Design of Spur Dikes
Design of Spur Dikes
Design of Spur Dikes
and Technology
Turner-Fairbank Highway
K STABILIZATION Research
6300 Georgetown
McLean,
Center
Virginia
Pike
22101
TRUCTURES
Report No.
FHWAIRD-84/101
USDepartment
of Transportation
Final Report
Federql Highway July 1985
, Administration
DO NOT REMOVE
VThis document is available to the U.S. public through the National Technical Information Service, Springfield, Virginia 22161
FOREWORD
NOTICE
The contents of this report reflect the views of the contractor who is
responsible for the accuracy of the data presented herein. The contents do
not necessarily reflect the official policy of the Department of
Transportation.
FHWA/RD-84/101
4. Title and Subtitle 5. Report Dote
July 1985
DESIGN OF SPUR-TYPE STREAMBANK 6. Performing Organization Code
STABILIZATION STRUCTURES
8. Performing Organization Report NO.
7. Author’s)
Scott A. Brown SCR-371-83-039
9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)
Sutron Corporation
2190 Fox Mill Road //
Herndon, VA 22071
13. Type of Report and Period Covered
16. Abstract
Multiple BY To Obtain
ounces 28 grams
pounds 0.45 kilograms
short tons 0.9 tonnes
(2000 lbs)
ii
TABLE OF CONTENTS
Page
CHAPTER 1. IMTRODUCTION 1
GENERALAPPLICABILITY OF SPURS
Function
Erosiotm Mechanisms
Rjlver Environment
Channel Size
Bend Radius
ChanneLbank Characteristics
System Impacts
Environmental Impacts
Esthetic Impacts
Construction-Related Considerations
costs
SPUR TYPES
Retardance Spurs 10
Fence Type 10
Jack/Tetrahedron Type 19
Retardance/Diverter Spurs 19
Light Fence Type 21
Heavy Diverter Spurs 24
Diverter Spurs 27
Hardpoints 28
Transverse-Dike Spurs 28
Spur Function/Purpose 32
Erosion Mechanism 34
Sediment IE$vironment 35
Flow Environment 37
Channel Velocity Environment 38
Flow Stage 38
Bend Radius 39
Debris and Ice-Load Environment 40
iii
LE OF CONTENTS(Continued.)
Page
OTHER COMSHDERATIglNS 41
costs 41
Charmel Size 44
Channelbed Fluctuations 44
Vegetation 45
Vandalisie and t4aintenance 45
PERMEABILITY 46
GEOMETRY ,52
STRUCWIIRE HEIGHT 81
CREST PROFILE 84
co 85
lbe 85
Channelban 90
REFERENCES 97
iv
LIST OF FIGURES
Figure Page
1 TIMBER PILE SPUR SHOWINGTHE IMPACT 7
OF EXCESSIVE FLOWDEFLECTION
15 FENCE-TYPE RETARDANCESPURS 18
V
LIST OF FIGURES (Continued)
Figure Page
25 TYPICAL ROCKHARDPOINTDESIGNS 29
34 FLOWOVER IMPERMEABLESPURS 51
36 SHIFT IN MAXIMUMCURRENTTHREADWITH 54
CHANGINGSTAGE
vi
LIST OF FIGURES (Continued)
Figure Page
52 LOCATION OF SECONDSPUR 79
vii
LIST OF FIGURES (Continued)
Figure Page
55 COMPARISON OF SCOURPATTERNSGENERATEDBY a4
(A) SUBMERGED,AND (B) NONSUBMERGED
IMPERMEAABLESPURS
61 HENSONSPUR SHOWINGOUTFLANKING 90
LIST OF TAEKES
viii
Chapter 1
INTRODUCTION
In the past, little guidance has been available for the design of
spur-type structures. Few design guidelines have been available; those that
are available are limited in scope and generally inaccessible to highway
design engineers. The design of these structures has been primarily based on
the designer’s 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
guidelines for the design of spur-type flow control and bank-stabilization
structures.
2
Chapter 2
Criteria for the selection of a specific spur type are presented in this
chapter. This includes a discussion of the general applicability of spurs,
the applicability of each of the major spur types, and a closer look at the
attributes of individual spur types.
GENERALAPPLICABILITY OF SPURS
Function
The functions or purposes for which spur-type structures are best suited
include protecting an existing bank-line, reestablishing some previous flow
path or alignment, and controlling or constricting channel flows. These
functions or purposes are discussed in detail in FHWA (1984). The primary
advantage of spurs over other countermeasure types is their ability to
provide flow control and constriction as well as the reestablishment of a
previous or new flowpath. While spurs also are effective at streambank
stabilization and protection in general, other countermeasure types can
provide equivalent or perhaps better protection against general bank erosion
(FHWA, 1984).
Erosion Heehanisms
3
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 erosion 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
channelbanks are discussed in FHWA (1984). Toe scour has been identified as
a primary cause of bank failure. By moving the flow forces responsible for
toe scour away from the bank, this erosion mechanism is effectively
countered.
Riwer Environment
Channel Size
Bend Radius
The use of spur-type structures for flow control and bank stabilization
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
spacing that would be required. Also, short-radius bends are typically found
on channels having small widths; the consequences of using spurs on small
channels has already been discussed.
Channelbank Characteristics
One advantage in the use of spur-type structures is that they have been
observed to provide an enhancing influence on bank vegetation. The erosive
action of currents impinging directly on the bank will often prevent or
hinder the natural volunteering of plant materials down the bank. Since
spurs shift these main flow currents away from the bank, a greater
opportunity exists for the natural volunteering of vegetation down the bank
and into the I1spur zone ,I’ helping to stabilize both the upper and lower
sections of the channelbank. In environments characterized by high sediment
loads, the vegetation will usually volunteer to the berm deposited between
the spurs, enhancing the stabilizing characteristics of the spur scheme. In
low sediment-yield environments, the reduced flow velocities within the spur
zone create a more acceptable environment for vegetative growth, therefore
allowing the advance of vegetative materials down the bank and into this zone
during low-flow periods. Again, the additional vegetative growth thus
created will enhance bank stabilization and help counter the lack of a
deposited sediment berm in low sediment-yield environments. It also helps
minimize the bank-scalloping characteristic of impermeable ’ spur
installations. The development of thick vegetation on the banks and between
spurs also provides a mechanism for flow retardance and energy dissipation
for spur-topping flow conditions, further enhancing bank stabilization. Bank
vegetation also enhances the appearance of the bank by presenting a more
natural-looking bankline.
System Impacts
5
Environmental Impacts
Environmental impacts include impacts on channel geometry, water
quality, and biology.
The location of the scour trough discussed above provides another point
of comparison between spurs and other countermeasure types. Because spurs
shift the flow current away from the bank, they also shift the scour trough
away from the bank, thus removing the immediate danger from undermining away
from the bank. Streambank-stabilization schemes that have their primary
component parallel to the channelbank (i.e., revetments, retardance
structures, longitudinal dikes, and bulkheads) must be designed to protect
against undermining along the entire length of the bank, adding significantly
to the cost of the stabilization schemes. Because only the riverward ends of
spur-type structures are impacted by the scour trough, only localized
protection at the spur heads is required. Also, the risk of a catastrophic
failure of the entire stabilization scheme as a result of toe erosion and
undermining is lower with spurs than with other structure types because only
the ends of the spur are impacted at any given time. Failure of the spur
head still leaves additional spur length to provide partial protection for
the bank until repairs can be made.
Several factors will affect the magnitude of the channel reshaping just
discussed. First, the more severe the channel constriction, the more
pronounced the resulting channel scour patterns will be. The channelbed
composition also plays a role in the magnitude of these erosion patterns;
channels cut in silt- and sand-size materials will exhibit greater depths and
extents of erosion than channels in gravel- and cobble-size materials. Since
impermeable spurs have a greater constricting effect on channel flows than
permeable spurs, the erosion patterns produced by impermeable spurs can be
expected to be more severe (assuming similar channel environments).
Impacts on channel geometry can also result from incorrect design and/or
construction of the spur scheme. The geometric layout of the scheme is of
primary importance. Misalignment of spurs can cause severe flow deflection
and could initiate an erosion problem on the opposite bank. Figure 1
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
6
FIGURE 1. TIMBER-PILE SPUR SHOWINGTHE
IMPACT OF EXCESSIVE FLOWDEFLECTION.
opposite channelbank as shown. Also, if the spurs produce too much flow
constriction i excessive channel deepening may occur, which can undermine and
cause the eventual failure of the spur structures. Time delays between
initial design surveys and construction can also result in a final spur
configuration whose geometric layout does not coincide with existing flow
conditions. The U.S. Army Corps of Engineers (1981) has documented several
cases where changes in stream pattern occurred between the time the initial
design survey was conducted and construction was started. The shifting
stream pattern resulted in a final spur configuration that was not compatible
with flow conditions after the scheme was constructed. The potential impacts
resulting from inappropriate spur-scheme layouts are the most significant
drawbacks to the use of spur-type flow-control and bank-stabilization
structures. The geometric layout of spur schemes is a more critical design
consideration for spur-type structures than for other countermeasure types.
This points out the need for careful and efficient planning, design, and
construction of spur schemes.
Water-quality impacts result from changes in turbidity together with
alteration of the local riverine habitat. The primary impacts are the
increased turbidity and stripping of bank vegetation during construction.
These activities can affect stream temperature and photosynthetic activities
that in turn may affect algae or aquatic plant populations, dissolved oxygen,
and other water-quality parameters. These are usually temporary impacts.
Also, since the construction of spur schemes produces less bank disturbance
Esthetic Impacts
The hazards associated with spur schemes are related to recreational use
of the river. The potential hazard spur-type structures can pose to boaters
is of primary concern. Besides obstructing flow, spurs can also obstruct
boats. Small boats can be pinned broadside along these structures,
particularly the permeable spur types, if flows are below the spur crest.
Also, when the spurs are just submerged, they can be hidden obstacles to
power boats. To avoid these hazards, adequate warning signs should be posted
to alert boaters and other recreational users to the potential hazard.
Spurs can also pose hazards in other recreational uses of a river, such
as swimming and fishing. The hazards discussed above for boats also apply to
people if they are swimming or fishing in the water around the structures.
In urban areas, there is also a potential hazard to children who might find
spurs attractive structures to play on or around. In general, permeable
spurs and spur structures with sharp or pointed edges create a greater hazard
than impermeable spurs. It is recommended that spurs not be used in areas
that are heavily used for recreational activities.
Construction-Related Considerations
Construction-related factors influencing the choice of a countermeasure
type include :
8
l required construction methods, and
costs
A cost analysis and comparison of the most common types of flow control
and streambank-stabilization structures is presented in FHWA (1984). This
comparison indicates that spur-type structures will often provide a
significant economic advantage over other countermeasure types for flow
control and bank-stabilization purposes. This has been found to be
particularly true where long reaches of gently curving meanders need to be
stabilized . Spurs have also been found to provide a significant economic
advantage where flow-control and/ or flow realignment are the primary
purpose(s) of the bank-stabilization scheme. The significant economic
advantage that can be realized through the use of spurs is often the deciding
factor in the selection of a spur scheme over some other countermeasure.
The data presented in FHWA (1984) indicate spur costs ranging from
$13/ft to $445/ft, with an average of $56.2/ft (1982 dollars). This cost
variance reflects the diversity of the spur designs available, as well as
site-specific costs such as channel environment, required site preparation,
etc. Cost data for individual spur types will be presented in later
sections. Note that all cost data reported herein have been adjusted to 1982
dollars.
SPUR TYPES
e RETARDANCE SPURS
-fence type (wood or wire)
e RETARDANCE/DIVERTER SPURS
-light fence (wood or wire)
-heavy diverter
8 DIVERTER SPURS
-handpoints
-transverse dike spurs
Common spur types from within these functional groups were illustrated in
Figures 2 through 14. Additional descriptions of the more common spur types
within each of these groups will be given below. The spur designs listed
below are based on typical designs that have been used in the past. Many
design variations of these spurs are possible using different materials and
configurations.
Retardance Spurs
Fence Type
The most common fence-type retardance spur is the Henson spur jetty,
which is illustrated in Figure 2. A typical design sketch of a Henson spur
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
sections are typically constructed of 2-inch by g-inch treated wood slats
mounted vertically to a frame on 18-inch centers. Individual fence units can
vary in size depending on the specific application, but they are typically 20
to 30 feet in length. The fence units, consisting of two pipe piles and one
fence panel, are then used in multiples to make up the spur structure. One
jetty can consist of any number of fence panels. The fence panels are
mounted to be movable in the vertical direction and rigid in the la!eral
direction. The purpose of the free-floating design is to allow the structure
to flex or shift with the channel bottom to maintain contact with the
channelbed during flow events that would otherwise scour under the fence
units. This is particularly important in channels having regime/low
threshold sediment environments. The design and function (vertical
flexibility) of these structures are patented by Hold That River Inc. under
U.S. Patent No. 3,333,320. A similar wood-fence retardance spur design was
reported by the COE (1978 1. The primary difference is that this design is
fixed rigidly in the vertical direction. This design alternative is
illustrated in Figure 15(b). Another spur type similar in function to the
Henson spur (vertical flexibility) is marketed by the Ercon Corporation:
patents are pending for this design. This structure is referred to as a
10
FIGURE 2. HENSONTYPE SPUR CJETTY; BARZOS RIVER
NEAR ROSHARON,TEXAS.
11
12
FIGURE 6. DOUBLE-ROWTIMBER PILE AND WIRE-FENCE SPUR.
(AFTER CALIFORNIA DEPT. OF PUBLIC WORKS, 1970)
14
FIGURE 10. TIMBER PILE/SUSPENDED LOG SPURS: ELKHORNRIVER WEST
OF ARLINGTON, NEBRASKA.
15
16
FIGURE 14. CRIB SPURS. (AFTER CALIFORNIA DEPARTMENT
Of PUBLIC WORKS, 1970)
WOOD SLATE
(TREATED)
TYPICAL SECTION
i ii i
ELEVATION
(cl
18
palisade and has a net section made of strapping material that is supported
by steel-pipe piles instead of the wood-fence unit. Additional variations on
the fence-type retardance spurs are also possible; for example, using
chainlink panels or other materials. A rigid chainlink design is shown in
Figure 15~. Chainlink panels that are vertically flexible could also be
used.
Jack/Tetrahedron Type
Jack and tetrahedron units have also been used to form retardance
spurs. The basic structural units of these spurs, the jacks and
tetrahedrons, are illustrated in Figure 17; part (a) illustrates a jack; part
(b) illustrates a tetrahedron. These structural units are skeletal frames
adaptable to permeable spurs by tying a number of similar units together in
longitudinal alignments. Cables are used to tie the units together and
anchor key units to deadmen. Struts and wires are added to the basic frames
as needed to increase impedance to flow (either directly by their own
resistance or indirectly by the debris they collect). Figure 3 illustrates a
typical tetrahedron spur unit. The basic frame of the jack [see Figure 17
(a> 1 is a triaxial assembly of three mutually perpendicular bars acting as
six cantilever legs from their central connection. Besides the
steel-membered jack illustrated, concrete jacks have also been used. The
tetrahedron frame [see Figure 17 (b)l is assembled 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.
Retardance/Diverter Spurs
19
SCALE
1000 0 1000 2000 FT
WIRE MESH
STREAM SIDE
PLAN VIEW .
ELEVATION
(a) (b)
20
TYPICAL LAYOUT
PART PLAU
A variety of both wood and wire or chainlink structures have been used
as light-fence type retardance/diverter structures. Figures 4 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.
21
.._ _,.“”I_x.xI_-..-....-.
,”_.., ,^ _.-. ....__ _,._“~.,.~~ . ,- -.- .; ..,_---- -._---. . ..---- ..----.--..-. ,-
2’ CROWN WIDTH STONE FOUNDATION
- ~_. .~
i-- i
BOARD-FENCE DIKE
TOP OF PlLES
, BRACE BOARD
END VIEW
22
GALVANIZED WIRE YESW
ELEVATION SECTION
FIGURE 20. DETAILS OF LIGHT-FENCE-TYPE SPUR.
OiAOONAL BR
k-l bj b-r
PART ELEVATION
PART PLAN
23
. , ^
..;i-~-._- _.,.
,T..--=“.-“--“...- ..-,.- _-, ,._ ~_/(” ,. - ---.- ..-.-,- .i .._ .._. ..^ ._ _.
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 spurs are illustrated in Figures 7
and 8. Typical design sketches for these structures are given in Figure 22.
These structures are the most permeable of the permeable diverter
structures. They are constructed by suspending a wire-mesh or welded-wire
fabric on a support frame of steel "1" or "HI? beams. Other materials such as
timber piles could .be used for the support frame. Part (a) of Figure 22
illustrates a structural design that has been used for the protection of high
channelbanks; part (b) illustrates a design for lower channelbanks. In both
design configurations a triple-pile header is used to provide sufficient
structural rigidity to the spur head to resist damage from large floating
debris. Here again, the welded-wire mesh is extended to below the channelbed
to minimize underscouring, and the structure is extended into the channelbank
to prevent outflanking.
Figures 9 and 10 illustrate two timber-pile spurs. Timber piles are the
basic component of most permeable diverter structures designed. Single piles
or pile clumps (three or more piles to a clump) constitute the basic
construction unit for these structures. Timber-pile spurs of various designs
have been used including single piles in line, single piles staggered, single
piles in multiple rows, single and multiple rows of pile clumps, and
staggered rows of pile clumps. Both single piles and pile clumps have been
spaced at various distances to provide various degrees of permeability. Rows
of piles or pile clumps are then usually braced with planks or additional
piles.
24
WELDED WIRE F
25
_.---_
.,"__
;...-__,-;.
..-_~--.- . j . . ----._.. --.-- ._ --.._
FIGURE 23. TYPICAL DESIGN OF TIMBER PILE DIVERTER SPURS
(A) DESIGN SKETCH FOR PILE CLUSTERSPUR
(B) DESIGN SKETCH FOR DOUBLE-ROW,SINGLE PILE SPUR
CC> DESIGN SKETCH FOR TIMBER PILE SPUR WITH SLASHEDTREES.
26
t.
Four by eight diagonal and horizontal bracing is used between the two
rows. Horizontal four by eight timbers are also used as horizontal sheathing
on the upstream face of the upstream row of piles. In this particular
design, pole screening is used on the upstream face of the downstream row of
piles. Other designs use the downstream row of piles for bracing and do not
include a facing material.
Diverter Spurs
Hardpoints
Transverse-Dike Spurs
Transverse-dike spurs are the most widely used impermeable spurs. These
structures are most commonly constructed of dumped rock riprap. Where rot k
of sufficient size is not available, however, gabion and crib designs have
also been used. Sheet-pile, asphalt, and concrete spurs have also been
designed. The cost of these structures will be prohibitive in most cases.
28
TYPICAL SECTIQN
ELEVATION
COREROCK
SECTION
PLAN
29
(a)
EXISTINQ SANU
APR/ON
lb) (cl
PART PLAN
ADJUSTABLE OR
FIXED BASKET
WITII ROCK FlLL
SECTION
30
FIGURE 29. SKETCH OF RECTANGULARTIMBER ROCK-FILL CRIB SPUR.
l sediment environment,
0 flow environment,
e bend radius/flow alignment, and
Spunr Function/Purpose
Flow-control and/or bank-stabilization schemes are generally constructed
to function in one of the following capacities:
32
TABLE 1. SPUR TYPE SELECTION TABLE.
SPUR TYPE
RETARDANCE
Fence Type 3 2 2 33"l 1 4 3 2 3 3 2 321 321 3 3 2
Jack/Tetrahedron 3 31 33 11 4 31 3 2 1 3 21 3 21 2 4 1
RETARDANCE/DEFLECTOR
Light Fence 3 3 3 33 2 2 3 3 2 3 3 2 3 3 2 3 3 2 3 4 2
Heavy Diverter 3 4 4 33 4 3 2 3 3 3 3 2 3 4 4 3 3 2 3 4 3
DEFLECTOR
Hardpoint 3 4 4 33 3 4 2 3 4 3 3 4 3 3 2 3 4 4 3 3 5
Transverse Dike 3 4 4 33 3 4 2 3 4 3 3 4 3 3 2 3 4 3 3 3 5
33
Erosion Mechanism
e transport by streamflow,
Abrasion occurs when solid materials, such as debris and ice, carried by
the flowing water collide with and dislodge surface soil particles.
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
34
structures have two significant advantages over other spur types. First 9
impermeable diverter spurs function by deflecting currents and any floating
debris away from the channelbank. Impermeable structures also have more
structural mass than most permeable structures and, therefore, are subject to
less damage from floating debris. The light retardance structures have a
history of being severely damaged by floating debris. This is because of
their small size and the fact that permeable structures will become clogged
with floating debris, increasing the hydraulic forces on the structure.
Therefore, these structures should not be used. Retardance/deflector spurs
are designed to deflect flow currents, as are the impermeable deflector
spurs. Their permeability, however, makes them debris skimmers like the
retardance structures. The light fence retardance/diverters are prone to
damage from the floating debris and therefore, are not recommended. However,
some of the heavier retardance/diverter structures have been found to be
effective at resisting abrasion forces.
Sediment Environment
Both permeable and impermeable spurs have been used in a wide range of
sediment environments. Sediment environments (or channelbed conditions) can
be defined as regime, threshold, or rigid. For purposes of identifying an
appropriate spur type, the sediment environment can be classified as
regime/low threshold, medium threshold, or high threshold/rigid. A regime
channel is one whose bed is in motion under virtually all channel-flow
conditions. Low threshold channels are those channels whose channelbeds are
in motion under all but some very low flow conditions. Therefore, regime/low
threshold environments are characterized by large suspended and bed-sediment
loads under most flow conditions. These channels are typically cut through
noncohesive sand- and silt-size materials. Medium threshold channels are
typically cut through sand- and gravel-size materials whose channelbeds are
mobile for moderate and high channel-flow conditions. Channels cut through
cohesive materials can also be considered medium threshold. High
threshold/rigid channels are typically cut through larger gravel-, cobble-,
and boulder-size materials. These materials will remain stable or rigid
under most flow conditions, but will become mobile during high flows.
Permeable spurs are best suited for regime/low threshold and medium
threshold environments. Permeable retardance spurs have been found to be
particularly effective in regime/low threshold environments. In fact, they
generally provide an advantage over other spur types in these environments.
The flow retardance created by retardance spur schemes creates a depositional
environment within the retarded flow zone along the channeibank for the
suspended and bed-sediment loads carried by these channels. This produces a
sediment berm or bench that will stabilize the base of the channelbank.
Also, by lowering flow velocities in this zone, permeable retardance spur
schemes will reduce or eliminate the transporting ability of channel flows
35
_. -:...(, ..,;“.~./Ij-l.~,,.X^_-..--X---.“-~.-r”
._=--_.-.
-- : ” ;_ ‘_( ,--.- .~ I -.~.i.- .i jl ,.,__. _
adjacent to the bank. This is important in cases where erosion resulting
from bank-weakening mechanisms (wave erosion, subsurface flow and drainage,
etc.) is occurring. As discussed previously, Henson-type spurs provide a
particular advantage in these highly dynamic environments because of their
vertical flexibility. Other fence-type structures will also function well.
Jack and tetrahedron structures have also been quite effective in these
environments except where there are high-flow velocities. In high-velocity
environments the jacks and tetrahedrons do not provide sufficient flow
retardance and are often lost to scour.
The above discussion is not meant to imply that permeable spurs should
not be used on channels that do not carry large sediment loads. In some
cases, the flow retardance produced by the spur scheme can be designed to
provide the desired level of bank protection. This is particularly true of
permeable retardance/deflector structures. These structures are designed to
function as flow deflectors as well as retardance structures. Permeable
retardance spurs and the light fence retardance/deflector structures are not
suited as well for use on high threshold/rigid channels.
36
results in a magnified potential for erosion of the channelbed in the
vicinity and just downstream of the tip of the impermeable structures. This
condition is much more pronounced in high-velocity environments and around
sharp bends than it is in low-velocity environments and around mild bends.
The occurrence of significant erosion at and downstream from the spur tip has
been observed by the authors at numerous field sites and is well documented
in reported laboratory studies (FHWA, 1983; Ahmad, 1951a and 1951b). Local
scour is a primary concern in alluvial environments because of the highly
erosive nature of the gravel-, sand-, and silt-size material comprising the
channelbed. The potential for excessive erosion at the scour tip, combined
with the high cost of providing protection against the erosion is a drawback
in the use of impermeable diverter spurs in alluvial environments.
The flow concentration and local scour conditions just described are
characteristic of impermeable installations in all river environments. In
high threshold/rigid channels (those cut through large gravel- and cobble-
size materials) ; however, these conditions pose less of a threat to the
stability of impermeable spur schemes. Flow concentration at the spur tip
will still cause erosion in these environments. Because of the lOW
transportability of the coarse materials making up the channelbed, and the
natural channelbed armoring that occurs in these environments, however y it
will be of a much smaller magnitude. In most cases, only a limited amount of
erosion (in comparison with truly alluvial environments) will occur. This
can usually be anticipated and adequately designed at little additional
cost.
Flow Emriroment
37
Channel Velocity Environment
As discussed above, retardance spurs are best suited for regime and
low-threshold sediment environments. Within these environments, however,
retardance spurs have not been successful in high-velocity environments, or
some of the higher medium-velocity environments. In these environments, the
retardance spurs generally do not provide sufficient flow retardance and are
often undermined or outflanked due to the dynamic nature of the channelbed
combined with the high flow velocities. This has been found to be
particularly true for jack and tetrahedron structures. Jack and tetrahedron
designs should not be used in the higher medium- or high-velocity
environments. Retardance spurs are also smaller and less structurally rigid
than other spur types and, therefore, are more susceptible to structural
damage in high-velocity environments than other types of spurs.
Deflector spurs have been found to be effective over the widest range of
flow conditions. Because of their structural rigidity, impermeable deflector
spurs are the least susceptible to damage in high-velocity environments of
any of the spur types. For this reason they are generally considered to be
applicable for low-, medium-, and high-velocity environments. It must be
remembered, however, that they are subject to limitations in regime and
low-threshold sediment environments.
Flow Stage
Flow stage must be considered in light of the height of bank to be
protected. For example, if the primary cause of erosion to be protected
against occurs at low stages (as defined above), or affects only the lower
portions of the channelbank, then spurs suitable for low-stage conditions
should be used. Conversely, if the primary cause of erosion occurs at high
stages, or impacts upper portions of high banks, spurs suited for countering
high flow stages should be used.
38
As indicated in Table 1, all of the major spur types are suited for use
under low stage conditions. Under medium stage conditions, retardance spurs
are at a slight disadvantage because at this point some of the outflanking
characteristics discussed above have been observed . However, this
disadvantage can be overcome in some cases by increasing the structure height
and ensuring that the retardance-spur structures are adequately tied to the
channelbank to prevent or minimize the potential for outflanking. Al though
spur-type structures are generally not well-suited to protecting against high
stage conditions, some large retardance/deflector spurs have been found to be
adaptable to these conditions. This is due to their structural design
carrying up and into the channelbank. For example, see Figure 22(a).
Bend Radius
39
retardance/deflector spurs have been used effectively on both large- and
medium-radius channelbends. Because of their permeability, however, they
have not been as effective as impermeable deflector spurs on small-radius
channelbends. As indicated in Table 1, impermeable deflector spurs provide
an advantage over other spur types on both medium and small channelbends.
This is primarily due to their capacity as positive flow-control structures.
On extremely small radius bends (bend radii less than 350 feet), the larger
transverse-dike impermeable structures will cause excessive flow constriction
and scour problems that will make them unacceptable. Impermeable hardpoint
spurs have however, been used effectively on some channelbends less than 350
feet in radius because they do not cause a significant flow obstruction.
40
OTHER CONSIDERATIOIS
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:
0 costs,
e channel size,
o channelbed fluctuations,
e vegetation,
o construction-related impacts,
Costs
o hydraulic conditions,
e right-of-way costs,
0 site-preparation requirements,
BOARD FENCE *
STEEL PILE AND
WELDED WIRE *
TIMBER PILE
DIVERTER SPURS
RIPRAP HARDPOlNTS -
RIPRAP DIVERTER
BABION DIVERTER
- COST RANGE
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
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
bridge-repair costs. For these reasons, cost data are not available for many
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
following information on spur costs should only be used as a rough guide in
any cost analysis. The actual cost of a spur scheme should be based on the
specific design being considered and the local cost of required construction
activities and materials. All cost data have been adjusted to a 1982 base
using Engineering News Record's average annual construction cost index.
Also, all costs are reported as dollars per foot of bank protected.
The only retardance spur for which reported cost data were available was
the Henson spur jetty, illustrated in Figures 2 and 15. The costs reported
ranged from $IlO/foot to $380/foot. All sites where costs were reported were
on medium-width channels with medium to high banks. Also, they all had
moderate channelbend radii. However, all Henson spur installations consist
of the same components and protect only lower portions of the bank.
Therefore, bank height is not a significant consideration. The component
primarily responsible for the cost variance reported was spur spacing.
Spacings reported ranged from 40 to 100 feet. Costs reported for sites
having spur spacings from 40 to 50 feet ranged from $300/foot to $380/foot;
42
at the other end of the scale, schemes having lOO-foot spacings had reported
costs in the neighborhood of $1 lO/foot to $150/foot~. Although less
expensive, the schemes designed with IOO-foot spacings have not been as
effective at stabilizing channelbanks as the YO- to 50-foot spacings.
Cost data were found for four of the retardance/diverter spurs. Data
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
were reported having an average cost of $5l/foot. These structures were on
small- to medium-width channels with medium-height banks and mild
channelbends. They were constructed at lOO-foot spacings and had lengths of
approximately 25 feet.
Cost data were also available for diverter spurs. Costs for riprap
hardpoints (see Figure 25) ranged from $13/foot to $1 IO/foot. The primary
factor affecting the reported costs is hardpoint spacing, which is dependent
on channeLbend radius. Other factors influencing the cost of these
structures are site preparation and bank height. The low end of the reported
range was for hardpoints spaced at 100 feet and having lengths of 68 feet.
The $1 IO/foot hardpoints were designed with lOO-foot lengths, spaced at 40
feet on mild channelbends in channels having large widths and medium bank. A
comparison of these costs indicates that hardpoint spacing is one of the
important design parameters that must be defined.
Costs for both gabion and riprap diverter structures were reported. The
costs reported for gabion spur installations (see Figures 22 and 13) ranged
from $32/foot to $126/foot. The low end of the scale was for IO-foot long
spurs 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
high channelbank heights. Also, cost data were not reported for larger
structures. Cost data for large riprap diverter structures ranged from
$%/foot to $226/foot. Here again, a major factor reflected in the cost
range is the spur length and spacing.
Channel Size
Channel size considerations were discussed earlier in this chapter in
relation to the applicability of spurs in general. It was stated that spurs
are generally unacceptable for use on small or narrow-width channels (widths
less than 150 feet). In general, this is true. Several spur types, however,
have been used effectively on some of the larger narrow-width channels. The
spur types that have been used effectively on narrow-width channels include
the smaller permeable fence structures and rock hardpoints. Actually, any
spur that can be designed only to produce a minimal flow constriction (less
than 10 to 15 percent of the channel width) could be used. However, spurs
should not be used at sharp bends on narrow channels.
ChannelCbed Fluctuations
44
Vegetation
The existence or lack of channelbank vegetation is another environmental
characteristic that should be considered during 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,
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.
45
PERMEABILITY
Considerations of spur permeability were discussed in relation to the
selection of an appropriate spur type (retardance structure,
retardance/diverter structure, diverter structure) in the last chapter m
However, for both the retardance and retardance/diverter structures, a
variety of spur permeabilities can be and have been designed. Spur
permeability as referred to in this report is defined as the percentage of
the spur’s surface area that is open or unobstructed. In environments where
the permeable structure can be reasonably assumed not to clog with floating
debris or other material, the determination of a particular spur Is
permeability only requires computation of the unobstructed flow area within
the structure. In most environments, however, the spur ‘s effective
permeability will be reduced as floating debris clogs the face of the spur.
An estimate of the amount of spur clogging that will occur must be considered
in the computation of a given spur’s permeability. The amount of spur
clogging that can be expected to occur is difficult to estimate and must in
most cases be based on experience.
46
are acceptable. 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 mildly curving channelbends, spurs having
effective permeabilities up to 80 percent have been used effectively.
However, these high degrees of permeability are not recommended unless
experience has shown them to be effective.
47
One area of comparison between spurs of different permeabilities is the
scour pattern produced downstream of the spur tip. As might be expected, the
laboratory data indicated that 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 decreases slightly
in size, but more significantly in depth. Figure 31 illustrates the
relationship between spur permeability and scour depth for spurs having
lengths equal to 20 percent of the channel?s width. As can be seen, the
scour depth decreases with increasing spur permeability regardless of the
spur angle to flow. Figure 31 also illustrates that impermeable spurs
produce the greatest change in scour elevation over a given range of spur
angles, indicating a greater variability of local scour at the spur tip for
the range of spur angles tested. Similar trends were also observed for other
spur lengths. Therefore, if an important design consideration is to minimize
the size and depth of local scour just downstream of the spur, spur
permeability should be maximized.
The type of vertical structural member used in the permeable spur also
has a bearing on the amount of scour produced downstream of the spur tip.
Round-membered verticals produced significantly less scour than square
vertical members. This implies that all vertical structural members should
be round or streamlined to minimize local scour where possible. Here again,
if minimizing local scour depth is an important consideration for a
particular design, spurs having round or streamlined vertical support members
should be used.
48
I.0
0.6
0.6
0.4
0.2 I
I P 20 40 60 60
SPUR PERYEABILITY
1.3
49
concentration off the spur tip is important to a particular spur design, a
spur with a permeability greater than 35 percent should be used.
Spur permeability was also found to impact the length of bank protected
downstream of the spur. An expansion angle downstream of the spur tip was
used as a measure of the length of bank protected during the FHWA laboratory
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
length parallel to the channelbank in the measure of length of bank
projected. Figure 33 illustrates the relationship between spur permeability
and the length of bank protected as measured by the expansion angle for spurs
having projected lengths equal to 20 percent of the channel’s width. Figure
33 indicates that the expansion angle increases with increasing spur
permeability 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 FHWA laboratory 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.
50
Oi I ,
0 20 40 60 60
SPUR PERMEABILITY
DLCFLERATIOM
ACCELERATIOW
/
- -
v
m -
51
permeable spurs allow a flow equalization on both sides of the structure this
acceleration/deceleration turbulence is only minimal for permeable spurs.
Because of the increased potential for erosion of the channelbank in the
vicinity of the spur root and immediately downstream when 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 protect the channelbank in this area.
52
FIGURE 35. EXTENT OF PROTECTIONREQUIREDAROUND
A CHANNELBEND. (AFTER U.S. ARMY CORPSOF ENGINEERS, 1981)
MAXIMUM CURRENT
THREAD LOW FLOW -- - - - -
indicates how these flow patterns change with flow magnitude, flow stage, and
whether or not the flow event is occurring on the rising or falling limb of
the runoff hydrograph. Figure 36 illustrates a typical shift in the location
of the main flow thread or thalweg between the low and high flow conditions.
The critical erosion zones for these flow conditions are also indicated.
Consideration of these critical erosion zones dictates the length of
channelbank that must be protected by a bank-stabilization scheme.
54
impractical during medium to high flow periods 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
sites under different flow conditions. Additional information can be
obtained by flying over the site during periods of high flow, or observing
the channelbend in question from a vantage point such as a bridge or nearby
hill. Accurate prediction of the location of shifting flow patterns in a
channelbend requires a thorough knowledge of flow processes in channelbends
and an understanding of the flow conditions characteristic of the bend in
question.
The above analysis will indicate the bank regions impacted by channel
flows under various flow conditions. Not all of these flow conditions,
however, will necessarily cause bank erosion problems. As discussed
previously, evidence of the upstream limit of erosion can usually be
identified by field observations. If no evidence of an initial point of
erosion can be discerned (either from field investigation or observations
from aerial photographs), other methods must be used. One such method is to
estimate the shear stress in the channelbend for various flow conditions.
Methods for estimating shear stress in channelbends are presented in FHWA
(1984). Comparing the actual shear stresses computed with critical shear
stresses for the channelbank will define the flow condition for which erosion
begins. The point where the flow pattern for this critical flow condition
impacts the channelbank would define the upstream limit of bank protection.
The downstream limit of channelbank protection would be defined as the
furthest downstream contact point for the design discharge being considered.
Normally, this downstream limit is extended to provide a factor or margin of
safety in the design.
l empirical methods,
o field reconnaissance,
55
o review of flow and erosion forces for various flow-stage
conditions.
Spunr Length
t o**.
a 0.8.
s 0.7.
0
3 0.8.
2 0.1.
3 0.4. 3IU
36% PERYL4BILITV
PERYE4BILITV
I 0.S-l
708 l LRYEABILlTV
LRYL*BILlTv
8fUR LENQTR
(IN PERCENT Df CWAYYLL WIDTbl)
04 1
0 10 SO 10 40 0 10 20 a0 40
SPUR LENOTW 8PUR LCYDTN
(IN PLRCCNT Of CHANNEL WIDTMI (IN PERCENT Of CNANNEL WIDT,,)
57
5s
a spur length of about 20 percent of the channel’s width. Therefore, to this
point there is a near linear relationship between the spur length and the
length of bank protected by the spur. For spur lengths greater than 20
percent of the channel’s width, LBP/PL drops off more rapidly indicating that
increasing the spur length beyond this point produces less of an increase in
length of bank protected. The significance of this is that a spur having a
length not greater than 20 percent of the channel width should be used to
maximize the length of channelbank protected per unit projected length of the
spur a Although not indicated in the figure, the laboratory data also
indicate that the greater the spur angle, the more rapid the drop in LBP/PL
with increasing spur length beyond 20 percent of the channel’s width.
59
Spur Spacing
The flow expansion angle is defined as the angle between a flow tangent
at the spur tip and a line between the spur tip and the point on the
channelbank where the flow reexpands to impact the channelbank. The
definition of expansion angle is illustrated in Figure 38. The results of the
FHWA laboratory study indicated that for a spur of given permeability, the
expansion angle downstream of the spur tip varied only with the spur’s
length . Figure 39 illustrates the relationships found between spur length
and the expansion angle for various spur permeabilities. As indicated in
Figure 38, the expansion angle for impermeable spurs is almost constant at a
value of 17 degrees. In contrast, the expansion angles for the permeable
spurs were found to increase exponentially with spur projected length.
Additionally, for spur lengths less than approximately 18 percent of the
channel width, spurs having a permeability of 35 percent produce
approximately the same expansion angles as impermeable spurs. This indicates
that they protect approximately the same length of channelbank. Also, as
spur permeability increases, the length of channelbank protected by the spur
decreases and is indicated by an increasing flow expansion angle.
60
FLOW RE-EXPANSION FOINT
SCOUR LINE\
/
FLOW LINE--r
e- EXPANSION ANGLE
0 - SCOUR ANOLE
oi
0 10 20 30 40
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 then projected to the spur tip as illustrated by
line A'B'. The point where this line intersects the channelbank (point 1)
defines the location of the root of the next downstream spur.
62
FIGURE 40. DEFINITION SKETCH FOR SPUR SPACING CRITERIA.
field sites where spur schemes have failed indicate that this failure usually
occurs near the downstream end of the scheme, which indicates a need for more
concentrated protection in this area.
Several additional comments can be made based on the results of the FHWA
studies. It was found that reducing the spacing between individual spurs to
spacings closer than the maximum indicated by the spacing criteria presented
above resulted in a reduction of local scour at the spur tips. Reducing the
spacing between spurs in this way reduces the magnitude of the
expansion/contraction between spurs and as such, minimizes the magnitude of
flow acceleration at the tip of the downstream spur in each of the two-spur
sets. Also, it was found that reducing the spacing between spurs caused the
stabilized thalweg to shift further away from the concave bank towards the
centerline of the channel. This finding is illustrated in Figure 41, which
provides a comparison of the flow thalweg resulting from wide and close
spacings of spurs oriented at 120 degrees. These findings indicate that some
spacing closer than the maximum recommended by the spacing criteria indicated
above should be used.
n
In summary, a spacing criteria based on the projection of a tangent to
the flow thalweg and projected off the spur tip is recommended. It is
----FLOW TltALWEG
(bl
Spur Orientation
Spur orientation refers to the spur's angle with respect to the
orientation of the main flow current within the channelbank. Figure 42
illustrates the definition of spur angle as used within the context of this
report. Historically, guidelines for spur orientation have been based
primarily on the personal experience and judgement of design engineers. Spur
angles used at documented spur sites range from 30 to 150 degrees. They are,
however, typically greater than 90 degrees.
64
FLOW DIRECTION
@- SPUR ANGLE
connection between the bank and the spur head, they are cheaper and should be
used where appropriate. Besides being cheaper to construct, spurs
perpendicular to the bank are less susceptible to damage from wave action.
65
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 protected by individual spurs, and flow deflection.
66
STAGNATION POINT ’
(b)
STAGNATION POINT
A (cl
STAGNATION POINT f
67
i""._ _..___
.r-,,-i -----..r .-,.. - --- _. ,.,--.^ ...-.
-*__ .-;._
..__.
-
depth is inversely proportional to the spur angle. That is, the smaller the
spur angle, the greater the scour depth. The greatest scour depths occur for
spurs angled upstream; the least local scour is associated with spurs angled
downstream.
Ahmad's findings with respect to scour depth were confirmed during the
recent FHWA study, during which it was found that scour depth always
decreases with increasing spur angle. It was also found that impermeable
spurs produce the greatest change in scour elevation over a given range of
spur angles, indicating the greatest variability of local scour at the spur
tip. Also, this variability in scour depth with spur angle decreases with
decreasing spur permeability. As spur permeability increases beyond 35
percent, it was observed that the rate of change of scour elevation with spur
angle and spur length becomes very small, indicating that permeable spurs are
not as sensitive to these parameters with regard to the magnitude of local
scour as are impermeable spurs.
The amount of flow deflection produced by spurs is another factor that
is controlled by the spur’s orientation. Figure 38 provides a definition
sketch of the flow deflection angle being discussed here. It was found
during the FHWA studies that for impermeable spurs and spurs with
permeabilities up to about 35 percent the deflection angle increased with
increasing spur angle. For spurs tested during the FHWA study with
permeabilities greater than 70 percent, no change in deflection angle with
changing spur orientation was found. Flow deflection angles ranged from
approximately 140 degrees to 160 degrees for impermeable spurs with spur
angles ranging from 90 degrees to 150 degrees. Impermeable spurs with a
permeability of approximately 35 percent had flow deflection angles ranging
from approximately 130 to 145 degrees for spurs having angles of 90 degrees
to 150 degrees. These findings were for single spurs in a straight channel.
However, because the magnitude of the flow deflection angle will be impacted
by the complex forces affecting flow in channelbends, the actual flow
deflection angles recorded during the FHWA laboratory study will not reflect
actual flow deflection angles in the field. However, the trends indicated
can be expected to hold.
It is interesting that the flow deflection angles found during the FH’UJA
study indicate a steeper flow deflection for permeable spurs than for the
impermeable spurs tested. An explanation for this lies in consideration of
the shape of the riverward tip of the spur. The impermeable spurs used in
the experiments had smoothly rounded tips, which allowed for a smoother flow
transition around the spur tip. However, the permeable spurs had sharp edged
or square tips. This difference in head form was seen to have a distinct
impact on the amount of flow deflection created by the spur.
Flow patterns observed when the spur crest is submerged are illustrated
in Figure 45 for two spur orientations. The flow component across the spur
crest is of primary concern with respect to spur orientation. As illustrated
in Figure 45, flow passes over the spur crest in a direction generally
perpendicular to the spur crest. Therefore, as the spur angle is increased,
the flow over the spur crest becomes aimed more directly towards the bank,
resulting in a more severe impact on the channelbank (compare Figures 45(a)
and (b). The magnitude of this upper-bank disturbance has been observed to
be much more severe for impermeable spurs and permeable spurs with
permeabilities less than 35 percent. For permeable spurs of greater
permeability, the impact of spur-topping flows becomes less severe with
increasing permeability. For permeable spurs with permeabilities greater
than 70 percent, very little impact on the upper channelbank was observed.
69
,
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
FLOW .
.
DlRECtlON FLOW
DIRECTIO#
t
Please note also that these comments are based on laboratory findings in
a test channel with highly erodible banks. Field observations indicate that
this upper-bank erosion is not a problem if upper portions of the bank are
well vegetated or otherwise stabilized. In arid regions, however, with
little upper-bank vegetation, these flow conditions could result in
upper-bank erosion if not otherwise stabilized.
Additional conclusions from the FHWA study indicate that spurs designed
to provide flow diversion should be designed to provide a gradual flow
training through the channelbend. This is accomplished by designing the spur
system so that the spur furthest upstream is at a flat angle (that is, a
large angle 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
70
(0) (b)
degrees. Subsequent spurs within the spur scheme had angles of 140, 130,
125, 120, and 110 degrees, respectively.
115, Reducing the spur angle as one
moves downstream provides stronger flow control at the downstream limit of
the scheme based on the findings presented above. It is recommended that
spurs within a spur scheme be set with 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. The actual angles
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
conditions. Local site conditions that should be considered include flow
constriction, local scour, flow concentration at the spur tip, flow
deflection, and the need to produce a relative shift in the channel thalweg
location. The impact each of these factors has on spur angles was discussed
above.
o The greater the spur angle the smaller the angle of flow
deflection.
Q 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,
The criteria for setting an appropriate spur orientation for spurs within a
stabilization scheme will be demonstrated in the following example.
72
DESIRE0 FLOW ALI
73
FIGURE 48. SETTING THE LIMITS OF PROTECTION.
74
FIGURE 49. SETTING MAXIMUMFLOW CONSTRICTION.
75
ACTUAL BANKLINE
MAX FLOW ENCROACHMENT
------ - 10% FLOW CONITRICTION
-- - LOW FLOW TNALWEO
P-m - MEDIUM FLOW THALWEO
- - - - - HIQH FLOW TNALWEO
76
Step 4. LOCATION AND ORIENTATION OF SPUR 81
Figure 51 illustrates the procedure used to locate and orient the first
upstream-most spur. First the bend radius line RI 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 illustrated in Figure 51 as line AA. The
flow tangent is then shifted along the radius line Rl until the 10 percent
flow constriction line is reached (see line A’A’). The spur angle of 150
degrees is then t.urned in an upstream direction (clockwise) from line A ‘A’,
to establish the line BB, which is parallel to the desired spur orientation
through the constricted width line where it intersects the radius line (Rl).
The line BIB’ is then drawn through the the point defining the upstream limit
of protection (spur location point) parallel to line BB. This line defines
the location of the center line of the spur, The spur length is then set
between the eroded bankline, and the 10 percent flow constriction line.
Setting the orientation of spur #2 and each subsequent spur is the same
as the procedure for orienting spur 81. As illustrated in Figure 53, the
first step is to draw a radius line, R3, through the spur location point
c*>. 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 (see line AlA’) The spur angle of 140 degrees is then
turned in an upstream direction from line A’A’ to establish the line BB. The
line B’B’ is then drawn through the spur location point (*>. Line BIB’
defines the centerline of spur 82. The spur length is then set between the
eroded bankline, and the 10 percent flow constriction line.
77
ACTUAL BANKLINE
MAX FLOW ENCROACHMENT
------- 10% FLOW CONSTRICTION
--- LOW FLOW THALWEG
--- - MEDIUM FLOW THALWEG
---- - HIGH FLOW THALWEG
78
ACTUAL BANKLINE
MAX FLOW ENCROACHMENT
---___ - 10% FLOW CONSTRICTION
-- - LOW FLOW T”ALWEQ
-__ -MEDIUM FLOW ,-“ALWEQ
- - - - - ,,lQ,, FLOW f”ALWEO
79
ACTUAL BANKLINE
MAX FLOW ENCROAO)IMENf
__---- - 10% FLOW CONSTRICTION
--- LOW FLOW THALWEQ
---- MEDIUM FLOW THALWEQ
- - - - -HIGH FLOW THALWEG
80
Step 7. LOCATION AND ORIENTATION OF SUBSEQUENTSPURS
Steps 5 and 6 are repeated until the downstream limit of protection is
reached. Figure 54 illustrates the final geometry developed in this way.
STRUCTUREHEIGHT
The height to which spurs 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:
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 given frequency), is lower
than the channelbank height, the design stage should be used to set the spur
height. If the design flow stage is higher than the bank height, spurs 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
reduce its effectiveness when overtopped; overtopping of spurs by as much as
3 feet does not affect the spurs’ efficiency. Impermeable spurs are
generally not constructed above bank height to eliminate the possibility of
out-flanking of the spur by flow concentration and erosion behind the spur at
high river stages. The most commonly advised height for spurs is that which
corresponds to bank height.
Designing spurs lower than flow stages that carry significant debris
loads is more important for permeable spurs than for impermeable spurs
because of the flow-skimming qualities of the permeable structures. The
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
the structure.
l 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
design flow stage.
l If the design flow stage is higher than the channelbank height, spurs
should be designed to bank height.
83
FLOW
PATTERN
(a) (b)
CREST PROFILE
84
A spur’s ability to maintain contact with the channelbed and bank is
fundamental to the spur ( s structural stability. Undermining and/ or
outflanking are the most commonly reported failure 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.
Channelbed 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.
85
Figure 56. ROCKRIPRAP SPUR ILLUSTRATING LAUNCHINGOF STONE
TOE PROTECTION. (A) BEFORELAUNCHINGAT LOWFLOW
(B) DURING LAUNCHING, AT HIGH FLOW CC> AFTER
LAUNCHINGAT LOWFLOW
86
(b)
(cl
57
FIGURE 58. PERMEABLEWOOD-SLAT,FENCE SPUR SHOWING
LAUNCHINGOF STONE TOE MATERIAL.
88
RlOlNAL BED
(a)
‘SCOURED BED
(b)
mesh is rolled down the upstream face of the support members into an
excavated trench. Some form of weighting mechanism can be attached to the
bottom to secure the wire mesh to the bottom. An alternative to placing the
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
sink the wire. This might require several additional vertical supports to be
driven on the upstream side of the wire roll to guide it as it drops.
89
FIGURE 61. HENSON SPUR SHOWING OUTFLANKING.
units can be placed on top of the old units to restore the structure’s
height. A similar mechanism could be designed for other fence-type
structures. However, care must be taken not to infringe on existing
patents.
Channelbank Contact
90
FIGURE 62. WIRE-MESH PERMEABLESPUR ILLUSTRATING
SPUR ROOTEXTENDING INTO CHANNELBANK.
Permeability
0 Where itis 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.
o Where it is necessary to provide a moderate reduction in flow
velocity , a moderate level of flow control, or where the
structure is being used on a mild to moderate channelbend, the
spurs with permeabilities up to 50 percent can be used.
o 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
decreases slightly in size, but more significantly in depth.
92
protect the channelbank in this area.
- empirical methods,
- field reconnaissance,
- evaluation of flow traces for various flow
stage conditions, and
- review of flow and erosion forces for various flow
stage conditions.
Spur Length
Spur Spacing
l The spacing of spurs in a bank-protection scheme is a function of
the spur’s length, angle, and permeability, as well as the
channelbend’s degree of curvature.
Spur Angle/Orientation
94
l 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.
Spur Height
96
REFERENCES
Ahmad, M., 1951a. "Spacing and Projection of Spurs for Bank Protection, Part
1" Civil Engineering (London), Vol. 46, No. 537, April.
Ahmad, M., 1951b. "Spacing and Projection of Spurs for Bank Protection, Part
2," Civil Engineering (London), Vol. 46, No. 538, April.
Ahmad, M., 1953. "Experiments on the Design and Behavior of Spur Dikes,"
Proceedings, Minnesota International Hydraulics Convention,
University of Minnesota, September 1-4, 1953. Published by St. Anthony
Falls Hydraulic Laboratory, August.
97
_i, -I_(_
-...__x_I_-ll_ll ---- -..^ -,_.,.- l^l_--,. .I .-- ._,-_.-..“_,..--__-- ..-.- --..-._-.___-__-.- -___-_ T--_
REFERENCES(Continued)
Keeley, J-W., 1971. "Bank Protection and River Control in Oklahoma,l' Federal
Highway Administration, Bureau of Public Roads, Oklahoma Division.
Simons, D.B, Li, R.M., Alawady, M.A., and Andrew, J.W., 1979. l'Connecticut
River Streambank Erosion Study, Massachusetts, New Hampshire, and
Vermont," Prepared for U.S. Army Corps of Engineers, New England
Division, Waltham, Massachusetts, November. (Available through the
National Technical Information Service; NTIS Publication ADA080466).
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