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February 2005

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 6

HA 113/05

COMBINED CHANNEL AND PIPESYSTEM FOR SURFACE WATERDRAINAGE

SUMMARY

This Advice Note gives guidance on the hydraulic andstructural design of combined channel and pipe systemsfor highway drainage. The type of system consideredconsists of a surface water channel and an internal pipeformed within the base of the unit that is able to carryadditional flow.

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HA 113/05

Combined Channel andPipe System for Surface

Water Drainage

Summary: This Advice Note gives guidance on the hydraulic and structural design ofcombined channel and pipe systems for highway drainage. The type of systemconsidered consists of a surface water channel and an internal pipe formedwithin the base of the unit that is able to carry additional flow.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 4 Section 2Part 6 HA 113/05

February 2005

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 6

HA 113/05

COMBINED CHANNEL AND PIPESYSTEM FOR SURFACE WATERDRAINAGE

Contents

Chapter

1. Introduction

2. Safety Aspects

3. Description of Combined System

4. Structural and Dimensional Requirements

5. Hydraulic Design Principles

6. Drainage Capacity of Channel

7. Drainage Capacity of Internal Pipe

8. Intermediate Outlets and Terminal Outfalls

9. Sub-Surface Drainage

10. Construction Aspects

11. Maintenance Aspects

12. Worked Example

13. References

14. Enquiries

Annexes

Annex A List of Symbols

Annex B Figures


February 2005


Chapter 1Introduction

1. INTRODUCTION

General

1.1 This Advice Note gives guidance on thehydraulic and structural design of combined channeland pipe systems for highway drainage. The type ofsystem considered consists of a surface water channeland an internal pipe formed within the base of the unitthat is able to carry additional flow. The system isparticularly suited to in situ construction in concreteusing slip-forming techniques. However, the hydraulicdesign procedures provided are generally applicableand are not limited to a particular method ofconstruction. Although the advice should be fully takeninto account in the design of new schemes (see 1.6),this Advice Note contains no mandatory requirements.

1.2 The function of the surface channel in thecombined system is to collect and convey rainwaterrun-off from the road surface. At suitable points alongthe channel, water is discharged into an integral pipeformed in the lower part of the channel unit. Use of acombined system can remove the need for a separatecarrier pipe in the verge and enables flow to be carriedlonger distances between outfalls to ditches or naturalwatercourses. By combining the surface drainagesystem into a single unit at the edge of the pavement,more space can be made available in the verge for otherservices.

1.3 This Advice Note should be read in conjunctionwith the following documents in DMRB 4.2:

• HD 33: Surface and Sub-Surface DrainageSystems for Highways

• HA 37: Hydraulic Design of Road-Edge SurfaceWater Channels

• HA 78: Design of Outfalls for Surface WaterChannels

• HA 83: Safety Aspects of Road Edge DrainageFeatures. Further details are given in TRL Report422 (Ref 3)

• HA 105: Sumpless Gullies.

1.4 The use of surface water channels should takeaccount of advice on safety given in HA 83.

February 2005

Scope

1.5 The principles outlined in this Advice Note applyto all schemes of Overseeing Organisations for TrunkRoads including motorways. They may also be appliedgenerally to other new highway schemes and by otherhighway authorities for use during the preparation,design and construction of their own comparableschemes. Combined channel and pipe drainage systemsmay be installed during major maintenance works or asa retro-fit.

Implementation

1.6 This Advice Note should be used forthwith for allschemes currently being prepared provided that, in theopinion of the Overseeing Organisation, this would notresult in significant additional expense or delayprogress. (In Northern Ireland, this Advice Note will beapplicable to those roads designated by the OverseeingOrganisation.) Design Organisations should confirm itsapplication to particular schemes with the OverseeingOrganisation.

1/1


Chapter 2Safety Aspects

2. SAFETY ASPECTS

2.1 When considering the use of a combineddrainage system having a surface channel and internalpipe, safety aspects relating to its location should betaken into account in accordance with the guidelinesgiven in HA 83 (DMRB 4.2).

2.2 Systems with triangular or trapezoidal channelswill usually be sited adjacent to the hardstrip orhardshoulder or at the edge of the carriageway and infront of the safety barrier, where one is provided.Layout details are given in the ‘A’ Series of theHighway Construction Details (HCD) (MCHW 3). Inthese locations, the maximum design depth of flow inthe channel should be limited to 150mm. In verges andcentral reserves, the side slopes of the channel shouldnot normally be steeper than 1:5 (vertical : horizontal)for triangular channels and 1:4.5 for trapezoidalchannels. In very exceptional cases, side slopes of 1:4are allowable for both types of channel.

2.3 Combined systems with rectangular channels, ortriangular channels of depth greater than 150mm,should be used only when safety barriers are providedbetween the channel and the carriageway. Systems ofthis type will normally only be justified when safetybarriers are warranted by other considerations. Inaddition, such systems should not be located in the zonebehind the safety barrier into which the barrier mightreasonably be expected to deflect on vehicle impact(because of the risk of the vehicle overturning due to itbeing too low relative to the safety barrier). Shallowerchannels of the types described in 2.1 may be located inthis deflection zone, or be crossed by the safety barrier(usually at a narrow angle), provided that the combinedlayout complies with the requirements of other relevantparts of the DMRB (Ref 1, note that Volume 1,Section 0 contains a contents list and index).

Further advice on such layouts should be sought fromthe Overseeing Organisation.

2.4 Co-ordination of the layout of safety barriers andchannels of combined drainage systems must bearranged at an early stage in design and not left tocompromise at later stages. Where safety barriers arenot immediately deemed necessary, sufficient spaceshould be provided in the verge or central reserve toallow for their possible installation. The combinedlayout must comply with the requirements of TD 19,Safety Fences and Barriers (DMRB 2.2), TD 32 Wire

February 2005

Rope Safety Fence (DMRB 2.2.3) and the HCD(MCHW 3) in terms of set-back and clearancedimensions and the mounting height of the safetybarrier.

2.5 The constraints on channel geometry given inthis document also apply to the outlet arrangementsused to discharge flow from the channel to the internalpipe of the combined system. For outlets and channelterminations, slopes exceeding 1:4 should not be usedon any faces, particularly those orthogonal to thedirection of traffic, unless such faces are behind asafety barrier.

2.6 If installed adjacent to the hardstrip orhardshoulder and not protected by a safety barrier,combined drainage systems with internal pipes shouldbe capable of withstanding the required loading whentested in accordance with the procedures specified inHA Clause 517 of the Specification for Highway Works(SHW) (MCHW 1) and European StandardBS EN 1433 (Ref 4).

2.7 Gully gratings used in a combined system todischarge water from the surface channel to the internalpipe (or to an outfall) should meet the geometrical andstructural requirements of BS EN 124 (Ref 5) andBS 7903 (Ref 6) and be of the appropriate load class(see Chapter 4).

2/1


Chapter 3Description of Combined System

INED SYSTEM
3. DESCRIPTION OF COMB
General

3.1 The drainage system described in this AdviceNote consists of a surface channel and an internal pipethat are constructed as a single unit and installed at thepavement edge to collect and convey rainfall run-offfrom the road surface. The cross-sectional geometry ofa typical combined system is shown in Figure 1.Recommendations on appropriate methods ofsub-surface drainage that can be used in conjunctionwith a combined system are given in Chapter 9.

3.2 The combined system is particularly suited toconcrete construction using slip-forming techniques.The recommendations on maximum pipe size andminimum cover given in Chapter 4 relate specifically toslip-formed or cast in situ concrete systems. It may bepossible to prefabricate hydraulically equivalentchannel and pipe units, but any such system will need tomeet the loading requirements in Chapter 4 and beapproved for use by the Overseeing Organisation.

Recommended Configuration

3.3 The recommended configuration of a combinedchannel and pipe system has the following principalfeatures:

(1) Surface water channel to be unslotted (as distinctfrom channels with continuous or regularlyspaced slots along the invert).

(2) Flow to be discharged from the channel to theinternal pipe at discrete points along a drainagelength (see Figure 2).

(3) Flow to be discharged to the internal pipe viaoutlets consisting of gully gratings in the base ofthe channel, with water dropping through thegratings into shallow benched chambersconstructed on the line of the pipe (see Chapter 8for details of chambers).

(4) The internal pipe to be unlined and formed as acylindrical void below the invert of the channel.With slip-form construction, the void may beproduced by an inflated flexible tube positionedat the appropriate height above the base of thechannel unit and with the face of the shield

February 2005

modified so that the tube is able to pass throughit. After the concrete has achieved a sufficientstrength, the tube can be deflated and removedfrom the void.

(5) Slip-formed combined systems to be constructedusing a minimum of class C28/35 air entrainedconcrete either unreinforced or with steel meshreinforcement in the base. (Addition ofreinforcement increases the maximum allowablesize of pipe, see Chapter 4.)

3.4 The recommended configuration has thefollowing advantages:

(1) The combined flow capacity of the channel andpipe can be significantly larger than the capacityof a conventional surface water channel of thesame overall width (see Chapters 6 and 7).

(2) Alternatively, a given flow capacity can beachieved with a channel of smaller overall width,which can be of benefit if space in the verge islimited (for example, in road-widening schemes).

(3) Use of an unslotted channel simplifiesconstruction, increases the structural strength ofthe unit, avoids maintenance problems associatedwith the clogging of slots, and facilitates the useof pressure jetting for cleaning of the internalpipe.

(4) Use of an unslotted channel and discrete entrypoints to the internal pipe allows a higher flowcapacity than an equivalent system with a slottedchannel (see item (3) in 5.3).

3.5 A combined system will generally have a greateroverall depth than a conventional surface water channeland therefore requires a modified sub-surface drainagesystem for removing seepage flows from the pavementconstruction (see Chapter 9). Due to its greater depth,the combined system requires more concrete than aconventional system, although the increase will bepartly offset by the volume of the void forming thepipe. Due to its higher flow capacity, a combinedsystem may reduce or eliminate the need for a separatecarrier pipe, with consequent savings in materials andconstruction costs.

3/1


Chapter 3Description of Combined System

Surface Channel

3.6 The channel functions in the same way as aconventional road-edge surface water channel(see HD 33 and HA 37, DMRB 4.2, for guidance onselection and design). The limiting dimensions andcross-sectional shape of the channel are determined byconsiderations of vehicle safety as described inChapter 2.

Internal Pipe

3.7 The hydraulic characteristics of the internal pipein a combined system are similar to those of aconventional carrier pipe in which flow enters the pipeat manholes spaced at intervals along its length.

3.8 The larger the size of pipe that can be used in acombined system, the longer the length of road that canbe drained without the need for an outfall.

Intermediate Outlets and Terminal Outfalls

3.9 At the point where the design rate of run-off froma length of road reaches the flow capacity of thechannel, it is necessary to discharge the water from thechannel into the internal pipe via an intermediate outlet.The outlet consists of one or more gully gratingsinstalled in the invert of the channel; beneath thegratings, a shallow benched chamber is constructed onthe line of the pipe. Information on the design ofintermediate outlets is given in Chapter 8.

3.10 At the point where the design rate of run-off fromthe road reaches the total capacity of the combinedsystem, the flow from the pipe and the flow from themost downstream section of channel need to bedischarged into a terminal outfall chamber. From thischamber the water can then be conveyed to awatercourse or toe ditch, or discharged into a separatecarrier pipe. Information on the design of terminaloutfall chambers is given in Chapter 8.

3/2

Outlets in Steep Roads

3.11 In steep sections of road with longitudinalgradients exceeding about 1:50, gully gratings may notbe able to collect a sufficiently large proportion of thehigh-velocity flow in the surface channel. Alternativedesigns of outlet, such as the weir outlet described inHA 78 (DMRB 4.2), can be used in these situations.However, the features that increase the hydraulicefficiency of these designs may also present a potentialhazard to vehicles so they will usually need to beprotected by a safety barrier. Further details are given in8.14 to 8.16.

February 2005


Chapter 4Structural and Dimensional Requirements

NSIONAL
4. STRUCTURAL AND DIMEREQUIREMENTS
4.1 Combined channel and pipe systems shall meetthe loading and safety requirements specified by theOverseeing Organisation. Loading requirements shouldbe based on the loading classes given in BS EN 124(Ref 5), with testing carried out in accordance with theprocedures specified in HA Clause 517 of the SHW(MCHW 1) and European Standard EN 1433 (Ref 4).

4.2 Surface channels at the edge of the pavement arenormally either triangular or trapezoidal in cross-section. Rectangular channels may also be usedprovided they are protected from traffic by a safetybarrier (see Chapter 2). The limiting dimensions forsurface channels are as follows:

• Maximum channel depth:Y

C= 150mm.

• Maximum side slopes for triangular channels:1:5 (vertical : horizontal).

• Maximum side slopes for trapezoidal channels:1:4.5.

• At outlets, maximum side slopes of 1:4 arepermitted locally in both triangular andtrapezoidal channels.

4.3 The limiting dimensions are given in 4.4 to 4.6for combined systems of concrete construction having atriangular surface channel and the type of cross-sectional profile shown in Figure 1. The dimensionscorresponding to a loading class of D400 in BS EN 124(Ref 4) were established from structural tests (see HRWallingford and TRL, Ref 7). Combined systems thatmeet the appropriate geometric limits may be deemed tosatisfy the corresponding loading requirement withoutfurther structural tests. For loading class C250 themaximum pipe size is limited to 300mm diameter ifunreinforced concrete is used. If diameters larger than300mm are required, reinforcement may be necessaryand guidance in this regard should be sought from theOverseeing Organisation. If it is wished to use analternative cross-sectional profile or constructionmaterial, approval must be sought from the OverseeingOrganisation in advance and structural testing of thesystem carried out in accordance with HA Clause 517of the SHW (MCHW 1) to demonstrate its ability tomeet the specified loading requirement.

4

TcB(c(

Ts

C

MMM

4R

TcB(c(

FrmotrTlpfl

Ts

D

MMM

February 2005

.4 Unreinforced C28/35 Mass Concrete

he combined channel and pipe sections should beonstructed from nominal C28/35 concrete toS EN 206 (Ref 8) and air entrained to BS 5931

Ref 9). All aggregate should be partially crushed orrushed in accordance with Clause 1103 of the SHWMCHW 1).

he limiting dimensions for combined channel and pipeections of this type are:

250 and D400 loading class:

aximum pipe size: Dmax

= 300mminimum vertical cover: U

min= D

max/2

inimum horizontal cover: Hmin

= Dmax

/2

.5 C28/35 Concrete with Light Mesheinforcement

he combined channel and pipe sections should beonstructed from nominal C28/35 concrete toS EN 206 (Ref 8) and air entrained to BS 5931

Ref 9). All aggregate should be partially crushed orrushed in accordance with Clause 1103 of the SHWMCHW 1).

or the purposes of this Advice Note, ‘light mesheinforcement’ is defined as a welded mesh formed ofild steel bars with a maximum spacing between bars

f 200mm. It is to be placed horizontally in the base ofhe combined channel and pipe block such that the barsun parallel and perpendicular to the line of the pipe.he minimum area of steel perpendicular to the

ongitudinal centreline of the pipe should be 385mm2

er metre run of pipe. The ends of the bars may be leftlat and the concrete cover to the steel should not beess than 40mm.

he limiting dimensions for combined channel and pipeections of this type are:

400 loading class:

aximum pipe size: Dmax

= 400mminimum vertical cover: U

min= D

max/2

inimum horizontal cover: Hmin

= Dmax

/2

4/1


Chapter 4Structural and Dimensional Requirements

4.6 C28/35 Concrete with Heavier MeshReinforcement

The combined pipe and channel sections should beconstructed from nominal C28/35 concrete toBS EN 206 (Ref 8) and air entrained to BS 5931(Ref 9). All aggregate should be partially crushed orcrushed in accordance with Clause 1103 of the SHW(MCHW 1).

For the purposes of this Advice Note, ‘heavier meshreinforcement’ is defined as a welded mesh formed ofmild steel bars. It is to be placed horizontally in thebase of the combined channel and pipe block such thatthe bars run parallel and perpendicular to the line of thepipe. The maximum spacing between the bars is to be100mm for the bars placed perpendicular to the line ofthe pipe, and 200mm for those placed parallel to theline of the pipe. The minimum area of steelperpendicular to the longitudinal centreline of the pipeshould be 1100mm2 per metre run of pipe. The ends ofthe bars may be left flat and the concrete cover to thesteel should not be less than 40mm.

The limiting dimensions for combined channel and pipesections of this type are:

D400 loading class:

Maximum pipe size: Dmax

= 500mmMinimum vertical cover: U

min= D

max/2

Minimum horizontal cover: Hmin

= Dmax

/2

February 20054/2


CIPLES

Chapter 5Hydraulic Design Principles

5. HYDRAULIC DESIGN PRIN

5.1 The methods given in Chapters 6 and 7 fordetermining the drainage capacities of the channel andinternal pipe of a combined system are based on thesame principles as those used in HA 37 (DMRB 4.2) forconventional surface water channels. An outlinedescription of the principles is given in this AdviceNote but, for more detailed information on individualtopics, reference should be made to May et al (Ref 7)and HA 37 (DMRB 4.2).

5.2 One of the key features of a combined system isthat the longitudinal gradients of the channel and theinternal pipe will normally be the same as thelongitudinal gradient of the pavement being drained.Any increase in flow capacity that might be gained byconstructing the pipe at a steeper gradient relative to thesurface of the road would be small and outweighed bythe increased cost and difficulty of construction. It istherefore assumed in this Advice Note that the road, thesurface channel and the internal pipe all have commonvalues of longitudinal gradient.

5.3 Subject to the limitations on maximum pipe sizegiven in Chapter 4, the designer of a combined systemcan consider the flow capacities of the channel and pipeseparately and has considerable freedom to vary theirrelative sizes. The following outline design procedureillustrates the case of a section of road of constantlongitudinal gradient (see Figure 2).

(1) Assume a suitable size and cross-sectionalgeometry for the surface channel and use themethod given in Chapter 6 to find the length ofroad, L

C (in m), that can be drained by the

channel before it reaches its design capacity. Thislength, L

C, determines the maximum allowable

distance between the upstream end of the systemand the first outlet and also the spacings betweensubsequent outlets.

(2) Assume a suitable diameter of internal pipe anduse the method in Chapter 7 to find the length ofroad, L

P(in m), that can be drained by the pipe

before it reaches its maximum flow capacity.

(3) If no additional flow is discharged into the pipefrom the surface, the internal pipe can becontinued downstream of the drainage length, L

P.

Therefore, as shown in Figure 2, the terminaloutfall of the combined system can be located a

(4)

(5)

5.4capa

•

•

•

•

February 2005

distance of up to one channel length, LC

,downstream (provided the flow from the pipe andthe flow from the last section of surface channelare able to discharge separately into the terminaloutfall chamber). Therefore, the maximum totallength, L

T, that can be drained by the type of

combined system considered in this Advice Noteis given by L

T = L

P + L

C. [Note that an equivalent

combined system with a slotted channel wouldonly be able to drain a total length of L

T = L

P].

The values of LC

, LP and L

T can be used to

determine the number and location of outletsrequired along a road drained by a section ofcombined channel and pipe system. Suitableoptions are described in 7.15 to 7.17. In practice,the layout of a system may also be affected bylocal features and possible requirements tostandardise outlet spacings within a scheme.

As an example, if the maximum lengths that canbe drained by the channel and the pipe arerespectively L

C = 110m and L

P = 420m, one

option would be to use four intermediate outletsand one terminal outfall. The spacings betweenadjacent outlets would be L

A = 105m and the total

length of road that could be drained by thesystem (from the upstream end to the terminaloutfall) would be 525m. The last intermediateoutlet would be located at a distance of 420mfrom the upstream end, which matches themaximum drainage capacity of the pipe.

The key factors that determine the drainagecity of a combined system are the:

longitudinal gradient of the road, S (vertical fallper unit distance along the road, in m per m);

effective width, WE (in m), of the catchment

drained by the combined system, taking accountif appropriate of any run-off from cuttings (seeChapter 12 and Annex C of HA 37, DMRB 4.2);

size, shape and cross-sectional area of the surfacechannel;

diameter, D (in m), of the internal pipe;

5/1


Chapter 5Hydraulic Design Principles

• hydraulic roughness values of the channel andthe pipe;

• statistical rainfall characteristics at the site, i.e.the relationship between the rainfall intensity, theduration of the design storm and its frequency ofoccurrence;

• variation of rainfall intensity with time during thedesign storm.

The effects of these various factors can be taken intoaccount using a method based on kinematic wavetheory. This method provides information about thevariation of flow conditions with time during a stormand enables the duration of storm that produces theworst flow conditions to be determined. More detailsabout the kinematic wave theory are given in HRWallingford Report DE 30 (Ref 10), TRRL ContractorReport 8 (Ref 11) and May (Ref 12).

5.5 The flow capacity of a channel or pipe can bedetermined from the Manning resistance equationwhich has the form:

n

SRAQ

2/13/2

= (1)

where Q is the flow rate (in m3/s), A is thecross-sectional area of flow (in m2), and n is theManning roughness coefficient of the channel or pipe.The hydraulic radius, R (in m), of the flow is given by:

P

AR = (2)

where P is the wetted perimeter of the channel or pipe(in m).

5.6 Rainfall statistics for short-duration storms in theUK can be approximated by the following equation:

)(2min0.4)(

0.4)(3.270.565

0.223 M5T

TNIo

−−= (3)

where IO is the mean rainfall intensity (mm/h) occurring

in a storm of duration T (minutes) with a return periodof N (years), such that a storm of this intensity willoccur on average once every N years. The quantity2minM5 is the depth of rainfall (in mm) occurring in astorm at the specified geographical location during aperiod of T = 2 minutes with a return period of N = 5years. The variation of 2minM5 with location in the UK

is(

5cgcapHatho

5/2

shown in Figure 3. Details of the basis for Equation3) are given in Annex A of HA 37 (DMRB 4.2).

.7 The rainfall intensity in a storm is not usuallyonstant but varies with time. The design equationsiven in Chapters 6 and 7 for the drainage capacities ofhannels and pipes assume that the intensity varies inccordance with what is termed the 50% summerrofile, as defined in Volume 2 of the Flood Estimationandbook (Ref 13). With this profile, the peak intensity

t the mid-point of the storm is approximately 3.9 timese mean intensity, I

O, averaged over the total duration

f the storm.

February 2005


CHANNEL

Chapter 6Drainage Capacity of Channel

6. DRAINAGE CAPACITY OF

Drainage Length

6.1 In a combined system, the surface channel isdivided into separate drainage lengths by theintermediate outlets and terminal outfall. For a givensize of channel, the maximum allowable distancebetween adjacent outlets will vary with the longitudinalgradient of the road and the effective width of thecatchment being drained. The outlets may not thereforebe equally spaced. Alternatively, it is possible to use astandardised spacing equal to the length of road that thechannel can drain in the most critical section of thecombined system.

6.2 For an individual section of surface channel, thedrainage length, L

C (in m), is defined as the maximum

length of road that can be drained by the channel underdesign conditions without the flow exceeding theallowable water depth in the channel. At thedownstream end of the drainage length, the flow needsto be discharged from the channel either to the internalpipe of the combined system via an intermediate outlet,or to a watercourse or other drainage system via aterminal outfall (see Chapter 8).

6.3 In addition to the physical properties of the roadand the channel, the value of L

C depends upon the

rainfall characteristics at the site and the selected valueof return period for the design storm. Based on thehydraulic principles described in Chapter 5, thefollowing equation can be used to determine values ofdrainage length for triangular surface channels ofsymmetrical cross-section:

( )( )

( )( )[ ] 62.1

362.02/1

3/122

29.26

5min2

4.0

41056.1

MW

N

n

S

YB

YBL

ECCC

CCC

−−

+×= (4)

where:

• YC

is the design depth of flow in the triangularchannel (in m) - see 2.2 and Figure 1.

• BC is the corresponding surface width of flow

(in m).

• S is the longitudinal gradient of the channel(in m per m).

•

•

•

•

Eqcrtrigi

M

6.suderetrieq

Q

Lo

6.suthreS

E

S

wchthgrm

February 2005

nC is the Manning roughness coefficient of the

channel.

WE is the effective width of the catchment

drained by the channel (in m) - see 5.4.

N is the return period of the design storm(in years).

2minM5 is a value of rainfall depth (in mm)characteristic of the geographical location of thesite - see 5.6 and Figure 3.

uivalent formulae for the drainage capacities of otheross-sectional shapes of channel (asymmetricangular, trapezoidal, rectangular and dished) areven in Chapter 5 of HA 37 (Ref 1).

aximum Flow Capacity

4 The maximum flow capacity, QC (in m3/s), of a

rface channel when it is just flowing full at its designpth of flow, Y

C, can be determined from the Manning

sistance equation (1). For the particular case of aangular channel of symmetrical cross-section, theuation has the form:

( )( ) C

CC

CCC n

S

YB

YB 2/1

3/122

3/5

4315.0

+= (5)

ngitudinal Gradient

5 If the longitudinal gradient of the road and therface channel varies along the drainage distance, L

C,

e value of S in Equations (4) and (5) should beplaced by the effective value of longitudinal gradient,. This can be estimated approximately from:

CE L

Z∆= (6)

here ∆Z (in m) is the difference in invert level of theannel between the upstream and downstream ends ofe drainage length, L

C. If the variation in longitudinal

adient is considerable, a more accurate estimate of SE

ay be obtained from Equation (15) in Chapter 7. Ineither case, an iterative procedure may be necessary to

6/1



determine SE if the longitudinal profile of the road has

been determined in advance of the hydraulic design.

Hydraulic Resistance of Channel

6.6 Recommended values of the effective Manningroughness coefficient for surface water channels aregiven in Table 1. The values for black top may beapplicable when determining surcharged flows onadjacent sections of flexible pavement (see thecalculation method referred to in 6.12), and may also beappropriate if overlays are applied to existing concretechannels as part of resurfacing work.

Channel Type Condition nC

Concrete Average 0.013

Concrete Poor 0.016

Black top Average 0.017

Black top Poor 0.021

Table 1 Values of Manning Roughness forSurface Channels

6.7 Factors that influence the effective value of nC

arethe surface finish of the channel, irregularities at joints,energy losses caused by the flow entering the channelfrom the road, and the presence of sediment or debrisdeposited in the channel.

Storm Return Period

6.8 Recommendations on the selection of designstorm return periods for highway drainage systems aregiven in Chapter 6 of HD 33 (DMRB 4.2). Surfacechannels should be designed so that flows produced bystorms with return periods of N = 1 year are containedwithin the cross-section of the channel (i.e. the designdepth, Y

C, in Equation (4) is not exceeded).

Surcharging of Channel

6.9 Limited surcharging of surface channels ispermissible during rarer storms. In verges, themaximum width of flow on the road surface duringstorms with return periods of N = 5 years should notexceed 1.5m in the case of hard shoulders and 1.0 m inthe case of hard strips. In central reserves, storms with

re

6cds

•

•

•

TsL

6(f

Q

wc

6edo

F

6EcminaoG

6/2

eturn periods of N = 5 years must not causencroachment of flow on to the carriageway.

.10 The following simplified method may be used toalculate the length of road that a surface channel canrain to an outlet when the channel is flowing in aurcharged condition:

First use Equation (4) to calculate the maximumlength of road, L

C(in m), that can be drained by

the channel when just flowing full at the designflow depth, Y

C, for storms having a return period

of N = 1 year.

Depending on the allowable width of flow, BS on

the adjacent hard strip or hardshoulder (see 6.9),use either Figure 4 or 5 to determine theappropriate value of the drainage length factor, φ.

The maximum length of road, LS (in m), that the

channel can drain to an outlet in the surchargedcondition for storms having a return period ofN = 5 years is given by:

CS LL φ= (7)

his simplified method assumes that the channel has aymmetrical triangular profile and provides estimates of

S that will tend to err on the conservative side.

.11 The maximum flow capacity of the channel, QS

in m3/s), under surcharged conditions can be estimatedrom:

CS Qφ= 575.1 (8)

here the value of QC is the design capacity of the

hannel obtained from Equation (5).

.12 A more accurate but more complex method ofstimating the maximum length of road, L

S, that can be

rained by a surcharged channel is given in Chapter 13f HA 37 (DMRB 4.2).

low By-passing at Intermediate Outlets

.13 The value of drainage length, LC

, given byquation (4) assumes that no water is entering thehannel at the upstream end of the drainage length. Thisay not necessarily be the case because making antermediate outlet large enough to collect 100% of the

pproaching flow may not result in the most economicverall solution, particularly in steeper channels.uidelines in HA 78 (DMRB 4.2) allow the use of

February 2005



collection efficiencies between 100% and 80% forintermediate outlets in surface water channels.

6.14 The following allowance for flow by-passing isrecommended in Chapter 14 of HA 37 (DMRB 4.2).Consider first the section of channel immediatelyupstream of the section being designed. Let the lengthof this upstream section be L

U(in m), and the collection

efficiency of its intermediate outlet be η (defined as theratio between the flow rate collected by the outlet andthe flow rate approaching it). Allowable values ofefficiency are in the range η = 1.0 to 0.8. For thesection of channel being designed, calculate fromEquation (4) the maximum length of road, L

C(in m),

that could be drained if there were no flow bypassingthe upstream intermediate outlet. Taking account of theactual bypassing, the maximum length of road, L

CB(in

m), that the channel can drain is estimated from:

( ) UCCB LLL η−−= 12

1(9)

If it is decided to adopt an equal spacing betweenintermediate outlets so that L

CB = L

U, Equation (9) can

be simplified to:

( )η−=

3

2 CCB

LL (10)

6.15 Similar considerations apply for surchargedconditions. If surcharged flow causes by-passing at anupstream outlet, the maximum length of road, L

SB

(in m), that can be drained by the channel may becalculated from either Equation (9) or (10) with L

CBreplaced by L

SB, and L

C replaced by the value of L

S

determined from 6.10 or 6.12.

Spacing of Outlets

6.16 The maximum allowable spacing, LA (in m),

between adjacent outlets (or between the upstream endof a combined system and the first outlet) is determinedby the following criteria:

(1) No flow by-passing at upstream outlet:L

A≤ L

C and L

A≤ L

S .

(2) With flow by-passing at upstream outlet:L

A≤ L

CB and L

A≤ L

SB.

February 2005 6/3


F INTERNAL PIPE

Chapter 7Drainage Capacity of Internal Pipe

7. DRAINAGE CAPACITY O

Drainage Length

7.1 The drainage length, LP (in m), is defined as the

maximum length of road that can be drained by asection of pipe under design conditions without theflow exceeding the allowable depth of water in the pipe.At the downstream end of the drainage length, the flowneeds to be discharged from the pipe to an outfall. Inaddition to the physical properties of the road and thepipe, the value of L

P depends upon the rainfall

characteristics at the site and the selected value ofreturn period for the design storm. Values of drainagelength for internal pipes of combined systems can bedetermined from the following equation:

(11)

where R (in m) is the hydraulic radius of the flow in thepipe (see Equation (2)), A (in m2) is the correspondingcross-sectional area of flow, n

P is the Manning

roughness coefficient of the pipe, and the otherquantities are as defined in 6.1. This equation is basedon the hydraulic principles and assumptions describedin Chapter 5 and so is fully consistent with theequivalent result for triangular surface channels givenby Equation (4).

7.2 Since the depth of the pipe below the invert ofthe surface channel will normally be small, the amountof additional flow capacity that can be obtained bysurcharging the pipe will also be small and should notbe relied upon for design purposes. It is thereforerecommended that the internal pipe should be sized sothat it is able to cater for flows produced by storms withreturn periods of N = 5 years when just flowing full atits downstream end (where the flow rate is largest). Onthis basis, Equation (11) can be rearranged as follows toprovide a direct relationship between the diameter ofthe internal pipe, D (in m), and the maximum length ofroad that can be drained:

( )[ ] 62.1

91.32/16

5min21024.1

MW

D

n

SL

EPP ×= (12)

( )

( )5min2

4.0100.8

62.1

362.02/13/2

6

×

−×= −

MW

A

Nn

SRL

E

PP

February 2005

7.3 If the pipe is designed to just flow full for stormswith a return period of 5 years, it can be shown fromEquation (11) that the maximum flow depths producedby storms with a return period of N = 1 year will notexceed about 60% of the pipe diameter.

Maximum Flow Capacity

7.4 The maximum flow capacity, QP (in m3/s), of the

internal pipe can be determined from the Manningresistance equation (1). For the particular case of a pipejust flowing full at its downstream end, the equation hasthe form:

PP n

SDQ

2/13/8

312.0= (13)

The corresponding formula for the flow velocity, VP

(in m/s), in the pipe at its downstream end is:

PP n

SDV

2/13/2

397.0= (14)

Longitudinal Gradient

7.5 Depending upon its size, the internal pipe of acombined system may have the capacity to drain aconsiderable length of road. Situations may thereforearise in which there is a significant variation inlongitudinal gradient along the length of the pipe. Inthese cases, it is recommended to replace the value of Sin Equations (11) and (12) by the effective value oflongitudinal gradient, S

E. This is found by determining

values of the local gradient, Sj, of the road at eleven

equally-spaced points (j = 1 to 11) between theupstream and downstream ends of the drainage lengthbeing considered. The value of effective gradient iscalculated from:

210

2

5.05.011

5.01 2400

−=

=

−−−

++= ∑

j

jjE SSSS (15)

An iterative procedure may be necessary to determineS

E if the longitudinal profile of the road has been

determined in advance of the hydraulic design.

7/1



7.6 If the longitudinal gradient is locally zero at theupstream end of the pipe (j = 1), the zero value of S

1should be replaced in Equation (15) by the modifiedvalue 9/21 SS =′ . Similarly, if the gradient is locallyzero at the downstream end, the zero value of 11Sshould be replaced by the modified value

9/1011 SS =′ .

7.7 The hydraulic design method is not valid if adrainage length has intermediate crest or sag points. Insuch cases, the drainage length should be divided intoseparate sub-lengths and the pipe in each sub-lengthsized separately. At intermediate sag points, the flow inthe internal pipe should be discharged via an outfall to awatercourse, toe ditch or separate carrier pipe.

Hydraulic Resistance of Pipes

7.8 Recommended values of the effective Manningroughness coefficient for smooth-bore pipes formed inconcrete by the slip-forming process are given inTable 2.

Condition nP

Average 0.014

Poor 0.016

Table 2 Values of Manning Roughness forInternal Pipe

7.9 The ‘average’ condition assumes that the surfaceroughness of the pipe walls is equivalent to a value ofk

S = 0.6 mm in the Colebrook-White resistance equation

(see HR Wallingford and Barr, Ref 14) but with anadditional allowance of ∆n = 0.0025 for energy lossescaused by flow entering the pipe at intermediate outlets.The ‘poor’ condition assumes a rougher surface finishof k

S = 1.5mm and a depth of sediment deposit in the

invert equal to 5% of the pipe diameter.

7.10 If the internal pipe is formed by a process otherthan slip-forming, appropriate values of n

P should be

assessed taking account of the surface finish of thepipe, the energy losses at intermediate outlets, theexistence of joints and the possible presence ofsediment deposits.

7/2

Self-cleansing Conditions

7.11 Run-off from roads can contain significantamounts of silt, sediment and debris. If flow velocitiesin pipes are not high enough, the larger and heaviersediment particles may form bed deposits that cansignificantly reduce the flow capacity of the pipes.

7.12 In the types of combined system considered inthis Advice Note, the diameter of the pipe will usuallybe kept constant along a drainage length in order tosimplify the construction process (see 7.18 to 7.20 forinformation on alternative tapered pipe systems). Forcases of constant pipe diameter, it will usually beimpossible to prevent sediment deposition occurring inthe upstream sections of pipe where the flow rates arerelatively low. However, due to the reserve capacity ofthe pipe, significant deposition may be able to occur atthe upstream end without causing surcharging problemsat road level.

7.13 In terms of the overall performance of acombined system, it is more important to ensure thatdeposition will not occur in the pipes at the downstreamend where the flow rate is greatest and it is necessary toprevent surcharging of the pipe. The value of minimumvelocity, V

min (in m/s), required to prevent deposition

depends on the diameter of the pipe, the depth of flow,and the concentration and size of sediment entering thesystem. Based on guidance on sediment problems inpipes given in CIRIA Report 141 (Ref 15) and HA 105(DMRB 4.2), it is recommended that combined systemsshould be designed so that the flow velocity, V

P, given

by Equation (14) at the downstream end of a drainagelength should not be less than the appropriate value inTable 3. Values of V

min for intermediate pipe sizes may

be obtained by interpolation.

Internal pipe diameter Minimum velocityD V

min(mm) (m/s)

200 0.71

250 0.73

300 0.79

350 0.84

400 0.89

450 0.97

500 1.05

Table 3 Minimum pipe-full velocities forself-cleansing flows

February 2005



Total Drainage Length

7.14 The maximum total length of road, LT (in m), that

can be drained by a section of combined channel andpipe system is given by:

APT LLL += (16)

where the drainage length, LP

, for the pipe is obtainedfrom Equation (11) or (12), and the maximum allowablespacing, L

A, between the last intermediate outlet and the

terminal outfall is determined from 6.16.

7.15 The number and positions of outlets along acombined system can be varied as appropriate providedthat the calculated maximum values of L

P, L

Aand L

T(see also 5.3) are not exceeded. Two possible optionsfor determining suitable layouts for systems aredescribed in 7.16 and 7.17.

7.16 This option is most appropriate for lengths ofroad of constant longitudinal gradient. In this case,construction considerations may favour the use of equalspacings between adjacent outlets, with the lastintermediate outlet being located at a distance L

P from

the upstream end of the system. This option requiresone terminal outfall and N

Iintermediate outlets, where

the value of NIis given by:

+=

C

PI L

LN INTEGER1 (17)

where the INTEGER function rounds down the ratioL

P/L

Cto the nearest whole number. The actual spacing,

AL′ , between adjacent outlets is given by:

I

PA N

LL =′ (18)

which will be smaller than the maximum allowablevalue of L

A. However, the pipe will be used to its

maximum capacity. The total length of road drained bythis section of combined channel is:

APT LLL ′+=′ (19)

A terminal outfall is required at this downstream pointto discharge all the flow from the combined system to awatercourse, toe ditch or separate carrier pipe.

7lI

(

(

(

(

T

7bUshaasss

7tnb

7potptt

February 2005

.17 The second option may be more appropriate forengths of road having a varying longitudinal gradient.n this case, the following procedure can be applied.

1) Starting from the upstream end of the system, usethe local value of longitudinal gradient tocalculate the maximum allowable length of road,L

A, that can be drained by the surface channel.

Locate the first intermediate outlet at this point.

2) Proceed downstream, locating an intermediateoutlet at each point at which the maximumdrainage capacity of the channel is reached.

3) Add together the individual values of LC to find

the cumulative drainage length, Lcum

, measuredfrom the upstream end of the system. The totallength of road, TL′ , that can be drained by thissection of combined system is equal to the firstvalue of L

cum that exceeds the maximum drainage

length, LP, of the internal pipe.

4) Locate a terminal outfall at this downstreampoint to discharge all the flow from the combinedsystem to a watercourse, toe ditch or separatecarrier pipe.

apered Pipe Systems

.18 The diameter of the internal pipe does not need toe kept constant along the length of a combined system.se of a smaller diameter in the upstream part of a

ystem increases the velocity of flow and can thereforeelp to prevent sediment deposition. However, thisdvantage may be outweighed by the added complexitynd cost of constructing a system with more than oneize of internal pipe. As explained in 7.12, a degree ofiltation can be accepted in an oversized pipe withouturcharging problems occurring at road level.

.19 If the diameter of the internal pipe is varied alonghe length of the system, the point at which it isecessary to increase from one pipe size to another cane found using Equation (12).

.20 Any change in pipe size should coincide with theosition of an intermediate outlet. The invert of theutgoing pipe should not be higher than the invert ofhe incoming pipe. Where possible the soffits of the twoipes should be set at the same level. This will requirehe vertical cover for the upstream pipe to be greaterhan the minimum value given in Chapter 4.

7/3


S AND TERMINAL

Chapter 8Intermediate Outlets and Terminal Outfalls

8. INTERMEDIATE OUTLETOUTFALLS

Intermediate Outlets

8.1 Intermediate outlets in combined channel andpipe systems consist of two components:

(1) gully gratings, installed in the base of thechannel, that discharge the flow from the surfacewater channel; and

(2) shallow chambers that enable flow from thesurface channel to enter the internal pipe.

8.2 The hydraulic design of the gully gratings (type,spacing, number) is described in HA 78 (DMRB 4.2).The following alternative geometries for intermediateoutlets, presented in HA 78 (DMRB 4.2), are generallysuitable for the combined channel and pipe system:

(1) In-line outlet, where the water is essentiallycollected symmetrically either side of the channelinvert (see Figures B5, B7 and B9 of HA 78,DMRB 4.2); or

(2) Off-line outlet, where the channel is widenedaway from the carriageway and the outlet isoffset from the centreline of the channel(see Figures B6, B8 and B10 of HA 78,DMRB 4.2).

For triangular channels the in-line design is generallymore efficient than the off-line design, but reasons forchoosing between them will mainly depend onconstructional aspects (see 8.4). Other factors beingequal, in-line outlets are preferable to off-line outletsbecause they can allow use of a narrower verge.

8.3 Recommended designs for the outlet chamberthat transfers flow from the surface channel to theinternal pipe were established through experimentaltesting and analysis (Escarameia and May, Ref 16;Escarameia et al, Ref 17) and are based on thefollowing principles:

(1) The plan shape of the chamber is determined bythe layout of the gratings forming the upper partof the outlet.

(2) The depth of the chamber is determined by theinvert level of the internal pipe (as well as by

(

8iFlF

8nwss

8(cibtmi

8ctcTtltbnpaf

T

8p

(

February 2005

construction requirements, see Chapter 10) andtherefore will usually be shallow.

3) The flow in the chamber should be containedwithin benching to minimise energy losses.

.4 Schematic diagrams of the chambers forntermediate off-line and in-line outlets are illustrated inigure 6 and 7. These Figures give cross-sectional

ayouts and should be consulted in conjunction withigures B5 and B6 of HA 78 (DMRB 4.2).

.5 The off-line arrangement (see Figure 6) willormally involve the construction of chambers withalls formed using either brickwork, precast concrete

ections or an internal liner, and with a partial coverlab to support one edge of the grating.

.6 The chamber for the in-line arrangementsee Figure 7) can be formed by removing some of theoncrete immediately after slip-forming so that thenternal shape of the chamber can be boxed out. A metaleam can be inserted to support inclined gratings wherehe channel is of sufficient width (see 10.1). Heavieresh reinforcement (as defined in 4.6) should be used

n the base of the channel unit.

.7 In order to minimise energy losses at thehambers, it is desirable to construct the benching up tohe soffit level of the incoming pipe so as to fullyontain the flow within the U-shaped benched channel.he vertical space available between the benching and

he underside of the cover slab of the chamber may beimited, particularly for smaller sizes of internal pipe. Inhese cases the level of the benching may be lowered,ut the depth of the U-shaped benched channel shouldot be less than 75% of the diameter of the incomingipe. It is recommended to construct the benching with minimum transverse slope of 1:10 to help direct thelow from the gratings towards the benched channel.

erminal Outfalls

.8 Terminal outfalls from combined channel andipe systems consist of two components:

1) gully gratings, installed in the base of thechannel, that discharge the flow from the surfacewater channel; and

8/1


Chapter 8Intermediate Outlets and Terminal Outfalls

(2) outfall chambers with catchpits that also receiveflow from the internal pipe, and convey thecombined flow to a suitable watercourse, toeditch or separate carrier pipe.

8.9 When not protected by a safety barrier, the uppersurface of the terminal outfall must terminate with asmooth transition, without abrupt changes in level orwidth (following the recommendations in 3.16 ofHA 78, DMRB 4.2). The flow collection efficiencyprovided by terminal outfall gratings will need to behigher than for intermediate outlets, and should be closeto 100%. The recommended hydraulic design procedureis described in 4.18 and 4.19 of HA 78 (DMRB 4.2).

8.10 The plan shape of the chamber will bedetermined by the layout of the gratings forming theterminal outfall. The invert level of the outgoing pipefrom the chamber should be governed by the followingtwo criteria:

(1) The invert level should be set at a minimum of300mm above the bottom of the chamber toprovide an adequate volume for sedimentretention.

(2) The invert level should be such that the waterlevel in the chamber does not rise high enough toprevent flow discharging freely from the surfacechannel into the chamber.

8.11 In order to meet criterion (2) in 8.10, it isrecommended that the water level in the chambershould be at least 150mm below the underside of thegratings when the chamber is receiving flow from thechannel under surcharged conditions. The height Z(in m) of the water surface in the chamber above theinvert of the outgoing pipe can be estimated from theequation:

4

2

23.02 D

QDZ T+= (20)

where D is the diameter of the outgoing pipe (in m) andQ

T is the total design flow rate (in m3/s) given by:

SPT QQQ += (21)

where QP is the flow rate from the internal pipe given

by Equation (13), and QS is the flow rate from the most

downstream section of surface channel when operatingunder surcharged conditions (see 6.10 to 6.12 andEquation (8)).

8/2

8.12 The gradient and diameter of the outgoing pipefrom the chamber should be determined from a suitableresistance equation or flow tables (such asHR Wallingford and Barr, Ref 14) assuming that thepipe is just running full at the design flow rate, Q

T,given by Equation (21).

8.13 An example of a suitable layout for a terminaloutfall chamber is given in Figures 8a to 8c.

Steep Roads

8.14 On steep roads (typically with gradients ofS > 1:50), the flow collection efficiency of gullygratings may be insufficient due to the effect of highwater velocities in the surface channel. In these cases,HA 78 (DMRB 4.2) recommends the use of weir outlets(see Figures B23 and B24 in HA 78). With the weiroutlet, the surface channel is locally widened on theverge side so that the flow can discharge smoothly overa longitudinal weir into an external sidespill channel. Asafety barrier must be installed locally to prevent thewheels of vehicles dropping into the sidespill channel.

8.15 In the case of an intermediate chamber with aweir outlet, flow from the sidespill channel can bedischarged into the internal pipe of the combinedsystem via a covered chamber formed on the line of thesurface water channel and downstream of the weiroutlet. A screen should be installed at the exit from thesidespill channel to prevent coarse debris being washedinto the chamber.

Access for cleaning the chamber may be providedeither by a removable manhole cover, or by a gullygrating installed in the invert of the surface channel(as described in 8.4 to 8.6). The latter arrangement willprovide some reserve flow collection capacity but thisshould not be taken into account when sizing the weiroutlet.

The chamber should be benched internally so as tominimise energy losses between the straight-throughflow carried by the internal pipe of the combinedsystem and the side flow entering from the sidespillchannel. Suitable layout details for shallow junctionchambers are given in Figure F5 of the HCD (MCHW3).

8.16 In the case of a terminal outfall, the flow in theinternal pipe and the flow from the most downstreamweir outlet can be discharged separately into a commonchamber, from which the combined flow can beconveyed to a suitable watercourse, toe ditch orseparate carrier pipe. See relevant sections of 8.8 to

February 2005

8.13.


E

Chapter 9Sub-Surface Drainage

9. SUB-SURFACE DRAINAG

General

9.1 Combined surface water channel and pipesystems will usually be significantly deeper than solidtriangular surface water channels, and so will befounded at a lower level of the pavement construction.Consequently, the base of a combined unit will usuallybe at a similar level to the base of the adjacentpavement. Therefore, the type of sub-surface drainageappropriate for solid channels (see F21 of the HCD,MCHW 3) will not be suitable for use with combinedsystems.

Drainage System

9.2 Since the channel of a combined system willform an effective barrier to the horizontal movement ofmoisture at the pavement edge, the sub-surface drainmust be located between the pavement construction andthe channel. This is a change from the drainagephilosophy for solid surface water channels in whichthe sub-surface drain is located at the extreme edge ofthe carriageway, where it is more readily accessible.

9.3 The recommended layout of sub-surface drainagesystem for combined channel and pipe systems isshown in Figure 9. Fin drains of Types 5, 6, 8 or 9(see the HCD, MCHW 3) are suitable for use with thislayout. However, it should be noted that the Type 10filter drain (see F21 of the HCD, MCHW 3) used inconjunction with solid surface water channels is notsuitable for use with combined systems.

9.4 The construction sequence for a combinedsystem will normally involve construction of thechannel and pipe prior to placement of sub-basematerial and any capping material. Hence any waterdraining from the carriageway construction maybecome trapped against the pavement side of thechannel. Since any sub-surface drainage must beprotected from this run-off, additional temporarydrainage will be necessary. It may be possible to makeuse of the excavation for the temporary drain in theconstruction of the permanent sub-surface drainage.

9.5 The Types 5 and 6 fin drain may be affixed to thepavement side of the channel, or Types 8 and 9 narrowfilter drain may be installed in the excavation. Damagemay occur during the pavement construction, in

February 2005

particular during the rolling of the sub-base material. Itis therefore recommended that the sub-surface drainageshould not be installed until placement of the sub-baseis complete.

9.6 The location of the sub-surface drainage willrender it inaccessible for maintenance and therefore it isessential that the diameter of the carrier pipe is sized toaccommodate some build-up of sediment during the lifeof the system.

9/1


S

Chapter 10Construction Aspects

10. CONSTRUCTION ASPECT

10.1 For combined systems constructed usingslip-forming techniques, many potential combinationsof channel size and pipe size are possible. Slip-formshields are expensive to manufacture but can potentiallybe used many times over. It is therefore recommendedthat designers and specifiers of combined drainagesystems (and also of conventional surface waterchannels) should co-operate with slip-form contractorsto limit the number of different designs required andthereby maximise the possibility of re-use. Three sizesof triangular channel and three sizes of pipe wouldcover a wide range of applications. Possible choiceswould be:

Top width of channel B (m)

0.901.201.50

Pipe diameter D (m)

0.300.400.50

If intermediate outlets of the in-line type are used(see 8.4 and Figure 7), the channel needs to besufficiently wide to accommodate two gratings andtheir frames with a minimum thickness of concretesurround of 100mm. For 500mm wide gratings, theminimum channel width will be about 1.35m. Therequired width may be reduced if inclined gratings in asingle-piece frame are produced for this type ofapplication.

10.2 In combined systems, the flow enters the pipe atdiscrete points along a drainage length so it is notnecessary to provide a pipe under the most upstreamlength of channel (see Figure 2). However, in practice,it may be simpler to construct the entire length of asystem with an internal pipe so as to allow use of thesame slip-forming technique throughout (or minimisethe number of different prefabricated units required).Where an unused section of pipe enters an outletchamber, the end of the pipe should be blanked off sothat incoming flow is directed smoothly into thedownstream section of pipe.

1bsa

1saTtlossbl

1ir

1ttim

C

1wfifctflbmptF

1soas

February 2005

0.3 If required, the diameter of the internal pipe cane varied with distance along the length of a combinedystem. The hydraulic factors to be taken into accountre described in 7.18 to 7.20.

0.4 The introduction of reinforcement into thelip-formed concrete channel can be readilyccomplished by the use of prefabricated steel mesh.he mesh should be cut into strips such that the

ransverse bar spacing is less than or equal to theongitudinal bar spacing. The mesh should be blockedff the blinding material to give a minimum cover to theteel of 40mm. To eliminate the risk of the slip-forminghield catching on the transverse bars, the mesh shoulde laid so that the transverse bars are beneath theongitudinal bars.

0.5 Splicing of the mesh sheets should be carried outn accordance with the manufacturer’secommendations.

0.6 The pipe void is formed by a flexible tube passedhrough the shield and anchored prior to inflation. Ashe shield moves forwards, concrete is fed in behind. Its important that the inflatable tube does not snag on theesh or ties.

ontraction and Expansion Joints

0.7 Contraction cracks can be dealt with in the sameay as for solid surface water channels. This involves

orming narrow slots about 25mm deep at suitablentervals across the top surface of the channel andilling the slots with a mastic sealant. Contractionracks may spread downwards through the full depth ofhe channel block. However, the amount of leakagerom the internal pipe through the hairline cracks isikely to be small, with the cracks gradually becominglocked by fine silt carried by the drainage flows. Anyinor leakage from contraction cracks will be

revented from reaching the pavement construction byhe sub-surface drainage system (see Chapter 9 andigure 9).

0.8 Where possible, any necessary expansion jointshould be formed at intermediate outlets or terminalutfalls. If an expansion joint is necessary part waylong a drainage length, a short length of plastic tubehould be inserted and sealed to the sections of internal

pipe either side of the joint in order to prevent leakage

10/1


Chapter 10Construction Aspects

into the joint.

Inspection

10.9 It is recommended that a CCTV inspection ofthe internal pipe (see Specification for SpecialistActivities, MCHW 5.9.2) be undertaken to ensure thatthe circular profile of the void is maintained over thelength of the channel. Deformations in excess of 10%of the nominal bore of the pipe will not be acceptable.

February 200510/2


Chapter 11Maintenance Aspects

11. MAINTENANCE ASPECTS

11.1 Channels should be regularly cleaned bysweeping in accordance with requirements set out in theTrunk Road Maintenance Manual (TRMM).

11.2 Gully gratings should be cleaned regularly toensure that they do not become blocked. There willnormally be no necessity to remove the grating from theframe.

11.3 Jetting of internal pipes shall be in accordancewith Clause 521 of the SWH (MCHW 1).

11.4 The frequency of cleaning of catchpits inchambers of terminal outfalls should not need to begreater than that for catchpits in systems withconventional concrete channels.

11.5 In the rare event that a section of combinedsurface channel and pipe system has to be removed andsubsequently replaced, the following procedure issuggested:

(1) The section to be removed should be cut bysawing through both the channel and the pipe tothe full depth of the block.

(2) If the reinforcement can be left in place then theconcrete should be broken out to expose it.

(3) Even if the reinforcement needs to be removed,care must be taken to ensure that it is not cutduring the sawing process. After removal of theconcrete, the reinforcement can then be cut so asto leave sufficient exposed steel to comply withthe splicing recommendations of the meshmanufacturer.

(4) Replacement mesh should be spliced to theexposed existing steel mesh in accordance withthe recommendations of the manufacturer.

(5) The exposed ends of the channel should bedrilled and dowelled in accordance with HD 32Maintenance of Concrete Roads (DMRB 7.4).The steel dowels should be to grade 460 ofBS 4449 (Ref 19) and have a length L

d = 600mm

if the dowels are of 16mm diameter, or Ld =

500mm if they are of 20mm diameter. Theembedded length of the dowels should be L

d/2.

(6)

(7)

(8)

(9)

(10

(11)

(12

(13

February 2005

Ideally the tie bars should be at 600mm centreswith at least two dowels in each face. Theminimum cover should be 40mm.

A section of plastic pipe with an internaldiameter similar to that of the pipe void, but notmore than 25mm smaller, should be cut to lengthand butted against each end of the channelsection.

Any gap between the pipe and the adjacentconcrete mass should be packed withcompressible filler and trimmed to minimiseintrusion into the pipe void.

The ends of the plastic pipe should betemporarily strutted off the dowels to hold thepipe in place.

) C35 concrete complying with the requirementsgiven in 4.4 should be placed andvibro-compacted around the steel mesh with carebeing taken not to dislodge the pipe. Theconcrete should then be brought up the sides andover the top of the pipe, working from the centreoutwards. The struts should be removed as theconcreting progresses.

In order to minimise the risk of flotation, a tiedstopper may be inserted into the pipe void at adownstream access point and the pipe void filledwith water as the concreting progresses. Caremust be taken since there is a significant risk ofwater escaping and washing cement from theconcrete, thereby allowing concrete to enter theresulting gap around the pipe.

) The surface channel should be provided with asteel trowel finish and have a cross-sectionalprofile similar to that of the original.

) 5mm deep saw cuts should be made across thewidth of the surface channel at the interfacesbetween the new and existing sections ofconcrete and the cuts filled with a waterproofsealant.

11/1


Chapter 12Worked Example

12. WORKED EXAMPLE

12.1 It is required to determine the spacing betweenthe intermediate outlets and terminal outfall for acombined surface channel and pipe system that willdrain a section of two-lane dual carriageway nearNorwich. The pavement is black top with a transversegradient of 1:40 on non-super-elevated sections. Thewidth of carriageway draining to the verge is 9.3m(including two 1.0m wide hardstrips). The section ofroad under consideration has a longitudinal gradient ofS = 0.8% (i.e. 1:125) and is on embankment so thecombined system will not receive run-off from anyadjacent pervious areas.

12.2 The combined system will be slip-formed inC28/35 mass concrete with light mesh to meet a D400loading requirement (see 4.5). The principal features ofthe system are as follows.

Surface water channel

• Symmetrical triangular channel with cross-fallsof 1:5 (vertical : horizontal)

• Design flow depth: YC = 0.120 m

• Corresponding flow width:B

C = 1.20m

• Average roughness condition: from Table 1(concrete), Manning roughness coefficientn

C = 0.013.

Internal pipe

• Diameter D = 0.400m

• Average roughness condition: from Table 2,Manning roughness coefficient n

P = 0.014.

Overall cross-sectional shape

• The surface channel is to be designed to allow amaximum width of surcharging of 1.0m on theadjacent hardstrip (see 6.9). For a straight sectionof road with a transverse gradient of 1:40, thiscan be achieved by setting the outer edge of thechannel 25mm above the level at the edge of thehardstrip (see HA 37, DMRB 4.2). Given that thesides of the channel have cross-falls of 1:5, itfollows that the overall width of the concrete unitforming the combined system will be equal toB + 0.125m = 1.325m.

•

R

•

1twb

W

1a

2

1doscfam

1ospdcobf

February 2005

C

To meet the structural requirements in 4.4, aminimum concrete cover equal to half the pipediameter needs to be provided. Therefore, theminimum depth of the concrete block is0.12m + 0.20m + 0.40m + 0.20m = 0.92m(measured from the edge of the hardstrip).

einforcement

The size of light mesh reinforcement required isdetermined from 4.5. The transverse bars of themesh need to provide a minimum steel area of385mm2 per metre run of pipe. This can beprovided, for example, by 7mm diametertransverse bars located at 100mm centres.

2.3 The effective width of catchment, WE

, draining tohe combined channel and pipe system is equal to theidth of the carriageway plus the width of the concretelock:

625.10325.130.9 =+=E m

2.4 The characteristic rainfall depth for the Norwichrea is found from Figure 3 to be:

min M5 = 4.0mm

2.5 The first step in the hydraulic design is toetermine the required spacing between intermediateutlets along the combined system. Flows produced bytorms with a return period of N = 1 year must beontained within the surface water channel with thelow depth not exceeding Y

C = 0.120m. Substituting the

bove values in Equation (4), it is found that theaximum drainage length is:

307=CL m

2.6 If some flow by-passing occurs at an intermediateutlet, the distance that can be drained by the nextection of channel will be reduced. Using the designrocedures given in HA 78 (DMRB 4.2), the selectedesign of intermediate outlet is calculated to have aollection efficiency of η = 0.90. Assuming that theutlets are equally spaced, the maximum spacingetween intermediate outlets allowing for by-passing isound from Equation (10) to be:

292=CBL m

12/1


Chapter 12Worked Example

12.7 It is also necessary to check that the width ofsurcharging on the hardstrip will not exceed 1.0mduring storms having a return period of N = 5 years.This can be done using the simplified calculationprocedure given in 6.10. For a design flow depth in thechannel of Y

C = 0.120m and a road cross-fall of 1:40,

Figure 4 shows that the drainage length factor forsurcharged conditions has a value of φ = 1.08. FromEquation (7), the maximum possible length of road thatcan be drained under surcharged conditions is therefore:

33230708.1 =×=SL m

If the flow collection efficiency of the intermediateoutlet under surcharged conditions is η = 0.85, it isfound from 6.15 that the maximum spacing betweenintermediate outlets taking account of by-passing is:

309=SBL m

12.8 The maximum allowable spacing, LA, between

intermediate outlets is the smaller of the two values LCB

and LSB

(see 6.16, item 2), which is:

292=AL m

12.9 The next step is to determine the length of roadthat can be drained by the internal pipe. As explained in7.2, it is recommended that the pipe should be designedto flow just full for storms with a return period of N = 5years. Using Equation (12) and the data given in 12.2 to12.4, it is found that maximum possible length of roadthat can be drained by the 0.40m diameter pipe is:

507=PL m

The flow velocity at the downstream end of the pipe isobtained from Equation (14) and has the value:

38.1=PV m/s

This will produce satisfactory self-cleansingconditions since it exceeds the minimum velocity ofV

min= 0.89m/s recommended in Table 3 for a pipe of

0.40m diameter.

12.10 The maximum total length of road that can bedrained by this section of combined surface channel andpipe system (i.e. measured from the upstream end to theterminal outfall) is:

799292507 =+=+= APT LLL m

12/2

In order to achieve a regular spacing between outletsand to simplify the setting-out and construction of thesystem, it is convenient to adopt the following slightlyconservative values:

500=′PL m and 250=′AL m

This results in a total drainage length for the section ofcombined system being considered of:

750=′TL m

Therefore, two intermediate outlets will be required atchainages of 250m and 500m (measured from theupstream end of the system) with a terminal outfalllocated at a chainage of 750m.

12.11 The dimensions and layouts of the intermediateoutfalls and the terminal outfall are determined usingthe recommendations in Chapter 8. Flow from theterminal outfall may be discharged to a watercourse ortoe ditch, or to a separate carrier pipe in the verge orcentral reserve.

12.12 The maximum design flow rate discharged fromthe terminal outfall is assumed to be equal to the sum ofthe flow rate, Q

P, from the internal pipe and the flow

rate, QS, from the most downstream section of surface

channel when flowing under surcharged conditions.From Equation (13):

173.0=PQ m3/s

and from Equations (8) and (5):

127.0=SQ m3/s

The design rate of flow discharged by the terminaloutfall is therefore:

300.0=TQ m3/s.

February 2005


Chapter 13References

13. REFERENCES

1. Design Manual for Roads and Bridges(DMRB), The Stationery Office.

HA 37: Hydraulic Design of Road-Edge Surface WaterChannels (DMRB 4.2).

HA 78: Design of Outfalls for Surface Water Channels(DMRB 4.2).

HA 83: Safety Aspects of Road-Edge Drainage Features(DMRB 4.2).

HA 105: Sumpless Gullies (DMRB 4.2).

HD 19: Road Safety Audit (DMRB 5.2).

HD 32: Maintenance of Concrete Roads (DMRB 7.4).

HD 33: Surface and Sub-surface Drainage Systems forHighways (DMRB 4.2).

TD 19: Safety Fences and Barriers (DMRB 2.2).

TD 32: Wire Rope Safety Fence (DMRB 2.2.3).

2. Manual of Contract Documents for HighwayWorks (MCHW), The Stationery Office.

Specification for Highway Works (SHW) (MCHW 1).

Highway Construction Details (HCD) (MCHW 3).

Specification for Specialist Activities (MCHW 5.9.2).

3. Walker C D. Safety aspects of road edge drainagefeatures. Report 422, TRL Limited.

4. BS EN 1433. Drainage channels for vehicularand pedestrian areas. Classification, design and testingrequirements, marking and evaluation of conformity.British Standards Institution, London.

5. BS EN 124. Gully tops and manhole tops forvehicular and pedestrian areas - Design requirements,type testing, marking, quality control. British StandardsInstitution, London.

6. BS 7903. Guide to selection and use of gully topsand manhole covers for installation within the highway.British Standards Institution, London.

February 2005

7. May R W P, Todd AJ, Boden DG and EscarameiaM. Combined surface channel and pipe system: ProjectReport. HR Wallingford, Report SR 624, 2004.

8. BS EN 206-1. Concrete. Specification,performance, production and conformity. BritishStandards Institution, London.

9. BS 5931. Code of practice for machine laid insitu edge details for paved areas. British StandardsInstitution, London.

10. HR Wallingford. Design of highway drainagechannels: Preliminary analysis. Report DE 30, 1976.

11. HR Wallingford. Motorway drainage trial on theM6 motorway, Warwickshire. Contractor Report 8,TRRL, 1985.

12. May R W P. Design of highway drainagechannels. OECD Symposium on Road Drainage, Bern,Switzerland, 1978, pp 450-459.

13. Institute of Hydrology. “Flood EstimationHandbook”, Volume 2: “Rainfall frequency estimation”by D Faulkner.

14. HR Wallingford and Barr D I H. Tables for thehydraulic design of pipes, sewers and channels. ThomasTelford, London, 7th Edition, 1998.

15. Ackers J C, Butler D and May R W P. Design ofsewers to control sediment problems. CIRIA Report141, 1996.

16. Escarameia M and May R W P. Surface waterchannels and outfalls: Recommendations on design.HR Wallingford, Report SR 406, 1995.

17. Escarameia M, Todd A J and May R W P.Combined surface channel and pipe system: InterimReport. HR Wallingford, Report SR 585, 2001.

18. Trunk Road Maintenance Manual (TRMM), TheStationery Office, London.

19. BS 4449. Specification for carbon steel bars forthe reinforcement of concrete. British StandardsInstitution, London.

13/1


February 2005 14/1

14. ENQUIRIES

All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:

Acting Divisional Director1A PED Federated HouseLondon RoadDorking GERRY HAYTERRH4 1SZ Acting Divisional Director

Chief Road EngineerScottish ExecutiveVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Directorate

Director of EngineeringThe Department for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 14Enquiries


February 2005

ANNEX A LIST OF SYMBOLS

Units

A Cross-sectional area of flow in channel or pipe m2

B Surface width of flow in channel or pipe m

BS

Allowable width of flow on hard strip or hardshoulder adjacent to channel during msurcharged conditions

D Internal diameter of pipe m

H Horizontal cover around pipe m

IO

Mean rainfall intensity mm/h

kS

Hydraulic roughness in Colebrook-White resistance equation mm

LA

Maximum allowable spacing between adjacent outlets in channel m

LC

Drainage length of channel, i.e. maximum length of road that can be drained by a msection of surface channel at design depth of flow

LCB

Value of LC allowing for flow by-passing at upstream outlet m

Lcum

Cumulative length of road drained by sections of surface channel, measured from mupstream end of combined system

Ld

Length of dowel rod mm

LP

Maximum length of road that can be drained by a section of internal pipe m

LS

Maximum length of road that can be drained by a section of surface channel under msurcharged conditions

LSB

Value of LSallowing for flow by-passing at upstream outlet m

LT

Total length of road that can be drained by a section of combined system m

LU

Drainage length of section of combined system immediately upstream of the drainage mlength being designed

N Return period of storm years

NI

Number of intermediate outlets in combined system -

n Manning roughness coefficient of channel or pipe -

Q Flow rate in channel or pipe m3/s

QS

Flow capacity of channel in surcharged condition m3/s

QT

Total flow rate discharged from a section of combined system m3/s

Annex AList of Symbols

A/1


February 2005

P Wetted perimeter of channel or pipe m

R Hydraulic radius of flow ( = A/P ) m

S Longitudinal gradient of road, channel or pipe (vertical fall per unit distance along m/mroad, channel or pipe)

SE

Effective value of S for road, channel or pipe of non-uniform gradient m/m

T Duration of storm minutes

U Vertical cover around pipe m

V Velocity of flow m/s

Vmin

Minimum pipe-full velocity for self-cleansing flow m/s

WE

Effective width of catchment drained by combined system m

YC

Design depth of flow in surface channel (from invert) m

Z Height of water surface in outfall chamber above invert level of outgoing pipe m

∆Z Difference in invert level of channel between upstream and downstream ends of mdrainage length

η Collection efficiency of outlet ( = flow rate collected by outlet/flow rate in channel -approaching outlet)

φ Surcharge factor (ratio between drainage length for surcharged channel and drainage -length for channel just flowing full)

2minM5 Rainfall depth occurring in 2 minutes with return period of 5 years mm

Subscripts

B value taking account of flow by-passing

C value for channel

j integer variable

min minimum value

max maximum value

P value for pipe

S value for surcharged conditions

′ actual value

A/2

Annex AList of Symbols


February 2005

ANNEX B FIGURES

Figure 1 Typical Cross-section of Combined Surface Channel and Pipe System

Figure 2 Longitudinal Profile of Combined Surface Channel and Pipe System

Annex BFigures

B/1


February 2005

Figure 3 Values of 2minM5 Rainfall Depth for UK(Reproduced from BS 6367:1983 by permission of British Standards Institution)

B/2

Annex BFigures


February 2005

Figure 4 Drainage Length Factor φφφφφ for Triangular Channelswith Surcharged Width of B

S = 1.0m

Annex BFigures

B/3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15

Design flow depth in non-surcharged channel Y C (m)

Dra

inag

e le

ng

th f

acto

r φφ φφ

Road cross-fall = 1:30




February 2005

Figure 5 Drainage Length Factor φφφφφ for Triangular Channelswith Surcharged Width of B

S = 1.5m

B/4

Annex BFigures

0.0

1.0

2.0

3.0

4.0

5.0

0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15

Design depth of flow in non-surcharged channel Y C (m)

Dra

inag

e le

ng

th f

acto

r φ φ φ φ Road cross-fall = 1:30




February 2005

Figure 6 Example of Intermediate Outlet with Off-line Grating

Annex BFigures

B/5

reinforced concrete slabextends beneath channelsection

insitu constructionreturned to edgeof channel

compactedgranular fill

concrete

verge

grating and frameinfill compound

25mm bedding compound

reinforced concrete slab keyed into channel

insitu construction

infill concrete

0

0

0

0

0

benching 0

0

0

0

0

0

CL

carriageway

5

1


February 2005

Figure 7 Example of Intermediate Outlet with In-line Grating

B/6

Annex BFigures

meshreinforcement

D400 (BSEN 124)V shape grating and frameinstalled to suit trafficdirection. (Frame depth 150mm)

0

min. D/2

varies

0

concrete removed duringslipform process to formvoid for inlet and rebatefor frame seating

verge carriageway

infill concreteafter framepositioned

25mm bedding compound

0

min. 1000

0

CL

min. D/2

min. 40

51

800


February 2005

Figure 8a Example of Terminal Outfall - Plan View

Annex BFigures

B/7

concrete channel0

internal pipe0

precast concrete ring0

insitu constructionbrickwork, corbelledto suit frame dimensions

gratings and frames0

terminal ramp0

0

0 600 x 600 openingthrough cover slab

0 infill concrete toform channel overframe box out

0

frame recessedinto channel

0

0

outgoingpipe

0

1:4precast concrete slab

0

infill concrete

0 0


February 2005

Figure 8b Example of Terminal Outfall - Longitudinal Cross-section

B/8

Annex BFigures

internal pipe

terminal ramp0

In situ constructionbrickwork, corbelledto suit frame dimensions

precast concrete ring(or in situ construction)

0

min. 600 x 600 opening

sw channel0

infill compound00 infill concrete

compacted fillmaterial

0

0


February 2005

Figure 8c Example of Terminal Outfall - Transverse Cross-section

Annex BFigures

B/9

vergecarriageway0

0

min. 600 x 600 opening

outgoingpipe

min. 300

0

insitu constructionbrickwork, corbelledto suit frame dimensions

infillconcrete

0


February 2005

Figure 9 Sub-surface Drainage for Combined System

B/10

Annex BFigures

Bound pavement construction

Sub-base

Capping layer

Formation

Type 5, 6, 8 or 9Fin Drains

Hard shoulderNon-pavement verge

Notes:

1. Where type 5 and 6 FinDrains are used the geo-texti leshould be affixed t o the side ofthe channel.

2. Pipe surround material shallbe as shown on drawing F18.

3. Top of pipe to be at least 50mmbelow t he bottom of the cappingor the base of the channel,whichever is the lower.

50mm

75mm