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 Limited
Woking, Surrey

pstewart@mayerbrown.co.uk


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.......

Timeline Publications Events

1998 Easter 1998 Floods
5 deaths, £400M damage, 1500
people evacuated
1999

2000 March 2000
First CIRIA SuDS Design Guide Autumn / Winter 2000 Floods
£1 Billion damage, 11,000
2001 July 2001 – PPG25 people evacuated

October 2001
2002 CIRIA Best Practice Guidance

2003

Autumn 2000 Floods

PPG25 - 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.


2004 July 2004 August 2004 – Boscastle
National SuDS Working Party Floods
Interim SuDS Code of Practice
2005 January 2005 – Carlisle Floods
3 Fatalities

2006 December 2006 - PPS 25

PPS25 - 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.”


2004 July 2004 August 2004 – Boscastle Floods
National SuDS Working Party
Interim SuDS Code of Practice January 2005 – Carlisle Floods
2005 3 Fatalities

2006 December 2006 - PPS 25

2007 February 2007 June / July 2007 Floods
CIRIA 697 – The SuDS Manual 13 fatalities
600 pages of guidance! Predominantly local flooding of
2008 urban areas
June 2008 – The Pitt Review
November 2009 – Cumbria Floods
October 2008 – Planning Regs 316mm in 24 hours – UK Record
2009 for driveways changed 1 Fatality – transport links
severed

So, 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?
No Legislation compelling the use of SuDS
...............................................................................................

No body made responsible for SuDS Adoption
...............................................................................................
Water Company ‘concerns’ about legal definitions in the Water Industry Act
preventing connections
...............................................................................................

Remember Sir Michael Pitt’s Review?
Pitt made 92 recommendations to the Goverment. These led
to the Flood & Water Management Act 2010.

RECOMMENDATION 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

What 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.

Does 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.

SuDS Approval & Adoption Timescales

(These are best guesses, as not fixed!)

Early 2011 Publish Consultation Draft National SuDS Standards

October 2011 Publish National SuDS Standards

April 2012 SAB Duties Commence?? – May be phased in

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.

A 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

Main Design Principles

• Levels of Service

• Infiltration

• Environmental enhancement

• Run-off treatment

• Run-off rate reduction

• Run-off volume reduction

Levels of Service

Technical definitions
Return 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.

Infiltration

•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.

Infiltration

Infiltration

• In Hampshire, much of the potable water is taken
from Chalk Aquifers. Revised EA groundwater
protection guidance was issued in 2008 - GP03.

• EA policy:

Infiltration

• 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.

Pollution 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

Run-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.

Run-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

Run-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

Environmental 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.

Run-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

Run-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:

2 5 10 20 30 50 100 100
Return +20%
Period
(1 in x)
Growth 0.88 1.28 1.62 2.00 2.26 2.62 3.19 3.828
Factor
50 Ha 220 320 405 500 565 655 798 957
discharge
(l/s)
Discharge 4.40 6.40 8.10 10.00 11.30 13.10 15.96 19.14
Rate
(l/s/Ha)

Run-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!

Run-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.

Masterplanning

• 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.

Common 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.

Land 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.
“L10 - Housing
•The worst case would be for a greenfield site on heavy clay. developments will be
permitted if they contribute
towards the provision of:
• An example of such a site is Newlands at Waterlooville. Original a open space in the area to
the National Playing Fields
Application1550 units + commercial in Hampshire on clay site. Association (NPFA)
standards;
SUDS is approximately 6% of the red line boundary, and falls b general amenity space
as part of the scheme. “
within the requirement for Public Open Space.

• Increased land values are often reported around well designed
and landscaped SUDS features.

Source 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

Strategic 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

Commuted 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)

Waterlooville 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 After Pond River
Runoff Swale Outflow Wallington

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

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