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What effect will the National Standards for
Sustainable Drainage Systems (SuDS) haveupon the design and implementation of
surface water drainage on residential
developments?
by Ryan Bruty
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Aboutme
Engineer at Taylor Wimpey East Anglia, been with thecompany for eight years
During which time completed Taylor Wimpeys
Management Trainee Scheme
Whilst completing Foundation Degree in Construction atSuffolk College
Since completed Bachelor of Science with Honours in CivilEngineering at University Campus Suffolk in May 2012
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Contents
Introduction
Literature ReviewKey Changes in Legislation
InterviewsKey Findings
Case StudyKey Findings
Conclusions
Questions?
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Introduction
National Standards for Sustainable DrainageSystems from The Flood and Water
Management Act 2010
Research based on consultation version ofStandards dated December 2011
Research carried out in same four sections as
Part 2 of the National Standards:
Runoff Destination
Peak Flow Rate and Volume
Water Quality
Function
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Literature Review
i. Runoff Destination
Hierarchy previously identified in Building Regulations:
1. Infiltration
2. SW Body3. SW Sewer
4. Combined Sewer
Enforced through amendments to Water Industry Act
removing automatic right to connect
Now dependant upon SAB (SuDS Approval Body)
Approval
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Literature Review
ii. Peak Flow Rate and Volume Three key elements:
1. Runoff rates
2. Runoff volumes for small events (
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Literature Review
iii. Water Quality
First mandatory regulations on water quality treatment
stages (although previously referred to in The SuDS
Manual)
Specifies minimum treatment stages dependent upon
sensitivity of hazards and outfall
For residential developments typically Low (roof
drainage) and Medium (car parking and roads)
Can be beneficial to separate roof and car parking runoffinto separate systems
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Literature Review
iii. Water Quality
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Literature Review
iv. Function Three topics under Function:
1. Design
2. Flood Risk3. Operation and Maintenance
Designing for exceedance (Flood flow routes) already
Mandatory requirement of CfSH. Maintenance responsibility of SAB but approved
drainage plan must include the safe operation and
maintenance for SuDS.
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Interviews
i. Runoff Destination
Indicated hierarchy already in place through
PPS25 (Planning Policy Statement 25)
(now superseeded) and the planningprocess.
As is but delay in obtaining right to connect.
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Interviews
ii. Peak Flow Rate and Volume
Approach 2 provides alternative to that
required for CfSH compliancebut
increases storage required.
Area of concern was No discharge off site
for 5mm rainfall event Infiltration,
evaporation and rainwater harvesting onlyoptions for compliance.
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Interviews
iii. Water Quality
Main concern to all partiescurrently only
considered if overlying aquifer.
Innovative products emerging providing off
the shelf options for treatment stages.
Prevention of 5mm event discharging off site
as less dilutedFirst Flush.
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Interviews
iv. Function
Many aspects already in place through CfSH
and PPS25.
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Case Study
Selected Project: Greenfield site of 1.5Ha in Cambourne, Cambridgeshire
52 No. residential dwellings
Underlying ground conditions Glacial Till Drainage designed using three methods (based on current
site layout):
1. Traditional - unrestricted outfall
2. PPS25pre-development rates3. National Standards for SuDS
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Case Study
i. Runoff Destination
Infiltration not possible due to poor
permeability of soil.
No surface water bodies in vicinity.
Surface water sewer only method as
previous have been discounted.
Same for all three design methods.
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Case Study
ii. Peak Flow Rate and Volume Over 2400% increase in storage required from
unrestricted outfall to PPS25.
Over 3900% increase in storage required from
unrestricted outfall to National Standards. 60% more storage required for National Standards
compliance (in accordance with Approach 2) over
that of PPS25.
Also requirement for storage of 5mm event(additional 50m for case study).
Alternative - Approach 1 but approx. 380m of
rainwater harvesting but storage required as PPS25
design.
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Case Study
iii. Water Quality
No treatment stages required for roof runoff.
Two stages required for road and car park
runoff.
Separate drainage runs could be used for
eachbut due to treatment options
incorporated was not. Stages selected had limited effect on the
development proposals and could be
incorporated with existing layout.
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To Conclude
Two key changes affecting Residential
Developers
Peak flow rates and volume on areaswith
increased impermeable areas (where infiltration cannot beused).
Must be restricted to either 2l/s/ha or as CfSH.
No discharge off site for 5mm rainfall event.
Minimum treatment stages to improve waterquality.
Number of stages dependant on destination and risk of
contamination.
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Any Questions?
Or email