CD 521 Hydraulic Design of Road Edge Surface Water Channels and Outlets Version 1.1.0
CD 521 Hydraulic Design of Road Edge Surface Water Channels and Outlets Version 1.1.0
CD 521 Hydraulic Design of Road Edge Surface Water Channels and Outlets Version 1.1.0
Drainage
Design
CD 521
Hydraulic design of road edge surface water
channels and outlets
(formerly HA 37/17, HA 78/96, HA 113/05, HA 119/06)
Version 1.1.0
Summary
This document gives requirements and guidance for the design of road edge surface water
channels and outlets, combined channel and pipe systems for surface water drainage, and
grassed surface water channels on motorways and all-purpose trunk roads.
Contents
Release notes 4
Foreword 5
Publishing information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Contractual and legal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Introduction 6
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Assumptions made in the preparation of this document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Mutual Recognition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Scope 13
Aspects covered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Associated documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Use of GG 101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Influencing factors 21
Health and safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Surface water channel (concrete or asphalt) and combined surfaced water channel and pipe . . . . . 21
Grassed surface water channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Environmental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Grassed surface water channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Structural . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Combined surface water channel and pipe (concrete) . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Grassed surface water channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Construction aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4. Design process 34
Step 1: Selection of channel type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
New highway construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Existing highway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Step 2: Determine design inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Step 3: Carry out calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1
CD 521 Version 1.1.0 Contents
Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Length of road to be drained by a surface water channel . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Surface water channel design flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Surcharged surface water channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Design of channel outlets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Drainage capacity of internal pipe for combined channels . . . . . . . . . . . . . . . . . . . . . . . . . 56
6. Normative references 62
7. Informative references 63
Appendix D. Roughness 75
D1 Channel roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
D2 Grassed surface water channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Appendix E. Rainfall 78
2
CD 521 Version 1.1.0 Contents
3
CD 521 Version 1.1.0 Release notes
Previous versions
Document Version Date of publication Changes made to Type of change
code number of relevant change
CD 521 1 March 2020
CD 521 0 August 2019
4
CD 521 Version 1.1.0 Foreword
Foreword
Publishing information
This document is published by National Highways.
This document supersedes HA 37/17, HA 78/96, HA 113/05, and HA 119/06, which are withdrawn.
5
CD 521 Version 1.1.0 Introduction
Introduction
Background
This document provides requirements for and advice on the design of surface water channels and their
outlets, including combined channels and pipes, and grassed channels.
This document represents the combination of four previous advice documents HA 37/17, Hydraulic
Design of Road-Edge Surface Water Channels, HA 78/96, Design of Outfalls for Surface Water
Channels, HA 113/05, Combined Channel and Pipe System for Surface Water Drainage, and HA
119/06, Grassed Surface Water Channels for Highway Runoff.
The superseded advice documents referenced above contained duplicate information, which is
resolved or removed from this amalgamated document.
The document compliments and expands on the information provided in MCHW HCD Series B [Ref
13.N] and MCHW HCD Series F [Ref 14.N] drawings.
Mutual Recognition
Where there is a requirement in this document for compliance with any part of a "British Standard" or
other technical specification, that requirement may be met by compliance with the Mutual Recognition
clause in GG 101 [Ref 10.N].
6
CD 521 Version 1.1.0 Abbreviations and symbols
Abbreviations
Abbreviation Definition
DMRB Design Manual for Roads and Bridges
FEH Flood Estimation Handbook
HCD Highway Construction Details
MCHW Manual of Contract Documents for Highways Works
SHW Specification for Highways Works
SuDS Sustainable Drainage Systems
Symbols
Symbol Definition Units
A Cross-sectional area of flow m2
Ap Cross-sectional area of post m2
b Effective cross-fall of channel -
Slope of side of channel remote from carriageway (1 unit vertical: b1 units
b1 -
horizontal)
Slope of side of channel adjacent to carriageway (1 unit vertical: b2 units
b2 -
horizontal)
Transverse slope of carriageway adjacent to channel (1 unit vertical: b3
b3 -
units horizontal)
B Surface width of flow m
Bc Channel width m
Bb Base width of channel m
Bd Surface width of flow for channel-full conditions m
Surface width of flow in surcharged channel neglecting the width of
Bs m
surcharge on hard-strip or hard shoulder
Bt The surcharged width of the channel at the downstream end of the transition m
Variable coefficient used in calculating the surface flow width of dished
c -
surface water channels
C Average plan width of the cutting m
Cd Drag coefficient -
D Diameter of pipe m
E Top width of flow in weir collecting channel m
Fd Non-dimensional number representing channel-full flow conditions -
Fs Non-dimensional number representing surcharged channel conditions -
g Acceleration due to gravity m/s2
7
CD 521 Version 1.1.0 Abbreviations and symbols
Symbols (continued)
Symbol Definition Units
Gg Width of outlet grating m
Gm Factor for channel shape -
H Grass height m
Hc Horizontal cover around pipe m
Hg Length of outlet grating m
I Mean rainfall intensity mm/hr
J Design flow depth in weir collecting channel m
K Hydraulic conveyance factor m3
8
LA Maximum allowable spacing between the last intermediate outlet and the
m
terminal outlet
L′A Actual spacing between adjacent outlets m
LB Length of road that can be drained when bypassing is permitted m
Lc Length of road that can be drained when N=1 year m
Lcum Cumulative drainage length m
Lip Average length between posts m
Lp Maximum length of road that can be drained by a section of internal pipe m
Lr Length of straight part of weir outlet parallel to the carriageway m
Ls Maximum length of road that can be drained under surcharge conditions m
Maximum length of road that can be drained under surcharge conditions
LSB m
where bypassing is permitted
Lt Length of transition for weir outlet m
LT Maximum total length of road that can be drained m
L′T Total length of road drained by this section of combined channel m
Lw Total length of weir outlet m
m Shape characteristic parameter -
mg Coefficient relating the grass type -
N Return period years
N1 Number of intermediate outlets in combined system -
1
n Manning's roughness coefficient s/m 3
1
nc Manning roughness coefficient of the carriageway s/m 3
8
CD 521 Version 1.1.0 Abbreviations and symbols
Symbols (continued)
Symbol Definition Units
1
nip Manning roughness coefficient of the internal pipe s/m 3
1
np Additional roughness coefficient due to posts s/m 3
P Wetted perimeter m
P IM P Percentage of impervious areas %
PR Percentage run-off from the whole catchment -
Q Flow rate m3 /s
Qc Design capacity of a surface water channel just flowing full m3 /s
Qd Approach flow for channel full conditions m3 /s
Qi Flow intercepted by outlet m3 /s
Qp Flow rate from the upstream internal pipe m3 /s
Qs Maximum flow capacity under surcharge conditions m3 /s
Qt Qp + Qs m3 /s
r Hydraulic radius factor (flow width/wetted perimeter) -
R Hydraulic radius of flow m
S Longitudinal gradient of a channel (vertical fall per unit distance measured
m/m
along the channel)
Se Effective value of S for channels with non-uniform slope m/m
Sj Local gradient determined at eleven equally-spaced points; j = 1 to 11 m/m
S1 Gradient at the upstream end m/m
S11 Gradient at the outlet m/m
SOIL Indices related to the infiltration potential of the soil -
T Storm duration minutes
Tc Critical storm duration minutes
U Vertical cover above and below the pipe m
U CW I Urban catchment wetness index -
V Velocity m/s
W Width of the impermeable part of the catchment m
We Effective catchment width m
Factor for use in the calculation of the shape factor for surcharged surface
X -
water channels
y Design depth of flow in surface channel - measured from the centre line of
m
the surface water channel invert
y1 Depth of channel from lower edge of carriageway to centre line of invert m
y2 Depth of channel from top edge of carriageway to centre line of invert m
y3 Overall depth of surcharged channel to centre line of invert m
Z Head of water above pipe invert m
9
CD 521 Version 1.1.0 Abbreviations and symbols
Symbols (continued)
Symbol Definition Units
α Run-off coefficient of the cutting -
η Efficiency -
ηd Efficiency of outlet for channel-full conditions -
ηs Efficiency of outlet for surcharged conditions -
ηD Efficiency of outlet grating with diagonal bar pattern -
ηL Efficiency of outlet grating with longitudinal bar pattern -
θ Angle of weir outlet -
Surcharge factor (ratio between drainage length for surcharged channel
ϕ -
and drainage length for channel just flowing full)
Depth of rainfall occurring at a particular geographical location in a storm
2minM5 mm
with T = 2 minutes and N = 5 years
10
CD 521 Version 1.1.0 Terms and definitions
Terms
Term Definition
A combined surface water channel and pipe system consisting
Combined (surface water) of a single unslotted unit, with flow discharged from the channel
channels into the internal pipe via gully gratings located over a shallow
chamber
The legislative body responsible for managing the quality of
Environmental protection agency
water bodies and flood risk in public watercourses.
A planar geocomposite arrangement designed to remove
Fin drain
sub-surface water from beneath the pavement.
Triangular or trapezoidal cross section channel, formed from
turf or seeded topsoil over a compacted subsoil, located near
Grassed surface water channel
the edge of the carriageway, used to collect, convey and
provide a degree of water quality treatment to surface water
Inlet cover with openings or bars
Grating
NOTE: Grating can be steel or concrete.
A chamber at the roadside connected to a drainage system to
receive surface water
Gully
NOTE: The chamber is usually surmounted by a surface
grating.
An outlet used to remove water part way along a surface water
channel to maintain channel capacity
Intermediate outlet
NOTE: Flow from the intermediate outlet is conveyed via a
carrier pipe or ditch to a suitable discharge point.
Point at which a drainage system discharges into a watercourse
or sewer
Outfall
NOTE: Usually, but not always, at the highway boundary.
These are for groundwater sources such as wells, boreholes
and springs used for public drinking water supply.
11
CD 521 Version 1.1.0 Terms and definitions
Terms (continued)
Approaches to manage surface water that take account of
Sustainable drainage systems water quantity (flooding), water quality (pollution), biodiversity
(SuDS) (wildlife and plants), and amenity are collectively referred to as
sustainable drainage systems (SuDS).
An outlet located at the low/end point of a channel to collect
flow carried along the channel
Terminal outlet
NOTE 1: Flow from the terminal outlet is conveyed to a suitable
discharge point.
Systems used to convey, store and treat highway runoff
12
CD 521 Version 1.1.0 1. Scope
1. Scope
Aspects covered
1.1 This document shall be used for:
1) the hydraulic design of concrete, asphalt, and grassed surface water channels;
2) the hydraulic design of outlets from triangular and trapezoidal surface water channels;
3) the hydraulic and structural design of combined channel and pipe systems; and,
4) the structural design of grassed surface water channels.
NOTE This document does not cover the structural design of concrete and asphalt surface water channels or
outlets and their chambers.
Associated documents
1.2 This document shall be read in conjunction with:
1) CG 501 [Ref 3.N];
2) CD 524 [Ref 6.N];
3) CG 502 [Ref 20.N];
4) CD 377 [Ref 16.N];
5) CD 532 [Ref 21.N];
6) CD 527 [Ref 18.N]
Implementation
1.3 This document shall be implemented forthwith on all schemes involving highway drainage on the
Overseeing Organisations' motorway and all-purpose trunk roads according to the implementation
requirements of GG 101 [Ref 10.N].
Use of GG 101
1.4 The requirements contained in GG 101 [Ref 10.N] shall be followed in respect of activities covered by
this document.
13
CD 521 Version 1.1.0 2. Surface water channel systems
14
CD 521 Version 1.1.0 2. Surface water channel systems
NOTE 1 For additional details of cross sectional shapes of channels refer to Appendix H.
NOTE 2 For further details of surface water channel profiles, dimensions and application see CD 524 [Ref 6.N].
2.3 When using a concrete or impermeable asphalt pavement, there shall be no step between the inner
edge of the channel and the top edge of the carriageway.
NOTE Where porous asphalt surfacing is used, a step can be necessary to allow water to drain from the
permeable layer.
2.3.1 The vertical distance between the top of the porous asphalt layer and the invert of the channel should
15
CD 521 Version 1.1.0 2. Surface water channel systems
16
Figure 2.4Na Example of intermediate outlet with inline grating
2.5 The surface water channel part of the combined channel and pipe system shall be triangular,
trapezoidal or rectangular in shape.
2.6 The internal pipe shall be unlined and formed as a cylindrical void below the invert of the channel.
2.6.1 The cylindrical void may be produced by an inflated flexible tube.
19
Figure 2.7N Typical cross section of triangular grassed surface water channel
3. Influencing factors
Health and safety
General
3.1 The position of surface water channels in relation to the pavement edge shall follow the guidance
provided in CD 524 [Ref 6.N].
3.2 Vehicle restraint systems shall be located outside the extent of the drainage channel.
NOTE 1 Fixed obstructions in a drainage channel, such as a longitudinal line of posts for a vehicle restraint
system, can reduce its flow capacity.
NOTE 2 Refer to CD 377 [Ref 16.N] and CD 127 [Ref 1.N] for additional requirements affecting the combined
layout of vehicle restraint systems and surface water channels, and other drainage infrastructure.
3.3 Where located adjacent to the hardstrip or hard shoulder and in front of a vehicle restraint system, the
side slopes of surface water channels, including channel outfalls, shall be 1:5 (vertical : horizontal) for
triangular channels and 1:4.5 for trapezoidal channels.
3.4 For both triangular and trapezoidal surface water channel terminations, the end ramp shall not exceed
1:4.
3.4.1 Where vehicle restraint systems are needed for other reasons and where located behind a vehicle
restraint system, the side slopes of surface water channels, including channel outfalls, may exceed 1:4.
3.5 Weir outlets shall be used on motorways and all-purpose trunk roads which exceed longitudinal falls of
1:50 (2%).
3.6 Weir outlets shall only be used where located behind a vehicle restraint system.
3.7 Gully gratings used for outlets in surface water channels shall meet the geometrical, structural and
loading requirements of BS EN 124:1994 [Ref 9.N] and BS 7903 [Ref 8.N].
Surface water channel (concrete or asphalt) and combined surfaced water channel and pipe
3.8 Surface water channels shall be limited to a maximum design depth of flow (dimension y1 in Figure 3.8)
of 150 mm where located adjacent to the hardstrip, hard shoulder or at the edge of the carriageway and
in front of the vehicle restraint system, where one is provided.
21
Figure 3.8 Cross section of surcharged trapezoidal channel
3. Influencing factors
CD 521 Version 1.1.0 3. Influencing factors
3.9 All rectangular channels, or triangular and trapezoidal channels with a design depth of flow greater than
150 mm, shall only be used when a vehicle restraint system is provided between the channel and the
carriageway.
NOTE A surface water channel on its own does not require the protection of a vehicle restraint system.
3.10 All rectangular channels, or triangular and trapezoidal channels with a design depth of flow greater than
150 mm, shall be located outside of the working width behind the vehicle restraint system.
3.10.1 All rectangular, or triangular and trapezoidal channels with a design depth of flow up to 150 mm, may
be located in the working width behind the vehicle restraint system.
NOTE Surface water channel depth is limited to mitigate the risk of vehicles overturning due to the channel
being too low relative to the vehicle restraint system.
3.11 Where a combined surface water channel and pipe is installed adjacent to the hardstrip or hard
shoulder and is not protected by a vehicle restraint system, the combined surface water channel and
pipe shall meet the loading requirements specified in MCHW Series 1100 [Ref 12.N] and BS EN 1433
[Ref 4.N].
Environmental
General
3.16 Assessment of pollution impacts from road runoff shall be determined in accordance with LA 113 [Ref
17.N].
3.16.1 The selection of surface water channel type should be based upon the need to mitigate pollution
impacts from road run off as determined by LA 113 [Ref 17.N].
NOTE Information regarding the environmental benefits associated with grassed surface water channels is
included in Appendix A and CD 532 [Ref 21.N].
23
CD 521 Version 1.1.0 3. Influencing factors
3.17.1 When a grass surfaced water channel is selected for use with an impermeable membrane (to protect
receiving ground water), the membrane should be installed below the subsoil to retain moisture to
maintain grass growth.
3.17.2 The impermeable membrane should extend over sub-surface drainage.
NOTE Refer to Figure 2.7N for a typical cross section of a triangular grassed surface water channel.
3.18 Grassed surface water channels shall be a minimum of 150 mm deep to ensure effective conveyance.
Structural
General
3.19 Surface water channel loading requirements shall be based upon the loading classes given in BS EN
124:1994 [Ref 9.N].
1) maximum spacing between bars of 100 mm for bars placed perpendicular to the line of the pipe;
2) maximum spacing between bars of 200 mm for those places parallel to the line of the pipe; and,
3) the minimum area of steel perpendicular to the longitudinal centre-line of the pipe of 1100 mm2 per
metre run of pipe.
3.23 The geometric limits, Dmax, Umin and Hcmin shall be deemed to meet the requirements of load class
D400 ( BS EN 124:1994 [Ref 9.N]) without the need for structural testing.
NOTE 1 The dimensions corresponding to a loading class of D400 in BS EN 124 ( BS EN 124:1994 [Ref 9.N])
were established from structural tests ( HRW SR 624 [Ref 2.I]).
NOTE 2 The geometric parameters D, U and Hc are shown in the typical cross section of a triangular combined
surface water channel and pipe system in Figure 3.23N2.
24
CD 521 Version 1.1.0 3. Influencing factors
Figure 3.23N2 Typical cross section of triangular combined surface water channel
and pipe system
3.24 Both light and heavier mesh shall be placed horizontally in the base of the combined channel and pipe
block such that the bars run parallel and perpendicular to the line of the pipe.
3.25 Concrete cover to the mesh reinforcement shall not be less than 40 mm.
3.25.1 Where mesh reinforcement is used, the ends of the bars may be left flat.
3.26 Where combined surface water channel and pipe systems are used, the sub-surface drain shall be
located between the pavement foundation construction and the channel.
NOTE Figure 3.26N shows a typical example of sub-surface drainage for a combined channel and pipe
system.
25
CD 521 Version 1.1.0 3. Influencing factors
Figure 3.26N Typical example of sub-surface drainage for combined channel and
pipe system
3.26.1 Where a combined channel and pipe system is used, the calculated flow capacity of the sub-surface
drainage pipe should be increased by 20%.
3.26.2 Where a type 5 or 6 fin drain is used, the geotextile should be fixed to the side of the channel.
3.26.3 Where a fin drain is used, the top of the fin drain pipe should be a minimum of 50 mm below the bottom
of the capping layer or the base of the channel, whichever is the lower.
NOTE 1 The location of the sub-surface drainage renders this less accessible due to intermediate chambers
being too shallow for connection from the fin drain. Therefore the diameter of the pipe is sized to
accommodate some build-up of sediment during the life of the system.
NOTE 2 Due to the depth of a combined surface water channel and pipe system a Type 10 filter drain MCHW
HCD Series F [Ref 14.N] cannot be used.
26
Figure 3.27N1a Typical intermediate outlet arrangement (plan view) for grassed surface water channel
3. Influencing factors
Figure 3.27N1b Typical terminal outlet arrangement (plan view) for grassed surface water channel
3. Influencing factors
CD 521 Version 1.1.0 3. Influencing factors
NOTE 2 The outlets from the grassed surface water channel form a solid obstruction within an area of
comparatively soft channel construction. Any vehicle wheel that impacts with the outlet structure or, if
travelling at a high velocity, the vehicle itself could be damaged.
3.28 The outlet apron concrete or plastic blocks shall incorporate holes for topsoil and grass growth, and be
inclined to slope downwards from the edge of the outlet to below the channel.
NOTE 1 Figure 3.28N1 shows a cross section through a typical grassed surface water channel outlet
arrangement:
29
Figure 3.28N1 Cross sections through typical grassed surface water channel outlet arrangement
3. Influencing factors
CD 521 Version 1.1.0 3. Influencing factors
NOTE 2 The design of the apron absorbs much of the energy and protects both the vehicle and the structure.
3.29 Where vehicle access across grassed surface water channels to communications equipment or other
infrastructure for routine maintenance is required, particularly where there is only a 1 metre wide
hardstrip, the channel shall be locally reinforced.
3.29.1 Locally reinforcing the channel in the vicinity of the communications equipment may take the form of a
200 mm thick layer of Type 1 sub-base on compacted formation, overlain by a 50 mm thick layer of
hydro-seeded topsoil and erosion control matting.
NOTE The suggested approach to locally reinforce the channel can be taken when early erosion control is
important to prevent the loss of topsoil, and encourage the establishment of vegetation.
3.29.2 Local reinforcement of the channel in the vicinity of the communications equipment may include
reinforcing mats within the topsoil and grass roots, or proprietary grass surface reinforcement products.
31
CD 521 Version 1.1.0 3. Influencing factors
3.30 Cohesive subsoil shall not be used in the construction of grassed surface water channels due to poor
performance when driven over.
3.31 Where there is percolation through the bed of the surface water channel, flows shall be intercepted by a
sub-surface drainage system before reaching the unbound pavement layers.
3.31.1 A Type 5 or 6 fin drain with a double cuspated core should be installed to provide sub-surface drainage,
with the top of the fin drain located above the top of the pavement sub-base level.
3.31.2 Where a fin drain is installed, no part of the drain should protrude into the grassed topsoil.
Construction aspects
3.32 Where in-line outlets are proposed for a combined channel and pipe system, the channel shall have a
minimum of 100 mm of concrete surround to the grating(s) frames (see Figure 2.4Na which shows a
typical example).
3.33 Where intermediate outlet chambers are proposed for a combined channel and pipe system, the depth
of concrete benching shall not be less than 75% of the diameter of the incoming pipe.
NOTE 1 Benching is normally constructed to the soffit level of the incoming pipe to fully contain flow, however
vertical space between the underside of the cover slab and the pipe can be limited.
NOTE 2 Appendix M includes details on construction related aspects of combined channel and pipe systems,
and grassed surface water channels.
Maintenance
3.34 Where long sections of barrier can impede maintenance of the grass, grassed surface water channels
32
CD 521 Version 1.1.0 3. Influencing factors
33
CD 521 Version 1.1.0 4. Design process
4. Design process
Step 1: Selection of channel type
New highway construction
4.1 Where a surface water channel is located within the central reserve and a carrier drain is proposed, or
an outfall is within the maximum length of road that can be drained by a channel, then a concrete,
asphalt or grassed surface water channel shall be used.
4.2 Where a surface water channel is located within the verge or central reserve and there is no carrier
drain proposed, or an outfall is not within the maximum length of road that can be drained by a channel,
then a combined concrete channel and pipe shall be used.
4.3 Where a surface water channel is located within the verge and there is a proposed carrier drain, or an
outfall is within the maximum length of road that can be drained by a channel, then a concrete, asphalt,
or grassed surface water channel shall be used.
4.4 Where a surface water channel is located within the verge on an embankment (fill) and there is a
proposed carrier drain, or an outfall is within the maximum length of road that can be drained by a
channel, then a concrete, asphalt, or lined grassed surface water channel shall be used.
Existing highway
4.5 Where a surface water channel is located within the central reserve and an existing carrier drain is
present, or an outfall is within the maximum length of road that can be drained by a channel, then a
concrete, asphalt or grassed surface water channel shall be used.
4.6 Where a surface water channel is located within the verge or central reserve and there is no existing
carrier drain present, or an outfall is not within the maximum length of road that can be drained by a
channel, then a combined concrete channel and pipe shall be used.
4.7 Where a surface water channel is located within the verge and there is an existing carrier drain, or an
outfall is within the maximum length of road that can be drained by a channel, then a concrete, asphalt,
or grassed surface water channel shall be used.
4.8 Where a surface water channel is located within the verge on an embankment (fill) and there is an
existing carrier drain, or an outfall is within the maximum length of road that can be drained by a
channel, then a concrete, asphalt, or lined grassed surface water channel shall be used.
34
CD 521 Version 1.1.0 4. Design process
NOTE Due to the nature of the calculations, there can be the need for some iteration, hence the work flow is
not always linear and can need some earlier steps to be revisited.
35
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Rainfall
5.3 The baseline value of 2minM5 for schemes in the UK shall be obtained from Figure 5.3 (taken from BS
EN 12056-3 2000 [Ref 7.N]).
36
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Figure 5.3 Values of 2minM5 rainfall depth for UK (reproduced from BS EN 12056-3:
2000 by permission British Standard Institution)
5.4 The baseline value of 2minM5 shall be increased to allow for the effects of climate change over the
lifetime of the development by the percentage stated in CG 501 [Ref 3.N].
NOTE Further information relating to rainfall data, rainfall intensity and storm duration can be found in
37
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Appendix E.
Catchment width
5.5 The effective catchment width We shall equal all impermeable surfaces draining to the surface water
channel (including the surface water channel) plus an allowance (if necessary) for runoff from a cutting.
5.5.1 Minor local variations in We may be allowed for by using an average width, calculated by dividing the
total effective area draining to an outlet by the drainage length L .
5.5.2 It should be assumed that 100% run-off occurs from concrete and black-top surfaces.
5.6 Field data shall be used to identify the amount of runoff from a cutting.
5.6.1 In the absence of suitable field data, We should be calculated from:
We = W + αC
where:
We effective catchment width (m)
W width of the impermeable part of the catchment (m)
α run-off coefficient of the cutting
5.6.3 Appropriate choices of antecedent wetness for Northern Ireland, Scotland, Wales and English counties
may be chosen from Table 5.6.3.
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CD 521 Version 1.1.0 5. Design of surface water channels and associat...
NOTE The basis of the data in Tables 5.6.2 and 5.6.3 is explained in Appendix C.
Channel geometry
5.7 Where an increase in the size of a surface water channel is desired part way along a drainage length,
transitions shall be gradual in order to minimise energy losses.
5.7.1 Where the invert is lowered, the length of the transition should not be less than 15 times the change in
depth.
5.7.2 The side of a surface water channel at a transition should not diverge outwards from the longitudinal
centre-line at an angle greater than 1:3 in plan.
5.7.3 Transitions for triangular and rectangular surface water channels shown in Figure 5.7.3 should be used.
39
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Figure 5.7.3 Transitions for triangular and rectangular surface water channels
NOTE In the case of a triangular profile, it is possible to deepen the channel without altering the cross-falls b1
and b2, and similarly a rectangular channel can be deepened while keeping the width Bb constant.
5.8 The base of trapezoidal and rectangular surface water channels shall fall away from the carriageway
edge at 1:40.
NOTE The channel base cross-fall is needed to provide self-cleansing characteristics that are similar to those
of conventional kerbed channels.
5.9 The factor Gm for the surface water channel shape shall be calculated using Equation 5.9.
40
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.10 The shape characteristics can be expressed in terms of a parameter which shall be defined using
Equation 5.10.
NOTE 1 The effect on flow capacity of a base section cross-fall for trapezoidal or rectangular surface water
channels is very small and can be neglected.
NOTE 2 For a triangular profile m = 1, and for a rectangular profile m = 0. Trapezoidal channels can have
values of m between 0 and 1.
5.11 For dished surface water channels, the surface flow width shall be calculated using Equation 5.11.
Equation 5.11 Surface flow width for dished surface water channels
B = cy m
where:
B surface width of flow (m)
c variable coefficient (-)
y design depth of flow (m)
m shape characteristic parameter (-)
5.12 The effective cross-fall of the surface water channel shall be defined using Equation 5.12.
41
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.14 The hydraulic-radius factor for a trapezoidal surface water channel shall be calculated using Equation
5.14.
Equation 5.14 Hydraulic radius factor for a trapezoidal surface water channel
Bb + (b1 + b2 )y
r= [( )1 ( )1 ]
Bb + 1 + b1 2 2 + 1 + b2 2 2 y
where:
b1 slope of side of channel remote from carriageway (1 unit vertical: b1 units horizontal)
b2 slope of side of channel adjacent to carriageway (1 unit vertical: b2 units horizontal)
Bb base width of channel (m)
NOTE 1 For a wide shallow surface water channel, r tends towards unity.
NOTE 2 The effect of the base section cross-fall on flow capacity is very small and can be neglected.
5.15 The hydraulic-radius factor for a triangular surface water channel shall be calculated using Equation
5.15.
Equation 5.15 Hydraulic radius factor for a triangular surface water channel
b1 + b2
r=( ) 1 ( )1
1 + b1 2 2 + 1 + b2 2 2
NOTE A triangular surface water channel with one side vertical can be catered for by putting b1 = 0.
5.16 The hydraulic-radius factor for a rectangular surface water channel shall be calculated using Equation
5.16.
Equation 5.16 Hydraulic radius factor for a rectangular surface water channel
Bb
r=
Bb + 2y
where:
Bb base width of channel (m)
y design depth of flow (m)
NOTE The effect of the base section cross-fall on flow capacity is very small and can be neglected.
Gradient
5.17 Where the gradient of a surface water channel (or pipe) varies with distance, an equivalent value of
uniform slope shall be calculated using Equation 5.17.
42
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
where:
Se equivalent value of uniform slope
Sj local gradient determined at eleven equally-spaced points (j = 1 to 11)
S1 gradient at the upstream end
S11 gradient at the outlet
5.17.1 Where the longitudinally-varying gradient is locally zero (but not adverse) at the upstream or
downstream end of a surface water channel or pipe, the zero value should be replaced in Equation
5.17 by the following.
Roughness
5.18 The hydraulic resistance of a surface water channel used in design shall depend upon its surface
texture, the standard of construction, and the presence of deposited sediment.
5.18.1 Manning's roughness coefficients n given in Table 5.18.1 should be used for design of concrete or
asphalt surface water channels.
Table 5.18.1 Values of Manning's roughness coefficents for surface water channels
Channel type Condition n
Concrete Average 0.013
Concrete Poor 0.016
Asphalt Average 0.017
Asphalt Poor 0.021
NOTE Further information about the factors influencing the hydraulic resistance is given in Appendix D.
5.19 For grassed surface water channels, the Manning's roughness coefficient shall be calculated using
Equation 5.19.
43
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Equation 5.19 Manning's roughness coefficent for grassed surface water channels
0.05
n= mg H
1− 5 1
R3 S 2
where:
grass height (assume H = 0.05m for fescues-dominated mixture and 0.075m for perennial
H
rye grass-dominated mixture)
coefficent relating to grass type ( mg = 0.0048 for perennial rye grass and 0.0096 for
mg
fescues-dominated mix.)
Calculations
Length of road to be drained by a surface water channel
5.20 The maximum length of road that can be drained by a section of surface water channel shall be
calculated using Equation 5.20.
Equation 5.20 Length of road that can be drained by a surface water channel
1 [ ]1.62
S2 2
−0.362 A
L = Gm (ry) 3 (N − 0.4)
n We (2minM5)
where:
length of road/ surface water channel between two adjacent outlets on a continuous
L
slope, or the distance between a point of zero slope and the downstream outlet (m)
Gm factor for channel shape (-)
S longitudinal gradient of the surface water channel (m/m)
r hydraulic-radius factor (-)
y design depth of flow (m)
N design return period (years)
We effective width of the catchment (m)
NOTE 1 Equation 5.20 is valid for all types of triangular, rectangular or trapezoidal channel provided the
cross-sectional shape factors b1, b2 and Bb (as appropriate - see Figure 2.2) do not vary with the depth
of flow or with distance along the drainage length.
NOTE 2 The effect of a base section cross-fall for trapezoidal or rectangular surface water channels on flow
capacity is very small and can be neglected.
NOTE 3 The total length of road that can be drained by a combined channel and pipe system is a function of
both channel and pipe capacity. Refer to the drainage capacity of internal pipes section.
NOTE 4 Use of Equation 5.20 to determine the maximum length of road to be drained does not take account of
bypassing at outlets.
44
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.20.1 Where a symmetrical triangular surface water channel is desired then the length of road that can be
drained may be calculated using Equation 5.20.1.
Equation 5.20.1 Length of road that can be drained by a symmetrical triangular surface water
channel
S 2 (N − 0.4)−0.362
1
(By)2.29
L = 1.56 × 10 6
1 1.62
(B 2 + 4y 2 ) 3 n [We (2minM5)]
where:
B surface width of flow (m)
y design depth of flow (m)
5.21 Where the size of a triangular surface water channel for a given length of road needs to be determined,
the design depth of flow shall be calculated using Equation 5.21.
S2 b
where:
b effective cross fall ( = b1 + b2 )
y design depth flow (m)
n Manning's roughness coefficient
L length of road to be drained (m)
S longitudinal gradient of the surface water channel (m/m)
r hydraulic radius factor (-)
N design return period (years)
NOTE Equation 5.21 is valid for all types of triangular surface water channel provided the cross-sectional
shape factors b1 and b2 do not vary with the depth of flow or with distance along the drainage length.
5.22 Where the size of a rectangular surface water channel for a given length of road needs to be
determined, the design depth of flow shall be calculated using Equation 5.22.
45
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
NOTE 1 To determine the design depth of flow, the unknown flow depth appears on both sides of Equation 5.22
and some iteration is needed to find the solution.
NOTE 2 Equation 5.22 is valid for rectangular surface water channels provided the cross-sectional shape factor
Bb does not vary with the depth of flow or with distance along the drainage length.
NOTE 3 The effect of a base section cross-fall for rectangular surface water channels on flow capacity is very
small and can be neglected.
5.23 For trapezoidal channels, flow depth, y , shall be determined using the following design procedure:
1) make an initial estimate of the size and shape of channel needed;
2) calculate the flow area A and the hydraulic-radius factor r ;
3) determine the values of the shape characteristic parameter m , and the factor of channel shape Gm ;
4) calculate the length L of road that can be drained and compare with the required length; and,
5) revise the channel geometry and repeat steps (1) to (4) until the required drainage length is
achieved.
NOTE No general equation for directly determining flow depth, y , in trapezoidal channels can be obtained
because different solutions are possible depending on the chosen values for base width and side slope.
5.24 Where a trapezoidal channel size varies part way along the length, the design of the smaller, upstream
channel shall be checked to ensure that it has the capacity to drain the length of road upstream of the
transition point.
5.24.1 The capacity of the larger, downstream channel should be similarly checked using the overall length of
road draining to the outlet.
5.24.2 The larger downstream channel should be designed assuming a drainage length 5% greater than the
actual length draining to the outlet.
NOTE For transitions in trapezoidal surface water channels, it is not possible to keep all the shape factors b1,
b2 and Bb constant. For this reason, the drainage length used when designing the downstream length
of the surface water channel is 5% greater than the actual length draining to the outlet.
46
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.25.1 The design flow for a trapezoidal channel should be calculated using Equation 5.25.1.
1
[ ] 13
S 2 (2Bb + by)5 2 5
Q= r y
n 32(Bb + by)2
where:
Bb base width of channel (m)
b effective cross-fall
y design depth of flow (m)
r hydraulic-radius factor
5.25.2 The design flow for a triangular channel should be calculated using Equation 5.25.2.
5.25.4 The design flow for a rectangular channel should be calculated using:
47
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Bb S 2 [ 2 5 ] 13
1
Q= r y
n
where:
Bb base width of channel (m)
y design depth of flow (m)
Equation 5.26.2 Maximum flow for symmetrical triangular surface water channels
Qs = 1.575ϕQc
where:
Qs maximum flow capacity under surcharge conditions when N=5 years ( m3 /s)
Qc design capacity of a surface water channel just flowing full when N=1 year ( m3 /s)
surcharge factor (ratio between drainage length for surcharged channel and drainage
ϕ
length for channel just flowing full)
NOTE ϕ can be estimated using Figure F.1 or F.2., or Table F.1 in Appendix F.
5.26.3 An alternative approach for estimating the maximum length of road that can be drained by a
symmetrical triangular surface water channel under surcharged conditions may be estimated from
Equation 5.26.3.
Equation 5.26.3 Maximum length of road that can be drained by a symmetrical triangular surface
water channel
Ls = ϕLc
where:
maximum length of road that can be drained under surcharge conditions when N=5 years
Ls
(m)
Lc length of road that can be drained when N=1 year (m)
48
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
NOTE 1 Equation 5.27 is defined for a trapezoidal surface water channel, but can be applied to the case of a
triangular surface water channel (with Bb = 0) or a rectangular surface water channel (with b1 =0= b2 ).
NOTE 2 Where the surface water channel can be constructed without a step between the inner edge of the
channel and the top edge of the carriageway, y2 = y1 .
5.28 The hydraulic-radius factor for an equivalent surcharged surface water channel shall be calculated
using Equation 5.28.
b1 y3 + b2 y1 + b3 (y3 − y2 ) + Bb + (y2 − y1 )
r= 1 1 1
(b21 + 1) y3 + (b22 + 1) 2 y1 + (b23 + 1) 2 (y3 − y2 ) + Bb + (y2 − y1 )
2
NOTE Equation 5.28 is defined for a trapezoidal surface water channel, but can be applied to the case of a
triangular surface water channel (with Bb = 0) or a rectangular surface water channel (with b1 =0= b2 ).
5.29 The shape factor for the equivalent surcharged surface water channel shall be calculated using
Equation 5.29.
Equation 5.29 Shape factor for the equivalent surcharged surface water channel
[ ( ) 12 ]
1 14
m= X − 1 + X2 + X + 1
2 3
NOTE Values of m for equivalent surface water channels can be greater than unity.
5.30 X for an equivalent surcharged surface water channel shall be calculated using Equation 5.30.
5.31 The hydraulic conveyance factor for an equivalent surcharged surface water channel shall be
calculated using Equation 5.31.
49
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
8 nc 3
where:
nc = Manning roughness coefficient of the carriageway
NOTE Equation 5.31 is defined for a trapezoidal surface water channel, but can be applied to the case of a
triangular surface water channel (with Bb = 0) or a rectangular surface water channel (with b1 =0= b2 ).
50
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.37 The off-line outlet geometry for triangular channels shall be as shown in Figure H.6 (see Appendix H).
NOTE In the arrangement for Figure H.6, the side slope on the road side is extended below the invert of the
channel to produce a ponding effect over the gratings which increases the efficiency of the outlet.
5.38 The spacing between pairs of off-line gratings in triangular channels shall not be less than 1.25Gg .
5.39 The width of gratings for off-line outlets in triangular channels shall be determined by Equation 5.39.
51
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.46.1 Calculation of the flow rate for channel-full conditions, Qd , and surcharged channel conditions, Qs ,
should be based on Manning's resistance equation, as set out in Equation 5.25.
5.46.2 When checking for surcharged conditions, an estimate of flow rate Qs may be taken from Figure H.3 for
triangular channels and H.4 for trapezoidal channels (see Appendix H).
NOTE The curves in Figure H.3 and H.4 in Appendix H are based on 1 m width of surcharging of the
carriageway at cross-falls of 1:30, 1:40 and 1:60. The value of Qs /Qd can be read from the curves and,
with Qd calculated using Manning's resistance equation, the value of Qs can be determined.
5.47 Values of outlet efficiency, ηd for channel-full and ηs for surcharged conditions, shall be determined from
design curves H.11 to H.22 in Appendix H.
5.47.1 The efficiency of an outlet may be defined as the ratio of the flow intercepted by the outlet, Qi , to the
total flow approaching it, expressed by Equations 5.47.1a and 5.47.1b.
52
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Equation 5.49.2 Length of road to be drained, for use in channel design where bypassing is
permitted at intermediate outlets
1
LB = L + (1 − η)LU
2
where:
LU length of surface water channel upstream of the intermediate channel
L length of surface water channel downstream of the intermediate outlet
LB the length of road that can be drained where bypassing is permitted
η collection efficiency of outlet (efficiency of 80% corresponds to η =0.8)
53
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.51.1 Where ηD is the efficiency corresponding to a diagonal bar pattern, the efficiency ηL corresponding to a
longitudinal bar pattern should be estimated by Equation 5.51.1.
ηL = 0.5 + 0.5ηD
NOTE 1 The design curves shown in Figures H.11 to H.22 (see Appendix H) are based on tests carried out with
gratings having a diagonal bar pattern. Comparing the performance of gratings equivalent in terms of
overall size and waterway area, longitudinal bars are more efficient that diagonal bars.
NOTE 2 Where the outlet is located adjacent to the carriageway and in front of any vehicle restraint system, use
of a longitudinal bar pattern grating can present a hazard to cyclists and provide a lower skidding
resistance than diagonal bars.
5.52 Where it is not possible to achieve minimum efficiencies of 80% for intermediate outlets and 97.5% for
terminal outlets (for both in-line and off-line arrangements) a weir outlet shall be used.
NOTE 1 The flow collecting efficiency of an outlet decreases as the steepness of the surface water channel
(steeper than 1:50) and velocity of flow within it are increased.
NOTE 2 A weir outlet functions by gradually directing water away from the carriageway and discharging it over a
side weir into a collecting channel (see Figures H.23 and H.24 in Appendix H for layout).
5.53 Where a weir outlet is used, a vehicle restraint system shall be installed along the carriageway side of
the collecting channel (see layout in Figure H.23 in Appendix H).
5.54 The procedure for designing weir outlets shall be as set out in the flow charts shown in Figures H.26
(for triangular channels), H.27 and H.28 (for trapezoidal channels), set out in Appendix H.
5.55 The total length, Lw , of the weir outlet shall comprise a straight length, Lr , parallel to the edge of the
carriageway and a length, La , at an angle to the carriageway (see Figure H.24 in Appendix H).
5.56 Lr shall be equal to Bt , the surcharged width of the channel at the downstream end of the transition,
so that in Equation 5.56:
Equation 5.58.1a Weir outlet transition length - triangular channel 1:5 side slopes
Lt = 25y1
54
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
Equation 5.58.1b Weir outlet base width - triangular channel 1:5 side slopes
Bb = 5y1
where:
is the design flow depth in the surface water channel upstream of the transition (see
y1
Figure H.1 in Appendix H)
5.59 For trapezoidal channels, in the transition section upstream of the weir outlet, the cross-falls and overall
depth of the channel shall remain constant but base width increased (from a value of 2y1 at the
upstream end) in order to reduce design flow depth approaching the weir outlet.
5.59.1 For trapezoidal channels with 1:4.5 side slopes, the length, Lt , of the transition and the base width, Bb
, at the downstream end should be determined by Equations 5.59.1a and 5.59.1b.
Equation 5.59.1a Weir outlet transition length - trapezoidal channel 1:4.5 side slopes
Lt = 25y1
Equation 5.59.1b Weir outlet base width - trapezoidal channel 1:4.5 side slops
Bb = 7y1
5.59.2 For trapezoidal channels with 1:5 side slopes, the length, Lt , of the transition and the base width, Bb ,
at the downstream end should be determined by Equations 5.59.2a and 5.52.2b.
Equation 5.59.2a Weir outlet transition length - trapezoidal channel 1:5 side slopes
Lt = 30y1
Equation 5.59.2b Weir outlet base width - trapezoidal channel 1:5 side slopes
Bb = 8y1
5.60 Where a weir outlet is used, the collecting channel into which flow is discharged shall be deep enough
to allow the outlet to discharge freely when the surface water channel is flowing under surcharged
conditions.
5.60.1 The design flow depth, J (in m), of the receiving channel may be estimated from Equation 5.60.1.
5.60.2 The overall depth of the channel should be determined by adding 0.15 m to the value of J .
5.61 A chamber shall be located below or immediately adjacent to an outlet (in-line, off-line or weir collection
channel) to enable flow from the surface water channel to be conveyed to a suitable discharge point.
55
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
NOTE 1 Possible configurations of outlet chambers are shown in Section 2, Figures 2.4Na and 2.4Nb
(combined surface water channel and pipe), Section 3, Figures 3.28N1 and 3.29N1 (surface water
channel - grassed), and Appendix I, Figures I.1 to I.5.
NOTE 2 Where a combined channel and pipe system is used, and in the case of an intermediate weir outlet,
flow from the collecting channel can be discharged into the internal pipe of the combined system via a
covered chamber formed on the line of the surface water channel and downstream of the weir outlet.
5.62 The outgoing chamber pipe shall be designed so that the water level in the chamber is not within 150
mm of the underside of the gratings when the outlet is receiving flow from the channel under
surcharged conditions.
5.62.1 The height, Z (in m), of the water surface in the chamber above the invert of the outgoing pipe may be
estimated from Equations 5.62.1a and 5.62.1b.
Qs is the flow rate (in m3/s) in the chamber corresponding to surcharged conditions in the
surface water channel
Equation 5.62.1b Height Z for combined surface water channel and pipe outlets
D Q2
Z= + 0.23 t4
2 D
where:
Qt is Qs + Qp (from Equation 5.73.1a)
5.62.2 The gradient and diameter of the outgoing pipe may be determined from standard flow tables or
resistance equation so that the pipe is just flowing full under surcharged channel conditions.
5.62.3 Where the chamber below the outlet is designed to trap sediment, the outgoing pipe from the chamber
may be connected directly to a carrier pipe by means of a 45 degree junction without the need for a
chamber at the junction position.
56
Figure 5.64 Longitudinal profile of combined surface water channel and pipe system
5.65 The diameter of the internal pipe in a combined surface water channel and pipe system shall be kept
constant along a drainage length.
NOTE 1 The diameter of the internal pipe is kept constant in order to simplify the construction process.
NOTE 2 Deposition of sediment at the upstream end where flow rates are low can be accommodated within an
oversized pipe without causing surcharging problems at road level.
5.65.1 Any change in pipe size should coincide with the position of an intermediate outlet.
5.66 Where more than one pipe size is used in a combined system, the invert of the outgoing pipe shall not
be higher than the invert of the incoming pipe.
5.66.1 Where more than one pipe is used in a combined system, the soffits of the two pipes should be set at
the same level.
5.67 When there is significant variation in the longitudinal gradient along the length of pipe, the longitudinal
gradient S shall be replaced by the effective longitudinal gradient Se .
NOTE 1 Refer to the Design inputs, Gradient sub-section, clause 5.17 onwards, for the method to calculate Se .
NOTE 2 Where the longitudinal profile of the road has been determined in advance of the hydraulic design, the
procedure to determine Se can be iterative.
5.68 Where a drainage length has intermediate crest or sag points, the drainage length shall be divided into
separate sub-lengths and the pipe in each sub-length sized separately.
5.69 At intermediate sag points, the flow in the internal pipe shall be discharged via an outfall to a
watercourse, or outlet to a toe ditch or separate carrier pipe.
5.70 The maximum length of road that can be drained by an internal pipe shall be determined using
Equation 5.70.
Equation 5.70.1 Drainage length for internal pipes when N=5 years
1
6S
2 D3.91
Lp = 1.24 × 10
nip [We (2minM5)]1.62
NOTE The maximum flow depths produced by storms with a return period of N = 1 year cannot exceed 60% of
the pipe diameter.
5.71 Manning's roughness coefficients for smooth-bore pipes formed in concrete by the slip-forming process
given in Table 5.71 shall be used.
Table 5.71 Manning's roughness coefficients for slipformed concrete internal pipes
Condition nip
Average 0.014
Poor 0.016
NOTE The 'average' condition assumes that the surface roughness of the pipe walls is equivalent to a value of
ks = 0.6 mm in the Colebrook-White resistance equation (see HRW Tables [Ref 19.N]) but with an
additional allowance of ∆ n = 0.0025 for energy losses caused by flow entering the pipe at intermediate
58
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
outlets. The 'poor' condition assumes a rougher surface finish of ks = 1.5 mm and a depth of sediment
deposit in the invert equal to 5% of the pipe diameter.
5.71.1 Where the internal pipe is formed by a process other than slip-forming, values of nip should be
assessed taking account of the surface finish of the pipe, the energy losses at intermediate outlets, the
existence of joints and the possible presence of sediment deposits.
5.72 Flow capacities of the surface water channel and internal pipe shall be assessed separately.
5.73 The maximum flow capacity of the internal pipe shall be determined from the Manning resistance
equation.
5.73.1 For an internal pipe flowing full at its downstream end, the flow capacity and corresponding velocity
should be calculated using Equations 5.73.1a and 5.73.1b.
Equation 5.73.1a Capacity of an internal pipe flowing full at its downstream end
8 1
D3S2
Qp = 0.312
nip
Equation 5.73.1b Velocity of an internal pipe flowing full at its downstream end
2 1
D3S2
Vp = 0.397
nip
5.74 The velocity of an internal pipe at the downstream end of a drainage length shall not be less than the
appropriate value given in Table 5.74.
NOTE 1 Interpolation can be used to obtain values of Vmin for intermediate pipe sizes.
NOTE 2 Derived from guidance on sediment problems in pipes given in CIRIA R141 [Ref 5.I] and CD 527 [Ref
18.N].
5.75 The maximum total length of road that a section of combined surface water channel and pipe system
can drain shall be calculated using Equation 5.75. :
59
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
5.76 The number and positions of outlets along a combined system shall not exceed the calculated
maximum values of Lp , LA and LT .
5.76.1 Where lengths of road have a constant longitudinal gradient and require equal spacing between
adjacent outlets, the following process should be followed:
1) calculate the number of intermediate outlets, N1 , using Equation 5.76.1a;
2) calculate the actual spacing between adjacent outlets, L′A , using Equation 5.76.1b, with the last
intermediate outlet located at a distance, Lp , from the upstream end of the system;
3) calculate the total length of road drained by this section of combined channel, L′T (where the
terminal outlet can be located) using Equation 5.76.1c.
Equation 5.76.1c The total length of road drained by the section of combined channel
L′T = Lp + L′A
NOTE L′A is smaller than the maximum allowable value of LA .
5.76.2 Where lengths of road have a varying longitudinal gradient, the following process should be followed,
starting from the upstream end of the system:
1) calculate the maximum allowable length of road, LA , that can be drained by the surface channel
using the local value of longitudinal gradient, and locate the first intermediate outlet at this point;
2) determine points downstream at which the maximum drainage capacity of the surface water channel
is reached and locate intermediate outlets at each point;
3) add together the individual values of channel length, L , to find the cumulative drainage length, Lcum
, measured from the upstream end of the system;
4) determine the total length of road, L′T , that can be drained by this section of combined system (this
is equal to the first value of Lcum that exceeds the maximum drainage length, Lp , of the internal
pipe);
5) locate a terminal outlet at this downstream point to discharge all the flow from the combined system
to a watercourse, toe ditch or separate carrier pipe.
5.77 The maximum allowable spacing, LA , between adjacent outlets (or between the upstream end of a
combined system and the first outlet) shall be determined by the following criteria:
60
CD 521 Version 1.1.0 5. Design of surface water channels and associat...
61
CD 521 Version 1.1.0 6. Normative references
6. Normative references
The following documents, in whole or in part, are normative references for this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
62
CD 521 Version 1.1.0 7. Informative references
7. Informative references
The following documents are informative references for this document and provide supporting
information.
Ref 1.I HR Wallingford. Escarameia M, Todd A J and May R W P. HRW SR 585,, 'Combined
surface channel and pipe system: Interim Report'
Ref 2.I HR Wallingford. May RWP, Todd AJ, Boden DG and Escarameia M. HRW SR 624,
'Combined surface channel and pipe system: Project Report'
Ref 3.I OECD Symposium on Road Drainage, Bern, Switzerland,. May R W P.. OECD ,
'Design of highway drainage channels'
Ref 4.I HR Wallingford. HRW DE 30, 'Design of highway drainage channels: Preliminary
analysis'
Ref 5.I CIRIA. CIRIA R141, 'Design of sewers to control sediment problems'
Ref 6.I National Highways. CD 535, 'Drainage asset data and risk management'
Ref 7.I Institute of Hydrology, UK. Faulkner D. FEH V2, 'Flood Estimation Handbook, Volume
2 Rainfall frequency estimation'
Ref 8.I Institute of Hydrology UK . Flood SR16, 'Flood Studies Supplementary Report 16'
Ref 9.I HR Wallingford. Escarameia M and Todd AJ. HRW SR 662, 'Grassed surface water
channels for road drainage,'
Ref 10.I Highway Research Board (USA). Izzard CF and Hicks WI. Highway Research Board,
'Hydraulics of runoff from developed surfaces'
Ref 11.I Highways England. MCHW HCD NMCS, 'MCHW Volume 3: HCD Section 3 -
National Motorway Communications System Installation Drawings'
Ref 12.I Transport Research Laboratory. HR Wallingford. TRL CR 8, 'Motorway Drainage Trial
on the M6 Motorway, Warwickshire' , 1985
Ref 13.I Science and Practice, CAB International. Adams WA and Gibbs RJ. Adams & Gibbs
1993, 'Natural turf for sports and amenity'
Ref 14.I Highways England. TD 131, 'Roadside technology and communications'
Ref 15.I BSI. BS 4449, 'Steel for the reinforcement of concrete. Weldable reinforcing steel.
Bar, coil and decoiled product. Specification'
Ref 16.I Highways England. CD 236, 'Surface course materials for construction'
Ref 17.I HR Wallingford. Escarameia M and May R W P. HRW SR 406, 'Surface water
channels and outfalls: Recommendations on design'
Ref 18.I National Water Council. Hydraulics Research Ltd.. HRW: Wallingford Procedure,
'The Wallingford Procedure: Design and Analysis of Urban Storm Drainage'
Ref 19.I The Sports Research Institute, London. Shildrick J. Turfgrass Manual, 'Turfgrass
Manual'
63
CD 521 Version 1.1.0 Appendix A. Grassed surface water channel - environmental...
A2 Flow attenuation
The increased surface roughness of the grassed channel in comparison with that of concrete can
reduce the corresponding flow velocity. Comparison between average flow velocities in the two types of
channel indicates velocities in grassed channels around 25% of those in concrete channels. A
reduction in velocity will increase the time of flow within the channel and thereby increase the time of
concentration. Consequently, the peak discharge flow rate to a receiving watercourse can be less from
the grassed channel.
A3 Sediment deposition
The lower flow velocity generates less energy thereby reducing the sediment transport ability of the
channel flow. Sediment can settle in the channel bed and be trapped by the grass blades.
A4 Pollution containment
Sediment is the prime constituent in the transport of heavy metals and polluting materials, such as lead,
copper, zinc, and cadmium. Metals are mainly contained in the suspended solids carried along by the
channel flow and are removed when the solids are deposited as sediment. Increased sediment
deposition can result in less of these pollutants reaching the receiving watercourse.
64
CD 521 Version 1.1.0 Appendix B. Worked examples
The width of flow corresponding to the design flow depth y1 is: B = 10 x 0.120 = 1.200 m
and the corresponding flow area is: A = 1
2 × B × y1 = 1
2 × 1.200 × 0.120 = 0.072m2
The shape parameter m of the channel (see Equation 5.10) has a value of:
m= 1.200×0.120
0.072 − 1 = 1.00
The width of the two-lane carriageway drained by the channel is 9.300 m (including two 1.000 m-wide
hardstrips). The overall width of the channel itself is:
B = b1 y3 + b2 y1 = (5 × 0.145) + (5 × 0.120) = 1.325 m
The road is on embankment and there is no run-off from the verge into the channel. The effective
catchment width is therefore:
We = 9.300 + 1.325 = 10.625 m
The road is located near Coventry and from Figure 5.3 it is found that the characteristic value of rainfall
depth occurring in 2 minutes with a return period of 5 years is:
2minM5 = 4.0 mm
Note: allowance for the effects of climate change contained in CG 501 [Ref 3.N] should be adopted for
all new designs.
The channel is to be designed so that the design flow depth y1 is not exceeded by run-off from storms
occurring once every year on average; the design return period is therefore: N = 1 year
The length of road that can be drained by the channel is calculated from Equation 5.20, in which the
factor Gm corresponding to the triangular shape of the channel is obtained from Equation 5.9 as: Gm =
2.90 x 106 (2.65 - 1.00) = 4.79 x 106
The maximum drainage length is therefore:
1 [ ]1.62
2
−0.362
L = 4.79 × 106 (0.005)
0.013 (0.981 × 0.120) × (1.0 − 0.4) ×
2 0.072
3
10.625×4.0 = 244 m
The critical storm duration corresponding to the design flow condition can be estimated from Equation
E.2 in Appendix E as:
[ ]
−2
Tc = 0.085 0.013×244
1 (0.981 × 0.120) 3 = 15.9 minutes
0.005 2
B2 Determining the length of road that can be drained by a surface water channel
constructed in a cutting
Consider the same road and channel as worked example B1, but constructed in a cutting which
contributes run-off to the channel. The road is located in Warwickshire so from Table 5.6.3 the
65
CD 521 Version 1.1.0 Appendix B. Worked examples
antecedent wetness is "medium". The soil in the cutting is a fairly heavy clay with a low permeability so
from Table 5.6.2 the run-off coefficient is α = 0.21. The average width of the cutting draining to the
channel is C = 15.0 m. Compared to the example in B1, the effective catchment width is increased to
the following value given by Equation 5.6.1: W e = 10.625 + 0.21 x 15.0 = 13.775 m.
All the other parameters in Equation 5.20 are unchanged so the revised length of road that can be
drained by the channel is:
( )1.62
L = 244 10.625
13.775 = 160m
66
CD 521 Version 1.1.0 Appendix B. Worked examples
The values of n , S and 2minM5 are as given in worked example B3. Since the channel is not permitted
to surcharge on to the carriageway that it drains, it is decided to determine the design depth of flow y of
the channel for storms with a return period of N = 5 years (see clause 5.1). The value of y for a
rectangular channel is determined from Equation 5.22, but it should be noted that y also appears on the
right-hand side of the equation. A short iterative procedure is therefore necessary as illustrated by the
following calculations.
Estimate a likely value for the design flow depth, e.g. y = 0.150 m, and substitute this on the right-hand
side of Equation 5.22 so that:
( )0.437 ( )0.292 0.158 [ 18.900 ]0.708
y = 9.75 × 10−4 0.013×300
1 1 + 2×0.150
1.000 (5 − 0.4) 1.000 × 4.1 = 0.168 m
0.005 2
Substituting this calculated value of y on the right-hand side of the equation gives a revised value of y =
0.169 m; one final iteration converges to the solution y = 0.170 m, which is the required design depth of
flow in the 1.0 m-wide rectangular channel.
B5 Determining the spacing between the intermediate outlets and the terminal
outlet for a grassed surface water channel
It is necessary to determine the spacing between the intermediate outlets and the terminal outlet for a
grassed surface water channel that will drain a section of dual, two-lane carriageway near Norwich.
The pavement is black top with a cross fall of 1:40 on non-superelevated sections. The width of the
carriageway is 9.3 m (including two 1.0 m-wide hardstrips). The longitudinal gradient of the road is 1 in
125, S =0.8%, and is at grade so cannot receive runoff from the adjacent pervious area.
The principal features of the system are as follows:
1) symmetrical triangular channel with crossfalls of 1:5 (vertical : horizontal);
2) design flow depth: y = 0.2 m;
3) corresponding flow width: B = 2.00 m;
4) grass type: perennial ryegrass (which gives mg = 0.0048 - see clause 5.20); and,
5) average grass height: H = 0.075 m (see clause 5.20).
The grassed surface water channel is to be designed to allow a maximum width of surcharging of 1.0m
on the adjacent hardstrip. For a straight section of road, with a crossfall of 1:40, this can be achieved
by setting the outer edge of the channel 25 mm above the level at the edge of the hardstrip. Also there
is to be an up-stand at the edge of the channel, nominally equal to 40mm. Given that the sides of the
channel have crossfalls of 1:5, it follows that the overall width of the channel can be equal to
B+(0.065x5) = 2.325 m (applies only to the side of the channel remote from the carriageway).
Using the above information and substituting this into Equation 5.19: n = 0.062
The effective catchment width, We , draining to the grassed surface water channel is equal to the width
of the carriageway plus the width of the grassed channel, including the additional width due to
surcharge: We = 9.30 + 2.325 = 11.625 m
The characteristic rainfall depth for the Norwich area is found from the map in Figure 5.3:
2minM5 = 4.0 mm
Note: the allowance for the effects of climate change contained in CG 501 [Ref 3.N] should be adopted
for all new designs.
The first step in the hydraulic design is to determine the required spacing between intermediate outlets
along the grassed channel. Flows produced by storms with a return period of N = 1 years should be
contained within the surface water channel with the flow depth not exceeding y = 0.20 m. Substituting
the values in Equation 5.20.1, it is found that the maximum drainage length is: L = 411 m.
The maximum length of road, Ls that the channel can drain to an outlet in the surcharged condition for
storms of return period N = 5 years, is determined from Equation 5.26.3 where the value ϕ can be
obtained from Table F.1:
67
CD 521 Version 1.1.0 Appendix B. Worked examples
B6 Determining the spacing between the intermediate outlets and terminal outlet
for a combined surface channel and pipe system
It is required to determine the spacing between the intermediate outlets and terminal outlet for a
combined surface channel and pipe system that will drain a section of two-lane, dual carriageway near
Norwich. The pavement is black top with a transverse gradient of 1:40 on non-super-elevated sections.
The width of carriageway draining to the verge is 9.3,m (including two 1.0,m-wide hardstrips). The
section of road under consideration has a longitudinal gradient of S = 0.8% (i.e. 1:125) and is on
embankment so the combined system cannot receive run-off from any adjacent pervious areas.
The combined system can be slip-formed in C28/35 mass concrete with light mesh to meet a D400
loading requirement.
The principal features of the system are as follows:
Surface water channel:
1) symmetrical triangular channel with cross-falls of 1:5 (vertical : horizontal);
2) design flow depth: y = 0.120 m;
3) corresponding flow width: B = 1.20m; and,
4) Average roughness condition: from Table 5.18.1 (concrete), n = 0.013.
Internal pipe:
1) diameter: D = 0.400 m;
2) average roughness condition: from Table 5.71, nip = 0.014.
Overall cross-sectional shape:
1) The surface channel is to be designed to allow a maximum width of surcharging of 1.0 m on the
adjacent hardstrip. For a straight section of road with a transverse gradient of 1:40, this can be
achieved by setting the outer edge of the channel 25 mm above the level at the edge of the
hardstrip. Given that the sides of the channel have cross-falls of 1:5, it follows that the overall width
of the concrete unit forming the combined system will be equal to B + 0.125 m = 1.325 m.
2) To meet the structural requirements set out in this document, a minimum concrete cover equal to
half the pipe diameter needs to be provided. Therefore, the minimum depth of the concrete block is
0.12 m + 0.20 m + 0.40 m + 0.20 m = 0.92 m (measured from the edge of the hardstrip).
The transverse bars of the mesh need to provide a minimum steel area of 385 mm2 per metre run of
pipe. This can be provided, for example, by 7 mm-diameter, transverse bars located at 100 mm centres.
The effective width of catchment, We , draining to the combined channel and pipe system is equal to
the width of the carriageway plus the width of the concrete block:
68
CD 521 Version 1.1.0 Appendix B. Worked examples
69
CD 521 Version 1.1.0 Appendix B. Worked examples
L′T = 750 m.
Therefore, two intermediate outlets will be required at chainages of 250 m and 500 m (measured from
the upstream end of the system) with a terminal outlet located at a chainage of 750 m.
The dimensions and layouts of the intermediate outlets and the terminal outlet are determined using the
recommendations in section 5. Flow from the terminal outlet may be discharged to a watercourse or
toe ditch, or to a separate carrier pipe in the verge or central reserve.
The maximum design flow rate discharged from the terminal outlet is assumed to be equal to the sum
of the flow rate, Qp , from the internal pipe and the flow rate, Qs , from the most downstream section of
surface channel when flowing under surcharged conditions. From Equation 5.73.1a:
Qp = 0.173 m3 /s
and from Equations 5.26.2 and 5.25.3:
Qs = 0.127 m3 /s
The design rate of flow discharged by the terminal outlet is therefore:
QT = 0.300 m3 /s
1) cross-falls 1:5;
2) design flow depth 0.120 m;
3) longitudinal channel gradient 1:200 = 0.005;
4) Manning's roughness coefficient (average condition) 0.013; and,
5) adopt an efficiency of 100% for the outlets and a carriageway cross-fall of 1:40.
The flow in the channel is calculated from Equation 5.25 but first it is necessary to calculate the
hydraulic radius R using Equation 5.13:
A (1.2×0.12)/2 0.072
R= P = 2(0.1202 +0.62 )0.5
= 1.224 = 0.0588m
70
CD 521 Version 1.1.0 Appendix B. Worked examples
The size of the gratings ( Gg is width and Hg is length) is calculated as described in Section 5:
4.5 ≤ Gg /0.120 ≤ 5.1
and Hg ≥ Gg
Taking the smallest dimensions allowed gives:
Gg = 0.120 × 4.5 = 0.540m
Hg = Gg = 0.540m
A commercially available grating, with width and length not smaller than 540 mm, should be chosen.
The total waterway area of the slots should not be less than 0.44G2g or 0.128 m2 (see Clause 5.33).
As shown in Figure H.5 (see Appendix H), the longitudinal distance between the two pairs of gratings
should be at least equal to 1.7 × 0.540m = 0.918m if a grating of width 0.540 m is chosen.
The flow in the channel is calculated from Equation 5.25 but first it is necessary to calculate the
hydraulic radius R using Equation 5.13:
1
A 2 (0.30+1.80)×0.15 0.1575
R= P = 1 = 1.830 = 0.0861m
0.3+2(0.152 +0.752 ) 2
71
CD 521 Version 1.1.0 Appendix B. Worked examples
72
CD 521 Version 1.1.0 Appendix B. Worked examples
73
CD 521 Version 1.1.0 Appendix C. Runoff from cuttings
A different runoff formula was finally adopted for the Wallingford Procedure, but Equation C.1 is more
suitable for application to roads in cuttings.
The effective width We of a road in cutting is defined as the equivalent width of road which can produce
the same total amount of runoff as a road of width W and a cutting of average width C . From Equation
C.1 it can be shown that:
74
CD 521 Version 1.1.0 Appendix D. Roughness
Appendix D. Roughness
D1 Channel roughness
The Manning resistance equation is appropriate when a flow is rough-turbulent (that is with its
resistance mainly determined by the surface texture of the channel). This is likely to be the case in
most road drainage channels, except perhaps near the upstream end where the velocity or depth of
flow is small and the flow may be smooth-turbulent (that is with its resistance mainly determined by the
viscosity of the water). It is often assumed that the roughness coefficient n depends only upon the
surface texture of the channel, but experimental evidence indicates that it can vary with the relative
depth of flow, the cross-sectional shape of the channel and the intensity of any lateral inflow.
A modified version of Manning's equation for shallow triangular channels was developed by the
Highway Research Board [Ref 10.I] and is recommended by the US Federal Highway Administration.
The equation has the form in Equation D.1.
Equation D.1 was obtained by applying Manning's equation to vertical elements in the cross-section
and integrating the discharge across the channel; no allowance is made for the resistance of a vertical
kerb. It gives capacities that are approximately 20% higher than the conventional version in Equation
5.25.2. Values of n quoted in the literature for triangular channels therefore depend upon which of the
two formulae were used to analyse the experimental data.
The conventional form of Manning's equation has been used in this document (that is in Equations
5.25, 5.25.1, 5.25.2, 5.25.3 and 5.25.4) so as to provide a common basis for all shapes of channel.
Values of n given in the literature vary typically from 0.010 to 0.017 for concrete channels and from
0.012 to 0.022 for asphalt channels. In Table 5.18.1 the recommended figures for "average" condition
are well correlated with the mean of the published values; the figures for "poor" condition are slightly
less than the corresponding maximum values.
Factors which can tend to increase the resistance of a channel are lateral inflow from the road surface
and the presence of silt and grit in the invert. Data on these effects are not available, but it is probable
that at the downstream ends of channels (which are the most critical points) they are not very large in
relation to the uncertainties in the basic n values. An "average" value of n = 0.013 for a concrete
channel could be appropriate if it has a trowel-type finish, no sharp discontinuities in line or elevation,
and is regularly cleaned.
An approximate procedure is given in the surcharged surface water channel section in Section 5 for
applying the design method for simple cross-sectional shapes to the case of surcharged compound
channels. It was found that a direct solution for compound channels could be obtained only when using
a development of the modified Manning's equation in Equation D.1. The relationships in Equations 5.26
to 5.29, between the relevant hydraulic characteristics (flow capacity and storage capacity) for a
compound channel and an equivalent "simple" channel, are therefore based on the modified form of
Manning's equation.
75
CD 521 Version 1.1.0 Appendix D. Roughness
The location of a longitundinal line of posts within a channel, such as a post-mounted vehicle restraint
system, can affect channel capacity and increase the risk of an obstruction or blockage. An estimate of
the extra flow resistance produced by a longitudinal line of posts is obtained by considering the drag
force acting on each post. The factor of 0.7 in Equation D.2 corresponds to a drag coefficient of Cd =
1.2 with an allowance for the effect of varying water depth between the upstream and downstream
ends of a channel. The effect of a longitudinal line of posts on channel capacity can be estimated by
increasing the appropriate value of n by np given by:
Allowance need not normally be made for one or two isolated posts located in the channel.
Equation D.3 Alternative Mannings equation for grassed surface water channels where
VR>0.002
H
n = 0.05 + 0.0048(1 + α)
VR
where:
α runoff coefficient (-)
H grass height (m)
V velocity (m/s)
R hydraulic radius of flow (m)
with α = 0 for Perennial Ryegrass-dominated grass mixtures and α = 1 for Fescues-dominated grass
mixtures.
Equation D.3 is applicable for VR>0.002. The great majority of design cases can be in that category.
However, where VR<0.002, n can be calculated using the following expression:
76
CD 521 Version 1.1.0 Appendix D. Roughness
Equation D.4 Alternative Mannings equation for grassed surface water channels where
VR<0.002
n = 0.05 + 2.4(1 + α)H
where:
α runoff coefficient (-)
H grass height (m)
77
CD 521 Version 1.1.0 Appendix E. Rainfall
Appendix E. Rainfall
Kinematic wave theory enables the peak depth of flow at the downstream end of a channel to be
determined for a given intensity and duration of rainfall. For design of surface water channels, it was
necessary to describe the rainfall characteristics by means of an equation relating mean intensity to the
duration and return period of the storm event.
This led to Equation E.1 being developed for predicting the mean rainfall intensity.
78
CD 521 Version 1.1.0 Appendix E. Rainfall
79
CD 521 Version 1.1.0 Appendix F. Surcharge factors
Figure F.1 Drainage length factor for triangular channels with surcharged width of
Bs = 1 m
80
CD 521 Version 1.1.0 Appendix F. Surcharge factors
Figure F.2 Drainage length factor for triangular channels with surcharged width of
Bs = 1.5 m
81
CD 521 Version 1.1.0 Appendix G. Outlet design tables
Table G.1 Triangular channels: Limiting values of Fd and Fs for terminal outlets
No of gratings (or pairs of gratings)
Type of outlet
1 2 3
Table G.2 Trapezoidal channel with cross-falls of 1:4.5: Limiting values of Fd and Fs for terminal
outlets
No of gratings
Type of outlet
2 3
In-line outlet:
Channel full ( Fd ) 0.55 0.85
Surcharged ( Fs ) 0.40 0.75
Off-Line Outlet:
Channel full ( Fd ) 1.0 1.3
Surcharged ( Fs ) 0.9 1.2
Table G.3 Trapezoidal channel with cross-falls of 1:5: Limiting values of Fd and Fs for terminal
outlets
No of gratings
Type of outlet
2 3
In-line outlet:
Channel full ( Fd ) 0.45 0.65
Surcharged ( Fs ) 0.30 0.50
Off-line outlet:
Channel full ( Fd ) 0.75 1.1
Surcharged ( Fs ) 0.65 1.0
82
CD 521 Version 1.1.0 Appendix H. Outlet design figures
83
CD 521 Version 1.1.0 Appendix H. Outlet design figures
Figure H.1 Cross-sectional shape of triangular channel
84
Figure H.2 Cross-sectional shape of trapezoidal channel
108
CD 521 Version 1.1.0 Appendix H. Outlet design figures
Figure H.26 Flow chart for design of weir outlets in triangular channels
109
CD 521 Version 1.1.0 Appendix H. Outlet design figures
Figure H.27 Flow chart for design of weir outlets in trapezoidal channels with
cross-falls of 1:4.5
110
CD 521 Version 1.1.0 Appendix H. Outlet design figures
Figure H.28 Flow chart for design of weir outlets in trapezoidal channels with
cross-falls of 1:5
111
CD 521 Version 1.1.0 Appendix I. Examples of collection chambers
Figure I.1 Example of collecting chamber for in-line outlet in trapezoidal channel
112
CD 521 Version 1.1.0 Appendix I. Examples of collection chambers
Figure I.2 Example of collecting chamber for in-line outlet in triangular channel
113
CD 521 Version 1.1.0 Appendix I. Examples of collection chambers
114
Figure I.4 Example of combined channel and pipe terminal outlet - longitudinal cross section (dimensions in mm)
Where grassed channels are to be established from seed, it is important that the seed germinates
quickly and that the grass cover develops rapidly. Roadside verges are unlikely to receive irrigation to
help establishment therefore the requirement is for grass types that can develop quickly.
117
CD 521 Version 1.1.0 Appendix J. Grass type selection
The speed of grass establishment varies according to the species. The establishment rates for
common UK turf grasses accoirding to Turfgrass Manual [Ref 19.I] are shown in Table J.2.
None of the grasses has a high salt tolerance, in contrast to some warm-season species, but the most
tolerant are slender creeping fescue, tall fescue and perennial ryegrass according to Adams & Gibbs
1993 [Ref 13.I].
118
CD 521 Version 1.1.0 Appendix J. Grass type selection
these grasses may eventually become more dominant in the lower lying central section of the grassed
channel, whereas the drier upper slope may retain grasses better adapted to the drier conditions, such
as fescues.
Growth rates can be modified by fertility and soil moisture content. Guidance on growth rates is given
by Turfgrass Manual [Ref 19.I] and Table J.4.
119
CD 521 Version 1.1.0 Appendix J. Grass type selection
120
CD 521 Version 1.1.0 Appendix K. Maintenance of surface water channels
K1.5 Patching
Where the grass has died or is severely damaged, the affected area should be removed, the topsoil
level reinstated and a section of appropriate turf inserted. The replacement turf should be watered
regularly until it becomes established in the channel.
121
CD 521 Version 1.1.0 Appendix K. Maintenance of surface water channels
122
CD 521 Version 1.1.0 Appendix L. Grassed surface water channel profiles
123
CD 521 Version 1.1.0 Appendix L. Grassed surface water channel profiles
124
CD 521 Version 1.1.0 Appendix M. Construction aspects
125
CD 521 Version 1.1.0 Appendix M. Construction aspects
Where possible, any necessary expansion joints should be formed at intermediate outlets or terminal
outfalls. If an expansion joint is necessary part way along a drainage length, a short length of plastic
tube should be inserted and sealed to the sections of internal pipe either side of the joint in order to
prevent leakage into the joint.
M1.3 Inspection
It is recommended that a CCTV inspection of the internal pipe, in accordance with CS 551 [Ref 5.N], be
undertaken to ensure that the circular profile of the void is maintained over the length of the channel.
Deformations in excess of 10% of the nominal bore of the pipe will not be acceptable.
126
CD 521 Version 1.1.0 Appendix M. Construction aspects
M2.5 Signage
Signs are generally remote from the pavement edge and are protected by safety barriers. The designer
ensures that no signs encroach into the channel.
Marker posts may encroach into the grassed channel and care is to be taken to ensure that their
installation will not result in any impermeable membrane being punctured.
127
CD 521 Version 1.1.0 Appendix M. Construction aspects
As a surfacing to existing filter (French) drains, sufficient stone filter medium and depth of adjacent
verge material is removed to permit the formation of the 200-mm channel and sufficient depth of
topsoil. A geotextile can be positioned over the filter material to prevent topsoil (and subsoil) from
contaminating the drain, but still permitting water to soak through.
It is beneficial for the geotextile properties to comprise:
1) porosity – not be impermeable but should have permeability no greater than the compacted subsoils
(channel needs to retain moisture to be able to support the grass);
2) woven or non-woven durable material and be clog resistant (not proof since the aim is to maintain
channel flow on the surface);
3) tensile strength: – have adequate tear and puncture resistance to permit compaction of the channel
subsoil and topsoil above it and be resistant to vehicle over run damage.
128
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