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Suffolk Surface Water Drainage (SuDS) Guidance, Standards and Information

Appendix A to the Suffolk Flood RiskManagement Strategy

Suffolk Flood RiskManagement Partnership

March 2016

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SUFFOLK LOCAL FLOOD RISK MANAGEMENT STRATEGY –SUFFOLK SURFACE WATER DRAINAGE (SUDS) GUIDANCE, STANDARDS AND INFORMATION

CONTENT

1 Introduction 3

2 Before Developing a Layout 6

3 High Level Standards and Guidance 8

4 Key Details Most Likely to Affect Development Layout 15

5 Land Ownership, Adoption and Maintenance 19

6 When are Underground Drainage Systems Acceptable? 20

7 Local Flooding Information, Planning policies and processes. 21

8 Design Tools & Guidance in other Publications, 21

9 Sustainable Drainage Systems (SuDS) Adoption 24

10 SuDS Maintenance 24

11 SuDS Gallery and Lessons 25

Flood & Water Management Act 2010 Sustainable DrainageSystems (SuDS) definition:-“SUSTAINABLE DRAINAGE” means managing rainwater (including snow and other precipitation)with the aim of:

(a) Reducing damage from flooding,(b) Improving water quality,(c) Protecting and improving the environment,(d) Protecting health and safety, and(e) Ensuring the stability and durability of drainage systems.

AcknowledgementsCIRIA Susdrain for permission to use pictures of SuDS from their website.

Suffolk Flood Risk Management Partnership

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1. Introduction

One of the actions in the Suffolk Flood Risk Management Strategy (SFRMS) is to produce this local guide tosurface water drainage and SuDS. It should be read in conjunction with the National Planning PolicyFramework1, Planning Practice Guidance2 and DEFRA Technical Standards3. Section 3 of this documentsummarises the effect of these on requirements for SuDS.

Suffolk County Council’s (SCC’s) Protocol4 (Appendix C of the SFRMS) provides further information onplanning polices, processes and bodies who may potentially be involved with SuDS.

Planning Practice Guidance5 Paragraph 50 states: “Local authorities and developers should seekopportunities to reduce the overall level of flood risk in the area and beyond. This can be achieved, forinstance, through the layout and form of development, including green infrastructure and the appropriate

The SFRMS states:The guiding principles for SuDS in Suffolk will be: -

Early consideration of sustainable flood and coastal risk management in production of LocalPlans and master planning– promoting and protecting ‘blue and green corridors’.

Wherever possible, the use of multifunctional, above ground SuDS that deliver drainage,enhancement of biodiversity, improvements in water quality and amenity benefits.

Ensuring that land owners realise both the importance of reducing flood risk and howproperly designed sustainable drainage systems can be an asset to their development.

Ensuring no increase in flood risk from new development wherever possible and contributingto reducing existing risk if feasible.

Ensuring water flows around properties when the design capacity of drainage systems isexceeded by extreme rainfall.

“Water is an essential part of our natural and built environment. The way we live, work and play tovarying degrees are influenced by the availability and quality of water. Increasingly we need toembrace water management as an opportunity rather than a challenge.

Successfully delivered sustainable drainage provides communities and wider society with benefitsset within the context of adapting to climate change, development and improving our naturalenvironment.”

Extracted from ‘Planning for SuDS – Making it happen’ (CIRIA report C687, 2010)

1 http://www.communities.gov.uk/documents/planningandbuilding/pdf/2116950.pdf2 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/3 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-techni-cal-standards.pdf4 http://www.suffolk.gov.uk/flooding5 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/reducing-the-causes-and-impacts-of-flooding/

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application of sustainable drainage systems,through safeguarding land for flood riskmanagement, or where appropriate, throughdesigning off-site works required to protect andsupport development in ways that benefit the areamore generally.”

Paragraph 51 says: “SuDS are designed to controlsurface water runoff close to where it falls andmimic natural drainage as closely as possible, theyprovide opportunities to:

Reduce causes and impacts of flooding

Remove pollutants from urban runoff at source,

Combine water management with greenspace with benefits for amenity, recreationand wildlife.“

Open SuDS will also help improve water quality andecological status in our rivers and streams asrequired by the Water Framework Directive.

Good quality open SuDS require more design input,however construction costs are lower, SuDS aresafer to build and maintain, and SuDS will enhancehouse values.

See also:Kent CCs SuDS Master Planning Process Guidereferences: “Whitehead, Tim, Simmonds, David andPreston, John (2006) The Effect of Urban QualityImprovements on Economic Activity. Journal ofEnvironmental Management, 80, (1), 1-12”

http://www.landscapeinstitute.org/PDF/Contribute/Waterbriefing14February2014_000.pdf and

http://www.crew.ac.uk/sites/www.crew.ac.uk/files/publications/crew%20research-drainage.pdf

Open SuDS need to be part of a fully integratedapproach to urban design, the object of which isgood place-making. SuDS should bemultifunctional, contributing to landscape design,public open space strategies, and biodiversityenhancement.

The key to success is for architects or planners toinclude SuDS in their earliest sketches or layoutsinformed by drainage engineers as described insection 2. The following terminology should beused in all Flood Risk Assessements (FRA’s),drainage strategies and supporting drawing toavoid confusion:-

Land drains or field drains - undergroundporous pipes which are intended drain waterfrom the soil. Usually these discharge intoditches and are small - circa 100mm

Culvert - underground conduit usuallyconveying flow from one side of a road orrailway to the other. Possibly along roads

Watercourse - clearly defined open channel,often natural or ditch often dug by farmers

Flood path - flow path or exceedance routeacross the surface

Piped Watercourse - was once an openchannel, watercourse or ditch

Highway drain - pipe draining runoff frompublic highway

Highway carrier drain - pipe carrying onlyrunoff from the highway, usually from severalgullies and routed along the road

An Assessment of the Social Impacts ofSuDS in the UKhttp://www.susdrain.org/files/resources/evidence/HRW_social_impact_summary.pdf 6

found well designed and managed SuDSappear to have a positive effect on housesaleability and on house prices. In areaswith well-established ponds, there isperceived belief among the residents thattheir properties would fetch a 10%premium, along with an increase insaleability. Where houses were sited closeto poorly designed and / or maintainedponds, it was felt that the saleability andprice may be compromised.

6 http://www.susdrain.org/files/resources/evidence/HRW_social_impact_summary.pdf

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Swale picture taken from Essex CC SuDS guide

Road side rills inVicar’s CloseWells, Somerset,built in 1342

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Typical detail of a road served by a single swale.

Surface water runoff, from homes on right hand side, drain to a rill then into the swale.

2 Before developing a layout

The following should reduce costs and land take.

Contact SCC and Local Planning Authority(LPA) and adopting bodies as soon aspossible for discussion. Appropriate soakagetest results at this stage will be helpful.

Determine LPA requirements for open spacesand whether these may be suitable for openSuDS.

Start with a contoured plan showing existingponds or watercourses, drainage systems,trees and hedges.

Topography will dictate location of flood flowpaths (exceedance routes) and strategicSuDS. Site appropriate open spaces in theselocations.

Avoid siting buildings in valley bottoms - theywould be liable to flooding. It will probably bepossible to site some strategic SuDS in theseareas.

Ensure building thresholds are abovesurrounding ground. Avoid basements.(Design vision)

Plan road and housing layouts usingcontoured plans considering topography &SuDS together – as follows:

Identify locations for shallow SuDS close tosource (e.g. 400mm deep alongsideroads).

Roads can be served by a single swalerather than one each side.

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If ground conditions are not suited toinfiltration, swales should be used toconvey flows towards a suitabledischarge point. If these swales arevegetated and not too steep they shouldprovide several functions - ieconveyance, interception, infiltration,attenuation and treatment, thus reducingthe total SuDS requirement.

Site SuDs in verges or communal areas,design visual appearance to enhancethe development saleability & biodiversityetc.

Avoid using underground pipes andgrates to drain buildings or roads. Useshallow rills/channels or verges to helpensure downstream SuDS are shallow andtake up less space. This will also avoid theexpense of pipes, manholes and gullies &risks associated with deep excavations &confined spaces. Shallow SuDS arecheaper & safer to build and maintain.

Wherever possible roads and roadsideswales should follow contours. They will bemore efficient at storing and treatingrunoff. On steep sites SuDS need to be

carefully planned to avoid erosion andmaintain effectiveness.

Wherever possible avoid having homeslower than adjacent roads. Where landone side of road is lower use excavatedmaterial from adjacent swale to build upground levels.

Where roads or driveways cross swales,use them as check dams. Use crossingpipes to control flows.

With road side swales, roads will not needto have conventional longitudinalgradients and this will reduce cut and fill.

Avoid too much excavation, avoid disposalof excavated material off site - re-use closeby or create tumps. Create bunds to holdback water in valley floors rather thanexcavating deep basins. If material doesneed to be taken off site, consider ways inwhich it can be put to beneficial use.

Plan details such as traffic calming along withSuDS (e.g. site speed humps at high points inroads).

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As a minimum, SCC Flood and Water team will require the following information to be submitted for eachtype of application or stage within the planning process:-

Additional information may be required under specific site conditions or development proposals

7= LASOO, 2015. Non-Statutory Technical Standards for Sustainable Drainage: Practice Guidance [Accessible on-line at http://www.lasoo.org.uk/wp-content/uploads/2015/06/155639-SUDS-Booklet-A4_HR.pdf]

(LASOO,2015)7

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SuDS design is an iterative process requiring severaldesigns to be produced. The initial preliminarydesign will determine the approximate size of theSuDS system. This design should aim tooverestimate by 10-20% the scale of the likely finalSuDS scheme to allow for errors/omissions. The firstiteration should identify areas where additionaldevelopment could be added. (or perhaps areaswhere insufficient space is provided for SuDS)

Subsequent designs will refine the SuDS design anddevelopment potential. The design submitted witha planning application should normally be the finaldesign. For outline applications the initial preliminarydesign described above may be sufficient.

CIRIA C687 “Planning for SuDS - Making it happen(2010)8 is free and provides useful guidance.

Watercourse at Westerfield. Side slopes too steep! Flood storage basin at Ravenswood

3. High Level Standards and GuidanceExtracts taken from the National Planning PolicyFramework9, Planning Practice Guidance10 and DEFRATechnical Standards11 at 30 March 2015 are includedin the table below. However these documents are likelyto be updated from time to time. Where critical,developers should check NPPF, PPG or TechnicalStandards using the above links. SCC welcomes anypre-application discussion on any issues.

Other documents which may be relevant are listedin Section 8 as well as BS 8582 2013 (Code ofPractice for Surface Water Management forDevelopment sites)

As at April 2015, for major developments the effect ofthe above standards and guidance is SuDs should:

Not increase flood risk off site (in all events upto 100 year return period);

Provide adequate standards of floodprotection on site - in most cases no floodinginside buildings in events up to a 100 yearreturn period and no flooding in other areas(apart from designated flood paths /storageareas) in events up to 30 year return period.

Take account of the construction, operationand maintenance requirements of bothsurface and subsurface components, allowingfor any personnel, vehicle or machineryaccess required to undertake this work.

Allowances for climate change to beincluded seehttps://www.gov.uk/government/uploads/system/uploads/attachment_data/file/296964/LIT_8496_5306da.pdf

8 http://www.ciria.org/ItemDetail?iProductCode=C687&Category=BOOK&WebsiteKey=3f18c87a-d62b-4eca-8ef4-9b09309c1c919 http://www.communities.gov.uk/documents/planningandbuilding/pdf/2116950.pdf10 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/11 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-techni-cal-standards.pdf

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SuDS specifically for improving water quality indownstream watercourses are most likely to berequired when runoff from significantdevelopments drain to small watercourses. Theprecise requirement would depend onenvironmental impact assessments and the RiverBasin Management Plan.

However, it is good practise to maximise treatment,amenity and biodiversity potential by using multi-functional open SuDS close to source. These willalso reduce capacities needed for attenuationand volume control.

The decision on whether a particular form or typeof sustainable drainage system would beinappropriate in relation to a particulardevelopment proposal, is a matter of judgement

for the local planning authority, based on advicefrom SCC including on what sort of sustainabledrainage system they would consider to bereasonably practicable.

The judgement of what is reasonably practicable isby reference to the Technical Standards publishedby the Department for Environment, Food andRural Affairs and takes into account design andconstruction costs which include maintenanceand lost opportunity costs. (NPPG para 082).

SCC has found that an open SuDS system canoffer a viable solution to many forms ofdevelopment.

Additional local guidance in (blue cells) andreferences to key elements of NPPF and PPG follows:

Requirement, or Guidance Source

When should SuDS be considered?

…..when considering major development, sustainable drainage systems shouldbe provided unless demonstrated to be inappropriate…..

Paragraph 079 ofPPG12

…expecting compliance with the technical standards is unlikely to be reasonablypracticable if more expensive than complying with Building Regulations -provided that where there is a risk of flooding the development will be safe andflood risk is not increased elsewhere. Similarly, a particular discharge route wouldnot normally be reasonable practicable when an alternative would cost less todesign and construct.

Paragraph 083 ofPPG13

Suffolk County Council view “a drainage system complying with BuildingRegulations” as being compliant with Sewers for Adoption 7th Edition, appropriateDistrict Council Local Plan Policies, the NPPF and Suffolk Local FRM Strategy. Suchsystems must not increase flood risk off site and will therefore usually needattenuation measures. The standards for flooding on the site can only bereduced if no runoff leaves the site in all events up to a 100 year RP.

SCC

Construction costs can include the opportunity cost of providing land for adrainage system above ground where the land utilised for the drainage system isnot also utilised for another land use. Design costs also include the resultingmaintenance and operation requirements arising from the design.

Paragraph 084 ofNPPG14

12 13 14 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/reducing-the-causes-and-impacts-of-flooding/why-are-sustainable-drainage-systems-important/

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Construction costs to include, where appropriate, preliminaries, disposal of surplusexcavated material, imported fill to trenches under roads, top soil stripping andreplacement, temporary works such as pollution control ponds, dealing withground water, trench support, design, supervision and adoption fees.

Maintenance costs to assume 100 year life, 3.5% discount rate and includereplacement of components with life expectancy <100 years and administrationcosts.

Rates and estimates should be agreed with SCC pre application. Potentialsources and tools include UK SuDS tools, SPON, DEFRA report WT1505 or SCCHighways Partnership schedules.

SCC

Runoff Destination

Generally, the aim should be to discharge surface run off as high up the followinghierarchy of drainage options as reasonably practicable:

1. into the ground (infiltration); 2. to a surface water body;3. to a surface water sewer, highway drain, or another drainage system;4. to a combined sewer.

Particular types of sustainable drainage systems may not be practicable in alllocations.


and Part H3 ofBuilding regulations

Contact SCC and / or see SFRA for further advice. It may be possible to drain sitesclose to large estuaries or the coast directly to those water bodies

Developers may wish to consider collection and reuse of surface water or groundwater.

SCC

Suffolk County Council will normally require full details of soakage tests includingsite records, location plans and calculations. Needs should be discussed withSCC at the pre app stages. Soakage rates need to be above about 5-10 mm/ hrfor infiltration to be the sole means of drainage.

CIRIA C753 – Chapter 25 describes requirements for soakage tests – large scaletest pits similar in size /proportion and depths to proposals, following BRE365.Requirements should be agreed with SCC.

SCC or District Council SFRA maps may indicate where infiltration drainage is likelyto be possible.

SCC

When should SuDS be considered? Continued

Continued

15 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/reducing-the-causes-and-impacts-of-flooding/why-are-sustainable-drainage-systems-important/

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Soakage rates for design will need to be reduced by an appropriate factor as setout in CIRIA Report 156 –table 4.6, reproduced below:

Consequences of failure

Suffolk County Council will not normally permit surface water discharges fromdevelopments into existing land drains, highway drains or piped watercoursesunless they have been constructed to an acceptable standard, have provenadequate capacity and clearly defined maintenance responsibilities. Owners ofsuch drainage will need to agree to connections.

SCC

Flood risk outside the development

See paragraph 103 - flood risk not be increased elsewhere… NPPF16, LocalPlanning policies &Suffolk Flood RiskManagementStrategy 2016

See S1 - no need to control peak flows or volumes discharged to the sea or largeestuary.

DEFRA TechnicalStandard17

Suffolk County Council expects drainage designs to take account of tidal orfluvial water levels where appropriate in designs. The UK SuDS Tools Web site - HRWallingford http://www.uksuds.com/ joint probability analysis tool will assist.

SCC

Peak Flow Control

See S2 and S3 …Green field or previously developed sites DEFRA TechnicalStandard18

Area drained(sq m)

No damage orinconvenience

Minorinconvenience –eg: flooding ofcar park

Damage to buildingsstructures or majorinconvenience egroads

<100 1.5 2 10

100 -1000 1.5 3 10

>1000 1.5 5 10

Runoff Destination Continued

16 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/6077/2116950.pdf17 18 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-technical-standards.pdf

Continued

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Peak Flow Control Continued

SCC expects green field runoff to be determined as described in theEnvironment Agency document SC030219, “Preliminary rainfall runoffManagement for Developments”, (2013) and the SuDS Manual. See:

https://www.gov.uk/government/publications/rainfall-runoff-management-for-developments

SCC recommends that discharge is restricted to Qbar or 2l/s/ha for all eventsupto the critical 100yr+CC.

The UK SuDS tools website; http://geoservergisweb2.hrwallingford.co.uk/uksd/index.htm includes tools forsurface water storage requirements, rain water harvesting tank sizing (andcapability for flood management), site evaluation, SuDS costs estimator, jointprobability analysis (for drainage with tidal or fluvial outfalls), soakaway designand a water quality calculator.

Green field rates should not include an allowance for climate change. Rainfallused to design the SuDS will need to be increased to allow for climate change.

Impermeable areas to include allowances for future added paving, extensions or verge hardening.

Where a site is previously developed, the proposed runoff rates should berestricted as close to the greenfield rates, or at the very minimum a bettermentof at least 30% should be considered over the Brownfield runoff rates – as persection 3.2.2 in Ciria SuDS Manual C753.

CIRIA SuDS ManualC754

Volume control

See S4,S5, S6 … volume discharged in 100 year Return period, 6 hour durationevents to highway drains, sewers or SW bodies not to be increased..…….


SCC will not normally accept flow control throttles with less than a 100 mmopening. However, smaller apertures may be accepted for carefully designedoutlets from beneath permeable pavements or if special measures are in placeto help prevent blockage. Where volume control requires a smaller throttle, thenthe volume control requirement may be waived.

Where the proposed discharge rate is greater than 2l/s/ha or Qbar for peak flowcontrol, there must be a separate area for volume control. Also known as LongTerm Storage (LTS) this must be provided on the site to counter the excess volumecreated by new impermeable surfaces. Volume control or Long Term Storagemust be discharged from the site at 2/l/s/ha even if a higher rate is permitted forpeak flow control. SCC recommend that for all sites discharging to awatercourse, the final permitted discharge rate for the entire site is 2l/s/ha or Qbarfor all events upto the 1in 100+CC event – this then accounts for any volumecontrol needed as per section 3.2 in EA document20.

SCC

19 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-techni-cal-standards.pdf20 http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/Rainfall_Runoff_Manage-ment_for_Developments_-_Revision_E.sflb.ashx

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Water quality

Para 109: planning system to contribute to and enhance local environment…..minimise impacts on biodiversity, preventing development from contributing tounacceptable risk of water pollution….

Para 120: …. effects (including cumulative) of pollution to be taken into account

NPPF23

See Paragraph 001,

Paragraphs 10, 11, River Basin Management plans, WFD

Paragraphs 16-18 -19……. Early engagement with LPA and EA to establish if WQis a significant planning issue… assessing impacts … environmental statement…..good status .. water framework directive etc.

Planning PracticeGuidance24

Planning PracticeGuidance 1025

Planning PracticeGuidance 16-1926

….Guidance on avoiding pollution and compliance with the law can be foundon a range of Pollution Prevention Guidance (PPG) notes originally produced bythe EA. The EA may take enforcement action against polluters, which couldinclude prosecution…

Topics include basic good environmental practises, oil separators, works nears orin watercourses, vehicle washing and cleaning.

Pollution preventionguidance27

One of the guiding principles for SuDS in Suffolk is “Wherever possible multifunctional above ground SuDS that deliver drainage,enhancement of biodiversity, improvements in water quality and amenitybenefits” should be used

The drainage system should be designed and constructed so surface waterdischarged does not adversely impact the water quality of receiving waterbodies, both during construction and when operational.

Surface runoff should be managed on the surface where it is reasonablypracticable to do so and as close to its source as is reasonably practicable.

Suffolk Flood RiskManagementStrategy 2016

scc

Continued

21 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/6000/2115548.pdf22 http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/Rainfall_Runoff_Manage-ment_for_Developments_-_Revision_E.sflb.ashx23 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/6077/2116950.pdf24 http://planningguidance.planningportal.gov.uk/blog/guidance/water-supply-wastewater-and-water-quality/water-supply-wastewater-and-water-quality-introduction/25 http://planningguidance.planningportal.gov.uk/blog/guidance/water-supply-wastewater-and-water-quality/infor-mation-about-the-water-environment/26 http://planningguidance.planningportal.gov.uk/blog/guidance/water-supply-wastewater-and-water-quality/water-supply-wastewater-and-water-quality-considerations-for-planning-applications/27 https://www.gov.uk/government/collections/pollution-prevention-guidance-ppg

Flood risk within the development

See S7, S8 & S9 ….. No flooding apart from designated areas in 30 year returnperiod rainfall event. No flooding inside buildings in 100 year return period,exceedance flows to be managed/routed as far as reasonably practicable ….


Where appropriate, (especially when designing SuDS for developments behindtidal defences), rising sea levels over the life of the development need to be takeninto account. Sea levels should be set out in SFRAs available on LPA web sites andannual allowances for sea level rise in the Technical Guidance to the NPPF21

SCC

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Watrer quality Continued

Water quality treatment components should be designed to ensure that theyfunction effectively during rainfall events more frequent than the 1 in 1 yearrainfall event.

The National SuDS tools web site, CIRIA C753 and Interim Non-Statutory Standardsfor Sustainable Drainage (SuDS) in Wales (consultation 12 Feb-30 April 2015)provide guidance on treatment.

scc

Where possible, interception storage should be provided to trap the first 5mm ofrainfall (CIRIA C793 para 4.3.1). This can take the form of initial losses into theground, vegetated surfaces or long term storage (semi-permanent pools).

Interim guidance:Green roofs assumed to absorb 5mm

Vegetated surfaces which are normally dry but receive runoff can be assumedto absorb up to 125 mm depth of water depending on permeability andsteepness:

Bases of swales flatter than 1 /100, where soil permeability k< 1X10 -6 m/sec canabsorb 25mm

Bases of swales flatter than 1 /100, where soil permeability k> 1X10 -6 m/sec canabsorb 125mm

Flat parts of detention basins absorb 25mm or 125mm depths dependant onpermeability.

Volumes intercepted can be increased by raising the level of the outlet.

Permanent ponds with constant controlled water levels provide no interception.

Interception can reduce volumes needed for controlling peak flows andvolumes.

Treatment storage in the form of a pond which retains water a volume of water=Vt during dry weather should also be provided where Vt = volume from at least15mm of rainfall. Various numbers of treatment stages are required dependingon pollution risk. See http://www.uksuds.com/Conveyance Swales can beassumed to be a treatment stage if velocities are <0.3m/s in a 1 year RP event.(usually flatter than a 1/100 longitudinal gradient). Swales might include semi orpermanent pools providing some interception.

Structural integrity

See S10 & S11 - components designed for anticipated loading over design lifetaking into account requirement for reasonable levels of maintenance.

Materials, products components etc. to be suitable nature and quality…..


Continued

28 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-technical-standards.pdf

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Structural integrity Continued

SCC suggests Sewers for Adoption 7th Edition provides suitable standards for mostmaterials/workmanship.

SCC is unlikely to accept plastic geo-cellular storage units where vehicle loadsmay be applied (this includes tractors/grass cutters) or where trees are plannedabove or within 5m. Details for any submission including geo-cells need toinclude the manufacturers assurances regarding life expectancy and structuraldesign calculation for the geo-cells and surrounding geotextile fabric. See CIRIAGuide C680 –Structural design of modular geocellualr tanks.

Erosion protection measures must be included where flow velocities exceed 1m/s in a 100 year event and at any locations where turbulence is likely such asinlets, weirs etc

Side slopes of swales, basins ponds etc. to remain stable (and safe) –see detailsin section 4.

SCC

Designing for maintenance considerations

See paragraph 085 …. designs need to take account of construction andmaintenance….access, climate change, increasing impermeable area …..maintenance and operation requirements…. economically proportionate.


See S12 regarding Pumping. DEFRA TechnicalStandard30

SCC will not accept underground storage which cannot be cleaned usingstandard jetting /vacuum equipment and will not normally accept confinedspaces which require man entry and pose potential risks to operatives.

For all applications SCC will require a copy of the CDM Health and Safety Planincluding risk assessments.

SCC

Construction

See S13 and S14 … connection to drainage systems… damage to drainageresulting from construction to be rectified.

See site handbook for the construction of SuDS (CIRIA C698)

Site management plans will need to address issues such as protection ofinfiltration basins from compaction by construction traffic, need to ensurevegetation is established in open SuDS before water is allowed in, keepingsediments/mud out of SuDS – especially soakaways and permeable paving etc.


29 http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/reducing-the-causes-and-impacts-of-flooding/why-are-sustainable-drainage-systems-important/30 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-techni-cal-standards.pdf31 https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-techni-cal-standards.pdf

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Different adopting bodies may have slightly varyingstandards for details. The following are likely toapply if SuDS are adopted by SCC Highways. Insuch cases materials and workmanship should beagreed with SCC Highways.

The maximum depth of water in open publiclyaccessible SuDS, which are normally dry, is 500mmin a 100 year return period (RP) event - typically thisequates to about 300mm in a 10 year RP event.

Reason: The Flood Risks to People Report FD2321EA/DEFRA (March 2005) classified flood hazards interms of flood depth and velocity. Static waterbetween 0.25m deep and 0.5m deep is classedas a “danger to some” (e.g. children). Ipswich BC’sdevelopment and Flood risk SPD accepts suchdepths on access routes for residentialdevelopments in FZ2 or 3 when the frequency thisis reached is 50-200 years

The above maximum allowable depths of watermay be reduced if velocities are high - For a 0.5-2m/sec velocity a depth of only 0.25m represents a“danger to some”.

See also CIRIA RP992 paper RP992/17 Nov 2013 -“Health and Safety principles for SuDS Frameworkand Checklists” – this describes needs for riskassessments for permanent or deeper water aspart of the Design process. The adopting body willneed to approve these.

Side slopes to open SuDS must be stable and allowsafe maintenance, access and escape. The slopewill normally also depend on the maximum depthof water in a 100 year return period (RP):

Water 0.5m – 0.3m deep - slope no steeperthan 1 in 4

Water 0.3m - 0.2m deep - slopes 1 in 4 to 1in 2 may be acceptable

Water <0.2m deep. Vertical -e.g. kerb faceof car park allowed to flood.

Side slopes may need to be flatter to avoid erosionwhere paved areas drain directly down grassedslopes or where the slope is intended to act as afilter strip.

Access to all SuDs to enable maintenance, repairsor replacement – minimum 3.5m wide fromadopted highway to SuDS; suitable for access bytanker/jetting machine, excavator or dumper asappropriate. See CIRIA C793 for further guidance.Sufficient headroom under trees or bridges needsto be provided.

For large SuDS basins with permanent water, a 3.5metre minimum width access (bench) around theperimeter is required with a maximum cross fall of1 in 15.

Soakaways to be > 5m from highway kerbs and>5m from buildings. Infiltration systems notnormally permitted under public highways butpermeable paving might be acceptable for somepublic highways.

Clearance between SuDS and buildings generally>5m but might be less if SuDS are impermeable.

Large trees are not normally acceptable wheresheet flows enter swales (i.e. in filter strips) but maybe acceptable on other side slopes.

Trees to be at least 3 m or > maximum canopyradius from underground drainage.

The minimum aperture on a flow control shouldnormally be 100mm, however this may bereduced if there is low risk of blockage – e.g. a flowcontrol for the outlet from beneath permeablepaving. Strategic flow controls should also normallyinclude a bypass valve that can be opened tolower the retained water level and enable theblockage to be easily removed. A high level

4. Key details most likely to affectdevelopment layout

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overflow is normally required to ensure a blockagedoes not result in flooding of buildings and helpcater for exceedance flows.

Where SuDS have to cross under highways, pipesshould be used with the minimum acceptablecover as shown in the following table. Multiplesmall pipes will be shallower than a single largepipe.

Alternatively it might be possible to provideshallower pipes or culverts in conjunction with trafficcalming rumble strips, with flood exceedanceroutes on the surface across the road.

SCC may accept crossing pipes designed as“inverted syphons” i.e. the invert of the pipe isbelow the swales. This enables swales either side ofthe road crossing to be 300-400mm deep. Thepipes should be at a gradient of say 1/50 fallingdown towards a sump which also prevents erosion.The levels should be set so floating oil is trapped inthe pipe. When the sump is emptied any floating

material will also be removed. The sump mayrequire a gulley grate cover.

SuDs crossing under a road at Upton, Northampton (picture from CIRIA Susdrain).

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The new regime for approval of surface waterdrainage from 6 April 2015 allows a range ofbodies to adopt and maintain surface waterdrainage systems. The following describes what areanticipated to be the usual arrangements.

Where open SuDS serve more than 1 property:

SCC Highways or perhaps Anglian Wateradopt SuDS in highway verges. These SuDSwill drain roads as well as private properties.Developers should contact SCC pre-application. E.g.a cross-falled road with nogullies or highway drains, draining directly intoa swale, potentially offers savings inconstruction and maintenance costs.

Anglian Water, LPA, or ManagementCompany adopts SuDS in Public OpenSpace (POS) owned by LPA or ManagementCompany.

Where a SuDS is within the private curtilage of aproperty (e.g. source control components such as raingardens, water butts and soakaways) the responsibilityof these assets is usually the individual propertyowner.These would normally need to be designatedunder the Flood & Water Management Act.

In general where SuDS receive runoff directly frompublic highway then SCC Highways would maintainmeasures to keep vehicles out of SuDS orstructures, including kerbs and channels thatsupport the highway and clear sediment traps, flowcontrols, de silt, repair erosion damage.

Where SuDS are sited in POS then it is possible abody would maintain elements of SuDS apart fromthose the Parks Department of the District/BoroughCouncil who would normally maintain if there wereno SuDS (e.g. normal grass cutting). In such casesParks Departments may also have requirements forslopes of basins or swales as well asplanting/landscaping.

Different adopting bodies may have slightlydiffering standards for access, slopes or depthswhich ultimately affect spatial requirements.

If an underground surface water system is acceptedby SCC it would normally need to comply withSewers for Adoption 7th edition (SFA7) and beadopted by Anglian Water. Normally pipes would beunder highways and any within open spaces wouldneed easements and adequate clearance frombuildings, trees and bushes as laid out in SFA7.

Before planning applications are made, as part ofthe decision process on the form of SuDS,developers should discuss and agreemaintenance options and costs with SCC, AnglianWater or other potential adopting bodies. NationalPlanning Practice Guidance require the drainagesystem to be designed to ensure maintenanceand operation requirements are “economicallyproportionate”. This could affect layout andsucceeding design stages of the development.

S106 commuted sums would be required by localcouncils for maintaining POS and or SUDS.

Management Companies should be funded byannual charges on owner occupiers but mayrequire some set up costs.

SCC Highways might adopt roadside swales withno commuted sum when their future costs are lessthan the costs of maintaining highway gullies,catchpits and pipes. They normally requirecommuted sums for soakaways or other SUDSdraining highways which are obtained through s38agreements.

5. Land Ownership, Adoption andMaintenance

High maintenance and replacement costs(e.g. for schemes involving undergroundstructures with limited lives) are likely toresult in such options being unviable.

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SCC will, through its new statutory consultee role inthe planning system for major developments andminor developments at high flood risk, seek toachieve surface water drainage that is compliantwith the NPPF, PPG, and DEFRA’s TechnicalStandards as well as guiding principles in the SuffolkLocal Flood Strategy i.e. wherever reasonablypracticable open multifunctional SuDS that deliverdrainage, enhancement of biodiversity,improvements in water quality and amenity benefit.

The most common situation where undergroundSuDS are likely to be accepted is whereunderground domestic soakaways serveconventional residential development denser thanabout 30 units per Ha (subject to local groundconditions, satisfactory design standards andsufficient space being available). Residentialgardens usually need to be a minimum of 9m longto provide the normal 5mclearance betweensoakaways and buildings.

For very dense development in urban areas orother development on steep slopes it may also beobvious that open SUDS are not viable. However ifsuch a development drains to a watercourse thenwater quality objectives would probably only beachieved through the use of some open SuDS.

Even if open SuDS are used it will be acceptable touse some pipes where necessary. e.g. where roadsor driveways cross.

Criteria for SCC or a developer to form ajudgement on whether other proposals forunderground drainage would be acceptable forthe majority of the surface water drainage systemare set out in PPG para 82. These are: constructioncosts, maintenance/replacement costs and lostopportunity costs.

Lost opportunity costs are the loss of profitsassociated with land required for SuDS that is notmultifunctional public open space. So LPA LocalPlan Policies for open space, garden size, housingdensity, size and numbers of homes are importantfactors in such judgements. In addition the costs ofmaintenance (and, where appropriatereplacement, over a 100 year life for residentialdevelopment) should also be” economicallyproportionate” by reference to surface watercharges incurred by consumers served by AnglianWater’s sewerage systems.

SCC has an Excel workbook developed to look atthese issues.

Open SuDS systems which take up more land thanrequired to be open spaces by LPA policies can befound to be viable.

Any proposals where the majority of the systemwould not be open SuDS will normally need to bedesigned along with an alternative design for openSuDS following the principles listed in Section 2.Costs and intangible benefits such as biodiversityand water quality for the two options will becompared to enable SCC to make commentsand recommendations on acceptability thatwould be passed on to the LPA.

Early discussion with SCC is essential to ensure costsand rates for construction, replacement andmaintenance used in such estimates areacceptable.

6. When are underground drainagesystems acceptable?

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SCC’s Protocol for advising Local PlanningAuthorities on surface water drainage aspects ofplanning and development control (Appendix C ofSuffolk Flood Risk Management Strategy) includesrelevant policies and describes useful informationthat can be supplied by SCC, such as floodrecords and maps as well as guidance on

drainage information that needs to be included inplanning applications. It also describes theplanning process for SuDS.

Strategic Flood Risk Assessments produced by LPAswill often provide information that needs to beconsidered when designing SuDS

7. Local flooding information, planningpolicies and processes

8. Design tools and guidance in otherpublications

This guide has been kept brief by avoiding repeating additional guidance listed in the following table.

National Planning Policy Framework, Communitiesand Local Government, March 2012

http://planningguidance.planningportal.gov.uk/blog/policy/

Planning Practice Guidance to the NationalPlanning Policy Framework, Communities andLocal Government, March 2014

http://planningguidance.planningportal.gov.uk/blog/guidance/flood-risk-and-coastal-change/

Technical Guidance to the National PlanningPolicy Framework (2012) (now Archived)

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/6000/2115548.pdf

Flood and Water Management Act 2010 http://www.legislation.gov.uk/ukpga/2010/29/contents

Suffolk Local Flood Risk Management Strategy 2016 www.suffolk.gov.uk/flooding

SCC Protocol for Advising LPAs on Surface WaterFlood Risk Aspects of Planning & DevelopmentControl

www.suffolk.gov.uk/flooding

Suffolk Local SuDs Guide www.suffolk.gov.uk/flooding

Environment Agency document SC030219,“Preliminary rainfall runoff Management forDevelopments”, (2013

https://www.gov.uk/government/publications/rainfall-runoff-management-for-developments

UK SuDS Tools Web site - HR Wallingford http://www.uksuds.com/

Continued

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Susdrain web site provides many guides andreports and case studies including the following:

http://www.susdrain.org/

Planning for SuDS – Making it Happen CIRIA C687freely

http://www.susdrain.org/resources/ciria-guidance.html

SuDS Manual CIRIA C793 http://www.susdrain.org/resources/ciria-guidance.html

Site handbook for the construction of SuDS (C698 http://www.susdrain.org/resources/ciria-guidance.html

Managing urban flooding from heavy rainfall -encouraging the uptake of designing forexceedance - Case studies (C738c)


Designing for exceedance in urban drainage:Good practice (C635)


Model agreements for sustainable drainagesystems (C625)


DEFRA report WT1505 SuDs Nov 2013

This compares construction, maintenance and lostopportunity costs of underground and open SuDS

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Frandd.defra.gov.uk%2FDocument.aspx%3FDocument%3D11852_FinalIssueSWDReport_November2013.pdf&ei=qdksVYzvL4WgPYPRgcgK&usg=AFQjCNHl_jBZL-BPh5Q5WrQu-ksy7O60bg&bvm=bv.90790515,d.ZWU

CIRIA paper RP992-17 Health and Safety Principles To Be confirmed

Source Control Using Constructed PerviousSurfaces. CIRIA C582 (2002)

Using SuDS Close to Buildings CIRIA http://skeletank.co.uk/wp-content/uploads/downloads/1209_fact_sheet_using_suds_close_to_buildings.pdf

CIRIA Report 156, Infiltration Drainage – Manual ofGood Practice

http://www.ciria.org/ItemDetail?iProductCode=R156&Category=BOOK

Soakaway Design Digest 365, Building researchEstablishment, 2007

http://www.brebookshop.com/details.jsp?id=844

Ground Water Protection: Principles and Practice, TheEnvironment Agency, Nov 2012, updated Aug 2013.

https://www.gov.uk/government/publications/groundwater-protection-principles-and-practice-gp3

Continued

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Towards Sustainable Water Stewardship –SuDSadoption manual, Anglian Water 2011

http://www.anglianwater.co.uk/_assets/media/AW_SUDS_manual_AW_FP_WEB.pdf

AW website – SuDs http://www.anglianwater.co.uk/developers/suds.aspx

AW web site - sewer requisitioning http://www.anglianwater.co.uk/developers/new-sewer.aspx

Building Regulations Part H

Sewers for Adoption 6 th Edition 2006, WRc (7thdue October 2012),

http://sfa.wrcplc.co.uk/

Ipswich Drainage and Flood Defence Policy, 2002 https://www.ipswich.gov.uk/sites/www.ipswich.gov.uk/files/Ipswich_Flood_Defence__Drainage_Policy_rev_Aug_20091.pdf

Ipswich Local Plan https://www.ipswich.gov.uk/taxonomy/term/588

IBC Level 2 Strategic Flood Risk Assessment, May2011

https://www.ipswich.gov.uk/content/strategic-flood-risk-assessment-sfra

Anglian River Basin Management Plan https://www.gov.uk/government/publications/anglian-district-river-basin-management-plan

EU Water Framework Directive, EEC directive2006/60/EC

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32000L0060:EN:NOT

Guidance on the permeable surfacing of frontgardens, Communities and Local Development

http://www.communities.gov.uk/publications/planningandbuilding/pavingfrontgardens

The Town and Country Planning (DevelopmentManagement Procedure) (England) Order 2010

http://www.legislation.gov.uk/uksi/2010/2184/schedule/5/made

Designing for Flood risk RIBA 2nd Edition http://www.architecture.com/Files/RIBAHoldings/PolicyAndInternationalRelations/Policy/Environment/2Designing_for_floodrisk.pdf

Flood Risks to People Phase 2 FD2312/TR2. DEFRAand Environment Agency (March 2006)

http://sciencesearch.defra.gov.uk/Default.aspx?Menu=Menu&Module=FJPProjectView&Location=None&ProjectID=12016&FromSearch=Y&FieldOfStudy=12&SearchText=FD2321&ShowDocuments=1&SortString=ProjectCode&SortOrder=Asc&Paging=10&FJP=1

Flood Risks to People Phase 1 FD2317 TR1 http://www.espace-project.org/part1/publications/reading/espace/docs/flood risks to people Final report in EA format.pdf

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Maintenance plans will normally be a planningrequirement. Plans should include schedules whichspecify when maintenance items are due. Owners/ maintainers of SUDS should record when theseactions are undertaken.

SCC will request the LPA to include planningconditions which prevent construction until SuffolkCounty Council have received and approved acompleted electronic copy of the asset registertemplate. (appended to back of this document).

Information supplied must include location,ownership and maintenance agreement details ofsurface water drainage including SuDS basins,swales, soakaways, pipes carrying surface water orground water and exceedance paths.

SCC will add SuDS records to its GIS system in orderto assist with its duty to investigate and reportflooding instances. If necessary, maintenancerecords / plans will be investigated andenforcement action by the LPA may be requested.

9. SuDS Adoption

The process and requirements will depend on whoadopts and maintains. Some of the above linksprovide details perhaps including model forms ofagreement, and requirements for supervision,period of maintenance, completion certificateand performance bond.

SCC expects that developers to undertakemaintenance until SuDS are adopted.

Examples of previous model adoption agreementscan be found:http://www.susdrain.org/resources/ciria-guidance.html

10. SuDS Maintenance

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11. SuDS gallery and lessons

(Above). Retrofit newly completed SuDS at HeathRd, Ipswich. Shallow basins rutted and damagedin wet weather by cars parking on new verges.

Heath Road, Ipswich with deterrent posts andplanting. This is with about a year of growth andthere is no evidence of parking on the verge.Timber posts have been utilised on thecarriageway side along with deterrent plantingon the footway side. A small gravel trench wasconstructed to provide additional storage andsoakage. The use of slimmer flexible “posts”,which would not rot could be investigated.

Poor inlet design causes erosion and depositionof sediment and a reduction in infiltrationcapacity.

The failure of a plastic geocellular tank under acar park in August 2012. The cause of the failureis not currently known. There are documentedexamples of failure of the plastic units andsurrounding geotextile failure It can be difficultto obtain sufficient design data to properlydesign these systems.

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An infiltration basin at Ravenswood is being usedas a BMX track. A small reduction in infiltrationcapacity has occurred due to compaction.Bunds or dividing walls need to be robust tocontinue to function. Consider “desire lines”when planning.

No road gullies (verge drainage or filter strip) - gentle side slope to avoid erosion and deposition of silt ininfiltration basin. Grass was established before allowing runoff into basin – temporary drainage, providedby a shallow trench at road side, was filled and turfed once remainder of grass was established.

Basins should not be within 5m of buildings. Shallow rills or channels might have been used to carryroof drainage into basin.

Posts or post & rail fence is expensive but needed to keep vehicles out – vehicles would cause ruttingin wet weather - However if there is no infiltration, soil might be engineered (reinforced & granular topsoil used) to reduce risk and avoid need for posts.

Planting in swales prevents parking; however,erosion control would be necessary due to thesteep side slopes in this example.

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Poor inlet design and planting: Swale taking flowfrom a car park during a storm event. Erosion likelydue to the concentration of flow at the inlet fromthe car park. Bark chips float away and blockgrates. Runoff should not have been permitted toenter the swale until planting was established.

Wooden check dam across a swale: These arelikely to quickly rot and become ineffective,causing unnecessary maintenance requirementsor the future and possibly flooding should itcollapse. Erosion likely when overtopped.

Good detail for draining roof water to swale butswale is too close to building. Rills / channelsneed to have flexible joints, especially in clay ormixed soils

This swale looks a bit small to be effective insevere events, but some flooding of edge of roadmay be acceptable in major events.

Works well on inside of bends where roads aresuper elevated. Need to keep vehicles out ofswale if a residential area.

For swale to convey water at low velocities, itneeds a relatively flat longitudinal fall. This alsoreduces the required land take and excavationand so reduces the overall cost.

House roof water could drain in from right handside but need for rills or pipes to cross footwayand driveways to cross-swales. Driveways couldbe used as check dams if slope is steep.

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Large rill or footway perhaps for excessive flows.

Stones used to reduce erosion risk. Stones shouldnot be at a higher level than the surface of thecar park as the voids are likely to quickly blockwith weeds and debris.

Example above is from Ravenswood. No gullies,runoff flows from edge of road into shallow basins.Posts prevent cars from parking in the basins.Timber posts will rot and can be broken or stolen!

A permeable car park at Lovetofts Drive, Ipswich.This has been in position for many years with noissues and minimal maintenance. The paving ison clay and so there is no infiltration. Surfacewater is stored in the permeable sub base underthe blocks. Discharge to the combined sewer,from under the blocks is controlled by a verysmall orifice (20mm).

Care needs to be taken with sub base specification,and surface gradients, IBC require a slight falltowards centre. A thicker sub base is required forsteeper slopes, or to allow for low infiltration orallowable discharges. See CIRIA guide.

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Shallow water, gentle side slopes for safety.Could look good but problems keeping shallowwater clear of excessive weed – regularmaintenance needed, and a minimum waterlevel will need to be maintained. Water qualityneeds to be good – this should be at the end oftrain, possibly only roof water.

If vegetation isallowed to growlong, boggy basinsare probably OK.Provides beneficialareas for habitatcreation, but needsto be safe.

Multifunctional shallow basin at Ravenswood,Ipswich provides capacity for exceedance flowsand is a play area. Soakaways under the surfacestore runoff from storms up to a 30-year returnperiod. The basin is dry most of the time.

Sloping permeable paving on clay. Normallypermeable paving should be flat or gentlysloped towards centre to contain runoff whenmaintenance is needed.

Shingle Driveway on sandy soil! 50mm of peashingle on 225mm of type 2 material. The type2sub base is impervious. It’s vital to use correctsub base material.

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