Experience with SuDS in Dublin December 2007 Padraig Doyle Senior Project Manager Mott MacDonald Pettit IEMA Conference Are 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 U27 It 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 U30 It 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  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