“What effect will the National Standards for
Sustainable Drainage Systems (SuDS) have
upon the design and implementation of
surface water drainage on residential
developments?”
About
me…
Engineer at Taylor Wimpey East Anglia, been with the company for eight years
During which time completed Taylor Wimpey’s
Management Trainee Scheme
Whilst completing Foundation Degree in Construction at Suffolk College
Since completed Bachelor of Science with Honours in Civil Engineering at University Campus Suffolk in May 2012
Contents
Introduction
Literature Review – Key Changes in Legislation Interviews – Key Findings
Case Study – Key Findings Conclusions
Introduction
National Standards for Sustainable Drainage
Systems from The Flood and Water Management Act 2010
Research based on consultation version of
Standards 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
Literature Review
i. Runoff Destination
Hierarchy previously identified in Building Regulations:
1. Infiltration 2. SW Body 3. SW Sewer
4. Combined Sewer
Enforced through amendments to Water Industry Act
removing automatic right to connect
Now dependant upon SAB (SuDS Approval Body)
Literature Review
ii. Peak Flow Rate and Volume
Three key elements:
1. Runoff rates
2. Runoff volumes for small events (<5mm) 3. Runoff volumes for large events
Two standard approaches provided for compliance (for
rates and volume of large events):
1. Restrict peak flow rate and volume of runoff 2. Restrict peak flow rate
Alternative approach for brownfield sites
Events up to 5mm event must not discharge off site No minimum flow rate (CfSH (Code for Sustainable
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 runoff
Literature Review
Literature Review
iv.
Function
Three topics under Function:
1. Design
2. Flood Risk
3. 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’.
Interviews
Interview Participants:
1. Engineer at Bloor Homes Eastern
(Residential Developer)
2. Design Engineer at Richard Jackson Plc
(Consulting Engineer)
3. Drainage Engineer at Ipswich Borough Council
Interviews
i.
Runoff Destination
Indicated hierarchy already in place through
PPS25 (Planning Policy Statement 25) (now superseeded) and the planning
process.
Interviews
ii.
Peak Flow Rate and Volume
‘Approach 2’ provides alternative to that
required for CfSH compliance – but increases storage required.
Area of concern was ‘No discharge off site
for 5mm rainfall event’ – Infiltration,
evaporation and rainwater harvesting only options for compliance.
Interviews
iii.
Water Quality
Main concern to all parties – currently only
considered if overlying aquifer.
‘Innovative’ products emerging providing off
the shelf options for treatment stages.
Prevention of 5mm event discharging off site
Interviews
iv.
Function
Many aspects already in place through CfSH
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. PPS25 – pre-development rates 3. National Standards for SuDS
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.
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 50m³ for case study).
Alternative - Approach 1 but approx. 380m³ of
rainwater harvesting but storage required as PPS25 design.
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
each – but due to treatment options incorporated was not.
Stages selected had limited effect on the
development proposals and could be incorporated with existing layout.
Case Study
iv.
Function
PPS25 and National Standards designed for
same events.
Flood conveyance route naturally falls in the
direction of the pond.
Spillway with additional inlet to system (after
flow restriction) specified to prevent flooding in event of blockage.
To Conclude
Two key changes affecting Residential
Developers
Peak flow rates and volume on areas with
increased impermeable areas (where infiltration cannot be used).
Must be restricted to either 2l/s/ha or as CfSH. No discharge off site for 5mm rainfall event.
Minimum treatment stages to improve water
quality.
Number of stages dependant on destination and risk of contamination.
Any Questions?
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