs and dikes held at the U.S. Army Engineer Waterways Experiment protected is one of economies; whether it is cheaper to construct a smaller
Station in Vicksburg, Hississippi, it was reported that spurs angled number of spurs at longer lengths, or a greater number of spurs at a shorter
downstream perform better than spurs angled upstream (Pokrefke, 1978). It was length for the spur type being considered must be determined.
also stated that spurs angled upstream are generally not used by the Corps of
Engineers because of their greater resistanee to flow and end scour and The angle of inclination of a spur also affects the magnitude of local
their tendency to accumulate debris and ice. lmpermeable spurs i~ New scour at the spur head. Sinee channelbed scour is determined in large part
Zealand have been designed normal to flow (90 degrees) and at various angles by the magnitude of flow veloeities, it would be expected that the higher the
up to 120 degrees (Acheson, 1968). Acheson also recommends that where spurs flow concentration the greater the local scour in the vieini ty of the spur
are to have a diversionary effect, the spur furthest upstream should have a tip. This is in fact the case. Figure 44 provides a comparison of scour
flat angle to the flow Hne; subsequent spurs should be plaeed at increasing hole patterns at the head of impermeable spurs angled from 30 to 150
angles; the last spur may be nearly at right a~gles to the bank. A similar degrees. This figure, which comes from experimental work done by Ahmad
design was developed by Brown (1919) for stabilization of the Loyalsock Creek (1953) indicates that the area impacted by scour increases sl1ghtly as the
in Pennsylvania using impermeable spurs, and a similar design orientation has orientation moves away from 90 degrees. However, the more important
been used with permeable spurs by the lowa Department of Transportation. indicator here is scour depth. The contours in the figure represent scour
depth divided by initial depth. The figure shows that the maximum scour

65 66
 I.)
ICOUJlOlnN
tNITIÁ~tIo ... nt ) L _

~'~'! ,.
-~
Ib)
,,, ,.,
u

.. -
u

~~ ,/

~
~ "~I
STAGNATION POINT '"
I.)

FIGURE44. LOCALSCOURPATTERNSAT THE TIP OF

IMPERHEABLE
SPURS. (AFTERAHHAD,1953)

STAGNATION POINT depth is inversely proportional to the "pur angle. That is, the smaller the
spur angle, the greater the scour depth. The greatest soour depths oocur for
FIGURE43. FLOWPATTERNSOBSERVED AROUND
SPURS spurs angled upstream; the least loc al "cour is associated with spurs angled
OF DIFFERENTORIENTATIONS. dOllnstream.
(AFTERAHMAD,1953)
Ahmad's findings with respect to scour depth were confirmed during the
recent FHWA study, during which 1t was found that scour depth always
decreases with increasing "pur angle. It was also found that impermeable
spurs produce the greatest change in scour elevation over a g1ven range of
spur angles, indicating the greatest variablli ty of local soour at the spur
tip. Also, this variability in scour depth with spur angle decreases with
decreasing spur permeab11ity. As spur permeab11ity increases beyond 35
percent, 1t was observed that the rate of change of soour elevation with spur
angle and spur length becomes very small, indicatlng that permeable spur" are
not as sensi ti ve to these parameters with regard to the magnitude of looal
scour as are impermeable spurs.

67
 The amount of flow deflection produced by apur s is another factor that
is controlled by the spur' s orientation. Figure 38 provides a defini t i on
sketch of the flow deflection ang l e being disoussed here. It was found
during the FHWA studies that for impermeabie spur s and spur a with
permeabllities up to about 35 percent the deflection ang.le increased with
increasing spur angle. For spurs tested during the FHWAstudy with
permeabilities greater than 70 percent, no change in deflection angle with
changing spur orientation wás found. Flow deflection angles ranged from
approximately 140 degrees to 160 degrees for impermeabie spurs with spur
angles ranging from 90 degrees to 150 degrees. Impermeable spurs with a
permeabili ty of approximately 35 percent had flow deflection
from approximately 130 to 145 degrees ror spurs having angles of 90 degrees
to 150 degrees. These findings were for single spurs in a straight
angles r ang mg

channel.
I--IANK
TOR
t FLOW
DIIIICTION +
~
FLOW
DIRICTION ~

However, because the magnitude of the flow deflection angle will be impacted
by the complex forces affectlng flow in chann~lbends, the actual flow ,al (b)
deflection angies recorded during the FHWAlaboratory study will not reflect
actual flow deflection angles in the field. However, the trends indicated
can be expected to hold. FIGURE 45. FLOW COMPONENTS IN THE.VICINITY OF SPURS
WHENTHEeREST IS SUBMERGEO.
It is interesting th at the flow deflection angles found during the FHWA
study indicate a steeper flow deflection for permeable apur s than for the
impermeable spurs tested. An explanation for this lies in consideration of
Please note also that these comments are based on laboratory findings in
the shape of the ri verward tip of the spur , The impermeable spur s used in
a test channel with highly erodible banks . Field observations indicate that
the experiments had smoothly rounded tips, which allowed for a smoother flow
transit ion around the spur tip. However, the permeable spurs had sharp edged this upper-bank erosion is not a problem if upper portions of the bank are
or square tips. This difference in head form was seen to have a distinct well vegetated or otherwise stabilized. In arid regions, however, with
impact on the amount of flow deflection ereated by the spur. li ttle upper-bank vegetation, these flow conditions could resul t in
upper-bank erosion if not otherwise stabilized.
Another factor that has been observed to be a function of spur
orientation is the effect of spur-topping flows on the channelbank behind and Ouring the FHWAstudy, consideration of multiple spurs within a
just downstream of the spur. Duri ng the FHWAstudies, i t was observed that bank-stabilization scheme on a meandering channel revealed additional inslght
there is a disturbance on the channelbank at the spur root and immediately into the impact spur orientation has on flow in channelbends. Ouring these
downstream that is eaused by the near-bank flows passing over the spur studies, spur orientation was found to have a direct effect on the position
crest. This disturbance impacts only the upper portions of the channelbank; of the channel thalweg (main flow current) in the channelbend. Spurs hav i ng
the lower portions of the channelbank rema1n protected by the spur. steeper orientations (around 90 degrees) were found to force the thalweg more
towards the center and inside portions of the channel through the
Flow patterns observed when the spur crest is submerged are illustrated channelbend. This correlates with the findings of the single spur
in Figure 45 for two spur orientations. The flow component acr css the spur experiments, and indieates that steeply angled spurs provide a more positive,
crest is of primary concern with respect to spur orientation. As illustrated or active, flow control. Spurs oriented at greater angles to the channel
in Figure 45, flow passes over the spur crest in a direction generally flow provide a less abrupt flow control , allowing the channel thaI weg to
perpendicular to the spur crest. Therefore, as the spur angle is increased, shift closer to the concave channelbank. Figure 46 compares the location of
t he flow over the spur crest becomes aimed more directly towards the bank, the channel thalweg produced by spurs angled at 120 degr ees and 150 degrees
resulting in a more severe impact on the ohannelbank (campare Figures 45(a) to the thaI weg.
and (b). The magnitUde of this upper-bank disturbance has been observed to
be much more severe for impermeable spur s and permeable spurs with Additional conclusions from the FHWAstudy indicate th at spurs designed
permeabilities less than 35 percent. For permeable spurs of greater to provide flow diversion should be designed to provide a gradual flow
permeability, the impact of spur-topping flows becomes less severe with training through the channelbend. This is accomplished by designing the spur
increasing permeability. For permeable spurs with permeabilities greater system so that the spur furthest upstream is at a flat angle (that is, a
than 70 percent, very little impact on the upper channelbank was observed. large angl e as defined here) and then reducing the spur angle for each
subsequent spur. For example, the optimum scheme found in the FHWA
laboratory study had the upstream-most spur oriented at approximately 150

69 70
 • The greater the spur angle the smaller the magnitude of flow
concentration at the spur tip.

• The greater the spur angle the smaller the angle of flow
deflection.

• The smaller the spur angle the greater the magnitude of flow
control as represented by a greater shift of the flow thalweg
away from the concave (outside) channelbank.

• It is recommended that spurs withln a spur scheme be set wlth

the upstream-most spur set at approximately 150 degrees to the
main flow current at the spur tip, and with subsequent spurs
having incrementally smaller angles approaching a minimum angle
of 90 degrees at the downstream end of the scheme.
lol 10'
The criteria for setting an appropriate spur orientation for spurs within a
FIGURE 46. COMPARISON OF THALWEG POSITIONS PRODUCED BY stabilization scheme will be demonstrated in the followlng example.
SPURS ANGLED AT (A) 120 DEGREE, AND (B) 150 DEGREES.
Geoaetric Design EXaBple
The following example is intended to provide a step-by-step approach for
degrees. Subsequent spurs within the spur scheme had angles of 140, 130, establishing the geometric layout of a spur scheme. Figure 47 shows a
125, 120, 115, and 110 degrees, respectively. Red)Jclngthe spur angle as one meandering channel that has encroached on a bridge abutment. In this
moves downstream provides stronger flow control at the downstream limit of situation, it is desired te establ1sh the bankline that existed prior to the
the scheme based on the findings presented above. It is recommended that erosion shown. Also, because of severity or sharpness of the channelbend and
spurs within a spur scheme be set with the upstream-most spur set at the need for a positive flow deflection, an lmpermeable spur scheme will be
approximately 150 degrees to the main flow current at the spur tip, and with designed.
subsequent spurs having incrementally smaller angles approaching a minimum
angle of 90 degrees at the downstream end of the scheme. The actual angles Step 1. ESTABLISH THE LIMITS OF THE FLOW CONTROL/BANK STABILIZATION SCHEME
used within the scheme are left to the judgement of the designer. Actual
spur angles should be set based on the designer's experience and local site Figure 48 illustrates the procedure used to set the lim1ts of the
conditions. Local site conditions that should be considered include flow flow-control scheme. First, the eroded bank area is defined. Del1neat10n of
constriction, local scour, flow concentration at the spur tip, flow this area can be determined from field surveys. It is important that the
deflection. and the need to produce a relative shift in the channel thalweg design engineer visit the site not only to establish the limits of the eroded
location. The impact each of these factors has on spur angles was discussed area, but also to become familiar with flow conditions at the site.
above.
Next, the minimum limits of protection are established. As illustrated,
The following is a summary of conclusions regarding spur orientation: a distance of 1.5 times the channel width is measured downstream of the
downstream limit of curvature of the bend to Iocate- the minimum downstream
• Retardance spurs should be designed perpendicular to the flow limit of protection. However, since the bridge abutment itself has acted as
direction. a channel control, the downstream limit of protection can be set at the
upstream side of the abutment.
• Retardance/diverter and diverter spurs should be designed to
provide a gradual flow training around the band. This is The upstream limit of flow control or bank protection is set by
accomplished by maximizing the flow efficiency wlthln the measuring a distance equal to 1 channel width upstream of the upstream
bend while minimizing any negative impacts to the reference line. The upstream reference line is set by projecting a tangent
channelbend. to the convex channelbank just upstream of the beginning of curvature for the
bend. In this case. however, bank erosion was observed upstream of this
• The greater the spur angle the smaller the magnitude of local limit. Therefore, the upstream limit of protection is set at the point of
scour at the spur tip. observed erosion.

71
 FIGURE 48. SETTING THE LIMITS OF PROTECTION.
FIGURE 41. CHANNELBEND SHOWING ERODED AREA,
DESIRED FLOW ALIGNMENT, AND DEPOSITED SANDBAR.

74
73
 ------ ACTUAL "U ...
------ lUX PLOW o.oAe ....... '
- - - - - - - '0.. 'La. CO"I'RIOTIOII
--- - -- Law 'LOW 'NAa.WIO
-- __ -_ "IDIU .. PLOW 'HAL.I.
-- - - - - HIQH 'LOW 'NALWIO

FIGURE 49. SETTING MAXIMUM FLOW CONSTRICTION.

Step 2. SET DESIRED FLOW ALIGNMENT AND MAXIMUM FLOW CONSTRICTION

The object here is to shift the channel-flow al1gnment to that which
existed prior to the bank erosion. Thls desired flow allgnment is
illustrated in Figure 49. The dashed line in the figure represents a 10
percent constriction of the channel width. This 10 percent constriction is
being used to establish the length of individual spurs. A 10 percent
constriction was selected here to m1nim1ze local scour and flow concentration
at the spur tip. Limiting the flow constriction to 10 percent also m1nimizes
the chance of spurs deflecting currents 1nto the opposite channelbank.
·Step3. ESTIMATE FLOW THALWEGS THROUGH BEND
The design criteria for spur spacing and orientation rely on a
prediction of the location of the channel flow thaIweg for various flow
conditions. Sketching three thalweg locations, one correspondlng to low, FIGURE 50. ESTIMATES OF THALWEG LOCATIONS FOR
medium. and high channel flow conditions, will usually provide sufficient VARIOUS FLOW CONDITIONS.
definition. Figure 50 illustrates these three thalweg locatlons for the
example conditions. A thorough knowledge of flow in natural channelbends is
required for accurate estimation of these thaIweg Iocatlons.

75
Step 4. LOCATION
ANDORIENTATION
OF SPUR#1

Figure 51 illustrates the procedure used to locate and orient the first
upstream-most spur. First the bend radius line Rl is drawn from the center
of curvature of the bend through the point defining the upstream limit of the
protection as defined in step 1. Next, a flow tangent to the estimated flow
stream-line at the spur tip is drawn. Typically, the low-flow thalweg
location should be used , since it will generally follow the desired flow
alignment. Such a flow tangent is 1l1ustrated in Figure 51 as line AA. The
flow tangent is then shifted along the radius line Rl untll the 10 percent
flow constriction line is reached (see line A'A'). The spur angle of 150
degrees is then turned in an upstream di rection (clockwise) from line A' A' ,
------ ACTUAL IANKLINE
to establish the line BB, which is parallel
through the constricted
to the desired spur orientation
width line where it intersects the radius line (Rl).
------y.x FLOW ENCROACHMENT
- - - - - - - 10'1 'lOW CONSTRICTlOM
The line B'B' is then drawn through the the point defining the upstream limit --- - -- LOW FLOW THALWEG
of protection (spur location point) parallel to line BB. This line defines -- - - -- MEDIUM FLOW THALWEQ
the location of the center line of the spur. The spur length is then set -- - - - - HIGH 'lOW THALWEQ
between the eroded bankline, and the 10 percent flow constriction line.

Step 5. LOCATION
OF SPUR#2

The approach to locating the second spur is illustrated in Figure 52.

This same approach will be used to locate each subsequent spur , First,
another radius line, R2 in Figure 51, is drawn through the tip of the
pr evt ous spur. The location of the next downstream spur depends on the
orientation of a tangent to the channel thalweg where it intersects line R2.
However, we have sketched three flow thaI weg lines representing different
flow conditions. The appropriate flow thalweg ls for the flow condition th at
intersects line R2 at one quarter of the distance from the flow constriction
line. Line AA in Figure 52 illustrates the tangent drawn to the
quarter-point thalweg curvature off the tip of Spur #1. Line AA is then slid
along line R2 to the tip of Spur 11 as indicated by line A'A' in the figure.
From line A'A', an expansion angle of 17 degrees (as determined for
impermeable spurs at 10 percent constriction in Figure 39) is turned tciwards
the concave bank line (counterclockwise). The location of the next
downstream spur is defined by the point at which the rotated line intersects
the maximum flow encroachment line. This point is indicated by an asterisk
(*) in the figure.

Step 6. ORIENTATION
OF SPUR'2

Setting the orientation of spur #2 and each subsequent spur is the same
as the procedure for orienting spur #1. As illustrated in Figure 53, the
first step is to draw a radius line, R3, through the spur location point
(0). Next, a flow tangent to the estimated flow stream-line at the spur tip
is drawn (line AA as discussed in step 4). Line AA is shifted along line R3
to the tip of the spur (seo line A'A') The spur angle of 140 degrees is then FIGURE51. LOCATION
ANDORIENTATION
OF FIRST SPUR.
turned in en upstream direction from line A'A' to establish the line BB. The
line B'B' is then drawn through the spur location point (.). Line B'B'
defines the centerline of spur #2. The spur length is then set between the
eroded bankline, and the 10 percent flow constriction line.

77 78
 _______ ACTUA" •• NKLlN,
_______ ACTUAL aANKLU'.
_______ M.JC 'LOW I"C"OAC*MINT
-------IIAI 'LOW ... CROACH .... T _______ 10" 'LOW CON'T'UCTION
- - - - - - - 10,. 'LOW CO ... TIUCTIOM _______ lOW 'LOW '"A"WIO
--- - --- LOW 'LOW 'HAL WIG ______ M&DIUM'LOW 'HALWIO
-- - - --III'DIUII PLOWTHALWRO -- - - - -HIOH 'LOW THALWIQ
-- - - - -- HfGH 'LOW THALWIG

FIGURE 52. LOCATION OF SECOND SPUR. FIGURE 53. ORIENTATION OF SPUR NUMBER 2.

79
Step 7. LOCATION
ANDORIENTATTON
OF SUBSEQUENT
SPURS

Steps 5 and 6 are repeated until the downstream limit of protection is

reached. Figure 54 illustrates the final geometry developed in this way.

Several additional comments can be made about the example presented

above. The spur angles used when setting out the example spur' scheme are
illustrated in Figure 54. Note that the spur angles decrease from 150 degrees
to 120 degrees and then remain constant. This was done to provide maximum
flow efficiency through the channelbend. This example documents a relatively
sharp bend curvature requiring a maximumin flow efficiency. For this reason
the spur s were not angled more steeply. The magnitude of this limiting spur
angle should be set based on conditlons particular to each site. _______ "L.N.
ACTU,U. I ••
_______ IIAI: , .. OW I"C"OACMIIIM'
Also, note the dogleg in the next to the last spur , This dogleg was _____ - -10' 'LOWCON.TltlenON
_______ LOW 'LOW 'HALWIO
designed into this spur to minimize the spur's total length and thus, its
______ MEDIUII FLOW TMALWIG
cost. This leg of the spur is not impacted by channel flows since it is ______ HIOH "LOW THALWla
inside the maximum flow encroachment 11ne. Doglegs such as this can be
designed where they will provlde an economie advantage without impacting the
effeetiveness of the stabilization scheme.

Tt is also interesting to note the relative spacing of the spurs.

Notice that the spurs on the downstream half of the bend are closer
together. As such , the scheme provides a more posi tive control of flow in
this area. The reduced spacing of the spur s in this area provided by the
spacing criteria presented correlates well with the need for greater flow
control in the downstream half of the channelbend (FHWA,1983).

STRUCTURE HEIGHT
The height to which spur s should be constructed is primarily a function
of the height of channelbank to be protected. Factors that influence the
appropriate height of bank protection are as follows:

• the mechanism causing the erosion,

• the existing channelbank height,

• the design flow stage, and

• the flow stage at which significant debris loads become aproblem.

The eros ion mechanism is important in establishing the spur height

because it defines the vertical regions of the bank that are impacted by the
erosion process and require protection. For example, if the channelbank is
to be protected against toe erosion , the spur s need only be high enough to
proteet the toe of the channelbank. On the other hand, if a mechanism FIGURE54. FINALSPURSCHEME
GEOMETRY.
causing er os ion of upper-bank materials i s the culprit, the spur should be
des i.gned to the height of the bank. Alternatively. if only the lower and
middle portions of the bank are being impacted, a spur height that covers
this region should be used.

81 82
 The existing bank height and design flow stage can be considered
together when establishing an appropriate spur height. If the flow stage to
be protected against (usually a design flow of gi ven frequency), is lower
tnan the channelbank height, the design stage should be used to set the spur
-
~/'COUR
FLOW

PATTIRN
~.",. PATTERN

height. Ir the design flow stage is higher than the bank height, spur s are
generally only designed to a height equal to the bank height. It has been ~.,," ~.,,"
found (Pokrefke, 1978) that constructing a spur to bank height does not /777~/777 /77777777
reduce its effectiveness when overtopped; over topping of spur s by as much as
3 feet does not affect the "purs' efficiency. Impermeable spurs are
generally not constructed above bank height to el1minate the possibility of (al (bI
out-flanking of the spur by flow concentration and erosion behind the spur at
high river stages. The most commonly advised height for spur" is that which FIGURE 55. COMPARISON OF SCOUR PATTERNS GENERATED BY
corresponds to bank height. (A) SUBMERGED, AND (B) NONSUBMERGED IMPERMEABLE SPURS.

Designing spur s lower than floN stages that carry significant debris
loads is more important for permeable spurs than for impermeable spurs
because of the flow-skimming qual1ties of the permeable structures. The CREST PROFILE
elevation of the top of these structures should be well below the high-water
level to allow the heavy debris to pass over the top and prevent damage to Spur crest profile is related to spur height. Permeable "purs are
the structure. usually designed with level crests, although in special cases where high
banks are to be protected, sloping crest designs have been used (see Figure
The effect of flow submergence on the behavior of a spur is related to 22) •
defining an appropriate spur height. Recently, it has been found (FHWA,
1983) that the behavior of impermeable "purs with respect to flow deflection Impermeable spurs have been constructed with both level orest! and
and local scour and flow concentration at the spur tip is Norse for flow crests sloping towards the head. Bath Acheson (1968) and J onsen et al.
stage conditions lower than the crest of the spur than when the spur crest is (1979) suggest that impermeable spurs be designed with a slight fall towards
submerged. For example, Figure 55 compares the scour patterns generated by the head. Richardson and Simons (1974) recommend that level crest spurs be
submerged and nonsubmerged spurs, As 1l1ustrated, the scour pattern placed normal to flow and sloping crest spurs be placed normal or angled
generated for the nonsubmerged case is larger and deeper. downstream to flow. Simons, et al. (1979) also recommend sloping crest
dikes for bank protection. The main advantage of sloping crest spur! is that
Based on the above statements, the following recommendations are made they allow different amounts of flow constriction w1th stage. The sloping
for establishing the height of spur systems: crest also allows the accommodation of changes in meander trace with stage.
Franco (1966) 1ndicates that sloping crest spurs are as effective as
• The spur height should be sufficient to proteet the regions of the level-crested designs.
channelbank impacted by the erosion process.
The following is a list of recommendations regarding crest profile:
• If the design flow stage is lower than the channelbank height, spurs
should be designed to a height no more than three feet lower than the • Permeable spurs should be designed w1th level crests unless bank
design flow stage. height or other special conditions dictate the use of a sloping
crest design.
• If the design flow stage is higher than the channelbank height, spurs
should be designed to bank height. • Impermeable spurs should be designed with a slight fall towards
the head, thus, al10N1ng different amounts of flow constriction
a Permeabie spurs should be designed to a height that will permit the with stage (particularIy important in narrow width channels), end
passage of heavy debris over the "pur crest and not cause structural the accommodation of changes in meander traoe with stage.
damage.
• When possible, impermeable spurs should be designed to be submerged by
approximately three feet under their worst design flow condition, thus
minimizing the impacts of local scour and flow concentration at the
spur tip. and the magnitude of flow deflection.

83
BED AID BAHK COlTACT

A spur' s abili ty to maintain contact with the channelbed and bank i5

fundamental to the spur ' s structural stabili ty. Undermining and/or
outflanking are the most commonly reported fallure mechanisms for spurs used
as flow control and streambank-stabilization countermeasures. Maintaining
bed and bank contact is primarily a problem in highly alluvial channel
environments where the channelbed surface fluctuates widely in response to
changing flow conditions.

Cbannelbed Contact

The mechanisms by which spurs maintain contact with the channelbed vary
with spur type.

Impermeable rock riprap spurs can be designed with excess stone in the
spur head to counter undermining at the spur tip in the event of streambed
elevation changes. As illustrated in Figure 56, as the streambed lowers, the
stone material will launch channelward, armoring the area around the spur tip
against future drops in the channelbed. In a design of this type, care must
be taken to size the riprap properly to provide a sufficient volume of
material for the launching process.

Gabion spurs can also be designed to counter changes in streambed

elevation at the spur head. This is done by extending the wire and stone
base course or mat channelward beyond the tip of the spur head to armor the
channelbed in the vicinity of the spur tip. Figure 57 illustrates that as
the streambed lowers, the base mat will drop with the bed to armor the area
around the spur tip against fut ure drops in the channelbed. Gabion spurs are
not as flexible as riprap spurs in this respect; therefore, they should be
used with caution in highly alluvial environments.

Several design techniques to protect against undermining of permeable

spurs are also available. The first technique, illustrated in Figure 58, is
to provide a rock-toe foundation for the spur , In a fashion simllar to that
of the rock riprap spurs discussed above, fluctuations in channelbed level
will cause the rock-toe material to launch and armor the area around the spur
preventing undermining. Note that sufficient material must be included in
the riprap blanket to armor against scour effects . This is particularly
important at the head of the structure, where an addi tional mass of material
might be needed (see Figure 58).
(c)
To avoid undermining of pile structures, the vertical support members
shou l d be driven to a depth significantly below the anticipated scour level. Figure 56. ROCKRIPRAPSPURILLUSTRATING LAUNCHINGOF STONE
It has also been found that round vertical pt Les induce a rnuch smaller depth TOE PROTECTION. (A) BEFORELAUNCHINGAT LOWFLOW
of local scour than do square vertical pt Ies (FHWA,1983). It has also been (B) DURINGLAUNCHING, AT HIGHFLOW(C) AFTER
observed (FHWA,1983) that extending the facing material of permeable spur s LAUNCHING AT LOWFLOW
to a depth below the channelbed surface and below anticipated scour depths
has a significant stabillzing effect on the channelbed in the vicinity of the
spur. This technique is illustrated in Figure 59. In this case, the wire

86
85
~i :
FIGURE 58. PERMEABLE WOOD-SLAT, FENCE SPUR SHOWING
LAUNCHING OF STONE TOE MATERlAL.

'10'
- ~
~ ~ ~

, ~
VW1RE
'.LOW
sr
IIE&H aXTEMDINO
IIAX ICOUR DE PTH

I - lilA X ICOUR DE !'TH

I:!:
-
Et ~
a 1-1--
r--X_.."",,,,,,,.,..·_
'c'
ITEEL ROD AMCHORI
FIGURE 57. GABION SPUR ILLUSTRATING FLEXIBLE HAT TIP PROTECTION.
(A) BEFORE LAUNCHING AT LOW FLOW (B) DURING
LAUNCHING. AT HIGH FLOW (C) AFTER LAUNCHING AT LOW FLOW
FIGURE 59. WIRE MESH SPUR WITH THE MESH SCREEN
EXTENDED BELOW THE MAXIMUM ANTICIPATED SCOUR DEPTH.

87 88
 OlllalMAL .IED

(.1

FIGURE 61. HENSON SPUR SHOWING OUTFLANKING.

(bI
units can be placed on top of the old units to restore the structure' s
FIGURE 60. HENSON SPURS (A) RESTING ON ORIGINAL CHANNELBED. height. A similar mechanism could be designed for other fence-type
AND (B) AFTER DROP IN CHANNELBED LEVEL. structures. However, care must be taken not to infringe on existing
patents.

The recommendation is that careful consideration be given to designing a

mesh is rol led down the upstream face of the support members into an spur that will maintain contact with the channelbed and not be undermined.
excavated trench. Some form of weighting mechani sm can be attached to the
bottom to secure the wire mesh to the bottom. An alternative to placing the Cbanaelbaak Coatact
wire in a pre-excavated trench is to lay a role of wire and an anchor weight
on the channelbed or in a small trench and allow natural scour processes to Another concern is the spur's ability to maintain contact with the
sink the wire. This might require several additional vertical supports to be channelbank. Spurs not adequately tied into the bank are susceptible to
driven on the upstream side of the wire roll to gUide it as it drops. outflanking. A case in point is illustrated in Figure 61, where spur-topping
flows continued to erode the upper portions of the channelbank, creating a
One additional technique for maintaining channelbed contact has been flow channel behind the spurs. In this case failure to tie the spurs
developed as a part of the patented Henson spur scheme marketed by Hold That adequately to the bank resulted in continued bank movement . In contrast,
River, Inc. of Houston, Texas. This technique is depicted in Figure 60. The Figure 62 illustrates a welded wire-mesh spur that was tied adequately to the
Henson spur jetties shown maintain contact with the channelbed by being free bank by running the wire mesh for a distance into the bank.
to move vertically with the bed. They are vertical wood-slat fence units
mounted on pipes that are driven to a dep th that prohibits fallure from The recommendation is that adequate consideration be given to tieing the
undermining. As the channelbed drops during a storm event, the wood slat spur structure adequately to the channelbank to avoid outflanking.
units slide on the pipes to maintain contact with the bed and provide
protection against undermining of the structure. If the vertical channelbed
drop during one flow event leaves the units buried or too low, additional

89 90
 • Where it is necessary to provide a moderate reduction in flow
veloei ty, a moderate level of flow oontrol, or where the
structure is being used on a mild to moderate channelbend, the
spurs with permeabilities up to 50 percent can be used.

• In environments where only a mild reduction in velocity is

required, where bank stabilization without a significant amount
of flow control is necessary, or on mild1y curving to straight
channel reaches, spurs having effective permeabilities up to 80
percent can be used. However, these high degrees of permeability
are not recommended un1ess experience has shown them to be
effective in a particular environment.
• It is recommended that jack and tetrahedron retardance spurs be
used only where it can be reasonably assumed that the structures
will trap a sufficient volume of floating debris to produce an
effective permeability of 60 percent or less.
• It is recommended that Henson-type spurs be designed to have an
effective permeability of approximately 50 percent.
• The greater the spur permeability, the less severe the scour
pattern downstream of the spur tip. As spur permeability
increases, the magnitude of scour downstream of the spur
FIGURE 62. WIRE-MESH PERMEABLE SPUR ILLUSTRATING decreases slightly in size, but more significantly in depth.
SPUR ROOT EXTENDING INTO CHANNELBANK.
• The vertical structural members of permeable spurs should be round
or streamlined to minimize local scour effects.

SPUR BEAD FORM OR DESIGN • The greater the spur permeability, the lower the magnitude of
flow concentration at the spur tip.
Numerous design shapes have been suggested for the head or riverward tip
of spurs. These have included straight, T-head, L-head, wing, hocky, • If minimizing the magnitude of flow deflection and flow
inverted hocky, etc. However, a simple straight spur head form is concentration at the spur tip is important to a particular spur
recommended. The only additional recommendation is that the spur tip be as design, a spur with a permeability greater than 35 percent should
smooth and rounded as possible. Smooth, well-rounded spur tips help minimize be used.
10ca1 scour, flow.
• The more permeable the spur, the shorter the length of
SUMMARY OF SPUR DESIGV RECOMMEWDlTIOWS channelbank protected downstream of the spur's riverward tip.

The following is a summary of the major recommendations presented in • Spurs with permeabilities up to approximately 35 percent proteet
this chapter; they are organized by design component for easy reference. almost the same length of channelbank as do impermeable spurs;
spurs having permeabilities greater than approximately 35 percent
Pe....
eability protect shorter lengths of channelbank, and this length decreases
with increasing spur permeability.
o Where it is necessary to provide a significant reduotion in flow
velocity, a high level of flow control, or where the structure is • Because of the increased potential for erosion of the channelbank
being used on a sharp bend, the spurts permeability should not in the vicinity of the spur root and immediately downstream when
exceed 35 percent. the flow stage exceeds the crest of impermeable spurs, it is
recommended that impermeable spurs not be used along channelbanks
composed of highly erodible material unless measures are taken to

91
 proteet the channelbank in this area. a major role in determining an acceptable spacing between
individual spurs in a bank-stabilization scheme.
E.tent or Channelbank Protection
• Reducing the spacing between individual spurs below the minimum
• A common mistake in streambank protection is to provide required to prevent bank erosion between the spurs results in a
protection too far upstream and not far enough downstream. reduction of the magnitude of flow concentration and local scour
at the spur tip .
• The extent of bank protection should be evaluated using a variety
of techniques, including: • Reducing the spacing between spurs in a bank-stabilization scheme
causes the flow thalweg to stabilize further away from the
- empirical methods, concave bank towards the center of the channel.
- field reconnaissance,
evaluation of flow traces for various flow • A spacing criteria based on the projection of a tangent to the
stage conditions, and flow thalweg, projected off the spur tip, as presented in the
_ review of flow and erosion forces for various flow above discussions, should be used.
stage conditions.
Spur Angle/Orientation
Information from these approaches should then be combined with
personal judgement and a knowledge of the flow processes • The primary criterion for establishing an appropriate spur
occurring at the local site to establish the appropriate limits orientation for the spurs within a given spur scheme is to
of protection. provide a scheme that efficiently and economically guides the
flow through the channelbend, while protecting the channelbank
Spur Length and minimizing the adverse impacts to the channel system.
• As the spur length is increased, • Spurs angled downstream produce a less severe constriction of
flows than those angled upstream or norm al to flow.
- the scour depth at the spur tip increases,
the magnitude of flow concentration at the spur tip • The greater an individual spur's angle in the downstream
increases, direction, the smaller the magnitude of flow concentration and
_ the severity of flow deflection increases, and local scour at the spur tip. Also, the greater the angle, the
- the length of channelbank protected increases. less severe the magnitude of flow deflection towards the opposite
channelbank.
• The projected length of impermeable spurs should be held to less
than 15 percent of the channel width at bank-full stage. • Impermeable spurs create a greater change in local scour depth
and flow concentration over a given range of spur angles than do
• The projected length of permeable spurs should be held to less permeable spurs. This indicates that impermeable spurs are mucp
than 25 percent of the channel width. However, this criterion more sensitive to these parameters than are permeable spurs.
depends on the magnitude of the spur's permeability. Spurs
having permeab11ities less than 35 percent should be limited to • Spur orientation does not in itself result in a change in the
projected lengths not to exceed 15 percent of the channel's flow length of channelbank protected for a spur of given projected
width. Spurs having permeabilities of 80 percent can have length. It is the greater spur length parallel to the
projected lengths up to 25 percent of the channel's bank-full channelbank associated with spurs oriented at steeper angles that
flow width. Between these two limits, a linear relationship results in the greater length of channelbank protected.
between the spur permeability and spur length should be used.
• Retardance spurs should be designed perpend1cular to the primary
Spur Spacing flow direction.
• The spacing of spurs in a bank-protection scheme 1s a function of • Retardance/diverter and diverter spurs should be designed to
the spur's length, engIe, end permeebility, as weIl as the provide a gradual flow training around the bend. This is
channelbend's degree of curvature. accomplished by maximizing the flow efficiency within the bend
while min1mizing any negative impacts on the channel geometry.
• The direction and orientation of the channel's flow thaIweg plays

93 94
 • The smaller the spur angle, the greater the magnitude of flow
control as represented by a greater shift of the flow thaI weg Cbannelbed and Channelbank Contact
away from the concave (outside) channelbank.
• Careful considerationmust be given to designing a spur that will
• It is recommended that spurs within a retardance/diverter or maintain contact with the channelbed and channelbank so that ft
diverter spur scheme be set with the upstream-most spur at
approximately 150 degrees to the main flow current at the spur
will not be undermined or outflanked. Methods and examples
tip. and with subsequent spurs having incrementally smaller presented herein can be used to ensure adequate bend and bank
contact.
angles approaching a minimum angle of 90 degrees at the
downstream end of the scheme.
Spur Read Form
Spur Syste. Geometry
• A simple straight spur head form is recommended.
• A step-by-step approach to setting out the geometry of a
retardance/diverteror diverter spur scheme was presented above. • The spur head or tip should be as smooth and rounded as possible.
The use of this approach wi'llyield an optima1 geometrie spur Smooth, well-rounded spur tips help minimize local scour, flow
system design. concentration,and flow deflection.
Spur Beight

• !he spur height should be sufficient to proteet the regions of

the channelbank impacted by the erosion processes active at the
particular site.
• If the design flow stage is lower than the channelbank hei ght ,
spurs should be designed to a height no more than three feet
lower than the design flow stage.
• If the design flow stage is higher than the channelbank height,
spurs should be designed to bank height.
• Permeable spurs should be designed to a height that will permit
the passage of heavy debris over the spur crest and not cause
structural damage.

• When possible, impermeable spurs should be designed to be

submerged by approximately three feet under their worst design
flow condition, thus minimizing the impacts of local scour and
flow concentration at the spur tip and the magnitude of flow
deflection.
Spur Crest Profile

• Permeable spurs should be designed with level crests unless bank

height or other special conditions dictate the use of a sloping
crest design.
• Impermeable spurs should be designed with a slight fall towards
the spur head, thus allowing different amounts of flow
constriction with stage (particularly important in narrow-width
channelsl, and the accommodation of changes in meander trace with
stage.

95
 REFEREIICES REFEREIICES (Continued)

Acheson, A.R., River Control and Drainage in New Zealand, Minlstry Franco, J.J., 1966. "Laboratory Research on Design of Dikes for River
of Works, New Zealand, 1968. Regulation," Miscellaneous Paper No. 2-860, U.S. Army Engineers
Waterways Experiment Station, Vicksburg, Mississippi, November.
Ahmad, M., 1951a. "Spaclng and Projection of Spurs for Bank Protection, Part (Available through the National Technical Information Service; NTIS
1" Clvil Engineering (London), Vol. 46, No. 537, April. Publication AD735846)
Ahmad, M., 1951b. "Spacing and Projection of Spurs for Bank Protection, Part Jansen, P., Ph.D, et aL, 1979. Principles of River Engineering:
2," Civil Engineering (London), Vol. 46, No.• 538, April. The Non-Tidal Alluvial River, Pitman Publishing Limited, London.
Ahmad, M., 1953. "Experiments on the Design and Behavior of Spur Dikes," Keeley, J.W., 1971. "Bank Protection and River Control in Oklahoma," Federal
Proceedings, Minnesota International Hydraulics Convention, Highway Administration, Bureau of Public Roads, Oklahoma Division.
University of Minnesota, September 1-4, 1953. Published by St. Anthony
Falls Hydraulic Laboratory, August. Littlejohn, B.J., 1969. "Investigation of Existing Dike Systems," Potomology
Investigations, Reports 21-23, U.S. Army Corps of Engineers District,
Apmann, R. P., 1972. "Flow Processes in Open Channel Bends," Journalof Memphis, Tennessee, May.
the Hydraulics Division, American Society of Civil Engineers Volume
98, No. HY5, Proceedings Paper 8886. Parsons, O.A., 1960. "Effect of Flow Flows on Channel Boundaries,"
Journalof the Hydraulics Division, American Society of
Brice, J.C., Blodget, J.C., et al., 1978. "Countermeasures for Hydraulic Civil Engineers, Vol. 86, No. HY4, Proceedings Paper 2443, April.
Problems at Bridges," Final Report, Federal Highway Administration,
FHWA-RD-78-162, Volumes 1 and 2. Pokrefke, T., No Title, Unpublished Paper, U.S. Army Engineer Waterways
Experiment Station, Hydraulics Laboratory, Vicksburg, Mississippi.
Brown, S.A., 1979. "Investigation of Meander Migration and Control on the
Loyalsock Creek Using a Physical Model," Masters Thesis, Submitted in Richardson, E. V., and Simons, D.B., 1974. "Spurs and Guide Banks," Open
Partial Fulfillment of the Requirement for the Degree of Master of File Report, Colorado State University Engineering Research Center, Fort
Science, The Pennsylvania State University, University Park, Collins, Colorado, February.
Pennsylvania.
Simons, D.B, Li, R.M., Alawady, M.A.. and Andrew, J.W., 1979. "Connecticut
Federal Highway Administration, 1983. "Laboratory Investigation of Flow River Streambank Erosion Study, Massachusetts, New Hampshire. and
Control Structures for Highway Stream Crossings," U.S. Department of Vermont," Prepared for U.S. Army Corps of Engineers, New England
Transportation, August. (Available through the National Technical Division, Waltham, Massachusetts, November. (Available throu%h the
Information Service, Springfield, Va.) National Technical InformationService; NTIS Publication ADA080466).
Federal Highway Administration,1984. "Selection and Design of Flow Control U.S. Army Corps of Engineers: 1978. "Interim Report to Congress; The
and Streambank stabil1zation Structures," Report No. FHWA/RD-83/099, Streambank Erosion Control Evaluation and Demonstration Act of 1974,"
U.S. Department of Transportation.Washington, D.C. Section 32 Program Interim Report, September.
Fenwick, G.B., ed., 1969. "State of Knowledge of Channel Stabilizationin U.S. Army Corps of Engineers, 1981. "The Streambank Erosion Control
Major Alluvial Rivers," Technical Report No. 7, Committee on Channel Evaluation and DemonstrationAct of 1974, Section 32, Public Law 93-251:
Stabilization,U.S. Army Corps of Engineers, October. Final Report to Congress," Main Report and Appendices A through H,
December.

97 98 "U.S. GOVf.1U'lMENT
PRINTINC OFFICE, 198$-461-816,20141
FEDERALLY COORDINATED PROGRAM (FCP) OF HIGHWAY RESEARCH,
DEVELOPMENT, AND TECHNOLOGY
The Offices of Research, Development, and maintenance, traffic services for maintenance
Technology (RD&T) of the Federal Highway zoning, management of human resources and
Adrninistration (FHWA) are responsible for a brood equipment, and identification of highway
research, development, and technology transfer pro- elements that affect the quality of the human en-
gram. This program is accomplished using numerous vironment. The goals of projects within thls
methods of funding and management. The efforts eategory are to maximize operational efficiency
inelude work done in-house by RD&T staff, con- and safety to tbe traveling public while conserv-
tracts using administrative funds, and a Federal-aid ing resources and reducing adverse highway and
program conducted by or through State highway or traffic impacts through protections and enhance-
transportation agencies, which include the Highway ment of environmental features.
Planning and Research (HP&R) program, the Na-
tional Cooperative Highway Research Program
4. Pavemenl Design, Conslruclloo, and
(NCHRP) managed by the Transportation Research
Managemenl
Board, and the ene-half of one percent training pro-
gram conducted by the National Highway Institute. Pavement RD&T is coneerned with pavement
design and rehabilititation methods and pro-
The FCP is a earefully selected group of projects, cedures, construction technology, recycled
separated into braad categones. formulated to use highway materiais, improved pavement binders,
research, development , and technology transfer
and improved pavement management. The goals
resources to obtain solutions to urgent national
will emphasize imprcvements to highway
highway problems.
performance over the netwerk 's life cycle, thus
The diagonal double stripe on the cover of this report extending maintenance-free operation and max-
represents a highway. It is cclor-ccded to identify imizing benefits. Specific areas of effort will in-
the FCP category to which the report's subject per- clude material characterizations , pavement
tains. A red stripe indicates category I, dark blue damage predictions, methods to minimize local
for category 2, light blue for eategory 3, brown for pavement defects, quality control specifications,
category 4, gray for category 5, and green for long-term pavement monitoring, and life cyele
category 9.
cost analyses.
Fep Cat.gory Descriptions
1. Highway Design and Operation tor Safety S. Structural Desigo and Hydraulic.
Safety RD&T addresses problems associated Structural RD&T is concerncd with furthering the
with the responsibilities of the FHW A under the latest technological advances in structural and
Highway Safety Act. It ineludes investigation of hydraulic designs, fabrication processes, and con-
àppropriate design standards, roadside hard- struction teehniques to provide safe, efficient
ware, traffic control devices, and collection or highway structures at reasonable costs, This
analysis of physical and scientific data for the
category deals with bridge superstructures, earth
formulation of improved safety regulations to
structures, foundations, culverts, river
better proteet all motorists, bicycles, and
mechanics, and hydraulics, In addition, it in-
pedestrians.
eludes material aspect. of structures (metal and
2. Traffic Control and Managemenl concrete) along with their proteetion from cor-
Traffic RD&T is concerned with increasing the rosive or degrading environments.
operational efficiency of existing highways by
advaneing technology and balaneing the 9. RD&T Maoagemeol and Coordinatloo
demand-capacity relationship through traffic
management techniques such as bus and carpool Actlvities in this category include fundamental
preferenrial treatrnent, coordinated signal tim- work for new concepts and systern character-
ing, motorist information. and terouting of ization befere the investigation reaches a point
traffic. where it is incorporated within other categories FHWA!R0-84/1 01/7-85(4oo)OE
of the FCP. Concepts on rhe feasibility of new
3. Highway Operalions
This category addresses preserving the Nation's technology for highway safety are includcd in this
highways, natura! resources, and community category. RD&T reports not within ether FCP
auributes. It includes activitles in physical project' will be published as Category 9 projects.