Experience with SuDS in Dublin

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IEMA Conference Are SuDS the answer for drainage?

• Experience with SuDS in Dublin

December 2007
Padraig Doyle Senior Project Manager Mott
MacDonald Pettit
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Stormwater and Urbanisation

• Runoff from greenfield sites - natural situation
• Effect of urbanisation
• Increase in rate, volume and pollutant load of
  runoff

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Greenfield
Urban
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Need for Stormwater Control

• Flooding
• Treatment costs
• Surcharging
• Lack of capacity for development
• Spills from Combined Sewer Overflows
• Pollution from stormwater runoff

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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

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Sustainable Drainage Systems (SuDS)

• Mimics Natural Behaviour
• Stormwater Control
• Removal of Pollutants
• Amenity Value

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SuDS Management/Treatment Train Concept
SuDS Train
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SuDS as an Environmental Solution

• Control of Flows
• Settlement of Solids
• Organic Breakdown
• INFILTRATION

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Climate Change

• 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

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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

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GDSDS - Recommendation of Policies

• New Development
• Stormwater Management
• Environment
• Infiltration/ Ex-filtration
• Basements
• Climate Change

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DCCs 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".

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"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.

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Stormwater Design Philosophy

• 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

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1) River Water Quality Protection

• 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.

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2) River Regime 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.

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3) Level of Service (flooding) for Site

• 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.

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4) River Flood Protection

• 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.

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Four Criteria
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Perceived risk to Groundwater

• 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!

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SuDS and 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

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SuDS can benefit Groundwater

• Ground water recharge combats lowering of
  groundwater levels in urban areas
• Urban trees
• Baseflows in streams
• Global warming

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SuDS are now reality here

• 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

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Application of SuDS policies

• 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

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Application of SuDS policies (2)

• 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

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Summary

• SuDS is compulsory under new Dublin Regional
  Drainage Policies
• Policies are spreading to other areas
• SuDS
• Stormwater Control
• Environmental Improvements
• Amenities

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www.mottmacpettit.ie