Surface Water Run-Off

Sur 1 - Management of surface water run-off from developments

To design surface water drainage for housing developments which avoid, reduce and delay the discharge of rainfall to watercourses

and public sewers using SuDS techniques. This will protect receiving waters from pollution and minimise the risk of flooding

and other environmental damage in watercourses

Assessor approach

• Not expected to know detail of this

• Engineer AQP who does this, knows SuDS

• Collating evidence• Assessor needs basic

understanding of principles only

RAIN

Stream or sewer

Infiltration

Surface water run off

Green field site /pervious ground

Sur 1 - Principles

This is our concern

Stream or sewer

Infiltration

No infiltration

Surface water run off – now greater as rain hitting housecannot infiltrate

Sur 1 - PrinciplesRAIN

Green field site /pervious ground

Stream or sewer

Sur 1 Possible Solutions – Sustainable drainage systems (SuDs)

Soakaway

Water goes here instead of running off

Run off is reduced to pre development level

PermeablePaving

Stream or sewer

Sur 1 Possible Solutions – Rainwater used in the dwelling

Water goes here instead of running off

Run off is reduced to pre development level

SuDS Management Train

• Source control (where the rain falls)– Soakaways– Porous/permeable paving– Roof water directed to garden– Rainwater harvesting– Green roofs– Small swales and ponds– Underground attenuation storage

• Site control (where the rain runs to)– Swales– Infiltration/detention basins– Larger soakaways

• Regional control– Balancing ponds and wetlands

(to encourage a holistic thought process)

Criteria Overview

Use SuDS management train:• 1 Peak rate of run off

– No greater than pre developed site

• 2A Volume of run off– Additional volume entirely reduced– Infiltration or other SuDS techniques – non ‘holding back’ solutions– Must do this first, only move to 2B if 2A cannot be satisfied

• 2B Volume of run off– Reduce run off rate to limiting discharge– Must do 2A first where possible– Underground attenuation tanks possible solution here (not in 2A)

Not acceptable in meeting 2A

• Source control (where the rain falls)– Soakaways– Porous/permeable paving– Roof water directed to garden– Rainwater harvesting– Green roofs– Small swales and ponds– Underground attenuation storage

• Site control (where the rain runs to)– Swales (in some cases)– Infiltration/detention basins– Larger soakaways

• Regional control– Balancing ponds and wetlands

Anything that holds water back will not comply with 2AegHolding pondsOversized pipes with hydrobreak

Details

Mandatory requirements

• Appoint appropriately qualified person– Professional or team of professionals– Knowledge of

• Site’s SWRO needs/opportunities• SuDS based solutions (+ experience)• ‘Champion’ SuDS• Provide robust hydraulic design calculations

• Report, flood risk assessment and drawings– Covering all mandatory requirements– In accordance with Development and Flood Risk: a practice guide

companion to PPS25– Concessions for smaller sites (see guidance)

• Peak Rate Run-off no greater– Than pre development– For 1 year and 100 year return period

events

• 5 l/s discharge is allowed – Post development– At a discharge point– To avoid blockage

• These rules do not apply – If no increase in impermeable areaor– If criteria 2A (next) cannot satisfied

Sur 1 Mandatory criteria 1- Peak run off rate

• If run off volume post development greater than pre

• (2A) Additional volume (m3) must be prevented from leaving site by– SuDS techniquiqes– ‘Holding back’ soluitons do not comply– 100 year 6 hour event

• If no increase in impermeable, criteria met by default

Sur 1 Mandatory criteria 2 - Volume of run off

SuDs

• Unless A cannot be satisfied– Responsibility of AQP to decide this– Full justification must be provided

In which case:• (2B) the peak rate of run off must be reduced to:

– 1 year peak flow rate– Mean annual flood flow rate (Qbar) – 2 l/s/ha (max)

• No single discharge point need be < 5 l/s

Sur 1 Mandatory criteria 2 - Volume of run off

} whichever is higher = ‘limiting discharge’

3. Designing for local drainage system failure• Demonstrate that the flooding of the property would not

occur in the event of the above• Caused by

– Extreme rainfall– Lack of maintenance

Sur 1 Mandatory criteria 3 - Volume of run off

Fag packet maths Poor site drainage

Good site drainage

Site size/increase in impermeable?Pre dev peak run off rate 1 / 100 yr

1 Required post dev run off rate 1 / 100 yr2A Peak flow rate increase due to dev. (pre

SuDs) 1 / 100 yrAdditional volume to be dealt with by ‘non holding back’ method (for 100 year event of 6 hours)

Equivalent to cube tank side length of2B 1 yr peak pre dev. flow rate

Qbar2 l / s / haLimiting discharge rateRate difference from 100 peak rate post dev.Additional volume to be dealt with in any wayEquivalent to cube tank side length of

2 ha / yes 2 ha / yes

5 / 10 l/s5 / 8 l/s

4 l/s3.51 m

2x60x60x6 = 43,200 litres

5 l/s4 l/s

4.76 m108,0005 l/s

22 / 44 l/s

4x60x60x6 = 86,400 litres

4 / 8 l/s20 / 40 l/s

4.42 m20 l/s18 l/s

4 l/s20 l/s24 l/s518,400

8.03 m

20 / 40 l/s

4 l/s

Information required

• A lot of detail– Confirmation of qualifications of consultant– Report

• Permeable/impermeable areas pre and post development• If impermeable increased, report to include:

– Permeability characteristics of site» Inc infiltration tests where appropriate

– Peak run off rate calculations for 1 and 100 year events– Methods used to reduce run off– Volume of run off pre, post development and with proposed

mitigation– etc

• FRA• Drawings

Special Cases

• Min flow rate set by sewerage undertaker• Max flow rate set by sewerage undertaker• Derelict sites• Planning Consent pre dates Code requirement• Infrastructure in place before Code requirement enforced• Planning Authority require a non-compliant flow rate• Supplementary guidance for Wales will be published by

WAG• For further guidance visit www.breeam.org/cshguidance

Assessing tips?

• Assessment methodology– Useful step by step approach

• Calculation procedures– Key publications– Allowance for climate change (post development only)– Greenfield sites

• calc method varies with size

• Using SUDS to improve quality of rainwater discharged or protect quality of receiving water

• Ensure no discharge for rainfall up to 5mm (1 credit)– and/or

• Run off from all hard surfaces receive treatment (1 credit)– In accordance with SuDS Manual– To minimise pollution

2 CreditsSur 1 – Optional credits

• Default Cases– Rainwater discharges directly into a tidal estuary or the sea

• Credits withheld if development goes against Environment Agency recommendations

Sur 1 - Management of surface water run-off from developments

Examples

Remineder of main requirementsThere are two key aspects to consider when

assessing surface water run-off in the Code:

• Peak rate of runoff– Ensuring the peak rate of runoff is no greater

post development than it was pre development

• Volume of runoff– Ensure that the post development volume of

runoff is no greater than it was pre development

Watercourse or sewer

Possible solutions to deal with volume where infiltration techniques are possible on site…

Soakaway

Water infiltrates rather than running off

Run-off is reduced to pre-development level

Green roof

PermeablePaving

Watercourse or sewerSome of the run-off

infiltrates rather than running off

Volume of run-off reduced with some residual run-off remaining

Swales

Holding Pond

Possible solutions where additional volumes of runoff are low, but infiltration rates are also very low (such as sites with clay soils), therefore only partial infiltration is possible…

ExampleSite has the following characteristics• Low permeability on the site pre development (therefore small

additional volume of runoff caused by the new development)• Additional Volume of Run-off: 6m3

• Fairly large open space• Surface SuDS were the cheapest option and were able to deal with all

the additional volumes of runoffPost development volume of runoff was no greater than pre development

WetlandSwale

Watercourse or sewer

Possible solutions where infiltration is not at all feasible on the site…

Water collected in the Rainwater Harvesting System rather than running off

Runoff is reduced to pre-development level

Watercourse or sewer

Possible solutions can be a combination of both infiltration and Rainwater Harvesting where there is a large volume to attenuate…

Water collected in the harvesting system rather than running off

Runoff is reduced to pre-development level

PermeablePaving

Example • Greenfield site pre development• Additional Volume of Run-off: 25m3

• Holding Pond/Swales were not an option due to size of site• RWH tank installed below the small communal garden• Permeable paving installed on the front drives

Permeable Paving

Communal Rainwater Harvesting Tank

Communal garden

Additional Volume entirely reduced.

RWH tank sized to also deal with ‘holding back’ the run off for the 100 year event before then being discharged at the Limiting Discharge rate