Post on 17-Dec-2015
Experience with SuDS in Dublin
December 2007
Padraig DoyleSenior Project ManagerMott MacDonald Pettit
IEMA ConferenceAre SuDS the answer for drainage?
Stormwater and Urbanisation
Runoff from greenfield sites - natural situation
Effect of urbanisation
Increase in rate, volume and pollutant load of runoff
Greenfield
Urban
Need for Stormwater Control
Flooding
Treatment costs
Surcharging
Lack of capacity for development
Spills from Combined Sewer Overflows
Pollution from stormwater runoff
Pollution from Stormwater Runoff Pollution from runoff
– Oils, antifreeze etc.
– Litter/ food stuffs
– Decaying grass, leaves and vegetable matter
– Animal waste
First Flush
– Pollutants build up and decay during dry weather - washed into streams under low flow conditions
Sustainable Drainage Systems (SuDS) Mimics Natural Behaviour
Stormwater Control
Removal of Pollutants
Amenity Value
SuDS Management/Treatment Train Concept
SuDS Train
SuDS as an Environmental Solution
Control of Flows
Settlement of Solids
Organic Breakdown
INFILTRATION
Environmental issues
Baseflows up to 40% lower
Combined Sewer Overflows
Wastewater Treatment Works
SuDS Benefits
Potentially more flexible than piped systems
Mimics natural cycle – can combat lowering of Groundwater Table and river baseflows
Reduced need for Combined Sewer Overflows
Climate Change
Introduction to Greater Dublin Strategic Drainage Study
Commissioned by DoEHLG Seven Local Authorities 150,000ha/ 1.2M Population
Study Budget in Excess of €10M Funded by DoEHLG under NDP Tolka Study Funded by OPW
GDSDS - Recommendation of Policies
New Development
– Stormwater Management
Environment
Infiltration/ Ex-filtration
Basements
Climate Change
DCC’s Development Plan
Policy U27It is the policy of Dublin City Council to promote the use of sustainable drainage systems, which balance the impact of urban drainage through the achievement of control of run-off quantity and quality and enhancing amenity and habitat.
Policy U30It is the policy of Dublin City Council to require that all new development must be constructed in compliance with the Greater Dublin Strategic Drainage Study document entitled "New Development Policy, Technical Guidance Document".
"New Development Policy, Technical Guidance Document"
All new development shall incorporate SuDS facilities, unless the developer can demonstrate that SuDS is impractical due to site circumstances. Where SuDS cannot be provided, the developer shall provide alternative means of dealing with pollutants.
Use of site storage to provide a mechanism by which the river regime can be maintained in its natural state, by minimising the difference between the developed and undeveloped local catchment runoff.
Storage must therefore limit the Flow Rate AND Volume of surface water runoff that drains directly off the developed site.
Interception storage and infiltration to groundwater of rainfall is important.
Four Design Criteria
Stormwater Design Philosophy
Small Events - Interception storage of at least 5mm, and preferably 10mm, of rainfall where runoff to the receiving water can be prevented.
Where initial runoff from at least 5mm of rainfall cannot be intercepted, treatment of runoff (treatment volume) is required.
Retention pond (if used) to have minimum pool volume equivalent to 15mm rainfall.
1) River Water Quality Protection
1 yr. storm - Discharge rate equal to 1 year greenfield site peak runoff rate or 2l/s/ha, whichever is the greater. Site critical duration storm to be used to assess attenuation storage volume.
100 yr. storm - Discharge rate equal to 1 in 100 year greenfield site peak runoff rate. Site critical duration storm to be used to assess attenuation storage volume.
2) River Regime Protection
30 yr. storm - No flooding on site except where specifically planned flooding is approved. Summer design storm of 15 or 30 minutes are normally critical.
100 yr. storm - Planned flood routing and temporary flood storage accommodated on site for short high intensity storms. Site critical duration events.
No internal property flooding. Floor levels at least 500mm above maximum river level and adjacent onsite storage retention.
No flooding of adjacent urban areas. Overland flooding managed within the development.
3) Level of Service (flooding) for Site
100 yr. storm - “Long-term” floodwater accommodated on site for development runoff volume which is in excess of greenfield runoff volume.
Temporary flood storage drained by infiltration on a designated flooding area brought into operation by extreme events only.
100 year, 6 hour duration storm to be used for assessment of the additional volume of runoff.
Infiltration storage provided equal in volume to “long term” storage. Usually designed to operate for all events.
Maximum discharge rate of QBAR or 2 l/s/ha, whichever is the greater, for all attenuation storage where separate “long term” storage cannot be provided.
4) River Flood Protection
Four Criteria
Higher pollutant risk from developed areas
SuDS can remove pollutants
Can use sealed systems near sensitive aquifers or areas with low permeability
Proper design is vital!
Perceived risk to Groundwater
Not adding any new water to ground water
Aim is to stop development from interfering with natural hydrological cycle
Restoration of natural hydrological cycle will benefit groundwater
SuDS and Groundwater
Ground water recharge combats lowering of groundwater levels in urban areas
Urban trees
Baseflows in streams
Global warming
SuDS can benefit Groundwater
Extensive use internationally
Traditional pipe and gully system has failed– Lack of maintenance - flooding– Lack of hydraulic capacity - flooding– Serious pollution problems
New policies have been adopted by seven Dublin Region local authorities and are also being used in other areas
SuDS are now reality here
Need for attenuation is widely accepted Good application of SuDS on large sites More resistance where footprint is small –but roof gardens/ infiltration
systems?
Introduction of SuDS website will help designers and checkers alike
www.irishsuds.ie NOTE: Website gives preliminary figures only
Application of SuDS policies
Implementation of Environmental criteria has proved most difficult Resistance to soft solutions in urban areas
- land take
- safety
- maintenance Still a lot of hard structural “solutions” Situation will evolve as knowledge increases
Application of SuDS policies (2)
SuDS is compulsory under new Dublin Regional Drainage Policies
Policies are spreading to other areas
SuDS
– Stormwater Control
– Environmental Improvements
– Amenities
Summary
www.mottmacpettit.ie