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