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Sustainable Drainage Systems (SuDS) and Integrated Constructed Wetlands (ICW)

The Greater Dublin Strategic Drainage Study (GDSDS) was completed in 2005 and involved the seven Local Authorities of the Greater Dublin Area.  The study carried out an in depth assessment of the Dublin’s drainage system. In addition to the extensive analysis of the drainage systems and the data collection that was carried out, the output also provided five policy documents.  One of these was an Environmental Policy, a second dealt with drainage of New Developments and a third on Climate Change.  These three documents focused on the design approach and criteria for new drainage.  The objective of these three policy documents was to ensure that any future development did not continue the trend towards increasing flooding in the city and pollution of the rivers.


Drainage design in the past has been extremely simple using a rational method to size pipes to ensure that surface water is removed as quickly as possible to ensure flooding does not take place.  Unfortunately this philosophy is flawed as, in transferring the surface water downstream, it provides the potential for flooding of other areas subject to the capacity further down the system.  In addition the pollution in the wash-off from the urban environment is conveyed into the natural environment. 


To provide an alternative method of drainage which does not have these failings therefore requires a completely new approach. “Best Management Practices” (BMPs) is a commonly used term throughout the world that refers to this new approach to drainage.  For some reason this term is not used in UK and Ireland, possibly because the term “drainage” is not included. In the UK and Ireland this practice is referred to as Sustainable Drainage Systems (SuDS). 

SuDS can best be summarised as offering a “total” solution to rainwater management while traditional drainage can be considered as only providing a “collection and disposal” approach.

 

SuDs also applies in the rural environment. The Water Framework Directive sets out the framework for achieving good status in all our rivers in Ireland. In the Eastern River Basin District 80% of our rivers are below good status. Currently most of the rural pollution comes from underperforming small treatment plants, overloaded communal septic tanks, clusters of single unit septic tanks, farmyard runoff and runoff from slurries spread on land.

In Ireland over the past 10 years, the National Parks and Wildlife Service of DEHLG has been uniquely developing a more robust and sustainable approach to the use of constructed wetlands. Categorised as surface-flow type wetlands they are similar to natural free surface water wetlands. Their holistic approach termed ‘Integrated Constructed Wetland’ (ICW) has been successfully applied to deal with a range of effluent types - farmyard runoff, industrial waste and sewage as well as urban pollution.

The concept is based upon the free surface-flow of water through a series of sequential linked shallow ponds vegetated with a range of emergent plant species. An ICW has often a long retention time of up to 90-100 days and it can be designed in many instances to have zero surface discharge. It’s diversity of plant species facilitates microbial and animal diversity and is generally more appealing for recreation and amenity. Due to the slow movement of water through the ponds suspended matter is deposited, and there is adequate time for both aerobic and anaerobic digestion of organic matter. There is good reduction of nitrates and phosphates which are generally greater than 95%. Reduction of faecal indicators is of the order of 99% due to the long retention times and the complex ecology of the aquatic system. ICWs also remove endocrine disrupters such as oestrogen and testosterone which are poorly removed with conventional wastewater treatment systems. An ICW effectively creates the important nexus between quality, quantity and amenity in a way that no other traditional Sewerage Treatment Works can.

 

This website lists a number of articles and papers on SuDs and ICWs. Dublin City Council commissioned HR Wallingford to develop the www.irishsuds.com web site to provide all the necessary information on SuDS and drainage design criteria and to make information as easily accessible as possible. In addition guidance on the application of the design criteria and two on-line tools are provided on the website free of charge. Using this website provides assistance on designing surface water drainage systems. A comprehensive list of hyperlinked web sites are attached to this www.irishsuds.com  web site.

 

 The GDSDS Policy Documents are available here

Greater Dublin Regional Code of Practice available here

 

SuDS Papers and Articles

 

ICW Papers and Articles

Constructed Farm Wetlands Design Manual
 

SEPA (The Scottish Environmental Protection Agency) recently published the Constructed Farm Wetlands Design Manual (prepared by Aila Carty). This can be accessed on the SEPA website:

http://www.sepa.org.uk/land/land_publications.aspx