Policy Context and Sustainable Drainage Issues and Application - Paul Stewart, Mayer Brown

solutions for a sustainable future

Sustainable Drainage

Presented By

Paul Stewart

Associate, Mayer Brown LimitedWoking, Surrey

[email protected]

What I will be discussing

• Define SuDS

• Timeline of Events & Planning Context

• Flood & Water Management Act – What it Means for SuDS

• Design Principles

• Masterplanning

• Costs

• Research


What Do You Think SuDS are?

• Sustainable (urban?) Drainage Systems

• A sustainable drainage system employs a range of

techniques, which vary depending on a range of variables to

achieve 3 main aims:

• Control of run-off (both rate and volume considered)

• Control & treatment of pollution

• Amenity (biodiversity / visual amenity / recreation?)

• There is rarely any one appropriate design to a given

situation.

•Details discussed later.......


Events

Easter 1998 Floods

5 deaths, £400M damage, 1500 people evacuated

Autumn / Winter 2000 Floods£1 Billion damage, 11,000 people evacuated

Publications

March 2000First CIRIA SuDS Design Guide

July 2001 – PPG25

October 2001CIRIA Best Practice Guidance

Timeline

1998

1999

2000

2001

2002

2003


Sustainable DrainageAutumn 2000 Floods

Sustainable DrainagePPG25 - July 2001

“Local Planning Authorities should, therefore, work

closely with the Environment Agency, sewerage

undertakers, navigation authorities and prospective

developers to enable surface-water run-off to be

controlled as near to the source as possible by the

encouragement of sustainable drainage systems”

Part H Building Regulations also updated to promote infiltration.

Sustainable DrainageEvents

August 2004 – Boscastle

Floods

January 2005 – Carlisle Floods3 Fatalities

Publications

July 2004

National SuDS Working PartyInterim SuDS Code of Practice

December 2006 - PPS 25

Timeline

2004

2005

2006

Sustainable DrainagePPS25 - December 2006

SuDS features heavily:

“RPBs and LPAs should further the use of SUDS by:

• incorporating favourable policies within Regional Spatial Strategies;

• adopting policies for incorporating SUDS requirements in Local

Development Documents;

• encouraging developers to utilise SUDS wherever practicable in the

design of development, if necessary through the use of appropriate

planning conditions or by planning agreements;

• developing joint strategies with sewerage undertakers and the

Environment Agency to further encourage the use of SUDS as an aid

to mitigating the rate and volume of

surface water flows; and

• promoting the use of SUDS to achieve wider benefits such as

sustainable development,

water quality, biodiversity and local amenity.”

Sustainable DrainagePublications

July 2004

National SuDS Working PartyInterim SuDS Code of Practice

December 2006 - PPS 25

February 2007CIRIA 697 – The SuDS Manual

600 pages of guidance!

June 2008 – The Pitt Review

October 2008 – Planning Regs

for driveways changed

Timeline

2004

2005

2006

2007

2008

2009

Events

August 2004 – Boscastle Floods

January 2005 – Carlisle Floods3 Fatalities

June / July 2007 Floods13 fatalities

Predominantly local flooding of urban areas

November 2009 – Cumbria Floods316mm in 24 hours – UK Record

1 Fatality – transport links severed

Sustainable DrainageSo, a question to see who’s still awake!

What has been the problem over the last 10 years?

Almost 10 years on from PPG25 and 4 years on from

PPS25, why has so little progress been made on the

implementation of SuDS?

...............................................................................................

...............................................................................................

...............................................................................................

Remember Sir Michael Pitt’s Review?

Pitt made 92 recommendations to the Goverment. These led

to the Flood & Water Management Act 2010.

No body made responsible for SuDS Adoption

No Legislation compelling the use of SuDS

Water Company ‘concerns’ about legal definitions in the Water Industry Act preventing connections

Sustainable DrainageRECOMMENDATION 9:Householders should no longer be able to lay impermeable

surfaces as of right on front gardens and the Government

should consult on extending this to back gardens and

business premises.

1st October 2008 – Change to permitted development rights

for paved surfaces >5m2

RECOMMENDATION 10:The automatic right to connect surface water drainage of

new developments to the sewerage system should be

removed.

Flood & Water Management Act 2010

RECOMMENDATION 20:

The Government should resolve the issue of which

organisations should be responsible for the ownership and

maintenance of sustainable drainage systems.

Flood & Water Management Act 2010


Main SuDS Issues

• A body will be created at Local level known as the SuDS

Approval Body (SAB).

• As drafted, the Act defines the SAB as the County Council

or Unitary Authority. So in your area the SABs will be:

• Hampshire County Council

• Portsmouth City Council

• Southampton City Council

• Isle of Wight Council

Sustainable DrainageWhat is the Remit of the SAB?

• Approval of ALL drainage works.

• “Construction work which has drainage implications

may not be commenced unless a drainage system for

the work has been approved by the approving body.”

How will the planning authority be involved?

• If the works require planning approval, the SuDS approval

application may either be free standing, direct to the SAB, or

combined with a planning application.

• If the latter, the planning authority must consult the SAB (if

different) and inform the SAB of its planning decision.

• When informing the applicant of the planning

determination, the planning authority should also inform

them of the SAB’s determination.

Sustainable DrainageDoes the Act sort out adoption?

Yes!!

The SAB must adopt and maintain an approved drainage

system, or part of a drainage system which serves more

than one property and meets the criteria. (except Public

Highways)

“You also ask whether shared areas of pervious pavement,

such as parking courts would be adoptable SuDS. Under the

FWMA, permeable surfaces that form part of a SuDS that

serve more than one property will be adoptable in new

developments and redevelopments.” - DEFRA

Also, the Sewerage Undertaker must accept water from an

approved (SuDS) drainage system.

Sustainable DrainageSuDS Approval & Adoption Timescales

(These are best guesses, as not fixed!)

Publish Consultation Draft National SuDS Standards

Publish National SuDS Standards

SAB Duties Commence?? – May be phased in

Early 2011

October 2011

April 2012

Another Question.....

So what happens to developments between now and then??

a)Continue Ad Hoc private / public adoption & resistance from

Southern Water.

b)Early negotiations with SAB organisations to agree

retrospective adoption.

Sustainable DrainageA couple of loose ends.....

• Funding for maintenance is uncertain. The Act makes provision for

checking fees and performance bonds, but is strangely quiet on how the

extra burden will be paid for.

• DEFRA claim SuDS maintenance will be funded in full, but have to

date given no details of this.

• In discussion with Southern Water regarding Waterlooville MDA, there

is some uncertainty over the upstream interface of SuDS & Sewers.

Southern Water currently claim that all surface water pipework upstream

of the final SuDS feature will be part of the Drainage System and

therefore adoptable by the SAB. (I disagree, but it needs to be clarified)

• This means that on the future Waterlooville MDA, there would be no

Southern Water surface water sewers on a development of 2550 (STP)

dwellings. They would be SAB maintained.


SuDS Components

Reference CIRIA C687

Planning For SuDS – Making It Happen

Sustainable DrainageMain Design Principles

• Levels of Service

• Infiltration

• Environmental enhancement

• Run-off treatment

• Run-off rate reduction

• Run-off volume reduction

Sustainable DrainageLevels of Service

Technical definitionsReturn period – The statistical probability of a particular storm occurring within a

particular period. 1 in 100 year storm will statistically occur once every 100 years.Storm duration – The length of a design rainfall event.

•BS EN 752 : 2008 / Sewers for Adoption 6th Edition

standard return period designs.

• Rainfall attenuated on site to the 1 in 100 year RP

(Not necessarily all in SUDS features)

• Overland flow checked against 1 in 200 year RP

(Association of British Insurers / Water Companies)

• Climate Change – PPS 25 suggests 10%, 20% or

30% increase in rainfall intensity depending on

design horizon.

Sustainable DrainageInfiltration

•The Perfect SUDS situation is where 100% of run-off

can be infiltrated back to ground.

• Consider infiltration first and test if borehole records

or site knowledge suggest that there is potential.

• Be sensible! Do not insist on infiltration tests of

boreholes show significant depths of dense clay.

• Groundwater recharge is desirable, however

groundwater should be protected from pollution.

• Where possible avoid traditional soakaways without

prior water quality treatment.

• Check with EA for groundwater Source Protection

Zones and consult on restrictions.


• In Hampshire, much of the potable water is taken

from Chalk Aquifers. Revised EA groundwater

protection guidance was issued in 2008 - GP03.

• EA policy:


• So what happens in Flood Zone 1? EA Interpretation

of policy in a recent consultation:

“In line with policy P4-12 of GP3, we will also object to

the use of Sustainable Urban Drainage Systems

(SuDS) at this location for the discharge of surface

water run-off. Surface water from areas of car parking

should be directed to the nearest surface water sewer.”

• This is slightly at odds with the normal EA support,

which SuDS enjoys.

• In chalk areas there are likely to be a lack of surface

water sewerage. I understand that the policy may be

clarified. in practice, SuDS could be used subject to EA

agreement, but would need to prevent direct infiltration

and achieve a high level of treatment.

Sustainable DrainagePollution Treatment

• A number of methodologies have been proposed by

academics to score the effectiveness of SuDS

features. The SuDS Manual (CIRIA C697) contains a

table suggesting minimum numbers of treatment train

components

Sustainable DrainageRun-off Treatment

• Treatment Train – multiple levels of SUDS are best for

treatment.

• Source Control – Initial treatment should occur as close to

the pollution source as possible. Examples:

• Pervious paving to car parks and private drives

• Filter Strips & Swales (alongside roads)

• Bioretention areas and rain gardens

• Green / Brown roofs (roofs attract airborne pollutants)

• Source control techniques are very effective at reducing

the pollutant loading on the downstream features, especially

where they trap the most polluted ‘first flush’ of a rainfall

event.

• Source control features are principally for pollutant

removal and initial volume losses. Although they can be

designed with additional volume storage if necessary.

Sustainable DrainageRun-off Treatment

• Local Treatment or Site Controls - Typically catchments

and sub-catchments up to 5 Ha

• Basins

• Ponds

• District Treatment or Regional Controls – Typically final

treatment and balancing from catchments over 5Ha.

• Lakes

• Wetlands

• Treatment Volume (Vt) – Permanently wet final treatment

is good. The permanent volume should be equivalent to at

least 10mm of Rainfall

• Conveyance features such as swales & reed channels

can provide additional treatment, particularly where they

are designed to settle sediments.*See CIRIA 697 Para 1.3.4 for processes and applications

Sustainable DrainageRun-off Volume Reduction (Interception Storage)

• In nature, very little of the initial rainfall runs off to

watercourses, it soaks into the upper soil. (Even in

Clay areas.

• Coincidentally in developed areas this initial ‘first

flush’ carries the most pollutants, particularly following

long dry periods

• CIRIA 697 introduces the concept of ‘Interception

Storage’. This is the aim to capture at least the first

5mm of rainfall and dispose of via filtration,

evaporation or evapotranspiration. Features used

include:

• Swales (check dams and underdrains assist)

• Green roofs

• Pervious pavements

• Bioretention areas

Sustainable DrainageEnvironmental Enhancement

• Vegetated SUDS systems should usually be given first

priority over pure engineering solutions, as their operation

is easier to observe and pollution incidents can be easily

detected. (Although underground tanks are sometimes

appropriate and should be given due consideration)

• Environmental Enhancement could be defined in a

number of ways. Such as:

• Habitat creation promoting ecological benefits

• Public space for rest or recreation

• Care should be taken to avoid designing unattractive or

inaccessible spaces which detract visually or attract

antisocial behavior

• SUDS designers should include or work closely with

Landscape professionals.

Sustainable DrainageRun-off Rate Reduction (Greenfield)

• SUDS on greenfield developments should aim to

discharge surface water at rates not exceeding the

calculated greenfield rate

• ‘Sites’ (catchments) under 200Ha – use Institute of

Hydrology Report 124 equation to calculate the

‘mean annual flood’ QBARrural. Then apply Flood

Studies Report Regional Growth Curves to derive

permitted rates for different return periods.

• However, if no long term volume storage applied

(explained shortly) a flat rate of discharge should be

applied at the greater of QBAR or 2l/s/Ha.

• Example – Newlands Common, Waterlooville

Sustainable DrainageRun-off Rate Reduction (Greenfield Example)

•Example – Newlands Common, Waterlooville

With urbanisation of less than 5%, the QBARrural equation (equation 7.1) in IOH 124 is appropriate, and can be expressed as

follows:

QBARrural = 0.00108 AREA0.89 SAAR1.17 SOIL2.17

Where,

QBAR = Mean annual flood

AREA = Catchment area

SAAR = Standard average annual rainfall (790mm from FEH CD-ROM)

SOIL = Soil index (assumed as 0.45, based on FSR WRAP class 4)

Using the above formula, a catchment area of 50 Ha results in a QBAR of 250l/s.

To produce greenfield run-off rates for a range of return periods the Flood Studies Report regional growth factors are applied as

follows:

Return

Period

(1 in x)

2 5 10 20 30 50 100 100

+20%

Growth

Factor

0.88 1.28 1.62 2.00 2.26 2.62 3.19 3.828

50 Ha

discharge

(l/s)

220 320 405 500 565 655 798 957

Discharge

Rate

(l/s/Ha)

4.40 6.40 8.10 10.00 11.30 13.10 15.96 19.14

Sustainable DrainageRun-off Rate Reduction (Previously Developed)

• PPS 25 seeks to mimic the pre-development surface

water flows and where practicable reduce flood risk.

• This is open to a certain amount of interpretation and

neither PPS 25 or CIRIA C697 describe how to treat

discharges from previously developed areas. Local

Planning Policies should consider the brownfield scenario

and provide guidance. In practice, the EA seek a (varying)

degree of betterment.

• My opinion is that the pre-development calculated flows

from frequent storms such as the 1 in 2 year should be

taken as the limiting discharge factor with SUDS

attenuating this rate to the 1 in 100 year return period. This

provides betterment for the more severe storm events.

• The other SuDS principles still apply!

Sustainable DrainageRun-off Volume Reduction (Long term storage)

• If we simply attenuate run-off rates, we are not actually simulating

greenfield conditions, as the total volume of run-off is not being reduced.

• This will lessen the benefits of SUDS to the downstream floodplains of

large rivers, where flood waters tend to build over extended time periods.

• Therefore, CIRIA 697 introduces the concept of long term storage, where

the aim is to capture the difference in volume between the greenfield and

developed situation for the 1 in 100 year 6 hour duration storm (arbitrary

figure).

• This volume is segregated and either infiltrated over an extended period

or discharged at a rate < 2l/s/Ha.

• If no volume storage is provided, extended attenuation’s required at a

rate of QBAR or 2l/s/Ha.

Sustainable DrainageMasterplanning

• Some controls can be incorporated in any development, regardless of

masterplanning issues – Pervious Paving / Green Roofs / Rainwater

Harvesting

• However, for best results SuDS design should be considered at an early

stage to make best use of the topography.

• Assess the existing drainage routes, overland flow, conveyance,

disposal. The best SuDS mimic this process.

• Allow space adjacent to impervious paving for linear SuDS.

• Overlook open space SuDS to enable residents to ‘take ownership’

• Slow water is best , follow contours with swales where possible, perhaps

align roads to contours. Cascade pools across contours.

Sustainable DrainageCommon Misconceptions

• My site’s on Clay, so is unsuitable for SUDS.

• Wrong. SUDS is not just soakaways. Applicants making statements

like this are ill-informed (or trying it on!)

• No space for SUDS, conflict with PPS3 densities.

• Wrong. SUDS can be incorporated as Public Open Space, or counted

as essential infrastructure and excluded from density calculations.

• SUDS is expensive and less houses affects profitability

• Debateable. Studies have shown that SUDS can be cheaper to build /

maintain and can add a premium to house prices around well designed

features

• SUDS is experimental. There’s no design guidance.

• Where have you been for the last 10 years. Design manuals from

CIRIA since 2000 (C522). Current best practice CIRIA C697 (2007) is

604 pages.

Sustainable DrainageLand Take Issues.

• Land take for SUDS varies considerably from site to site, and

depends primarily on infiltration potential and prior use.

• In addition the wider employment of source control SUDS, such

as green roofs, pervious pavements and bioretention areas will

reduce the space requirement for strategic SUDS.

•The worst case would be for a greenfield site on heavy clay.

• An example of such a site is Newlands at Waterlooville. Original

Application1550 units + commercial in Hampshire on clay site.

SUDS is approximately 6% of the red line boundary, and falls

within the requirement for Public Open Space.

• Increased land values are often reported around well designed

and landscaped SUDS features.

“L10 - Housing

developments will be

permitted if they contribute

towards the provision of:

a open space in the area to

the National Playing Fields

Association (NPFA)

standards;

b general amenity space

as part of the scheme. “

Sustainable DrainageSource Control Approximate Costs

• Green Roofs (cost of coverings)

• Extensive green roof (50mm) - £65/m2

• Welsh slate - £65/m2

• Eternit fibre slate - £45/m2

• Concrete Tiles - £25/m2

(Loadings to consider, 0.7kN/m2 for extensive green roof / 0.4kN/m2 for tiles)

• Pervious Pavements light duty (1msa)

• Pervious block paving (Clay) - £70/m2

• Porous asphalt (car park) - £40/m2

• Standard block paving (Clay) - £60/m2

• Standard asphalt - £40/m2

•Plus possible savings on drainage?

• Bioretention area

• Intentional ponding in landscaped area.

•Same cost as verge / small landscaped area

Sustainable DrainageStrategic SUDS costs

• Swales

• Large Swale type - £73/m (7m wide)

• Grass Verge - £18/m

• Carrier drain (say 225mm) - £60/m

• Ponds

• Newlands Main Access (267m3) £71k - £266/m3

• Newlands Pond 9 est (1485m3) £240k - £162/m3

• Newlands Pond 10 est (7500m3) £334k - £44/m3

•Underground plastic cells - £200/m3

Sustainable DrainageCommuted Sums

• Newlands agreed rates for 20 year commuted sums (per m2) were as follows

(May 2006):

• Swales £6.54*

•Attenuation Basins £5.70*

•Wetlands £10.08 (£12.59)

•Lake £6.52*

* Rates increased to normal Public Open Space rate of £7.43 (£9.28)

Sustainable DrainageWaterlooville SuDS Research

• In 2003 the EA, Grainger and the Local Authorities set up a group to study

the effectiveness of SuDS for the Waterlooville MDA.

• The aim was that the research would be unique in that it would look at the

baseline conditions, construction phases and post construction and was part

funded by developer contributions and the EA.

• The baseline conditions of the River Wallington and the flow from the site

has been undertaken by the EA.

• Due to EA financial pressures Mayer Brown took on greater responsibility for

the research and applied for government funding to recruit a research

associate in cooperation with the University of Portsmouth.

• The continuation of the project will depend on future funding, but we would

like to continue with the original aims of the project.


• Hampshire Example – West of Waterlooville

• What has been constructed is the main access

junction and associated SUDS, including:

• Bioretention Area






•Swale






• Swale

• Pond


•Ponds well oxygenated with low BOD – typical of road runoff ponds

•Pond outlet NH4+, TON, sPO4 very low and less or same as river


•Heavy Rainfall Event – 22nd January 2010

•COD very high in runoff.

•Progressively reduced across

treatment train.

•Pond outlet less than river.


•Heavy Rainfall Event – 22nd January 2010

•Ammonia reduced less in the initial stages.

•Soluble pollutant.

•Good removal efficiency in pond components.

Road

Runoff

After

Swale

Pond

Outflow

River

Wallington

Policy Context and Sustainable Drainage Issues and Application - Paul Stewart, Mayer Brown

Technology

Transcript of Policy Context and Sustainable Drainage Issues and Application - Paul Stewart, Mayer Brown