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May 2006

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 1

HA 103/06

VEGETATED DRAINAGE SYSTEMS FORHIGHWAY RUNOFF

SUMMARY

This Advice Note gives guidance on how vegetateddrainage systems may be used to convey, store and treathighway runoff. They are described as vegetateddrainage systems because they contain a significantelement of vegetation and they are designed for useespecially in highway drainage networks. They aresimilar to systems described elsewhere as SustainableDrainage Systems (SUDS). Designers should note thatnot all SUDS are appropriate for highway use.

INSTRUCTIONS FOR USE

This revised Advice Note is to be incorporated in theManual.

1. This document supersedes HA 103/01, which isnow withdrawn.

2. Remove existing Contents pages for Volume 4,and insert new Contents page for Volume 4, datedMay 2006.

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4. Insert HA 103/06, in Volume 4, Section 2, Part 1.

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HA 103/06

Vegetated Drainage Systemsfor Highway Runoff

Summary: This Advice Note gives guidance on how vegetated drainage systems may beused to convey, store and treat highway runoff. They are described asvegetated drainage systems because they contain a significant element ofvegetation and they are designed for use especially in highway drainagenetworks. They are similar to systems described elsewhere as SustainableDrainage Systems (SUDS). Designers should note that not all SUDS areappropriate for highway use.


THE HIGHWAYS AGENCY

TRANSPORT SCOTLAND

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 4 Section 2Part 1 HA 103/06

May 2006

REGISTRATION OF AMENDMENTS

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

amendments amendments

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VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 1

HA 103/06

VEGETATED DRAINAGE SYSTEMS FORHIGHWAY RUNOFF

Contents

Chapter

1. Introduction

2. Description of Vegetated Drainage Systems

3. Runoff Constituents and Treatment Processes

4. Selection of Vegetated Drainage Systems

5. The Design and Construction of VegetatedDrainage Systems

6. Maintenance and Management of VegetatedDrainage Systems

7. References

8. Glossary

9. Enquiries


May 2006


1/1

Chapter 1Introduction

May 2006

1. INTRODUCTION

General

1.1 This Advice Note gives guidance on howvegetated drainage systems may be used to convey,store and treat highway runoff. It is based on a reviewof recent research and practice, and a survey of a rangeof balancing ponds, both wet and dry, that are fed byrunoff from trunk roads or motorways. They aredescribed as vegetated drainage systems because theycontain a significant element of vegetation and they aredesigned for use especially in highway drainagenetworks. They are similar to systems describedelsewhere, e.g. Planning Policy Guidance Note 25:Development and Flood Risk (PPG 25), as SustainableDrainage Systems (SUDS). Designers should note thatnot all SUDS are appropriate for highway use.

Background

1.2 There is growing awareness by theEnvironmental Protection Agencies (EPA’s) thathighway runoff may, under certain circumstances, havean adverse effect on receiving waters. This has arisenboth because of improving knowledge of the pollutingcontent of highway runoff and because of improvedtreatment of other sources of pollution. There is alsoconcern at the possible impact that roads can have onlocal hydrology and therefore on flood risk. A list ofEPA’s and their contacts can be found on one of thesewebsites:

www.environment-agency.gov.ukwww.sepa.org.ukwww.ehsni.gov.uk

1.3 Overseeing Organisations have a duty underpollution protection legislation to ensure that highwayrunoff does not pollute receiving waters. This can ariseboth from the effects of routine runoff and fromspillages on the highway. For discharges to ground, TheGroundwater Regulations 1998 (SI 1998 No 2746) inEngland Scotland and Wales (ref. 6) or TheGroundwater Regulations (Northern Ireland) 1998(Statutory Rule NI 1998 No 401) (ref. 8) include lists ofsubstances whose entry to groundwaters must beprevented or controlled. Because consents are notrequired for the discharge of highway runoff, theresponsibility is on the Overseeing Organisations todischarge their duty not to pollute receiving waters byensuring that systems for the treatment of highway

runoff are installed when necessary. EPA’s should,however, be consulted about proposals for discharginghighway runoff, as they have the power to serveprohibition notices in respect of discharges that are inbreach of pollution legislation. Implementation into UKlaw of the Water Framework Directive (2000/60/EC)has given added weight to the need to protect existingwater bodies.

1.4 The treatment of storm water runoff usingvegetated systems is still a developing technology. It isrecognised that advice given in this Advice Note maytherefore be overtaken by developments in currentresearch and practice. There is also limited data on thedegree of effectiveness of such systems in the UK.However the Advice Note is provided as the bestavailable advice, because the use of these systems isbeing advocated by the EPA’s, and because it isconsidered that vegetated systems, when designed,constructed and maintained as suggested in this AdviceNote, can be used to provide a significant degree oftreatment to highway runoff and protection to receivingwaters.

Purpose

1.5 The purpose of this Advice Note is to provideadvice on the design of highway drainage, both new andupgraded, so Designers or Specifiers may consider howvegetated systems may be incorporated in the overalldrainage network and how they may be maintained.These systems can provide both treatment of thehighway runoff and attenuation of discharge flow rates.They may also contribute to the landscape and natureconservation value of the surrounding area. The AdviceNote discusses how these qualities may complementeach other, but also where they may conflict. This canoccur because the prime function of the vegetatedsystems will be to protect the natural water regime intowhich the highway runoff discharges. Maintenance ofthe proper operation of the systems is essential forcontinuing protection and must take priority. Anylandscape or nature conservation value should becompatible with this function and should not inhibit themaintenance work required for the correct functioningof the system.

1.6 The Advice Note discusses a range of vegetateddrainage systems. Many of these are in common use inthe UK, some are used for other types of non-highway


Chapter 1Introduction

waste water or storm water treatment and some are inroutine use in mainland Europe and the USA. Theeffectiveness, economy and advantages of these systemsare discussed from the point of view of the Designer orSpecifier. The Advice Note then discusses how suchsystems can be selected, designed, constructed andmaintained in such a way that the whole drainagesystem, while satisfactorily performing its main purposeof removing surface water rapidly from the road, canalso provide some degree of both attenuation andtreatment of the flow. Such measures will thus reducethe risk of storm water either affecting the naturalhydrology of the catchment area (causing localflooding) or polluting the downstream surfacewatercourse or ground waters.

1.7 Having first determined that some form oftreatment is necessary, it is fundamental to the processof selection of systems to determine what it is thatvegetated drainage systems are required to treat. TheAdvice Note should therefore be read in conjunctionwith HA 216 Road Drainage and the WaterEnvironment (DMRB 11.3.10) which provides anoverview of the current knowledge of highway runoff inthe UK, the factors that are believed to affect its natureand a guide to the assessment process and toolsavailable for calculating the potential risk of pollutionfrom routine highway runoff and accidental spillages.

1.8 The selection of vegetated systems cannot beprescribed. Each situation must be consideredindividually as there are very many factors to be takeninto account, ranging from the local landscape, geology,hydrology, climate and local river catchment andquality, to the actual or predicted traffic levels on theroad, the risk of accidental spillages and the practicalityof provision for maintenance. It is important thatDesigners and Specifiers consider the widest possiblenumber of factors before considering what systems toincorporate. In many situations, particularly whereroads are being improved, the availability of land willbe a major factor in the selection process. Selection isan iterative, rather than a linear, process and is notamenable to illustration by simplified design guides,such as flow charts, to aid the choice of suitablesystems.

Scope

1.9 Vegetated drainage systems may be used in newschemes and for upgrading installations on existingroads. Designers and Specifiers of DBFO schemesshould note that it is not considered appropriate tospecify performance standards for these systems.

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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. In making such an assessment, it must beborne in mind that the Overseeing Organisations have aduty not to cause the pollution of receivingwatercourses and groundwaters. Design Organisationsshould confirm its application to particular schemeswith the Overseeing Organisation.

May 2006


Chapter 2Description of Vegetated Drainage Systems

ATED DRAINAGE
2. DESCRIPTION OF VEGETSYSTEMS
General

2.1 Vegetated drainage systems can be used ascomponents of a drainage network to convey, store andtreat water running off the highway, before it outfalls tothe receiving waters. They may be designed tosupplement or replace conventional drainage systems.By their nature, they are part of the surroundinglandscape and can contribute to the nature conservationor landscape amenity value of an area. This can be donewithout diminishing their primary purpose ascomponents of the drainage system.

2.2 In a strict sense, vegetated systems are thosetreatment measures which employ vegetation as aprimary treatment component. However, for thepurposes of this Advice Note, systems which do notnecessarily include vegetation, but may be enhanced bya vegetated component, are also considered. Thesystems can be considered as

(i) those that convey water, such as swales andgrassed channels; or

(ii) those that treat water while it flows slowlythrough the system such as wetlands andinfiltration basins; or

(iii) those that treat water at rest, such as ponds.

2.3 In practice there may be an overlap betweenthese functions. These systems are described below andtypical examples are shown in Figure 2.1.

Swales

2.4 Swales are wide, shallow, gently slopingdepressions designed to convey water. They areparticularly effective in controlling pollution whenthere is a degree of infiltration to the subgrade and canbe used to reduce spillage risk by the addition of checkdams, which also can attenuate the flow. Swales arewell suited to areas where the road is on a gentlysloping embankment as the embankment slopes can bedesigned to be part of the swale, and are most effectivewhen the water flow is slowest, immediately after itleaves the road. Used in this way they could alsooperate as the first form of attenuation and treatment ina drainage system. Their effectiveness in removing

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llutants will depend upon the detailed design.spended solid loadings can be reduced by over 50%,t levels of soluble pollutants will not be greatlyuced.

assed Channels

Grassed channels are a development of swalesr use as road edge channels. They differ from normaladside channels in that the flow of water is designedbe slower, so that filtration and, to a lesser extent,dimentation may occur as the water flows across andough the grass. Guidance on the design, constructiond maintenance of grassed channels can be found in 119: Grassed Surface Water Channels for Highwaynoff (DMRB 4.2).

There is no clear distinction between swales andassed channels though historically the term swale hasen used to describe a wide grassed channel. Thevice in this Note generally relates to swales, asscribed in 2.4, though some of the properties of aale may be exhibited by a grassed channel. The speed

the water along the channel is the critical factor, as aw flow at the top of the run with good grass coverll encourage a degree of sedimentation, but if there is possibility of faster flows from stormy conditions,suspension of the particles can occur. The flow ineper sided channels tends to be less stable and suchannels are harder to clean and maintain, unlike gentlydulating swales which encourage smooth flows andy be mowed.

filtration Basins

Infiltration basins store and treat water, and canrefore provide a degree of risk reduction for bothoding and pollution downstream. They are designedretain storm water flows and allow the water torcolate through a filter layer which may typicallymprise porous material, such as gravel. The watery then be directed to a surface water outfall, or ity continue to percolate through to the groundwaters.

gure 2.2 shows a possible design with water flowingan outfall. The shape of the basin will be determined the land available, and for small catchments arrow trench-like basin will be more appropriate.filtration Basins have the potential to remove

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Chapter 2

Description of Vegetated D

rainage Systems

Wetland

Pond
May 2006
Swale/Grassed Channel Surface Flow

SedimentationBalancing Pond

Figure 2.1



Figure 2.2

May 2006 2/3



suspended solids and reduce metal loads, but likeswales soluble pollutants may not be significantlyreduced. Their effectiveness in removing pollutants willdepend on their design for storm flows, particularly assuspended solids are prone to remobilisation, resultingin high sediment and metal loads being discharged.

2.8 Infiltration basins are not suitable for retainingaccidental spillages, because of the difficulty incleaning them, and should be sited downstream of somedevice to contain the spillages, unless this risk isperceived to be low. They may be used to receive the‘first flush’ flows (see paragraph 4.5) and to hold andtreat these, with the rest of the flow bypassing the basinto other systems.

Wetlands

2.9 Wetlands can be defined as areas that arepermanently saturated by surface water or groundwaterso that they are able to support aquatic and/or semi-aquatic (emergent) vegetation such as reed swamps,marshes, or bogs, depending on the degree of saturationand inundation. Natural wetlands are of relatively rareoccurrence and generally of high nature conservationvalue. They should not therefore be used to treat water,nor should treated highway runoff be discharged intothem, unless they have low nature conservation value orhighway runoff could provide additional water ofsufficient quality and quantity. In both cases agreementhas to be reached with the EPA after the appropriatenature conservation body has been consulted. As aconsequence, wetlands required to treat runoff willnormally be of the constructed type.

2.10 Wetlands can be categorised according to thepredominant flow pathway of water through the system:either as Sub-Surface Flow (SSF) wetlands where theflow of water is primarily through the growing mediaand plant root zone; and Surface Flow (SF) wetlandswhere the primary flow is across or close to the surfaceof the growing medium and through the above groundparts of the plants.

Constructed SSF Wetland

2.11 Constructed SSF wetlands are essentially basinsfilled with porous material through which water flows.The porous material is kept at saturation, up to anappropriate water level, and is usually planted withcommon reed swamp vegetation. This type of wetlandis increasingly being used to provide final treatment tomunicipal wastewaters and domestic sewage byremoving nutrients (particularly nitrates and

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sphates) through biological and chemical processeshe supporting porous media and the root zone.vided there is a relatively long residence time (24hrs

more), the hydraulic resistance and large surface areavided by the media and the vegetation promoteher levels of adsorption, microbial degradation andlogical uptake (particularly of metals) than occurs iner systems. SSF wetlands are well suited totment of relatively small and constant volumes of

ter, and have the advantage of taking relatively littlece. These systems are, however, dependent on theous medium being kept saturated and free frompended solid loads, which rapidly clog the pores andsform them into surface flow systems.

2 Highway run-off is markedly different fromage effluent in both water quality and flowracteristics. Most important of these are theentially high (peak) suspended solid loads and lowrient loads of highway runoff, and the intermittentcharge in the summer months (which may beufficient to maintain the wetland). Hence, SSFtlands are rarely appropriate for highway runofftment, except possibly where soluble metals are aticular problem and a very high level of protection ofsitive receiving waters is required. Where they ared there will be a requirement for pre-treatmentvisions to remove suspended solids and controlctures to regulate in- and out-flow rates. Designers

Specifiers should also be satisfied as to the viabilitySSF wetlands during dry periods, possibly by thevision of secondary flows through the system. They more costly to build and require a higherintenance input than other vegetated systems.

nstructed SF Wetland

3 Constructed SF wetlands are essentially similarhe sub-surface type being permanently saturatedn ended or closed basins, or low lying level ground.

e growing medium material is kept at the saturation inundation appropriate for the type of vegetation

ablished. This type of wetland is increasingly beingd to provide treatment of highway runoff in theA. They are effective in the removal of suspendedids and associated heavy metals through the physicalcesses of settlement and filtration. Provided there islatively long residence time (24 hours or more),orption and microbial degradation and biologicalake of metals and nutrients can occur. SF wetlands well suited to treatment of highway runoff as they able to deal with the high suspended solid loads inhway runoff. However they need to be designed sot they remain sufficiently wet in the summer months.

May 2006



They are relatively less costly to build but require aspecialist maintenance input. Figure 2.3 shows a sketchof a typical SF wetland. Their effectiveness in removingpollutants is dependent on their design for storm flows,particularly as suspended solids can remobilise near tothe outfall structure.

Ponds

2.14 Ponds may be designed primarily to attenuateflows, and are then customarily known as BalancingPonds, accepting large inflows, but discharging slowly.Alternatively they may be designed primarily to treatthe water by allowing suspended solids to settle out:these are customarily known as Sedimentation Ponds.In practice they will normally perform both functions tosome extent.

2.15 They may be designed to retain water at all times– particularly if there is an alternative constant watersource, or if they have been engineered from existingponds. These are sometimes known as wet ponds orRetention Ponds. Such ponds can both store water thusreducing flood risk downstream, and treat it by allowingsuspended solids to settle out. The smallest particlestake longer to settle out, but available evidence suggeststhat particles of less than 63 µm may carry more than50% of the pollution load. Figure 2.4 shows a sketch ofa typical wet pond. Ponds have a high potential toremove suspended solids, and a potential dependent ontheir design for storm flows to remove metal andhydrocarbon pollutants, although their effectiveness inremoving the latter two pollutant types is less.

2.16 Ponds that are designed to empty after rainfallevents, or to be dry for extended periods arecustomarily known either as dry ponds or DetentionPonds. They are usually cheaper to build, but they willhave less effect in treating the runoff (unless they arealso infiltration basins as in paragraph 2.7 above).

2.17 The ability of a balancing pond to attenuate theflow will depend on its capacity, and rate of outflow.Such devices should be designed so that the naturalhydrology of a catchment area is not affected by largequickly-drained areas of highway. Ponds may be themost practical and effective measures for treatment ofhighway runoff if sufficient detention time is allowedfor both sedimentation and biological processes withinthe pool.

May 2006

Hybrid Systems

2.18 In many instances, the vegetated system may be ahybrid, being neither a pure wetland nor a balancingpond, but a combination of the two. The pond-wetlandhybrid system is particularly favoured in the USA andhas been used in the UK. These may take the form ofsmall pools of open water within the wetland ormarginal wetland associated with a pond, or othervariations. These hybrids are considered to be moreeffective than pond or wetland systems alone. Figure2.5 shows a sketch of a typical hybrid system.

Combinations of Systems

2.19 In many instances in the UK the treatment systemis a combination of a Pond and an SF wetland. Anotherexample is to use a swale to reduce the suspended solidload followed by a wetland to treat the dissolvedcomponent of runoff. If the estimated spillage risk issignificant (the annual probability of a major spillageincident being greater than 1%), the combined systemshould be designed so that spillages are containedwithin the swale before the flow reaches the wetland, ascontamination of the swale will more easily beremedied than damage to the wetland. Control of thespillage could be by check dams, which provideautomatic spillage containment, backed up by a secondcontrol system further downstream. Other systems totrap suspended solids may be used if swales cannot bedesigned to fit. Provided there is sufficient availableland, a combination of treatment systems may be themost appropriate. Combination systems have theadvantages of both SF wetlands and Ponds, and subjectto design and storm flows, without their respectivedisadvantages. They are simpler to design, and operateand maintain, and are particularly appropriate wherethere is a high risk of spillages. Table 7.1 inHD 33 Surface and Sub-Surface Drainage Systems forHighways (DMRB 4.2) shows which vegetated systemsare compatible with various conventional drainagesystems.

2.20 An SF wetland could be fed from the outflow ofa balancing pond, which has the ability to containspillages. In some locations additional containmentmeasures may have to be located upstream of thewetland to provide automatic protection against minoroil spillages.

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Figure 2.3

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Figure 2.4

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Figure 2.5

May 20062/8


Chapter 3Runoff Constituents and Treatment Processes

S AND TREATMENT
3. RUNOFF CONSTITUENTPROCESSES
General

3.1 Although vegetated systems may be classifiedinto the three types as described in paragraph 2.2 whentheir hydraulic properties are being considered, it ismore appropriate to assess their role as treatmentsystems on the basis of the runoff constituents andtreatment required. The principal constituents requiringtreatment are suspended solids, heavy metals, and awide range of organic compounds, which include oilsand grease.

3.2 The most important treatment processes invegetated drainage systems are:

(i) settlement and filtering of particulateconstituents;

(ii) adsorption of organic compounds to vegetationand soils;

(iii) microbial degradation and assimilation of organiccompounds;

(iv) uptake of nutrients and metals by higher plants;

(v) precipitation, bacterial oxidation and adsorptionof heavy metals.

These processes, as applicable to the various types ofvegetated systems, are set out in Table 3.1.

Suspended Solids

3.3 Suspended solids are the principal road runoffconstituents requiring treatment because of the harmfulphysical effect they can have on aquatic habitats. Manyof the other potentially harmful constituents of runoff(heavy metals, organic compounds) are in particulateform or attached to particles which can accumulate inthe water course, leading to chronic pollution. Research(refs. 3, 5) indicates that typically more than 50% ofboth metal and organic contaminants associated withsuspended particles are contained within the fractionfiner than 63µm.

3.4 The main removal mechanisms of solids byvegetated systems is due to enhanced settlementthrough reduced flow velocities and filtering effects

May 2006

(ref. 9). Settlement is the main process in very slowlyflowing standing water as in ponds and contained typesof inundated SF wetland. It is not, however, asignificant process in swales, infiltration basins andthose SF wetlands without standing surface water,where there is a through flow and short residence time.Settlement will be most effective in systems wherethere is a long residence time or where there arestructures that induce particulate collisions and arrestthe flow thereby promoting settlement. 24 hours is therecommended minimum duration, as described in 3.15,and this should be exceeded where possible. Thefiltering process will only take place where there is asignificant flow through vegetation, as in swales andgrassed channels, infiltration basins, and open types ofSF wetland without standing surface water. The SSFtype of wetland is not an appropriate choice for theremoval of solids, because of the potential to clog thesubstratum pores and change the hydraulic functioningof the wetland.

Heavy Metals

3.5 As much of the metal content of the runoff isparticulate, the principal removal processes withinvegetated systems are the same as for the suspendedsolids (settlement and filtering). Precipitation andformation of metal hydroxides and other insolublecompounds can also occur in certain oxidation/reduction conditions. Vegetated drainage systems suchas swales and grassed channels, SF wetlands andinfiltration basins, and ponds with a vegetation coverwill remove heavy metals in the particulate form eitherby filtration and/or by settlement. SSF wetlands wouldnot be expected to be effective in this role for thereason given in paragraph 3.4. Settlement columnstudies indicate that, for runoff from a single stormevent, the majority of heavy metal pollutant removaloccurs within the first 6 hours. Removal of copper andzinc, the two metals most likely to be present insignificant concentrations, improves if there is up to24 hours residence time (see paragraph 3.15).

3.6 Adsorption of metals to soil and vegetation, andassimilation by microbes and plant uptake of solublemetals, also take place in vegetated systems, but areconsidered to be of less significance than the physicalprocesses and are reliant on a relatively long residencetime. Plant uptake is dependent on active vegetative

3/1



growth and microbial activity on temperature; both arelikely to be significantly lower in the late autumn-winter period and metals may be released ondecomposition.

3.7 Precipitation of metal oxide solids is a secondaryprocess that removes metals from the runoff water.Insoluble metal oxides typically form under aerobicconditions in retention systems such as ponds, and arethen precipitated. Aeration of water is therefore animportant part of standing/flowing water systems suchas ponds. Precipitation as a removal mechanism may beless in soft water and is unlikely to occur in the SSFtype of wetland owing to the prevailing anaerobicconditions in the root zone.

3.8 There is now substantial evidence (refs. 15, 16)that re-mobilisation of metals may occur especially withhigh inputs of sodium chloride into sandy non-calcareous substrates, due to displacement of the metalsby sodium ions. This is caused by the use of salt as ade-icing agent.

Organic Constituents

3.9 This group contains a wide range of chemicals.Adsorption to suspended solids and subsequentsettlement is a significant removal process for oils andgrease. Lighter aromatic fuel additives may volatiliseprior to degradation, in which case prolonged exposureof runoff to the air is beneficial. Other aromaticcompounds such as PCBs (Poly-Chlorinated Biphenyls)and PAHs (Polynuclear Aromatic Hydrocarbons) aretoxic and may be more persistent, but have a tendencyto adsorb to suspended solids.

3.10 As much of the organic content of the runoff isassociated with particles, the principal removalprocesses within vegetated systems are the same as forsuspended solids (settlement and filtering). Significantamounts may therefore be removed by settlementprocesses. Vegetated drainage systems such as swalesand channels, SF wetlands and infiltration basins, andponds with a vegetation cover will remove it in theparticulate form. SSF wetlands would not be expectedto be effective in this role, as the heavy particulatefractions may cause clogging of the porous media.

3.11 Biological degradation of oils and grease isrelatively slow, taking several days even in idealconditions. The biodegradation process is thereforeconsidered to be of less significance in the removal oforganic constituents from runoff than settlement. Theprocess is likely to be of significance, however, in the

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n-situ degradation of accumulating material. Ponds andF wetlands are probably the best systems for such

n-situ transformation as they are more likely to supporthe requisite aerobic microbes.

utrients

.12 Nitrogen and phosphorus are usually the primaryutrients of concern, but are not normally present inignificant concentrations in highway runoff, and areherefore not a major consideration for treatment whereigh levels of urea are applied for road de-icing. Whereignificant concentrations occur, uptake by vegetationnd microbes in vegetated systems are likely to reducehese concentrations, but only in the growing season.he most effective system is the SSF wetland wheree-nitrification of nitrates takes place under anaerobiconditions, and nitrogen and phosphate are removed bylant and microbe uptake. Vegetated systems sometimesontribute to higher nutrient discharges than areormally associated with road runoff owing toccumulation and degradation of plant material, but thishould be significantly less than other sources,articularly agriculture.

odium Chloride and Other Dissolved Constituents

.13 Other than heavy metals and nutrients, theignificant dissolved constituent of highway runoff inhe UK is sodium chloride (NaCl), applied as de-icingalt. Vegetated systems are unable to reduceoncentrations of common salt to any significant extent.odium chloride can cause damage to vegetation, andan potentially trigger the release of accumulatedutrients and heavy metals adsorbed to the suspendedolids into solution. Where use of de-icing salt is likelyo be very frequent and the dilution of runoff byeceiving waters is low, either the ‘first flush’ (seearagraph 4.5) should be diverted to infiltrationacilities with groundwater protection or only pondshould be used.

he Importance of Residence Time and Loading

.14 Residence time (sometimes referred to asetention time, Rt, where Rt = V/Q where Q is theutflow and V is volume of the system) is the perioduring which the runoff is retained within the drainageystem. It is probably the overall ‘key’ design parameteror all treatment systems that depend on the settlementrocess (e.g. ponds and SF wetlands) for removal ofediment, metals and hydrocarbons. Permanent pooltorage capacity (min 10-15%), hydraulic gradient

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<1%, length-width ratios >3:1, uniform cross-section,flow path (zig-zag), flow arrest structures (submergedislands/weirs, vegetation across flow), and outfallstructures are often cited (ref. 11) as important designand influencing features. Where appropriate, theseapply to separate, hybrid or combination systems.Hydraulic loading has also become recognised as animportant factor, where the efficiency of the treatmentsystems declines with increased loading. Flow ratesunder storm conditions are also significant inremobilisation of sediment and bypass structures arefrequently recommended.

Tab

Treatment of Highway Runoff: Pri

Runoff Swales Infiltration SF WeConstituent Basins

Suspended Filtering Filtering FilteriSolids Settlement Settlem

Heavy Metals Filtering Filtering Filteri(particulate) Adsorption Plant uptake Settlemand soluble) Settlement Adsorp

Adsorption Plant uPrecipitation

Organic Filtering Filtering FilteriCompounds Adsorption Settlement Settlem(particulate Adsorption Adsorpand volatile) Biodegradation Biodeg

Volatilisation Volatil

Nutrients Plant uptake Plant uptake Plant u

Oil & Grease Filtering Filtering Filteri(particulate) Adsorption Adsorption Adsorp

Settlement SettlemBiodegradation Biodeg

Note: bold type indicates dominant processes• see paragraph 2.14 for definitions of Ponds

May 2006

3.15 Residence times of about 24hr are therecommended minimum, but longer is required for veryfine sediment where there is reliance on ponded wateralone, and times less than one hour may result inremobilisation of sediments. In these contexts vegetatedwetlands promote sedimentation and therefore appear torequire shorter residence times than ponds orinfiltration basins and result in relatively greaterreductions in pollution loads. For those simplydepending on filtration (e.g. swales), the factors thatshould be used to determine pollutant removalefficiency include: the maximum depth and velocity ofthe water, length of channel and hydraulic loading.

le 3.1

ncipal Processes in Vegetated Systems

tlands SSF Wetlands Balancing SedimentationPonds • Ponds •

ng Filtering Settlement Settlementent

ng Adsorption Settlement Settlementent Filtering Adsorption Plant uptaketion Plant uptake Adsorptionptake Precipitation

ng Adsorption Settlement Settlementent Biodegradation Adsorption Adsorptiontion Filtering Biodegradation Biodegradationradation Volatilisation Volatilisationisation

ptake Plant uptake Plant uptake Plant uptake

ng Filtering Settlement Adsorptiontion Adsorption Adsorption Settlementent Biodegradation Biodegradation Biodegradationradation

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Expected Pollutant Removal Performance ofVegetated Systems

3.16 There is currently insufficient data to be able toascribe precise treatment performances to vegetatedsystems under UK conditions. The degree to whichdrainage systems and a combination of systems canreduce the pollution in routine runoff is illustrated inTable 3.2, which is taken from a study carried out byWRc (ref. 10) to determine treatment efficiencies of arange of systems. The table highlights the performance

Table

Indicative Treatment Efficien

Road Site/Treatment Devices

A34 Bypass oil separator/surface flow wetland/wetbalancing pond

A34 Filter Drain

M4 Oil trap manhole/Sedimentation Tank

M40 Full retention oil separator/wet balancing pond

A417 Bypass oil separator/dry balancing pond

3.17 Systems can therefore only be rated in relativeterms and have therefore been classified as having low,moderate or good performance, roughly correspondingto removal efficiency categories of <30%, 30 - 60% and>60%. Because performance figures will depend onhow efficiency is calculated and on the duration andintensity of the rainfall event considered, theperformance guidelines in Table 3.3 below apply to therelative removal from the runoff within vegetated

3/4

efficiencies of individual components of a drainagesystem as well as overall system efficiency for 5 sites inEngland. It will be noted that in some instances anegative efficiency was recorded within a discretecomponent of the system. This indicates the potentialfor pollutants to remobilize within component systemsas a result of high velocity flows entering a system.Whilst the findings from this study are instructive thescope of the study was limited and the figures are givento indicate rather than prescribe the range of treatmentefficiencies of certain systems.

3.2

cies of Drainage Systems

% Reduction: Inlet to Outlet

Initial Second TotalForm of Form of System

Treatment Treatment Treatment

Metals 15 11 24PAHs -1 99 99TSS 37 73 83

Metals 7 7PAHs 52 52TSS 38 38

Metals -7 41 30PAHs -30 -26TSS -19 43 33

Metals 19 35 48PAHs 13 50 57TSS -9 62 58

Metals 27 39 56PAHs 4 16 22TSS 56 -37 40

systems that are designed for the control of runoff fromrainfall events with short recurrence intervals (typically6 months).

May 2006



Table 3.3

Expected Pollutant Removal Performance of Vegetated Systems

Runoff Swales Infiltration SF Wetlands SSF Wetlands Balancing SedimentationConstituent Basins ** Ponds Ponds

Suspended Good Good Good Good Moderate GoodSolids andassociatedheavy metals

Heavy Metals Moderate- Moderate- Moderate- Good Poor Poor-Moderatein solution* Good Good Good

Oil and Good Moderate- Good Good Moderate ModerateGrease Good

Nutrients* Poor Poor Moderate- Good Poor Poor-ModerateGood

* in growing season

** very limited operational life due to rapid ‘clogging’ of wetland substratum

May 2006 3/6


Chapter 4Selection of Vegetated Drainage Systems

ED DRAINAGE
4. SELECTION OF VEGETATSYSTEMS
General

4.1 In considering how best to convey highwayrunoff from the road surface, the Designer or Specifiermust consider not only the requirement to ensure thatwater is removed as quickly as possible from the roadsurface, but also the effect on the local hydrology of thearea and the threat of pollution to local waters. If, indesigning systems to convey the runoff to the localwatercourse, it is also possible to maintain or enhancethe landscape or nature conservation value of the area,that will be an added benefit, but it should not beconsidered essential.

4.2 Opportunities for environmental benefits shouldbe carefully considered at the beginning of the designprocess. A review of the planning context should becarried out to establish the presence of anyinternational, national or local designations within,adjacent to or in the vicinity of the proposed site area.The references to wildlife legislation, designated sitesand areas of potential nature conservation interest inDMRB 11.3.4 – Ecology and Nature Conservation maybe helpful in this regard. The review will indicate therelative importance of the landscape and ecology of thearea, and whether it is likely that environmentalmitigation will be a significant part of a road scheme. Insuch circumstances, vegetated systems may beparticularly appropriate and the acquisition of anyadditional land required for them may be justified onaccount of the benefits they can bring. Ease of accessfor maintenance of the systems should also beconsidered early in the design process.

4.3 A detailed landscape assessment, combined witha hydrological study and an ecological survey, shouldbe used to influence the nature, scale and location of avegetated drainage system. This will help to secure awell integrated system within its site context; protectingexisting landscape features (natural/man-made)wherever desirable, and reflecting the character of thesurrounding landscape. Further advice on this can befound in DMRB 11.3.4 and DMRB 11.3.5 – LandscapeEffects. Guidance is also available from the Statutoryconsultees: English Nature, Scottish Natural Heritage,the Countryside Council for Wales or the Environmentand Heritage Service: Natural Heritage, NorthernIreland. Further information can also be obtained fromthe Wildlife Trusts.

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May 2006

ighway Runoff

.4 Designers or Specifiers should carry out anssessment of the highway runoff to be discharged.A 216 (DMRB 11.3.10) gives a review of the

haracteristics of highway runoff. Factors that canffect its water quality include:

i) the amount of traffic, and proportion of HGV’s;

ii) the area of road surface drained to one outfall;and

iii) the deposition from surrounding land andatmospheric sources.

.5 The quality of runoff in urban areas has beenhown to be markedly different from that in rural areas.his appears mainly due to the debris on urban roads

including oil and fuel from parked vehicles) which isashed into the drains. Air quality has also been

dentified as a contributing factor in the urban context.

he intensity of runoff will be affected by the climate.reas in the south-east of the UK, although having the

owest annual rainfall, experience the most intensetorms. When such rainfall events follow long dryeriods, the build-up of pollutants on the road surface isashed off first, and the runoff from the first 10mm of

ainfall is often the most seriously polluted. This isometimes referred to as the ‘first flush’ effect of atorm. For this reason, discharges from short intensetorms with long antecedent dry periods are generallyore likely to pose a pollution threat than discharges

rom longer rainfall events which provide higherilution.

.6 Systems should normally be designed to treat allhe water in a rainfall event but, where this is notossible, Designers or Specifiers should attempt to treathis first 10mm separately, for by so doing they will beikely to be dealing with the most polluted flow.aragraph 5.30 describes how this can be done. In suchituations it will be important to ensure that the timeaken for the runoff to reach the outfall from theurthest point (the time of concentration) is similar forll drain runs entering any one outfall. If any one run isuch longer, it will continue to discharge the more

olluted ‘first flush’ for longer, and separate treatment

4/1



will not be readily achievable. Similarly practicallimitations, such as land availability, may mean that,whilst a system can be designed to provide storage forthe design event, its treatment efficiency will reduceduring extreme events. In these circumstances pollutantconcentrations are likely to be reduced because of thehigh volume of runoff related to the antecedentpollution build-up, and because dilution of theattenuated flow will be high.

4.7 Loadings of heavy metals and organiccompounds (especially aromatic hydrocarbons) andsometimes suspended solids, tend to be of greaterconcern than loadings of nutrients and BiochemicalOxygen Demand (BOD). A high proportion of thesepollutants are found to be adsorbed to the fine siltfraction of the suspended solids. By collecting, filteringand treating the fine sediment which is subsequentlyremoved, the system will be dealing with a significantpart of the runoff pollution.

Receiving Waters

4.8 Having established the likely quality and quantityof the runoff from the highway, the Designer orSpecifier will have to consider the characteristics of thereceiving waters. These will be surface watercourses orgroundwater or both. Exceptionally they may be tidalwaters.

4.9 The factors affecting the likely pollutant impactof highway runoff on surface watercourses are theirquality and flow. A discussion of river quality is givenin HA 216 (DMRB 11.3.10). The river flow determinesthe available dilution to the runoff. The greatest risk ofpollution is in the summer, not only because of the ‘firstflush’ phenomenon described in paragraph 4.5, but alsobecause the river flows are then at their lowest. Agenerally accepted standard for measuring dry weatherflows is the river flow which is exceeded for 95% of thetime – this is known as the 95 percentile flow, and dataon many rivers can be obtained from the appropriateEPA.

4.10 There is little evidence available to date thatgroundwaters have been adversely affected by highwayrunoff, but the consequences of any pollution incidentcould be severe as remedial measures will be verydifficult to achieve. A precautionary approach shouldtherefore be adopted when designing systems to protectdischarges to ground.

4/2

Other Factors Affecting Selection of Systems

4.11 Once the Designer or Specifier has establishedthe design parameters for the highway runoff and thecharacteristics of the receiving waters, the requiredlevel of treatment of the runoff, if any, can bedetermined, as well as the degree of attenuationrequired to avoid an effect on the local hydrology. Indeciding what systems to select in a drainage design,the other factors may be considered in turn as describedbelow.

The Availability of Land

4.12 This is probably the most significant factoraffecting the choice of systems. For new road schemes,the optimum drainage systems should where possible beidentified before decisions on land acquisition aremade. In certain situations, it may be necessary toconsider the acquisition of additional land to achieve abetter scheme in landscape, visual and ecological terms.Such a decision would be based on a review of theconsiderations described in paragraph 4.2. This landmay need to be acquired, or rights of access obtainedfor maintenance, which could impose their ownconstraints on the landscape.

4.13 Where land availability is limited, as forimprovement schemes, vegetated systems may have tobe designed to fit into the road corridor, using landwithin interchanges where this is suitably located. Evenfor these schemes, however, the possibility of acquiringextra land should always be considered where it isnecessary to achieve a good standard of design. Bymaking landowners aware of the powers available toconstruct highway drainage, it may often be possible toobtain the necessary land by agreement. Installation ofinfiltration trenches and long linear ponds may befeasible if designed sympathetically. If existing pondsare available, it may be possible for these to be adapted,provided that their use for treatment is ecologicallyacceptable. Where land is further restricted, as it willcommonly be in urban contexts, restriction of treatmentto the ‘first flush’ (see paragraph 4.5) may be necessaryin order to reduce the volumes to be stored and treated,and to maximise the benefit from the use of vegetatedsystems. The designs shown in this Advice Noterepresent an ideal to be aimed at: in restricted sites itwill usually be better to design a compromise solutionthan not to design any vegetated system. HD 33(DMRB 4.2) gives advice on how conventionaldrainage systems, which usually occupy less space thanvegetated ones, can be designed to reduce pollution andflood risk. Advice on the design of retro-fitted systemsis given in paragraph 5.44.

May 2006



The Climate and Rainfall Characteristics

4.14 These factors need to be considered along withthe catchment area of road drained to each outfall. Theclimate will determine whether the area is likely to havelong dry periods with occasional intense storms (e.g.South East England) or whether it will have higher andmore uniform rainfall (North West England, WestScotland, Wales, Northern Ireland). These factors whenconsidered with the local catchment hydrology, willenable the Designer or Specifier to determine the extentto which attenuation of the flow from particular outfallsis required. Attenuation can be achieved by largebalancing ponds, or infiltration basins, or by a series ofsmaller containment devices, possibly located inswales, and located at the top of the drainage system.The size of the pond or basin will be determined by thedegree of attenuation required. Seasonal climatic andrainfall characteristics, and the availability of a baseflow for irrigation, will also determine theappropriateness of selecting a viable wetlandvegetation. Where a base flow is used, itscharacteristics will have to be estimated and included inthe determination of the size of balancing facility.

Soil Permeability

4.15 For the purpose of selecting drainage systems,the Designer or Specifier needs to know thepermeability of the natural subsoil and that used tocreate embankments. On permeable soils, swales,grassed channels and infiltration basins/trenches arepotentially effective solutions, but consideration mustbe given to the percolation of water to ground and tothe effects of the possible transmission of pollutants tothe underlying subsoil, particularly in areas of existingcontamination. The use of impermeable membranesmay be necessary in some circumstances to preventseepage to ground. They may also be necessary tomaintain the quality of the treatment system. Whereproposed sites for drainage systems are situated aboveaquifers the vulnerability of the ground water should beconsidered by reference to the groundwatervulnerability maps, information on abstraction sourceprotection zones and by consultation with the EPAs.Designers or Specifiers must satisfy themselves that theinstallation will comply with the requirements of TheGroundwater Regulations 1998 in respect of direct orindirect discharges to groundwater. Unless reliableinformation is available on the attenuating properties ofthe subsoil in respect of pollutants routinely found inhighway runoff, it is recommended that a high factor ofsafety be used when designing and constructing systemsabove vulnerable groundwaters. Appropriate protection

May 2006

measures may include placing thicker membranes orimported impermeable material (possibly a wasteproduct from elsewhere on site), to provide animpermeable barrier above the vulnerable strata.

4.16 In areas of impermeable soils, swales will be lesseffective and wetlands and ponds will generally bemore suitable. Balancing ponds will tend to retainwater, and infiltration basins/trenches will have to beconstructed above the impermeable soil, using suitablegranular materials.

4.17 Soils are also important in determining thestability of a vegetated system. In general, soils with agravel, sand and clay mixture may be considered to beerosion resistant, while fine sands and silts aresusceptible to erosion. Soil type thus affects the designof the system, particularly for swales and grassedchannels or ditches which experience greater flows thanponds and basins etc. Velocities must either be lowenough to prevent erosion, or the system must beaugmented by rip-rap, sheet spreaders and concreteaprons to reduce input velocities and prevent scour.

4.18 Although the Designer or Specifier will primarilybe concerned with highway runoff, there may be otherwatercourses affected by the road design, andgroundwater levels may be close to the surface. In suchsituations it may be possible to use such water flows,which will probably be far less intermittent thanhighway runoff, to provide water to wetland and pondsystems to prevent them drying out.

Topography

4.19 The nature of the surrounding topography shouldhave an influence on the choice of systems, to enablethe design to be well integrated within its landscapesetting. In flat areas with little or no gradients, largersystems such as wetlands or ponds may be appropriate,if suitably designed and planted, and sufficient water isavailable to prevent drying out. It may be possible toincrease the number of outfalls, to reduce the risk ofspillage at any one outfall, and design smaller features.In undulating countryside, treatment and attenuationsystems will probably have to be smaller (and morefrequent) to blend into the landscape. Use of ponds andinfiltration basins will be possible, but swales, whichrequire gentle, even slopes, are likely to be harder todesign sympathetically. Figures 4.1 and 4.2 showindicative sections of these systems in undulating andflat landscapes.

4/3

Volume 4 Section 2

Part 1 HA

103/06

4/4

Chapter 4

Selection of Vegetated Drainage System

s

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Figure 4.1

Volume 4 Section 2

Part 1 HA

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May 20

Chapter 4

Selection of Vegetated Drainage System

s

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Figure 4.2



Spillage Risk

4.20 When selecting drainage systems to reduce therisk of pollution to downstream watercourses, the riskof accidental spillage must also be considered. Amethod is given in Annex I of HA 216 (DMRB 11.3.10)for estimating the probability of pollution occurringfrom a major spillage event. The need for spillagecontainment and control facilities should be determinedin accordance with the guidance given in HA 216(DMRB 11.3.10) and after consultation with the EPA.Systems should be designed and adapted to includethese measures and should usually include a method tocontrol the discharge of any spillage at the outfall,regardless of whether designated containment provisionis made. Means of control should be simple to find,readily accessible from the highway and capable ofbeing operated easily and quickly by the emergencyservices. Details of signs to be used to locate controldevices are given in HD 33 (DMRB 4.2).

4.21 In some cases the vegetated drainage system maybe used for containment but, where reinstatement wouldbe difficult and costly, the system will have to beprotected. Swales, for example, could have check damsand balancing ponds could have penstocks, flap valvesor outlets designed to be blocked with sand bags.Wetlands could be fitted with an inlet control device tobe closed in the event of spillage, or be siteddownstream of a dry containment area. Infiltrationbasins will have to be designed downstream of asuitable containment system to collect any spillage: themain problem in this case is to ensure that clean up ofany spillages does not impair the basin’s effectiveness.

4.21 The system chosen for control of the spillage atany location should be appropriate to its estimatedprobability. Where this is assessed at less than 1%annually for a major spillage, methods such as checkdams, booms or notch weirs will be more suitable thanmechanical devices such as penstocks or gate valves,which are more susceptible to vandalism and maybecome ineffective through lack of use. Theconsequence of any spillage will determine theimportance to be attached to providing some form ofspillage control. In particularly sensitive areas thatinclude designated sites, there should be some form ofcontainment prior to the outfall, no matter how low theprobability. It only takes one incident to causeconsiderable damage. As noted in Annex I of HA 216(DMRB 11.3.10), there may be exceptional locationswhich require dual containment measures. The lowerthe probability of a spillage, the more appropriate it willbe to design a low cost containment solution, that needs

4/6

little or no maintenance. Examples include dry ditches,small bermed areas or simple oil traps.

Summary

4.23 Whilst it can be seen that there are many factorswhich can influence the choice of vegetated systems foruse in drainage systems, only certain drainage systemsmay be appropriate in some locations. It is, however,likely that at any one site there will be more than onesolution. Also in some circumstances where floodcontrol and spillage containment is required, more thanone system may be necessary. It is therefore importantfor Designers or Specifiers to consider as many relevantfactors as possible before deciding on a suitabledrainage design.

May 2006


UCTION OFYSTEMS

Chapter 5The Design and Construction of Vegetated Drainage Systems

5 THE DESIGN AND CONSTRVEGETATED DRAINAGE S

General

5.1 Attention to detail in the design and constructionof vegetated systems is very important, if the benefits ofwater treatment and attenuation are to be fully utilised,and higher levels of overall integration with thesurrounding landscape are to be achieved. The advicebelow makes suggestions for good design principles,and indicates where possible problems may occur.Further details on the design and construction of thesesystems can be found in the referred texts in Chapter 7.

5.2 Prior to the commencement of any design, adetailed survey of the site and the adjoining area shouldbe carried out. This would provide baseline informationon the presence and nature of existing landscapefeatures/habitats/species. Wherever possible, suchinformation should influence the type, location andscale of vegetated drainage systems and planting to beused. All access points, including those formaintenance, should be sensitively located to fit withthe flow and pattern of the surrounding landscape.Alternative surface treatments for access roads shouldalso be considered to minimise their visual impact. Thelayout and surface treatment should be designed toaccommodate the size and weight of the relevantmaintenance plant.

5.3 Stability of the vegetated systems is paramount.As treatment depends on the integrity of the wholesystem, should any part of the system fail thensignificant reductions in performance will occur.Stability is often related to the storm events for whichthe systems are designed. When designing a vegetatedsystem to cater for a particular storm event, theprobability of that event should be half that used forhydraulic design, as given in Chapter 6 of HD 33(DMRB 4.2). This will give an adequate factor of safetyagainst failure of an individual system. As the mostimportant treatment processes are settlement andfiltering, a basic requirement of the design of a systemis to slow and spread the flow, to encourage theseprocesses. The hydraulic characteristics of a system willtherefore be critical to its effectiveness.

Ground and Structural Stability

5.4 Designers and Specifiers must also ensure thatthe design of vegetated systems does not in any way

prejotheconallostru

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May 2006

udice the stability of embankments, cuttings orr highway structures, by requiring that the systems

tain the water flowing through them, and by notwing leakage from the systems to destabilise suchctures.

s

Central to the aims of establishing and supportingetated systems are the careful handling, storage andof available soil materials on site. Special careuld be taken during the excavation and placement of materials, to prevent damage to the soil structure compaction of the topsoil and subsoil. Carefulning of the site works should therefore consider theng of operations, location of storage areas andss routes. Where possible, soil materials on site

uld be fully utilised and a proposed schemegned with the prevailing site conditions in mind,eby obviating the need to import materials.

les

Swales are most effective in treating water whenspeed of the water flow is slow. In most rainfallnts this will occur naturally, but the more intensems may lead to situations where the previouslymulated suspended solids are washed downstream.les should, therefore, be designed for a 5 year 24r event, and checked against a 10 year event. Thennel velocity should not exceed 0.25m/sec, with theitudinal slope being ideally less than 2% and noe than 6%. The flow speed should be less if thele length is less than 120m, as an 8 - 10 minutedence time is ideal for maximum effectiveness.r 10 minutes, removal efficiency is expected tol off. Residence time is best extended by increasing width and length. Decreasing slope or increasing

density of vegetation would also increase thedence time but would result in greater depths due toeased impedance.

Swales should be designed without any abruptnges in vertical or horizontal direction, so that the of erosion is reduced. This will also make theme attractive features and easier to maintain. Swalesbest located as close to the road as possible, where rates are slowest. They are well suited to areas

5/1



where drainage from the road flows over the edge. As itis hard to channel water from a piped flow to a swalewithout the risk of rivulets or channels occurring, a

flsh

Figure 5.1

5/2

ow spreader, similar to that shown in Figure 5.1ould be used.

May 2006



5.8 Stability of the channel is of the utmostimportance and should be the primary concern in thedesign process. If the channel should form rivulets, theeffectiveness of the swale in treating the water will begreatly reduced. A smooth sheet flow is ideal. Anydesign changes to improve pollutant removal (e.g.wider or longer channels) will also aid stability. Rip-rap, concrete aprons and spreaders should all beconsidered, particularly at the outfall of the swale toachieve an even flow of low velocity and minimise thepossibility of scour. Swales work best at encouragingsettlement if there is a small vertical component to theflow, caused by infiltration, and are therefore mosteffective if constructed in areas where the underlyingmaterial is permeable and discharge to ground can bepermitted. However they must be designed to minimisethe risk of water penetrating the carriageway subgradeand Designers or Specifiers must satisfy themselves thatthe swale will not promote slippage of the highwayembankment.

5.9 In certain conditions, the design of swales mayobviate the requirement for a system of gullies andpiped drainage with the swale discharging to a suitablecollection point. They should be considered in all casesas they represent a cost effective control measurewhich, even if insufficient for all the treatmentrequirements in themselves, may provide significantprimary treatment prior to secondary systems such asponds or wetlands. Their potential for significanttrapping of suspended solids may extend the periodsbetween major maintenance for those downstreamsystems.

Spillage Containment

5.10 Installation of check dams will not only slow theflows but will also act as a form of automatic spillagecontainment for the less serious accidental spillages.Other possible controls include locations that can beblocked by sandbags, and outlets designed to retainflows. The anti-scour details mentioned in paragraph5.7 could include a V-notch weir designed to containthe early flow and minor spillages Although thegradient of most swales will make them unsuitable to beused to contain major spillages, it may be possible todesign the lowest part of the swale with a shallowergradient and surrounded by a berm. The outlet could bea notched weir. Care will be needed in these cases toensure that any spilt liquid cannot percolate togroundwaters, and an impermeable liner may beneeded.

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May 2006

gration into the landscape

Swales should be carefully integrated into theeral landscape of the highway, incorporatingsting and proposed landscape elements such as treesedges as appropriate. It may be possible to design

m within an interchange, or even to amend the roadnment, so that the water from the carriageway flowswales. The optimum gradient for channel slopes isilar to that of slip roads.

nting

2 The growth characteristics of the grass species important to the effectiveness of the swales inining suspended solids. A dense and even sward isuired consisting of species with a combination ofid establishment and recovery, some salt tolerance tolerance of wet conditions and periodicndation. Species to consider are perennial ryegrasslium perenne) and creeping bent (Agrostisonifera). Where salt tolerance is a particularuirement, salt tolerant cultivars of fescue speciesstuca spp.) are available. Rush species (e.g. Juncus.) should also be considered. Vegetation heightuld be approximately twice the depth of water to beted and ideally 100-200mm. Shorter vegetation will treat greater flows and taller vegetation will have adency to be flattened. Seeding to establish vegetationuld be uniform and carried out early in thestruction process as soon as soils are ready, and in appropriate season - ideally late summer (August/tember). Where quick establishment is required, turf

ing would be appropriate to prevent the creation of, although this is more expensive than conventional

ding. Geotextiles may be employed to preventsion in early stages or as temporary measures inance of seeding, but should not be relied on in theg term as they will degrade.

iltration Basins

3 The size of the infiltration basin will beermined by the catchment area. The basin should be enough to accept the first 10mm of a storm, thest flush’ as described in paragraph 4.5, withoutrtopping. The total catchment to any one basinuld ideally be between two and ten hectares, with arow trench-like infiltration basin being used forller catchments. The basin must have a level base, be located sufficiently far above the groundwaterel (at least 1.5m) to ensure that the filter layer is notrated and that water percolates through at a

sonable speed. The basin should empty within 72

5/3



hours. It should be noted that such basins may act assources of pollutants rather than sinks if subsequentstorms cause scouring of suspended solids, which arethen released by overtopping to the receiving waters. Itmay be better to isolate the first flush in a smaller basinand use a second basin for attenuation if required.

5.14 The filter layer should consist of a free draininggranular material to permit dispersal. There should be a10 - 15% sand fraction to ensure that treatment of thewater is effective. A material such as EarthworksMaterial Class 6D, as detailed in Series 600 MCHWVolume 1, would be particularly suitable. Geotextilesshould be used to prevent contamination of the filterlayer from either the subgrade or topsoil. The designshould allow for occasional removal and replacement ofthe filter material. This could either be at predeterminedmajor maintenance intervals or following an accidentalspillage.


5.15 Infiltration basins and trenches are vulnerable todamage resulting from major accidental spillages. If therisk of spillage is sufficiently high to warrant theprovision of containment it is best to choose anothersystem. If that is not possible, it is preferable to use twobasins as described in paragraph 5.13 above with acontrol system to divert the spillage into the secondbasin, or else to protect the infiltration basin by using aseparate containment area. Filter layers can be replacedif damaged, but the operation may not always bestraightforward and, if not carried out promptly, there isthe risk of pollution from the spilled substance beingwashed out of the filter medium.

Integration into the landscape

5.16 The shape of the basin will be determined byprevailing environmental considerations, with the aimbeing to fit the basin into the surrounding landscape toachieve the most natural appearance. The slopes andshape of the basin should reflect those that occurlocally. Where it is considered that the basins maybecome unattractive elements in the landscape,carefully designed low mounds, combined with plantingcould provide screening for passing motorists. Wherepossible, local materials should be used for engineeringfeatures and boundary treatments.

Planting

5.17 Vegetation types for basins, where standing wateris likely for only a short duration, can be creeping bentand rush species, the same as for swales (see paragraph

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.12). Where standing water occurs for longer periodsays) wetland species such as common reedhragmites australis), reed canary grass (Phalaris

rundinacea) and amphibious bistort (Persicariamphibia) should be considered. The inlet should beesigned to prevent scouring of the basin. Rip rap orow spreaders should be included if necessary.

ub-Surface Flow Wetlands

.18 As stated in paragraph 2.12 above, SSF Wetlandsill rarely be suitable for use in highway drainagestems. Where, exceptionally, they are considereditable, guidance on their design and construction can

e found in various sources (refs. 4, 17, 18).

urface Flow Wetlands

.19 SF Wetlands are ideally located on gently slopinground of about 0.5%, but can be on level ground orround sloping to 1%. The required area will depend one area drained and the level of treatment required. As

guide, they should at least be designed to treat therst 10mm of runoff without overtopping. They shoulde surrounded by berms with slopes of 20% or less,hich are at least 0.5m above the permanent watervel. The depth of water should be between 0.15 and.3m, and a variation in depth, as shown in Figure 2.3,ill assist in creating a range of habitats to improveeatment potential. The existing sub-grade may be usedr the base, but if groundwaters need protection, a liner

s described in paragraph 5.40 may be needed. The inletould be designed to produce an even flow toinimise the risk of scour: a flow spreader, similar toat in Figure 5.1 may assist, as may the use of stone

itching to stabilise the entry channel. The outlet coulde a weir, or high level pipe to maintain the requiredater level, and to help prevent the drying out of thestem. Where groundwater levels are high it may be

ossible to utilise ground water flows to keep theetland moist, provided that arrangements fortroducing the groundwater prevent any possibility of

ontamination of the aquifer by pollutants from theetland. Ideally the wetland should be designed tovertop only exceptionally, but if this is not possible, aypass may be needed to take flows in excess of therst 10mm stipulated above.

pillage Containment

.20 A SF Wetland, designed as a shallow basin, canct as an effective spillage containment facility. Outfallsill need to be designed so that spills can be contained,

ither mechanically, by a penstock or similar, or by

May 2006



emergency personnel using sandbags or booms to blocka suitable channel. Although the wetland may need tobe thoroughly renovated following a serious spillage,this is considered acceptable for events where theestimated probability is less than, say, 1% annually, orwhere the wetland is used as the second system wheredual containment has to be provided. For higherprobability events, the Designer or Specifier shouldweigh the cost of providing separate containmentagainst the cost of renovating the wetland following aspillage, also taking into account the value andvulnerability of the receiving waters.

Integration into the Landscape

5.21 The shape and layout of SF wetlands should aimto reflect the overall pattern and scale of thesurrounding landscape. Geometric shapes and steepuniform banks should be avoided where possible withside slopes generally being no steeper than 1 in 3.Smoothly flowing shapes that fit with the flow of thelandform can create more natural and attractivefeatures. Where possible local materials should be usedfor engineering features and boundary treatments.

Planting

5.22 A range of vegetation is appropriate to this typeof wetland and selection depends on the depth of watercover and its periodicity. For permanently full or partlyinundated wetlands, reed mace (Typha latifolia),

Worke

A wetland is to be designed to treat runoff from 20,000

From Table B.1 in HA 216 (DMRB 11.3.10) Annex I,

This equals 5 x 2 x 1000 / 365 = 27.4 gm / day.

Required area of planting in wetland to remove zinc =

From Table 3.1 in HA 216 (DMRB 11.3.10) Annex I, M1.2 kg / ha / yr. This equals 1.2 x 2 x 1000 / 365 = 6.6

Required area of planting in wetland to remove copper

The greater area is required to remove zinc, so area of

May 2006

branched bur-reed (Sparganium erectum), pond weeds(Potamogeton spp.) and flote-grass (Glyceria fluitans)are appropriate. In seasonally wet parts common reed,reed canary grass and amphibious bistort areappropriate. In drier parts rush species should beconsidered. Establishment of aquatic and emergentspecies should be from planted rhizome/root stock inthe spring. The grass and rush component can be sown,ideally in late summer. The extent of planting should bedesigned so there is vegetation across the entire flowpath. In hybrid systems, vegetation may be planted instrips, as shown in Figure 2.5, but this would not beappropriate in areas where there is a permanent waterbody. As a guide to the sizing of wetlands in relation tometal removal (particularly copper and zinc), thefollowing removal rates, based on various studies (refs.13, 14), could be used:

Removal of metals: gm/day/m2 Vegetated SystemsTotal Zinc 0.04

Soluble Copper 0.05

These can be used to estimate the minimum size of reedbeds in wetlands where there is a reliance on vegetationrelated removal. This can be determined in conjunctionwith the detailed assessment method in HA 216(DMRB 11.3.10) as well as to assess the notionalmitigation effect of the wetland on the water quality ofthe receiving water course. The following workedexample shows how this can be done.

d Example

m2 of a road with AADT > 30,000.

Method B, the build up rate for total zinc is 5 kg / ha / yr.

27.4 / 0.04 = 685 m2.

ethod B, the build up rate for soluble copper is gm / day.

= 6.6 / 0.05 = 132 m2.

planted wetland should be at least 685 m2.

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Balancing Ponds

5.23 Balancing Ponds are primarily designed toattenuate storm flows from the highway, so thatdownstream watercourses are not exposed to damagingflows that can cause erosive or flooding damage. Indesigning ponds to attenuate flows, consideration mustbe given to the hydrological characteristics of the localcatchment area. The capacity of the pond shouldnormally be such as to retain an event with an annualprobability of 1% for that catchment and discharge thewater into the downstream watercourse at a rate thatwould occur in those rainfall conditions if no road werepresent. Advice on the local catchment characteristicsmay be obtained from the local land drainage authority,who will also be able to advise on any history of localflooding, and the risks associated with an increased rateof surface runoff from the road. This is also to avoid therisk of damage to habitats and species caused by therapid discharge of runoff into sensitive receivingwaters. The EPA should be consulted about both pondcapacities and discharge rates and agreement reachedwith them. EPA’s are able, if they wish, to stipulatemaximum discharge rates as a condition of the consentneeded for the construction of the outfall, provided thatthe requirement is reasonable, because such consentsmay not be unreasonably withheld. Control of the rateof discharge from a balancing pond may be by means ofa small diameter pipe, an orifice or a notched flowcontrol device as shown in Figure 5.2. The section inFigure 2.4 shows how the outfall design can allowdifferent discharge rates for storm events of variousprobabilities.

5.24 Although runoff treatment is a secondaryfunction of balancing ponds, Designers or Specifiersshould take the opportunity to maximise the potential ofthe systems to treat runoff. By retaining storm water fora period, some treatment will be achieved by thesettlement of suspended solids. The longer theresidence time, the more effective will be the treatmentas the smaller particles settle out last. The treatment ofwater may be made more effective by establishingvegetation (as described below) and designing the pondto allow a variety of plants to grow, ideally with a rangeof depths. Direct pathways or channels between theinlet and outlet should be avoided if possible. Byretaining the runoff and only permitting a controlleddischarge, the available dilution by the receiving waterswill be greater than would occur without the pond. Thiswill reduce the risk of pollution of those waters.

5.25 Balancing ponds should be located within thehighway land so that maintenance can be fully

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controlled by the highway authority. It may be possibleto locate the pond within an interchange, or a linearpond may be necessary, running parallel to the road.Balancing ponds can be designed to be either ‘wet’,holding water throughout the year, or ‘dry’, having atendency to dry out in the summer months. This is mosteasily achieved by adjusting the invert level of theoutflow pipe or channel. For wet ponds the outlet invertlevel should be at least 0.5m above the pond surface,but for a dry pond it may be flush with, or less than0.3m above, the pond level. It may be that even in a wetpond, shallower areas will dry out with a permanentbody of water remaining in the deeper sections.


5.26 Balancing ponds can act as effective forms ofspillage containment if the outlet is designed so that thesmaller flows typical of spillages and the “first flush”(see paragraph 4.5) are contained in the first instance.This could be achieved automatically by a siphon sothat the outflow only operates when a certain waterlevel is reached, or manually by a penstock. Where riskof spillage is not high (annual probabilities less than1%), it may be more appropriate to use controls such assandbags or a notched weir as shown in Figure 5.2. Thedesign must allow easy access and straightforward useof the specified spillage control device. In emergencies,the quicker such devices can be used, the lower will bethe risk of pollution. There is a risk that devices such aspenstocks may be more susceptible to vandalism andthey may deteriorate over time due to prolonged periodswithout use.

Integration into the Landscape

5.27 New pond landforms should aim to reflect theoverall character shape and scale of the prevailingtopography. They should generally aim to avoidgeometric shapes with steep, uniform banks/margins.Curvilinear or indented shapes that ‘marry in’ smoothlywith adjoining contours, tending to create moreinteresting and attractive features as illustrated inFigure 5.3.

5.28 Allowing margins to develop with vegetationwhere land meets water can create an area for wildlife.Where possible, side slopes to balancing ponds shouldbe in the range of 1:3 to 1:10, to allow for ease ofmaintenance as well as ensuring a safe environment.These side slopes should be top dressed withappropriate soil materials, as shown in Figure 5.4.Gently sloping shelves should be incorporated withinponds to provide appropriate conditions for emergentand marginal vegetation For a given volume of storage,

May 2006



shallow pools are better than deep basins in terms ofpollutant removal efficiency and within the landavailability constraints the basin depth should bebetween 0.5m and 2m. For the deeper ponds, safetymust be a design consideration, as discussed inparagraph 5.46.

Planting

5.29 In balancing ponds water levels are likely to varyconsiderably. Some ponds may be designed to becomedry and others to retain some depth of water. Inpermanent wet parts of ponds up to 0.5m deep, flote-grass (Glyceria fluitans), branched bur-reed(Sparganium erectum), reed mace (Typha latifolia),common club-rush (Schoenoplectus lacustris) andcommon reed (Phragmites australis) are appropriate asa belt between 2 and 6m wide leaving an area of openwater. In dry ponds or drier parts of ponds commonreed (Phragmites australis), reed canary grass (Phalarisarundinacea) and amphibious bistort (Persicariaamphibia) are particularly appropriate where there arelong or frequent periods of inundation. Whereinundation is infrequent creeping bent and rush speciesmay be more appropriate. Vegetation may also beplanted close to the inlet to promote sedimentation.Rhizome cuttings of reed mace, common reed andbur-reed are recommended for establishment of thesespecies.

Sedimentation Ponds

5.30 The primary function of a Sedimentation Pond isto allow suspended solids to settle out of the runoffprior to its discharge into the receiving watercourse. Bycontrolling the rate of discharge, the dilution by thereceiving waters will be increased, thus reducing thepollution risk. Typically sedimentation ponds would beused where there is a high pollutant level and a highquality receiving surface watercourse or groundwater.For optimum performance the design of the pond needsto ensure a long slow flow path, providing a generousretention time, but avoiding stagnant areas. Long ponds,with the inlet at the opposite end from the outlet areideal, as shown in Figure 5.5. The inlet should allowwater to enter without scouring the pond surface, andthe outlet should have a flow control device (which willmean the pond will also attenuate flows). This system isone where it may be appropriate for the ‘first flushflows’ (see paragraph 4.5) to be diverted, with thebalance of the flow flowing to a separate treatment area(or diverted to the receiving watercourse). One way ofdoing this would be to use an inlet device as shown inFigure 5.6 to divert the flow away from the treatment

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May 2006

ond once it is full. By doing this the required area ofond will be less, as it will only need to be sized to treatunoff from the first 10mm of rain. If it is possible toonstruct an outlet with variable control, as shown inigure 2.4, the design should aim to detain the first

nflows for the longest period and allow subsequentnflows (when the level of the pond is higher) to flowut at a quicker rate. That way runoff from most rainfallvents will be detained for the maximum period.

.31 Stability of the system is essential to preventank erosion from overland flow and wave action.anks slopes should be 1:3 or less. If steeper byecessity, then they should be reinforced by gabions,or example. Short circuiting of the flow and increasingocal flow speeds are other potential problems.ncreasing pool length is an obvious way of increasinghe flow path length but, if space is restricted, baffleslose to inlet and outlets, islands and long or irregularhorelines are all ways of increasing the path lengthrom inlet to outlet. Care must be taken not to cause thelow speed to increase in localised areas, as not onlyould this reduce the effectiveness of the system, itould also increase the risk that re-suspension ofarticles may occur during intense storms.

pillage Containment

.32 Sedimentation Ponds are well suited toontaining accidental spillages in the same way asalancing ponds if the outlet is designed so that, in allut very wet conditions, the spill will be retained untilumped out. The comments in paragraph 5.26 aboveill apply equally to sedimentation ponds.

ntegration into the Landscape

.33 Sedimentation ponds may be long and relativelyarrow to increase the flow path. For this reason carefulonsideration needs to be given to their integrationithin the surrounding landscape. The shape and layout

hould aim to create a natural effect with a smoothlylowing outline reflecting the character of the area.eometric shapes and steep uniform banks should be

voided with side slopes being no steeper than 1 in 3.here possible local materials should be used for

ngineering features and boundary treatments.

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Figure 5.2

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Figure 5.4



Figure 5.5

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Figure 5.6

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Planting

5.34 These ponds are designed to remain water-filledthroughout the year. Here, reed mace (Typha latifolia),common reed (Phragmites australis) and branched bur-reed (Sparganium erectum) are appropriate as marginalspecies, with spiked milfoil (Myriophyllum spicatum),pond weeds (Potamogeton natans & P. pectinatus) andcommon club-rush (Schoenoplectus lacustris) suitable

Table

Water Level and Supply Requireme

Species tolerant of fluctuating Species requiring shwater levels and duration standing water at al

Phragmites australis Sparganium erectum(common reed) (branched bur-reed)

Phalaris arundinacea Glyceria fluitans(reed canary grass) (flote-grass)

Persicaria amphibia Typha latifolia(amphibious bistort) (reed mace)

Hybrid Systems

5.35 Vegetated systems can be designed and built as acombination of two or more of the above systems. Thisis usually done to maximise the treatment of the runoff.One such combination could be a system containing SFwetland and a balancing pond. Similarly it may bepossible for an infiltration basin to act as a balancingpond, with minor modifications to the design. In suchcases the advice given above for both of the abovesystems should be followed. Care should be taken toensure that in creating a hybrid system the benefits ofthe two constituent systems are not lost.

Pre-treatment Systems

5.36 With the exception of SSF wetlands, which willonly rarely be appropriate for treating highway runoff,vegetated systems are considered sufficiently robust notnormally to require separate pre-treatment facilities forroutine runoff from rural highways. Consideration may,however, have to be given to the provision of oilseparators in urban areas, or in other locations such asbusy laybys where significant oil pollution is likely to

May 2006

for deep water (>0.5m). The water level and supplyrequirements for these plant species are summarised inTable 5.1. These aquatic species can perform a limitedrole in uptake of soluble metals, but importantly cancontribute to oxygenation of the water and enhance theprocesses of precipitation and biodegradation of organicmaterial. These species can be established in bio-degradable pots/bags containing a suitable media toallow rooting into the pond sediments.

5.1

nts of Wetland and Pond Species

allow Species requiring moderate tol times deep standing water at all times

Schoenoplectus lacustris(common club-rush)

Myriophyllum spicatum(spiked water-milfoil)

Potamogeton species(pond weeds)

occur from standing vehicles. Where spillage isassessed as a high risk (an annual probability greaterthan 1%), systems such as infiltration basins may needsome additional protection. In many situations, it willbe more economic to accept the risk of an occasionalspillage and the consequential remedial cost to thesystem than to construct the additional facility, and thisshould be considered when the system is designed orspecified.

Considerations for Integration into the WiderLandscape

Pond Details and Engineering Features

5.37 In many circumstances, the engineering featuresassociated with vegetated drainage systems can have asignificant visual impact on the surrounding area. Incertain high profile situations where engineeringfeatures are visible above ground (e.g. headwalls orlined channels), it would be appropriate to use of localbuilding materials to reflect the character of thesurrounding area. Careful attention to detailing, forexample matching stone size and jointing patterns oflocal walls, can create convincing and attractive

5/13



features that will quickly weather and mellow. Theangle of inlet/outfall header walls should follow theprofile of the ground (e.g. pond margins) to reduce theirvisual prominence where possible. Figure 5.7 showsexamples of both good and bad practice.

Boundary Treatments

5.38 Boundary treatments, such as fences and walls,can have a significant visual impact in the landscape.The use of hard ‘urban’ materials can be quiteinappropriate in a rural situation. The use of local stylesand materials and their careful alignment (both verticaland horizontal) should therefore be considered. The useof thorny plants, possibly combined with low fences orwalls and a suitably planted narrow fringe, could alsobe considered as a boundary deterrent whereappropriate.

Retaining Structures

5.39 Where abrupt changes in level are requiredwithin more confined areas, the sensitive use ofstepped, stone-filled gabions, as shown in Figure 5.8,can be very effective. These can be quickly vegetatedby spreading soil on top of the gabions and thenseeding. The use of appropriate local stone within thegabions will also reflect the character of the area. Alining will be required where it is necessary to preventpassage of water through the gabions.

Pond Lining

5.40 Where the pond/wetland needs to be lined, theuse of puddled clay is preferable if there is a localsource available. The puddled clay should be at least300mm thick and not be allowed to dry out, as this mayresult in cracking and leakage. Artificial pond liners(e.g. butyl rubber, membranes, concrete) should be wellburied and designed with a shallow batter. Where butylrubber linings and membranes are used, these should behigh quality to ensure an appropriate design life and toavoid damage during maintenance operations. A markerlayer of coloured sand, or other similar marker, shouldbe installed above flexible membranes to provide anindication of their presence to avoid damage duringperiodic removal of sediment. A shelf should beincorporated to allow for the colonisation of the pondand its margins by vegetation. Figure 5.4 illustrates a

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5/14

otection of Existing Features

1 Whilst considering the design and location of aetated system, every effort should be made totect and retain existing features of landscape andure conservation value. The proposals should avoid negative effects on such features, whether natural

man made, that contribute to the character of thetem itself. Examples of such features includetected species, biodiverse habitats, landmark trees hedgerows. In order to protect such features, aeful site survey and evaluation should be undertakenidentify opportunities and constraints and to helplise the site’s wildlife potential. A careful analysis of landscape pattern and character should influence theeral layout of a particular site and the choice ofcies appropriate to the locality.

ndscape Enhancement/Habitat Creation

2 The construction of vegetated systems canvide opportunities for enhancing the landscape andure conservation value of the site beyond themediate limits of the selected drainage system. Theign should consider a broad variety of habitat types landscape features in addition to those specifically

ated to the vegetated system. These could include:

Hedgerows – New hedges and hedgerow trees tolink with and enhance existing wildlife corridors.

Woodland areas – New native woodland areas toreflect the surrounding landscape character andprovide new habitats.

Shrubs/woodland edge – New habitats andenhancement of the surrounding area.

Specimen/landmark trees – Individual or smallgroups of trees to provide features within thelandscape.

Grassland/wildflowers – Landscape enhancementand habitat creation.

Islands – For larger pond/wetlands it may beappropriate to incorporate islands to provide asafe refuge for wildfowl.

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5.43 Locating a pond or wetland drainage system nearan existing wetland environment/watercourse can helpto encourage the rapid establishment of flora and fauna.This is further aided by retaining as much of theexisting vegetation as possible (e.g. turf, trees andhedges). Professional guidance should be sought forproposals to restore or enhance existing habitats. Suchproposals may overlap or be integrated with new habitatcreation works, particularly marginal and wetlandhabitats, and often new woodland planting. Specieswhich could be potentially invasive and cause negativeimpacts on surrounding features should not be used. Inall cases, however, the carrying out of the maintenanceof the primary functions of the drainage system must beconsidered and must not be prejudiced.

Retro-fitting of Vegetated Systems

5.44 Where space allows, existing ditches may bewidened, their slopes slackened and emergent reed typevegetation introduced to create a linear system,resembling a natural stream. The principles ofparagraph 5.22 should be applied. If pools and areas ofreed can be incorporated, that will have added benefit.It may be possible to replace a section of piped drainwith an open ditch or swale, particularly if a form ofspillage containment is to be introduced. Wherever suchretro-fitting of systems is proposed, the hydraulicperformance of the whole drainage system should bechecked to ensure this work does not increase the riskof flooding.

Health and Safety Considerations

5.45 As required by the Construction (Design andManagement) Regulations 1994 (SI 1994 No 3140),Designers or Specifiers should be aware of the risks toboth the public, highway maintainers and thoseimplementing the Contractor’s design as a result ofdesigning or specifying vegetated systems. A riskassessment should be made at the time the systems aredesigned or specified. Because of their role in thetreatment and control of runoff, it is recommended thatthere should not normally be public access to thosevegetated drainage systems that may contain standingwater, and boundary treatments should be designedaccordingly.

5.46 Where, exceptionally, it is proposed to allowpublic access to a system, for amenity and recreationaluse, the design should ensure that water levels adjacentto the pond edge are shallow and gently graded. If deepwater is unavoidable the public should be excluded, orin exceptional circumstances suitable fencing erected to

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duce the risk of persons falling into the water.hambers, penstocks, valves and weirs should all besigned to deter unauthorised access and to avoid thessibility of the public being endangered.

47 Ease of access for the appropriate maintenanceant is important, not only to encourage regularaintenance, but also for the safety of the maintenanceeratives. It will also aid emergency personnel inrrying out any measures to mitigate the effects of aillage. Appropriate access to systems remote from theain carriageway is essential in all locations, and inany locations a gated access could be provided.here systems are to be constructed adjacent toisting watercourses, a margin of about 8 metresould be allowed for maintenance and access by the

parian owner.

stablishment and Aftercare

48 Careful establishment and aftercare of theoposed system will be required in order to ensure theccess of the scheme and its integration into the widerndscape. Set out below are some design principlesat should be considered where appropriate.

ecification

49 A comprehensive written specification should beoduced by an appropriately qualified professional.his should be keyed to detailed site plans at anpropriate scale (generally no smaller than 1:500).

his will cover the planting operations and thetablishment and aftercare during the contract. Theecification should include a schedule of all plantaterial, giving species, cultivars, sizes (pot size wherepropriate), numbers and densities, and should alsoclude details of plant handling and deliveryocedures.

efects and maintenance

50 The aftercare (defects and maintenance period) isitical to the success of any scheme and a maintenanceriod of no less than 3 years should be included.uring this period, maintenance should include plantotection (shelter guards, rabbit fencing etc.),atering, weed treatment, fertiliser and replacement ofiled plant material.

5/17


AGEMENT OFYSTEMS

Chapter 6Maintenance and Management of Vegetated Drainage Systems

6 MAINTENANCE AND MANVEGETATED DRAINAGE S

General

6.1 Vegetated Systems are likely to require a morefrequent level of inspection than conventional drainagesystems, as the growth of vegetation will need to beinspected and controlled to ensure the system continuesto operate as designed. For all vegetated systems amaximum inspection interval of six months isrecommended, preferably at the start and end of thegrowing season. Further inspections should be carriedout after any storm events equal to or greater than aone-year event. These are needed to check for signs oferosion or flooding, which would indicate whether thesystem has been affected by the storm. Advice shouldbe sought from an ecologist, or other appropriatelyqualified environmental specialist, about the inspectionof plants, their removal and trimming, the removal ofsediment and organic material, and the maintenance ofthe ecological health and operation of the systems. Inparticular advice should be sought concerning thepotential presence of protected species and breedingbirds.

6.2 In maintaining these systems, it will be importantto ensure that their primary hydraulic and treatmentfunctions are performing satisfactorily and protectingthe receiving surface watercourses or groundwater.Particular problems likely to be found include scouringof channels and the formation of rapid pathways thatallow the runoff to bypass the drainage systems.Conservation of the landscape and any habitats withinthe systems, whilst important, must not be allowed toimpede in any way the primary maintenancerequirements. It may, however, be possible to adapt theprogramme of maintenance to minimise disruption tohabitats and species. If the presence of protectedspecies or breeding birds is suspected, advice should besought from qualified personnel before maintenanceworks are planned and carried out. They will advisewhether a survey is needed and any special provisionsrequired. The statutory consultees listed in paragraph4.3 will be able to give guidance, and they should beconsulted if protected species are located.

6.3 Accumulation of sediment and plant waste islikely to occur in these systems. When removing suchdebris, care must be taken to avoid damage to flexibleliners below. Information should be provided tooperatives on the presence and depth of liners and on

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existence of any depth markers, which should belaced if damaged. Consideration should be given to impact that disturbance of the sediment will have on short-term migration of fines and contaminants from system and maintenance operations plannedordingly.

One purpose of a vegetated system is to trap andt contaminants in highways runoff. By definition,

refore, it is an area of land that is contaminated andl require appropriate de-contamination at the end ofuseful life, or whenever it becomes redundant.nsideration must be given to the disposal of sediment plant waste as these will retain contaminants from highway runoff. Sediment and plant waste may bessified as special waste or hazardous waste andposed accordingly. If there is doubt as to the status of waste the advice of the EPA should be sought. Inition, sediment and plant waste is likely to require-treatment prior to disposal at a landfill site. This take place either as the material is extracted or at landfill site itself.

The disposal of special or hazardous waste isensive and disposal facilities are limited, and theefits of testing, screening, separation andchanical de-watering of the sediments using mobilent, should be considered. This not only facilitates thearation of the materials into high and lowtamination levels (thereby minimizing disposalts) but also reduces the volume and weight of anyterial that has to be landfilled by removing excesster. The sand and pressed cake so produced is in am that can be accepted by landfill sites under the

s of the Landfill (England and Wales) Regulations2 which bans high moisture content wastes.

nagement Plan

A Management Plan/ Manual should be preparedet out a system’s objectives, formulate an annualgramme of maintenance operations and provideortunities to review the behaviour of the system.

is may be undertaken by inspection of the operationhe various elements of the system, by monitoring thenagement of pond vegetation and by follow uplogical surveys where warranted by the degree ofure conservation interest. Meaningful monitoring ofter quality to determine the effectiveness of the

6/1



system is a complex and costly operation and must notbe specified in the management plan without theagreement of the Overseeing Organisation. Themanagement plan should prescribe the variousmaintenance operations which may be required. Wherespecified operations are unable to be undertaken, forexample due to adverse weather conditions, the effectsshould be assessed and recovery measures indicated.Properly implemented, a management plan would help

tnfsitmai

Table 6

Table of Inspection and Maintenance Re

Swale Infiltration SFBasin

INSPECTIONS:

• Inflow/outfalls Quarterly or Quarterly Quafter each aft

• Integrity/erosion major storm ma

• Debris/rubbish

• Build-up of Annually Twice Ansediment or annuallyinvasive weeds

• Vegetation cover/ Monthly or Annually Anvigour after each

major storm

• Check for protected Specialist advice to be sought, as descspecies/breedingbirds

ROUTINE WORKS:

• Clearance of Monthly or Quarterly Qurubbish/debris after each

major storm

• Cutting vegetation Monthly or Annual 10after each andmajor storm

• Removal of plant N/A N/A N/Alitter

• Removal of To be To be Tosediment determined determined det

annually annually ann

6/2

o ensure the continuity and stability of a system’satural resources. Specific maintenance requirementsor particular types of system are suggested below andummarised in Table 6.1. The suggested requirementsn this table should be not be interpreted rigidly. Ashere is relatively little experience in the UK of

anaging vegetated systems on highways, a proactivepproach, based on local knowledge and site specificssues, is to be encouraged.

.1

quirements for Vegetative Systems

Wetland SSF Wetland Balancing Pond/Sedimentation Pond

arterly or Monthly or Monthlyer each after eachjor storm major storm

nually Annually Annually

nually Annually Annually

ribed in paragraph 6.2

arterly Monthly or Quarterlyafter eachmajor storm

year cycle 1-5 year cycle 5-10 year cycle and remove and remove remove

5-10 year cycle N/Aif required

be To be To be determinedermined determined annuallyually annually

May 2006



Swales

6.7 Swales are most efficient at treating water whenthe grass sward is dense and about 100 – 200mm long.Much shorter than that and the suspended solids in thewater will not be retained; much longer and the flow ofthe water will be too slow or the vegetation will becomeflattened, although in some locations this may not affectthe efficiency of the drainage. Regular mowing to100mm will therefore be needed. This may be three orfour times a season, depending on growth and thelocation. Appropriate machinery able to cut grassefficiently to this length will need to be selected. Grassshould not be mowed when ground conditions are wetand soft as this could compact soils, create ruts andresult in erosion. Removal of litter and debris will beneeded on a regular basis and swales should be checkedafter major storm events. The effectiveness of theswales will be greatly reduced if there is a build up ofdebris, as this will tend to create channels, rather thanthe even flow necessary for optimum efficiency.

6.8 Sediment removal may be required in particularfrom upstream areas of the swale and also at checkdams if these are present. Long term siltation over theremainder of the swale is unlikely to be a problem,although ‘crusting’ may be observed. This should beremoved mechanically and will result in subsequentneed to reseed. Swales should be inspected at least fourtimes a year for structural repairs, especially to the inletareas and side slopes where erosion may occur. Repairshould include infill, reshaping of the slopes andreinforcement if necessary. Bare areas should bereseeded and fertilised if necessary.

Infiltration basins and Trenches

6.9 To work efficiently these systems must be keptclear of debris. Signs of ponding will indicate thatinfiltration is not taking place and that cleaning out ofthe filter is needed. Litter and sediment removal will beneeded twice a year. This should include a visit in lateautumn if there is the possibility of leaf fall renderingthe system less effective. Slopes and spillways shouldbe checked twice annually to ensure there is no erosion,settlement, slope failure, tree growth, wildlife damageor vehicular damage.

SSF Wetlands

6.10 The maintenance of substratum hydraulicconductivity is essential in SSF wetlands and thisrequires regular inspection of the inflow and outflow on

May 2006

at least a monthly basis. The main threat appears to beclogging by silt and grease, hence the pre-treatmentsystem, a prerequisite of this type of wetland, needs tobe checked and cleaned/emptied. The design life ofconstructed SSF wetlands in the highway situation iscurrently unknown, but is anticipated to be between 15– 20 years where silt and grease inputs are minimal.After this period, the substratum may need to bereplaced. Inspection should involve taking cores toestablish the state of the substratum and identifyclogged or severely contaminated areas. There mayonly be a need to replace sections of the bed.

6.11 The reed vegetation may need to be cut and thestems removed to maintain vigour and prevent thebuild-up of excessive vegetation litter on the surface. Itis suggested that the vigour and litter build-up isreviewed annually and cutting takes place asappropriate. Cutting should be by hand and not machinein order to avoid damage to the substratum and itsporosity, and it is recommended that only half the bed iscut at a time. Where replacement substratum isprovided the area would be replanted.

SF Wetlands

6.12 The anticipated maintenance and inspection ofSF wetlands is anticipated to be less onerous than theSSF type. It is recommended that these systems areinspected on a quarterly basis to ensure the integrity ofthe system, especially after periods of heavy rain. Thedesign life of constructed SF wetlands is anticipated tobe 10 or more years, depending on size and level ofinputs. After this period, accumulated silts may need tobe removed and any ponds may require to be dredged.These should be undertaken on a cyclic basis perhapsinvolving only a third or a half of the system at a timethereby ensuring there is mature vegetation to promotesedimentation while the newly planted areas arebecoming established. In this type of system it isanticipated that the vegetation would not be cut. Wheresediment removal is undertaken, the area would bereplanted.

Ponds

6.13 Non-routine maintenance should includesediment removal from the base of the pond. The buildup of sediment will vary with local conditions, but as aguide, this operation may be needed every ten years.Sediment removal should be undertaken one third at atime to avoid removal of all vegetation.

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Volume 4 Section 2Chapter 6Maintenance and Management of Vegetated Draina

6.14 Cutback and removal of vegetation may berequired to retain the area necessary for maintenance oa permanent pool of open water and to preventexcessive build-up of organic detritus and nutrients.Periodic removal of vegetation may also increasegrowth rates and nutrient removal and also improvediversity of the flora when competitive species such asreedmace (Typha latifolia) threaten to dominate.

6.15 Consideration of vegetation removal should onlybe necessary at 5 – 10 year intervals for these purposesNot all the vegetation should be removed each time.The root systems of the plants should not be entirelydestroyed or removed as it will take a considerable timto re-establish. Instead roots or rhizomes may beremoved in small patches to promote new root/rhizomegrowth. Removal should follow a cycle allowing twothirds of the vegetation to remain on each occasion inorder that its treatment capacity is maintained. Removashould consist of a thinning process rather than removaof large stands of vegetation in order to avoid release otrapped pollutants and sediments and to preventerosion. The organic detritus is also an important sinkfor both metals and organic pollutants. Willow, alderand birch may all become established around the pondmargins and may need to be removed to preventshading of the treatment vegetation.

6.16 Structural Repairs to the inlet/outlet and otherexposed elements should be undertaken as soon asdamage is observed. Deterioration of these items islikely to be more rapid than usual (unless specialmeasures were taken during construction) due to thevariable water levels, which will create an aggressiveenvironment. Areas of erosion should be filled,compacted and reseeded as soon as possible. Erodedareas near inlets and outlets may require rip-rap fill toprevent further erosion.

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Management of the Vegetated System Landscape

6.17 Beyond the routine maintenance of a vegetateddrainage system, there may be opportunities to managethe broader landscape around the system in the interestof nature conservation and landscape visual amenity.This could include operations such as laying maturehedges or thinning/coppicing areas of regeneratingwoodland. Such operations, including routinemaintenance, would ideally be carefully prescribed inthe form of a written management plan produced withprofessional guidance, commencing from initial sitework through to longer-term operations for a period ofapproximately 10 years. Such an approach would helpto avoid any potential conflicts between maintaining theeffective functioning of the drainage systems with thatof nature conservation and visual amenity wherefeasible or appropriate.

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Chapter 7References

7. REFERENCES

1 Design Manual for Roads and Bridges(DMRB)

HD 33 Surface and Sub-surface DrainageSystems for Highways (DMRB 4.2)

HA 37 Hydraulic Design of Road EdgeSurface Water Channels (DMRB 4.2)

HA 39 Edge of Pavement Details (DMRB 4.2)

HA 41 Maintenance of Highway GeotechnicalAssets (DMRB 4.1)

HA 55 New Roads Landform and Alignment(DMRB 10.1)

HA 56 New Roads Planting, Vegetation andSoils (DMRB 10.1)

HA 78 Design of Outfalls for Surface WaterChannels (DMRB 4.2)

HA 106 Drainage of Runoff from NaturalCatchments (DMRB 4.2)

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

HA 118 Design of Soakaways (DMRB 4.2)

HA 119 Grassed Surface Water Channels forHighway Runoff (DMRB 4.2)

HA 216 Road Drainage and the WaterEnvironment (DMRB 11.3.10)

Environmental Assessemtn Techniques: Ecologyand Nature Conservation (DMRB 11.3.4)

Environmental Assessment Techniques:Landscape Effects (DMRB 11.3.5)

2 Trunk Road Maintenance Manual: Volume 2Routine and Winter Maintenance Code TRMM2

3 Water Framework Directive (2000/60/EC)

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Dorman M E, George T S, Hartigan J P &Quasebarth T F. (1996)Retention, detention and Overland Flow forpollutant removal from highway stormwaterrunoff US Department of Transportation: FederalHighway Administration report nos. FHWA-RD-96-095 Volume I (Research Paper) and FHWA-RD-96-096 Volume II (Design Guidelines).National Technical Information service (NTIS)

Nuttall PM, Boon A G & Rowell M R (1997)CIRIA Report 180: Review of the Design andManagement of Constructed Wetlands

Young G K, Stein S, Cole P, Kammer T,Graziano F & Bank F. (1996)Evaluation and management of highway runoffwater quality. US Department of Transportation:Federal Highway Administration report no.FHWA-PD-96-032

The Groundwater Regulations 1998 (SI 1998 No2746)

The Groundwater Regulations (Northern Ireland)1998 (Statutory Rule NI 1998 No 401)

The Construction (Design and Management)Regulations 1994 (SI 1994 No 3140)

Humphries R N & Walker A 1994: Modelling thefiltration of surface mineral extraction wastewater by reed bed treatment systems, pp 691 –701 5th International Minewater Congress,Nottingham

Moy F et al, 2003. Long term Monitoring ofPollution from Highway Runoff. EnvironmentAgency R&D report No. P2-038

Wong T H F et al, 1998. Managing urbanstormwater using constructed wetlands. IndustryReport 98/7, Co-operative Research Centre forCatchment Hydrology

Pontier H, 2002. An evaluation of combinedconventional and wetland systems for the controland management of road runoff. PhD Thesis,University of Portsmouth

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Chapter 7References

14 Younger P L et al , 2002. Mine Water: Hydrology,Pollution, Remediation. Kluwert AcademicPublications, Dordrecht

15 The PIRAMID Consortium, 2003. Engineeringguidelines for the passive remediation of acidicand/or metalliferous mine drainage and similarwaste waters. European 5th Framework RTDProject EVK1-CT-1999-000021 University ofNewcastle-upon-Tyne

16 Norrstrom AC & Jack G 1998 Concentration andfractionation of heavy metals in roadside soilsreceiving deicing salts. The science of the TotalEnvironment, 218: 161-174

17 Novotney V, Muehring D, Zitmer D H,Smith D W & Facey R 1998 Cyanide and metalpollution by urban snowmelt: impact of deicingcompounds. Water Science & Technology, 38:223-230

18 Ellis J, Shutes R and Revitt D, 2003 GuidanceManual for Constructed Wetlands R&DTechnical Report P2-159/TR2 EnvironmentAgency

19 Ellis J, Shutes R and Revitt D, 2003 ConstructedWetlands and Links with Sustainable DrainageSystems R&D Technical Report P2-159/TR1Environment Agency

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Chapter 8Glossary

8. GLOSSARY

Absorption substances taken into plant, soilor other material and loosely heldwithout binding or physical orchemical incorporation

Adsorption (of particles or droplets)collected, and sometimes bound,on surface of soil, vegetation orother matter

Aerobic in the presence of oxygen (air)

Anaerobic in the absence of oxygen (air)

Assimilation conversion into complexconstituents of the organism

Biodegradation decomposition by microbes

Biological Oxygen the rate of removal of oxygen byDemand (BOD) bacterial organisms using the

organic matter in water

Precipitation deposition of soluble substancein an insoluble form (e.g. oxide,carbonate)

Re-mobilisation (of metals) change to solubleform, or displacement as ionicform, from cation exchange siteson clay minerals or organicmaterials

Rhizome an underground plant stemwhich produces roots and shoots

Species:

Aquatic plants living wholly within thebody of water or directly on itssurface

Emergent plants which are rooted in waterand have parts (shoots, leavesand or flowers) which growabove the surface of the water

Marginal plants which live at the margin ofwater bodies

Sem

Subs

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i-aquatic plants which are capable ofliving in both water and on wetland

tratum subsurface material providingsupport for plants and/or filteringfunction and main pathway forwater flow (usually soil orgravel)

tilisation changing of liquid to gaseousphase

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9. ENQUIRIES

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

Divisional Director(Safety & Information)Highways AgencyRoom 4B, Federated HouseLondon RoadDorking A J PICKETTSurrey RH4 1SZ Divisional Director

Chief Road EngineerTransport ScotlandVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport WalesWelsh Assembly GovernmentCathays Parks M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Wales

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

Chapter 9Enquiries

ha 103/06 - standards for highways · ha 103/06 vegetated drainage systems for highway runoff...

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