DMRB VOLUME 4 SECTION 2 PART 1 - Standards …

1 February 2004 DESIGN MANUAL FOR ROADS AND BRIDGESVOLUME 4 GEOTECHNICS ANDDRAINAGESECTION 2 DRAINAGEPART 1HA 106/04 drainage OF runoff FROMNATURAL CATCHMENTSSUMMARYThis 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 FOR USEThis is a new Advice Note to be incorporated into existing Contents pages for VOLUME new Contents pages for VOLUME 4, datedFebruary HA 106/04 into VOLUME 4, SECTION 2, part archive this sheet as : A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

2 HA 106/04 drainage of runoff fromNatural CatchmentsSummary: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 MANUAL FOR ROADS AND BRIDGESTHE HIGHWAYS AGENCYSCOTTISH EXECUTIVEWELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRUTHE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELANDV olume 4 SECTION 2 part 1 HA 106/04 February 2004 REGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsRegistration of AmendmentsVolume 4 SECTION 2 part 1 HA 106/04 February 2004 REGISTRATION OF AMENDMENTSA mendPage NoSignature & Date ofAmendPage NoSignature & Date ofNoincorporation ofNoincorporation ofamendmentsamendmentsRegistration of

3 AmendmentsVOLUME 4 GEOTECHNICS ANDDRAINAGESECTION 2 DRAINAGEPART 1HA 106/04 drainage OF runoff FROMNATURAL catchment to the Collection of of Design of and AFiguresAppendix BDetermination of Design Return PeriodAppendix CRoughness Values for the HydraulicDesign of DitchesDESIGN MANUAL FOR ROADS AND BRIDGESF ebruary 2004 VOLUME 4 SECTION 2 part 1 HA 106/04 February 20041/1 Chapter 1 Introduction1. 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 water runoff to highway drainagesystems is conventionally assumed to derive from theroad cross- SECTION .

4 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 recommendations may be applied to other roads withrural ( natural ) catchments and traffic conditions, 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.

5 However, highway drainage systems were, insome cases, overloaded by additional water draining tothe roads from the surrounding natural 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 recommendations on earthworksdrainage, both surface water and sub-surface water, aregiven in HD 33, Surface and Sub-Surface DrainageSystems for Highways ( dmrb , 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 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.

6 HD 33 alsohighlights the importance of ensuring a coordinatedanalysis of the horizontal and vertical road profilesbefore the final alignment is 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 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 km2, or 1ha) can beneglected. Smaller sites with a history of frequentflooding may need to be considered on a one to onebasis.

7 Catchments with surface areas greater than25 km2 are outside the scope of this 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 ). 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 , Ref. 1). 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.

8 This is not covered inthe present document but information can be obtainedfrom the Environment Agency and the BritishGeological Survey, among other 4 SECTION 2 part 1 HA 106/04 February 20041/2 Chapter 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 4 SECTION 2 part 1 HA 106/04 February 20042/1 Chapter 2 Methodology2. 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 4 SECTION 2 part 1 HA 106/04 February 20043/1 Chapter 3 natural catchment Identification3.

9 natural catchment 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 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 determine the natural catchment dimensionsthe following definitions width:a)For valley catchments the distance between thetop 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).

10 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 flat areas definition of the natural catchmentboundary is not always obvious, and engineeringjudgement should be applied. The maximum catchmentwidth should not exceed 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 informationon local features. Due to the linear nature of roads, thedischarge points for the natural catchment flows will inmost cases be the same as those used for the dischargeof road runoff .