Transition experiments in the context of E2STORMED were undertaken by applying the DST to two stormwater management scenarios -- retrofitting drainage into an existing development and a new development. Conventional stormwater infrastructure was then compared with appropriate sustainable drainage (SuDS) solutions for each location. The DST uses an established process and allows users to choose qualitative and quantitative criteria to be used in a multi criteria analysis which includes the use of weighted and scored matrices to assess objectives (EA, 2013). The criteria which can be compared include: financial, energy consumption, CO2 emissions, environmental factors, water quality, other quantitative criteria such as CSO reduction / volume of water reused and social criteria.
Stormwater harvesting solutions were chosen by both Hersonissos in Crete and Zagreb in Croatia as part of the energy efficient solution for two locations. The Hersonissos example is a small hamlet which has no formal drainage infrastructure and currently experiences frequent pluvial flooding during large storm events due to incapacity in the local watercourse. The Zagreb example is in a neighbourhood located in a combined sewer area to the southwest of the city centre that on occasion experiences fluvial flooding when river and groundwater levels increase during exceptional rainfall events.
Zagreb SuDS solutions were designed to maximize soil permeability, increase WWTP energy efficiency through reduced pumping and treatment costs and increase the area's economic and aesthetic value. These include permeable pavement, detention basins and green roofs where roof slope gradient is favourable. As part of the City Gardens initiative (growing food for personal use), water butts will also be used to collect stormwater from roofs and be used for irrigation purposes.
Analysis of outputs delivered for Zagreb indicate that disconnecting stormwater from the combined sewer network reduced flows, achieved a reduction in pollutant loading and reduced flood damage risk to properties. Peak outflow for the design storm was also reduced which reduced pipes diameters to the existing network. Both energy consumption and CO2 emissions were lower in the SuDS development which included water butts for irrigation of local produce. However Economic benefits from water reuse, water treatment, flood protection and lower energy consumption were not high enough to make costs lower than the conventional solution. In order to compare results and to choose the most beneficial solution six different criteria were selected: net costs; net energy consumption; net emissions; water quality; social acceptance; and evaluation of ecosystem services.
Hersonissos SuDS solution is designed to reduce local flooding and provide aesthetic and recreational benefits, which are highly valued in a touristic area. In addition, it will offer relief to the main stream and reduce erosion in the watercourse. It will also provide capacity for a new development to go ahead and deliver water re-cycling for crop irrigation purposes of nearby olive grove. Initial analysis of outputs is ongoing in Hersonissos but indications are: high energy savings and reduction in CO2 emissions due to decreased conveyance costs of water for potable use and irrigation purposes. This offsets, the increased construction and maintenance costs for the SuDS solution, particularly when considering a 50 year return period for which the systems are designed.
Conclusions and Recommendations
A key issue for both Hersonissos and Zagreb was availability of data. In many cases the information was difficult to obtain as the responsible organisations did not have the data or did not collect it in a way that could be used without significant data processing which proved to be extremely time consuming. For the SuDS solutions local rates were difficult to define with default values used which may have decreased accuracy of financial results. Criteria selection in Zagreb was initially undertaken without input from all stakeholders so the weighting initially applied may be unrealistic for this first attempt at comparing the two scenarios. The DST is in the final stages of revision based on feedback from all partners and application will soon be finalised for Hersonissos and Zagreb. Results to date provide a clear indication of energy efficiency in the case of sustainable drainage solutions, which are also associated with cost savings in the long run, while the positive results observed for rainwater harvesting techniques are promising, particularly in areas facing water scarcity.
Environment Agency (2013). Multi Criteria Analysis (MCA). [online]. Available from: http://www.environment-agency.gov.uk/research/policy/32929.aspx
European Commission. 2000. Water Framework Directive. Directive 2000/60/EC of the European Parliament.
European Commission. 2012. Communication Document. A Blueprint to Safeguard Europe's Water Resources. Brussels, 14.11.2012. COM(2012) 673 final.
UNEP and Global Environment Centre Foundation (GEC). 2004. Water and Wastewater Reuse: An Environmentally Sound Approach for Sustainable Urban Water Management. UNEP Publications.