Water sensitive urban and building design (2016)
Water Sensitive Urban Design (WSUD) is an emerging urban development paradigm aimed to minimise hydrological impacts of urban development on environment. In practice, the WSDU integrates stormwater, groundwater water supply and wastewater management to:
- protect existing natural features and ecological processes;
- maintain natural hydrologic behaviour of catchments;
- protect water quality of surface and ground waters;
- minimise demand on the reticulated water supply system;
- minimise wastewater discharges to the natural environment;
- integrate water into the landscape to enhance visual, social, cultural and ecological values.
WSUD aims for an integrated approach across various scales, from individual allotments to large subdivision and major catchments. In practice, to apply WSUD principles means to:
- protect natural creeks and other waterways on site;
- reduce potable water demand through measures such as water efficient fittings and appliances, rainwater harvesting and wastewater re-use;
- treat in a decentralised manner urban stormwater for re-use and/or discharge to receiving waters;
- match the natural water runoff regime as closely as possible;
- minimise wastewater generation and treating wastewater to a standard suitable for effluent re-use opportunities;
- integrate stormwater management into the landscape, creating multiple use corridors that maximise the visual and recreational amenity of the development;
- support water utility innovations.
A comprehensive strategy for WSUD, should consider the following technical aspects: (i) planning for water conservation (optimise water distribution amongst various uses, investigate potable water conservation, wastewater re-use and storm water harvesting opportunities); (ii) improve quality of storm water (including storm water treatment measures to reduce pollutants); and (iii) integration with elements of urban design. Institutional aspects such as collaboration with watershed authorities, alternative approaches to community involvement, and ways to drive innovation are as important and should frame the whole process of WSUD implementation.
Reducing hardened, impervious surfaces and accurately design drainage of urban spaces, in combination with the use of pervious roads, penetrable concrete and water passing pavements helps to enhance the infiltration of storm water in underlying surface, reducing runoff into sewerage systems and urban spaces, attenuating flood peaks, reducing the urban pollution load in run-off), as well as reduce the risk of damages due to drainage system failure by flooding. facilitating groundwater recharge. Sustainable Urban Drainage Systems (SUDS) are made up of one or more structures built to manage surface water runoff; they tend to mimic natural drainage. SUDS often incorporate soil and vegetation in structures that are usually impermeable (e.g. green rooftops); the uptake and passage through soil and vegetation reduces runoff velocity and improves water quality. Surface permeability in urban areas can be increased by using permeable paving where appropriate (e.g. footpaths, car-parking areas, access roads), thus reducing surface run-off and increasing groundwater recharge. The harvesting and use of rainwater can reduce the pressure on drinking water resources. Infiltration devices, such as “soakaways”, allow water to be drained directly into the ground; basins, ponds, and urban infrastructure such as children’s playgrounds can be designed to hold (excess) water when it rains. Measures for rainwater utilization for non-potable uses and design of urban public spaces can help meet water efficiency targets and improve environmental quality.
- Stakeholder participation
- Success and Limiting Factors
- Costs and Benefits
- Legal Aspects
- Implementation Time
- Life Time
Inter-organisational collaboration and collaboration with the watershed administration, and community participation is a key factor for city planning. The importance of collaborative stakeholder participation and information systems is highlighted in the literature for the successful implementation of these measures, as well as for the achievement of integrated urban drainage management (IUDM).
Success and Limiting Factors
Rainwater and stormwater management in urban areas is considered a tangible and risk-free solution, because of the low side effects and the efficiency and the effectiveness considered high. The reduction of water consumption of urban, private and public sectors, by the development of programmes to promote the efficient use of water, has been also considered highly valuable, pointing to benefits even in case of less pronounced climate change impacts. Key transition factors for cities are:
- inter-organisational collaboration and collaboration with the watershed administration, and community participation;
- uniform regulatory framework and processes;
- organisational capacity;
- and organisational commitment.
The importance of institutional frameworks (governance and management) for successful and widespread implementation of these measures is considered central. The bottlenecks are of institutional and social nature. Planning processes require earlier and more intense consultation with different planning authorities. Choices relating to sustainable urban surface water drainage are seen as rather about the resolution of conflicts between different interests than choice being reducible to technical optimisation. As with the retrofitting of SUDs, careful consideration of institutional arrangements, regulations and codes, making information available, and possible incentives can be key to making a business case for the adaptation of this measure.
Costs and Benefits
The measure reduces stormwater flood risks (area and people flooded) in urban areas by decreasing state indicator (excess) water availability; it also reduces water stress (impact) by decreasing the sensitivity (state) of water use and increasing water availability (state). In infiltration areas, the measure contributes to groundwater repletion. Cost-effectiveness should be investigated in the local context since it depends on local precipitation, prices of water, urban density etc. further to reducing the demand for tap water, water storage facilities can contribute to flood prevention in urban areas. Whereas in water abundant European areas rainwater harvesting for private household uses appears as a viable option, a case study in Spain indicates that in dense urban areas under Mediterranean conditions rain water harvesting appears to be economically advantageous only if carried out at the appropriate scale in order to enable economies of scale, and if complemented with water pricing strategies that provide incentives for increasing water efficiency.
Studies on costs of SUDS and SUDS retrofitting suggest these measures are attractive in economic terms. Studies performing comparative cost analysis between traditional drainage and SUDS are supportive of SUDS: if well designed and maintained they would be more cost effective to construct, and would cost less to maintain, than traditional drainage solutions. A Cost-Benefit Analysis approach is hampered by the comparative lack of studies, but first studies show that the benefits of SUDS in new developments significantly outweigh costs, e.g. by factors of 2,3 or 1,5 (depending on assumptions) over 20 years. A cost-benefit analysis of retrofitting of different SUDS techniques performed by the UK’s Environment Agency suggested that 2 of 4 techniques can always be considered to provide net financial benefits, whereas for the remaining 2 local conditions will determine if the benefit-cost ratio is larger or smaller than 1. Due to methodological difficulties, the economic valuation of the benefits of SUDS has yet to incorporate hard-to-monetise benefits such as reduction in diffuse pollution, additional recharge to aquifers, deferred investments in sewage treatment capacity, and amenity value.
Whereas for new Improved Drainage Design systems the economic case for a response during flood events which reacts to the conditions as a result of Climate Change are clear, the case is not as clear-cut for retrofitting. For instance the cost of pipe retrofitting, e.g. replacing existing pipes with ones with 15 % more capacity, is usually considered prohibitive. However, this measure can be implemented during normal rehabilitation / maintenance programmes, thus making it economically viable.
Water Framework Directive, Floods Directive, Communication on Water Scarcity and Droughts, Urban Wastewater Treatment Directive, Eurocodes, European Regional Development Fund, the European Social Fund, Cohesion Fund, Rural Development Fund. The possibilities of EU policies promoting these decentralized measures seem limited; implementation of this measure is more strongly related to national and regional regulations regarding building codes, drainage codes, flood prevention, and water quality. EU policies (European Regional Development Fund, the European Social Fund, Cohesion Fund, EU Rural Development policies) could promote these measures in the case of larger infrastructures such as those related to drainage systems, both in the construction of new ones or renovating existing systems. Future policy and regulatory efforts addressing building standards, such as the current Eurocodes, could foster the use of SUDs such as green rooftops in their specifications when appropriate.
More than 25 years.