Drainage of Runoff from Natural Catchments

February 2004

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 1

HA 106/04

DRAINAGE OF RUNOFF FROMNATURAL CATCHMENTS

SUMMARY

This Advice Note gives guidance on how to deal withsurface water runoff from natural catchments drainingtowards trunk roads (including motorways), in order tolimit the frequency and severity of flooding incidentscaused by runoff from beyond the highway boundary.

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This is a new Advice Note to be incorporated into theManual.

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3. Insert HA 106/04 into Volume 4, Section 2,Part 1.

4. Please archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

HA 106/04

Drainage of Runoff fromNatural Catchments

Summary: This Advice Note gives guidance on how to deal with surface water runoff fromnatural catchments draining towards trunk roads (including motorways), inorder to limit the frequency and severity of flooding incidents caused by runofffrom beyond the highway boundary.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 4 Section 2Part 1 HA 106/04

February 2004

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VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 1

HA 106/04

DRAINAGE OF RUNOFF FROMNATURAL CATCHMENTS

Contents

Chapter

1. Introduction

2. Methodology

3. Natural Catchment Identification

4. Approaches to the Collection of Runoff

5. Estimation of Runoff

6. Hydraulic Design of Ditches

7. Worked Examples

8. References and Bibliography

9. Enquiries

Appendix A Figures

Appendix B Determination of Design Return Period

Appendix C Roughness Values for the HydraulicDesign of Ditches


February 2004


Chapter 1Introduction

1. INTRODUCTION

General

1.1 This Advice Note gives guidance on how to dealwith surface water runoff from natural catchmentsdraining towards trunk roads (including motorways), inorder to limit the frequency and severity of floodingincidents caused by runoff from beyond the highwayboundary.

1.2 Surface water runoff to highway drainagesystems is conventionally assumed to derive from theroad cross-section. This includes the road surface,verges and adjacent cuttings or embankments (termedInterior Catchment). Additional surface flow may alsobe produced by runoff draining to the road from landoutside the highway corridor (termed ExteriorCatchment). Exterior catchments can be rural, urban ora combination of both. This Advice Note deals solelywith rural (natural) catchments since exterior urbancatchments have their own specific drainage systems.Its recommendations may be applied to other roads withrural (natural) catchments and traffic conditions, asappropriate.

1.3 Following the Autumn 2000 floods, a review ofroad flooding incidents showed that approximatelytwo-thirds were associated with the lack of capacity ofthe drainage systems. Existing HA guidance on how todeal with runoff from road surfaces was found to beadequate. However, highway drainage systems were, insome cases, overloaded by additional water draining tothe roads from the surrounding natural catchment.Others were unable to convey the flows because ofsubmergence of the outfalls or blockage within thesystem. This document is aimed at minimising theflooding problem associated with runoff from roadadjacent catchments.

1.4 General recommendations on earthworksdrainage, both surface water and sub-surface water, aregiven in HD 33, Surface and Sub-Surface DrainageSystems for Highways (DMRB 4.2, Ref. 1). HD 33recommends that cut-off drains are constructed at thetops of cuttings and at toes of embankments wherewater from adjoining land may flow towards the road.HD 33 states that intercepting drains or ditches shouldbe sufficiently deep to collect these flows, includingthose from any severed agricultural drainage systemsbut no quantitative guidelines are given. HD 33 alsohighlights the importance of ensuring a coordinated

February 2004

analysis of the horizontal and vertical road profilesbefore the final alignment is chosen.

Scope

1.5 The guidance given in this Advice Note isapplicable to new road projects as well as to existingroad schemes where reduction of the flood risk isconsidered necessary to ensure resilience of the roadnetwork to extreme weather conditions.

1.6 Natural catchments adjacent to roads varysignificantly in size, shape, type of soil and vegetationcover, and the amount of runoff contributed to roaddrainage systems can range from negligible tosignificant. In defining the areas of natural catchment toconsider, the contribution of smaller catchments (ingeneral terms less than 0.01 km2, or 1ha) can beneglected. Smaller sites with a history of frequentflooding may need to be considered on a one to onebasis. Catchments with surface areas greater than25 km2 are outside the scope of this document.

1.7 The design of culverts and outfalls to preventflooding of roads due to high water levels at the pointof discharge is given in HA 107, Design of Outfall andCulvert Details (DMRB 4.2).

1.8 Design guidance concerning prevention offlooding of roads constructed in the flood plain by highwater levels in adjacent rivers or streams, is given inHA 71, The Effects of Highway Construction on FloodPlains (DMRB 4.2, Ref. 1).

1.9 In certain areas of the UK and in periods of highgroundwater levels, springs may appear at the surface incatchments adjacent to roads. These springs cangenerate significant flows and potentially cause orincrease the risk of road flooding. Since natural springsare dependent on local geological conditions andgroundwater levels, they will require specificassessments to determine their likely location, flow rateand impact on road performance. This is not covered inthe present document but information can be obtainedfrom the Environment Agency and the BritishGeological Survey, among other sources.

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Chapter 1Introduction

Implementation

1.10 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. Design Organisations should confirm itsapplication to particular schemes with the OverseeingOrganisation.

February 20041/2


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Chapter 2Methodology

2. METHODOLOGY

2.1 The overall procedure for dealing with runofffrom natural catchments is summarised in the flowchartof Figure 1 (in Appendix A). The most important stages,which are described in detail in the next chapters, are:

i) identification of flood-prone areas andcharacterisation of natural catchment;

ii) estimation of runoff;

iii) hydraulic design of ditches/culverts and/orupgrade of existing road drainage system;

iv) formulation of maintenance programme.


Chapter 3Natural Catchment Identification

IDENTIFICATION
3. NATURAL CATCHMENT
3.1 The size, shape and other characteristics ofnatural catchments, such as gradients, are likely to varyconsiderably along the highway alignment. Theircontributions in terms of runoff are also likely to varyfrom negligible amounts from catchments of smalldimensions to large flow rates. The latter should bedischarged into ditches or through culverts, or dealtwith by modifications to the highway drainage system.

3.2 There are essentially two types of naturalcatchment that may be encountered alongside roads(see Figure 2, in Appendix A):

Valley CatchmentsCatchments formed by a well-defined valley, either dryor drained by a watercourse (including ephemeralstreams).

Strip CatchmentsCatchments with no defined valley, forming a strip offairly uniform width along the highway boundary.

3.3 To determine the natural catchment dimensionsthe following definitions apply.

Catchment width:a) For valley catchments – the distance between the

top end of the catchment and the top of thecutting, or the pavement edge, measured alongthe valley, perpendicular to the ground contours(distance A-B in Figure 2 of Appendix A).

b) For strip catchments – the distance between thehighest point of the catchment and the top of thecutting, or the pavement edge (distance C-D inFigure 2 of Appendix A).

Catchment length:This is defined as the distance of natural catchmentadjacent to the highway boundary, measured parallel tothe road.

3.4 In flat areas definition of the natural catchmentboundary is not always obvious, and engineeringjudgement should be applied. The maximum catchmentwidth should not exceed 10km.

3.5 When defining the extent of natural catchmentsadjacent to roads, Ordnance Survey maps at 1:25 000scale should be consulted. Site inspections are alsorecommended as they can provide useful information

odmoirpwvpbsa

3p

•

•

F

•

•

February 2004

n local features. Due to the linear nature of roads, theischarge points for the natural catchment flows will inost cases be the same as those used for the discharge

f road runoff. However, the criteria that need to be metn terms of pollution loads may be less stringent forunoff from natural catchments and therefore moreoints of discharge may be considered suitable. Areashere the amount of silt in the runoff is expected to beery high can still be associated with a significantollution risk. Catchment widths smaller than 50m cane neglected, unless there is information specific to theite indicating the need to take the local runoff intoccount (e.g. history of frequent local flooding).

.6 A checklist to aid the identification of flood-rone areas is given below:

Road configuration:

- low points/areas (sag);

- inner areas of bends in road alignmentwhere accumulation of flow can occur dueto adjacent catchment;

- connection with other roadways (e.g. sliproads) that can act as a drainage pathway;

Catchment features (see Figure 3 in Appendix A):

- large fields adjacent to the road (ExamplesA.1, B.1 and C.1);

- slopes intercepted by the road (ExamplesA.2 and A.3);

- areas of well defined stream catchment(even if stream is ephemeral) producingconcentrated flows;

- presence of natural springs.

or existing schemes:

poor condition of existing cut-off ditches, landdrainage and culverts (i.e. overgrown vegetationin ditches, blockages in culverts and ditches,collapsed drains);

level of outfalls that do not allow free discharge(see guidance in HA 107, DMRB 4.2);

3/1


Chapter 3Natural Catchment Identification

• poor condition of road drainage system(blockages, siltation);

• signs of erosion (gullies) in cutting slopes; poorestablishment of vegetative protection in steepcuttings; in cultivated land, furrows running inthe direction of the slope (rather thantransversely).

3.7 Examples of catchments that can producesignificant runoff are given in Figure 3 of Appendix A.They refer to two situations (roads in cutting and inshallow embankments) and need to be considered inconjunction with the checklist given in 3.6 for theidentification of flood-prone areas.

February 20043/2


February 2004

Chapter 4Approaches to the Collection of Runoff

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4. APPROACHES TO THE COLLECTION OFRUNOFF

4.1 Slopes containing a well defined watercourse thatare intercepted by roads will usually require theprovision of a culvert to ensure that the runoff isadequately conveyed away from the road construction.The design of culverts is covered by HA 107,DMRB 4.2).

4.2 Slopes intercepted by roads but without a well-defined watercourse path can also produce significantamounts of runoff, which need to be discharged bymeans of ditches (or pipes). Where possible, theseshould be located at the top of cuttings or at the toe ofembankments, see HA 44, Earthworks: Design andPreparation of Contract Documents (DMRB 4.1.1,Ref. 1). If a large ditch is used at the top of a cutting, ageotechnical assessment of the cutting should be carriedout (as specified in the DMRB HA 43, 44, 48 and 68and in SH4), taking into account possible groundwaterseepage from the cutting. If necessary to avoid stabilityproblems, locating the ditch at road level should beconsidered.

4.3 In some locations of the road network,particularly in older schemes, there may be no spaceavailable for the construction of ditches, either at thetop of cuttings, bottom of embankments or at road level.In these cases ad-hoc solutions need to be considered.Among possible measures are:

- upgrading of the road drainage system to receiverunoff flows from the natural catchment;

- changes in the land-use patterns adjacent to theroad to minimise gradients and potential for soilerosion (through negotiations with the landowners);

- protection of slopes against soil erosion by meansof mats, for example (also through negotiationswith the land owners).

4.4 When designing and constructing ditches, careshould be taken not to affect the long-term drainagecharacteristics of environmentally sensitive soils, suchas peat bogs.


F

Chapter 5Estimation of Runoff

5. ESTIMATION OF RUNOF

Background

5.1 The published methods for estimating runoffhave been reviewed. The criteria used for the selectionof the recommended method(s) were based on technicalas well as practical considerations. The recommendedmethod should:

i) be applicable to small catchments (comparedwith river catchments, the linear dimension ofnatural catchments draining to roads can beorders of magnitude smaller);

ii) be applicable to catchments with no well-definedwatercourse;

iii) not require hydraulics modelling software;

iv) be simple to use by non-river engineers.

5.2 Of the range of methods available for theprediction of runoff from pervious surfaces, most arenot suited to the case of catchments alongside roadswhich, see Chapter 3, have specific characteristics.Some of the most recent and sophisticated methods,such as those described in the Flood EstimationHandbook, FEH (Ref. 2), were developed forpre-defined river catchments of a certain dimension andcannot deal with smaller catchments. The FEHapproach also requires the use of a software packageand considerable knowledge of hydraulics. Othermethods assume the presence of watercourses, which isnot always the case in highway applications.

Design return period

5.3 Highway drainage systems are designed tointercept and remove rainfall from short duration, highintensity events with return periods of 1 year (for nosurcharge of piped systems or road-edge channels) or5 years for no flooding of the carriageway. Flood flowsfrom natural catchments can have durations of severalhours so the potential for traffic disruption is greaterthan that produced by runoff from paved surfaceslasting only a few minutes. For this reason, it isrecommended that flow rates from natural catchmentswithout defined watercourses should be assessed fordesign storms with a return period of 75 years (seeAppendix B for background on the choice of this return

February 2004

period). For culverts that convey permanentwatercourses beneath roads, the flow rates should beassessed for return periods that can vary between 25and 100 years depending on the implications offlooding (see HA 107, DMRB 4.2).

Recommended methods

5.4 For rural catchments larger than 0.5km2 (or 50ha)flood flows should be estimated using a methoddeveloped by the Centre for Ecology & Hydrology,CEH, (then Institute of Hydrology) and described inreport IH 124 (Ref. 3). This was based on a study of 71small rural catchments and was derived fromcatchments drained by a well-defined watercourse.

The mean annual flood Qa (in m3/s) is calculated by:

Qa = 0.00108 AREA0.89 SAAR1.17 SOIL2.17 (1)

where

AREA (in km2) is the catchment plan areaSAAR (in mm) is the standard average annual rainfallfor the particular location (see Figure 4 of Appendix A)SOIL is the soil index, defined as:

( )( ) (2)0.50.450.400.300.15

54321

54321

SSSSSSSSSSSOIL

++++++++=

S1, 2….denote the proportions of catchment covered byeach of the soil classes 1 to 5. Soil class 1 has a lowrunoff potential and soil class 5 has a high runoffpotential. The parameter SOIL for a natural catchmentcan vary between 0.15 (very low runoff) and 0.5 (veryhigh runoff). This parameter can be estimated from soilmaps reproduced in HA 71/95 (DMRB, Ref. 1) or, for asimplified approach, through consultation of Table 5/1(adapted from the Agricultural Development andAdvisory Service, ADAS, Ref. 4):

5/1



Table 5/1 Runoff potential and soil classes

General soil description Runoff Soilpotential class

Well drained sandy, loamy or earthy Very low S1

peat soilsLess permeable loamy soils over clayeysoils on plateaux adjacent to verypermeable soils in valleys

Very permeable soils (e.g. gravel, sand) Low S2

with shallow groundwaterPermeable soils over rocksModerately permeable soils some withslowly permeable subsoils

Very fine sands, silts and sedimentary Moderate S3

claysPermeable soils (e.g. gravel, sand)with shallow groundwater in low lyingareasMixed areas of permeable andimpermeable soils in similar proportions

Clayey or loamy soils High S4

Soils of the wet uplands: Very high S5

Bare rocks or cliffsShallow, permeable rocky soils onsteep slopesPeats with impermeable layers atshallow depth

Note: Chalky soils can have a wide range ofpermeabilities, and runoff potentials that vary betweenthose of clay loams and those of coarse sands.

The parameter SAAR for the site under considerationcan be obtained from a map of the annual averagerainfall in the UK (see Figure 4 in Appendix A).

The mean annual flow can be scaled to the requiredreturn period of 75 years, by applying a scaling factor,F, based on the regional growth curves suggested by theFlood Studies Report (Ref. 5) for the area underconsideration (see Figure 5 in Appendix A):

Q = F Qa (3)

where

Q is the design flow (in m3/s)F is the scaling factor, dependent on the regionQa is the mean annual flow.

5/2

5.5 The Agricultural Development and AdvisoryService, ADAS (Ref. 4) developed a method primarilyfor the sizing of field drainage pipes, which was basedon the Transport and Road Research Laboratory, TRRL,method (Ref. 6). The ADAS method is applicable tovery small catchments, having been developed forcatchment areas up to 0.3km2 (or 30ha). This methodtakes into account the design storm rainfall and time ofconcentration for the required return period by usingthe Bilham formula. For the required 75 year returnperiod the design flow, Q (in m3/s) can be determinedfrom:

∗−=2.0

11.19)(0.0443 SOILSAARAREAQ(4)

ùùú

ø

ééê

è −T

T10

118.790.28

where

AREA (in km2) is the catchment plan areaSAAR (in mm) is the standard average annual rainfallfor the particular location (see Figure 4 in Appendix A)SOIL is the soil index, defined as:

( )( ) (5)1

0.50.450.400.300.15

u

54321

S-SSSSSSOIL ++++=

S1, 2….denote the proportions of catchment covered byeach of the soil classes 1 to 5 and Su is the unclassifiedarea of the catchment covered by water or pavement.Soil class 1 has a low runoff potential and soil class 5has a high runoff potential. The parameter SOIL for anatural catchment can vary between 0.15 (very lowrunoff) to 0.5 (very high runoff). This parameter can beestimated from soil maps reproduced in HA 71/95(DMRB, Ref. 1) or, for a simplified approach, throughconsultation of Table 5/1 (adapted from the AgriculturalDevelopment and Advisory Service, ADAS, Ref. 4):

T is the time of concentration (in hrs) and is given by:

(6)0.1677 0.39

0.78

Z

WT =

where

W is the maximum catchment width in metres (seedefinition of width in 3.3)Z is the average height of the catchment divide inmetres (see Figure 2 in Appendix A) above thedischarge level (ditch level).

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Application of methods

5.6 It is recommended to use:

• the IH 124 Method (described in 5.4) forcatchments > 0.4km2

• the ADAS Method (described in 5.5) forcatchments ≤ 0.4km2.

5.7 The design flows estimated with therecommended runoff methods are surface runoff flowsthat take into account saturation of the soil.

5.8 Worked examples are presented in Chapter 7.

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DITCHES

Chapter 6Hydraulic Design of Ditches

6. HYDRAULIC DESIGN OF

Location and type

6.1 Ditches should be located where they can fullyintercept the flow from the natural catchments adjacentto the road. The location of ditches is mainly dependenton the space available. Possible locations are: i) alongthe edge of the road; (ii) along the top of cuttings or iii)at the toe of embankments. In cuttings, ditches shouldpreferably be positioned at the top of the cuttings toavoid potential erosion of the slope by surface water.Large sized ditches may create stability problems in thecutting slope and, therefore, appropriate measuresshould be taken (see 4.2).

6.2 Where ditches are located alongside the road,they may be designed to convey the runoff from thecarriageway as well as that of the natural catchment.HA37 (DMRB, Ref.1) can be used to determine therunoff rate from the road (including the contribution ofthe plan area of the cutting), which would be added tothat of the natural catchment.

6.3 Ditches should preferably consist of earthchannels lined with a native grass species (orcombination of species), in order to provide adequateresistance to flow erosion.

Sizing

6.4 The size of ditches can be calculated usingManning’s resistance equation:

(7)2/31/2RS

QnA =

where

A is the cross-sectional area of the flow (m2)Q is the flow rate (m3/s)n is the Manning roughness coefficient - values ofManning’s n are given in Appendix CandS is the longitudinal gradient of the ditch (m/m).

The hydraulic radius R is defined by:

February 2004

(8)PAR =

where P is the wetted perimeter, i.e. the perimeter of thechannel in contact with the water flow.

6.5 Ditches should be sized for conditions at theirdownstream end, where flow rates are highest. It is alsorecommended to carry out checks at intermediatelocation(s), where flow rates are smaller but gradientsmay be flatter. This may lead to the required size ofditch varying along its length.

6.6 The appropriate gradient, S, for use in the designshould be determined from the conditions at thedownstream end and, if intermediate conditions arechecked, the gradients should be adjusted accordingly.The minimum design gradient for ditches should be1/500 to ensure flow conveyance in flat areas.

6.7 To achieve stability and high flow capacity, thecross-sectional shape of ditches should beapproximately trapezoidal. For a trapezoidal shape withequal side slopes, base width B, side slopes 1:b(vertical: horizontal) and flow depth y, the hydraulicradius R is given by:

(9)2

0.5222

2

ö÷õæ

çå ++

+=ybyB

byByR

6.8 In some cases the design flow rate will be suchthat the required size for the ditch may be too large tobe accommodated within the available space on theverge or the top of cuttings. The designer may thenconsider discharging part of the flow into the roaddrainage system to keep the ditch size within certainlimits. In this case the road drainage system must bechecked to determine its ability (or otherwise) toconvey the additional flow. To check this it isrecommended to estimate the runoff from the roadcross-section (including areas in cutting) using theguidance in HA 37 (DMRB, Ref.1) but with a rainfallintensity of 30mm/hr. This lower rainfall intensity willreflect the low probability of simultaneous occurrenceof the design storm for the natural catchment and forthe road cross-section.

6/1


Chapter 7Worked Examples

7. WORKED EXAMPLES

7.1 Example 1

Determine the runoff from a natural roadside catchmentin Dorset, near Lyme Regis having the followingcharacteristics:

Catchment area: 1km2

Catchment slope: S = 1/12.5 = 0.08Soil type: Clay

The IH 124 method will be applied to this catchment, asthe catchment area is greater than 0.4km2.

The value of SOIL index for the catchment can becalculated from Equation (2). It can be assumed that thecatchment is uniform in terms of soil characteristics andfrom the soil maps in HA 71 the soil class can be takenas S4 (alternatively, from consultation of Table 5/1, thesoil class for clay soils is given as S4). The proportionsof the catchment with soil classes S1 to S3 and S5 are nil,so:

SOIL = 0.4

The average annual rainfall (SAAR) for the location isobtained from Figure 4 (in Appendix A) and it can betaken as:

SAAR = 900 mm

The mean annual flood is then calculated from Equation(1) to be:

Qa = 0.00108 x (1.0)0.89 x (900)1.17 x (0.4)2.17

= 0.423m3/s

For the design return period (75 years) the mean annualflow is scaled up using Figure 5. The site lies in Region7 and therefore for a 75 year return period the scalingfactor F is 2.91. The design flow can then be calculatedas:

Q75 = 2.91 x 0.423 = 1.23m3/s

7.2 Example 2

Determine the runoff from a natural roadside catchmentin Yorkshire, between Manchester and Huddersfieldhaving the following characteristics:

CaMAvabSo

Thth

Thcacafroassoprar

SO

Thob

SA

Thca

T

Usye

Q

Ascaca

February 2004

tchment area: 0.14km2

aximum drainage width: 250merage height of catchmentove discharge level: 38mil type: Upland peat

e ADAS Method will be applied to this catchment, ase catchment area is smaller than 0.4km2.

e value of SOIL index for the catchment can belculated from Equation (2). It can be assumed that thetchment is uniform in terms of soil characteristics andm the soil maps in HA71 the soil class can be taken

S5 (alternatively, from consultation of Table 5/1, theil class for upland peats soils is given as S5). Theoportions of the catchment with soil classes S1 to S5e nil, so:

IL = 0.5

e average annual rainfall (SAAR) for the location istained from Figure 4 and can be taken as:

AR = 1400 mm

e time of concentration for the catchment, T, islculated from Equation (6) as:

3.01hr38

2500.1677 0.39

0.78

=×=

ing the information above, the design flow for 75ars return period is determined from Equation (4):

( )

s/m45.101.310

101.379.18

0.511.1914000.04430.14

328.0

2

=ùùú

ø

ééê

è

×−×

××−××=

can be seen from this example, a small catchmentn generate higher rates of runoff than largertchments such as that of Example 1.

7/1


Chapter 8References and Bibliography

OGRAPHY
8. REFERENCES AND BIBLI
1. Design Manual for Roads and Bridges (DMRB)(The Stationery Office)

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

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

HA 43 Geotechnical considerations and techniques forwidening highway earthworks (DMRB 4.1)

HA 44 Earthworks: Design and preparation of contractdocuments (DMRB 4.1.1)

HA 48 Maintenance of highway earthworks anddrainage (DMRB 4.1.3)

HA 68 Design methods for the reinforcement ofhighway slopes by reinforced soil and soil nailingtechniques (DMRB 4.1.4)

HA 71 The Effects of Highway Construction on FloodPlains (DMRB 4.2)

HA 107 Design of Outfall and Culverts Details(DMRB 4.2)

SH 4 Geotechnical certification procedures: trunk roadground conditions.

2. FEH (1999). “Flood Estimation Handbook,Volume 2 Rainfall frequency estimation” by FaulknerD., Institute of Hydrology, UK, ISBN for volume 2:0948540 90 7.

3. IH Report 124 (1994). “Flood estimation forsmall catchments” by Marshall D.C.W. and BaylissA.C., Report No.124, Institute of Hydrology, UK,ISBN 0 948540 62 1.

4. Agricultural Development and Advisory Service,ADAS (1980). “Pipe size design for field drainage” byBailey A.D., Dennis C.W., Harris G.L. and HornerM.W. Report 5, Land Drainage Service, Research andDevelopment, MAFF, December 1980.

5. Natural Environment Research Council (1975).“Flood Studies Report”. In five volumes.

6(cR

7PD

February 2004

. Transport and Road Research Laboratory, TRRL1973). “The estimation of flood flows from naturalatchments” by Young C.P. and Prudhoe J. TRRLeport LR565.

. HR Wallingford (1981). “The Wallingfordrocedure: Design and Analysis of Urban Stormrainage”.

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February 2004 9/1

9. ENQUIRIES

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

Divisional DirectorResearch & Development of Standards DivisionThe Highways AgencySunley TowersPiccadilly Plaza A JONESManchester M1 4BE Divisional Director

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

Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings J R REESCardiff Chief Highway EngineerCF10 3NQ Transport Directorate

Assistant Director of EngineeringThe Department for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street D O’HAGANBelfast BT2 8GB Assistant Director of Engineering

Chapter 9Enquiries


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APPENDIX A FIGURES

Figure 1 - Methodology Flowchart

Appendix AFigures

A/1

Existing road scheme?

History of flooding?

Consult records; assess flooding causes

Site visits to flood-prone areas → use checklist (Section 3.6)

Identification of possible flooding causes and critical areas

Assessment of capacity of existing features (e.g. land drainage, cut-off ditches, culverts, connecting roads)

Identification of areas at risk → use maps & checklists (Section 3.6)

Estimate runoff from natural catchment - Chapter 5

Design ditches/culverts (location, sizing, outfalls) – Chapter 6

Required capacity results in excessively large ditches for the space available?

Consider discharging part of the natural runoff flow into road drainage system: check capacity (Section 6.8)

Define maintenance programme to ensurecontinued capacity of drainage system.

N

N

N

Y

Y

Y

Y

Volume 4 Section 2

Part 1 HA

106/04

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Figure 2 - Types of Natural C

atchment

A/2

Appendix A

Figures


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Figure 3 - Typical road catchment profiles that can generate significant runoff

Appendix AFigures

A/3


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Figure 4 - Annual Average Rainfall in the UK (Originally from the Met Office;Available from Ref 7, The Wallingford Procedure)

A/4

Appendix AFigures


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Figure 5 - Values of the Scaling Factor F for UK Regions (Derived from theFlood Studies Report, Ref. 5)

Appendix AFigures

A/5


TION OF RETURN

Appendix BDetermination of Return Period

APPENDIX B DETERMINAPERIOD

Introduction

B.1 The primary purposes of road drainage systemsare to minimise water depths occurring on road surfacesduring heavy storms and to prevent seepage causingdamage to the pavement construction. Since runoffoccurs rapidly from roads, the most critical stormconditions for the design of surface water drainagesystems are normally associated with heavy rainfallevents typically lasting between 2 and 15 minutes.

B.2 Design Standards (HD 33, DMRB 4.2, Ref. 1)require that edge-of-pavement drainage systems shouldbe able to convey flows produced by storms with areturn period of N = 1 year without any surcharging orsurface flooding. Limited surcharging onto hardstrips orhardshoulders is permitted for storms with returnperiods between N = 1 year and N = 5 years providedthat the water does not encroach onto the carriageway.It follows that in rarer storms having return periodsexceeding N = 5 years there is likely to be someflooding of carriageways on roads designed inaccordance with Design standards. However, suchflooding will last only a few minutes and causerelatively little delay or inconvenience to road users(particularly since, in very heavy rain, drivers are likelyto slow down due to poor visibility).

B.3 Flooding from natural catchments is verydifferent in character from flooding caused by highrates of runoff from road surfaces. The most criticalstorm duration for design is usually equal to the time ofconcentration of the catchment (i.e. the time needed forthe whole catchment to contribute runoff); for naturalcatchments draining to roads this time can typically beof the order of 5 to 50 hours. The excess volume offlow from a natural catchment can be very large andlead to widespread inundation of a road. The flooding isalso likely to last several hours. As a result, the delaysand inconvenience caused to road users can be veryconsiderable, even though the rainfall intensity duringthe long-period storm would not itself cause asignificant problem to drivers.

B.4 These considerations indicate that drainagesystems dealing with runoff onto roads from naturalcatchments should be designed so that flooding occursvery infrequently (since closure of a section of

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otorway or trunk road approximately once every fiveears would not be a satisfactory level of service).owever, the current Design Standards in HD 33 are

onsidered to provide a satisfactory degree ofrotection against flooding for the case of systemsealing with runoff from the road surface. Theollowing sections describe an analysis that was carriedut to develop a quantitative measure of theerformance provided by the HD 33 guidelines. Thiserformance measure was then used to estimate auitable design return period for the case of drainageystems dealing with runoff from natural catchments.

looding Index (FI)

.5 The description in B.2 and B.3 of the differentlooding characteristics produced by runoff from roadurfaces and from natural characteristics indicates thathe degree of inconvenience caused to road usersepends on the following factors:

the magnitude of the flooding

the time for which the flooding lasts

how frequently the flooding occurs.

.6 These factors can be described quantitatively byhe Flooding Index (FI) which is defined as:

(B.1)N

dN)TQ(QFI 2ON

1000

NO

−= ñ

here

O = return period (in years) of the storm that isused to determine the flow capacity of thedrainage system (such that no flooding ofthe carriageway occurs for storms withreturn periods up to and including NOyears).

N = average flow rate from a catchment per mlength of road (in m2/s) produced by astorm having a return period of N years.

B/1


Appendix BDetermination of Return Period

QO = value of QN for the design return period ofNO (and proportional to the flow capacityof the drainage system).

T = duration (in s) of the design storm(proportional to the time for which anyflooding persists).

dN = probability of occurrence of a storm havingN2 a return period between N years and

N + dN years.

The value of FI takes account of the magnitude of theflood and the period for which it lasts. The Index,therefore, provides a measure of the cumulative volumeof flooding per m length of road caused by all possiblestorms having return periods between NO years (belowwhich no flooding will occur) and an assumed upperlimit of 1000 years.

Comparisons

B.7 The value of FI was first calculated for a typicalsurface water channel receiving only runoff from theadjacent road surface (longitudinal gradient of 1/100,transverse gradient of the carriageway of 1/40). It wasassumed that the duration of the design storm wasT = 300s (5 minutes) and that the channel was designedto cater for storm return periods up to NO = 5 yearswithout any flooding of the adjacent carriageway.

B.8 The analysis was then repeated for tworepresentative cases of natural catchments draining toroads. The two catchments were located in the south-east and the north-west of England. Values of flow rate,QN, and design storm duration, T, were determinedusing the method described in 5.4. The values obtainedfor T were 8 hours and 13.5 hours respectively.

B.9 The objective of the analysis was to determineappropriate values of the design return period for thesystems receiving runoff from natural catchments suchthat they would have the same numerical value of FI asthe surface water channel considered in B.7. By meansof a trial-and-error procedure, it was found that thedrainage systems for the two catchments needed to besized for storm return periods of NO = 60 years and 140years.

Btocilomd

B/2

.10 Based on this method of analysis, it was decidedo adopt a return period of NO = 75 years for the designf drainage systems dealing with flow from naturalatchments. It should be noted that the Flooding Indexs a measure of the cumulative effect of flooding that isikely to occur on a road over a long period. The valuef FI does not have a direct physical meaning but is aeans of comparing the long-term performance of

ifferent types of drainage system on a common basis.

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APPENDIX C ROUGHNESS VALUES FOR THEHYDRAULIC DESIGN OF DITCHES

The flow capacity of a channel is dependent to asignificant extent on the surface texture. Grass can getestablished easily in the UK and offers good protectionagainst flow (and wind) erosion. For these reasons it isrecommended that flow from natural catchments bedrained by grassed ditches whenever possible. Thegrass height and the presence of weeds should becontrolled to maintain the capacity of the ditch andtherefore regular maintenance will be necessary.

The table below gives values of the Manning’sroughness coefficient for use in the hydraulic design ofditches. For the design of new ditches it isrecommended to use n=0.050 in schemes where amaintenance programme will be put in place; wheremaintenance is doubtful or irregular, higher values mustbe used.

Type of Condition of Manning’schannel the ditch n

Grassedchannel, Average, 0.050regularly good

maintained

Grassed Good 0.050channel, notmaintained, Average 0.080with dense

weeds Poor 0.120

Appendix CRoughness Values for the Hydraulic Design of Ditches

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Drainage of Runoff from Natural Catchments

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