water-sensitive urban design water-sensitive urban design water-sensitive urban design (wsud) is a land planning and engineering design approach which integrates the urban water cycle, including stormwater, groundwater, and wastewater management and water supply, into urban design to minimise environmental degradation and improve aesthetic and recreational appeal. wsud is a term used in the middle east and australia and is similar to low-impact development (lid), a term used in the united states; and sustainable drainage system (suds), a term used in the united kingdom. comparing the natural and urban water cycle and streetscapes in traditional and blue-green cities background a tram running on green tracks in adelaide, australia. replacing impermeable surfaces with grassy surfaces reduces stormwater runoff, among other environmental benefits. traditional urban and industrial development alters landscapes from permeable vegetated surfaces to a series of impervious interconnected surfaces resulting in large quantities of stormwater runoff, requiring management. like other industrialized countries, including the united states and the united kingdom, australia has treated stormwater runoff as a liability and nuisance, endangering human health and property. this resulted in a strong focus on the design of stormwater management systems that rapidly convey stormwater runoff directly to streams with little or no focus on ecosystem preservation. this management approach results in what is referred to as urban stream syndrome. heavy rainfall flows rapidly into streams carrying pollutants and sediments washed off from impervious surfaces, resulting in streams carrying elevated concentrations of pollutants, nutrients, and suspended solids. increased peak flow also alters channel morphology and stability, further proliferating sedimentation and drastically reducing biotic richness. increased recognition of urban stream syndrome in the 1960s resulted in some movement toward holistic stormwater management in australia. awareness increased greatly during the 1990s with the federal government and scientists cooperating through the cooperative research centre program. increasingly city planners have recognised the need for an integrated management approach to potable, waste, and stormwater management, to enable cities to adapt and become resilient to the pressure which population growth, urban densification and climate change places on ageing and increasingly expensive water infrastructure. additionally, australia's arid conditions mean it is particularly vulnerable to climate change, which together with its reliance on surface water sources, combined with one of the most severe droughts (from 2000–2010) since european settlement, highlight the fact that major urban centers face increasing water shortages. this has begun shifting the perception of stormwater runoff from strictly a liability and nuisance to that of having value as a water resource resulting in changing stormwater management practices. australian states, building on the federal government's foundational research in the 1990s, began releasing wsud guidelines with western australia first releasing guidelines in 1994. victoria released guidelines on the best practice environmental management of urban stormwater in 1999 (developed in consultation with new south wales) and similar documents were released by queensland through brisbane city council in 1999. cooperation between federal, state, and territory governments to increase the efficiency of australia's water use resulted in the national water initiative (nwi) signed in june 2004. the nwi is a comprehensive national strategy to improve water management across the country; it encompasses a wide range of water management issues and encourages the adoption of best practice approaches to the management of water in australia, which include wsud. differences from conventional urban stormwater management wsud regards urban stormwater runoff as a resource rather than a nuisance or liability. this represents a paradigm shift in the way environmental resources and water infrastructure are dealt with in the planning and design of towns, and cities. wsud principles regard all streams of water as a resource with diverse impacts on biodiversity, water, land, and the community's recreational and aesthetic enjoyment of waterways. principles protecting and enhancing creeks, rivers, and wetlands within urban environments protecting and improving the water quality of water draining from urban environments into creeks, rivers, and wetlands restoring the urban water balance by maximizing the reuse of stormwater, recycled water, and grey water conserving water resources through reuse and system efficiency integrating stormwater treatment into the landscape so that it offers multiple beneficial uses such as water quality treatment, wildlife habitat, recreation, and open public space reducing peak flows and runoff from the urban environment simultaneously providing for infiltration and groundwater recharge integrating water into the landscape to enhance the urban design as well as social, visual, cultural, and ecological values easy, cost-effective implementation of wsud allowing for widespread application. objectives reducing potable water demand through demand- and supply-side water management incorporating the use of water-efficient appliances and fittings adopting a fit-for-purpose approach to the use of potential alternative sources of water such as rainwater minimising wastewater generation and the treatment of wastewater to a standard suitable for effluent reuse and/or release to receiving waters treating stormwater to meet water quality objectives for reuse and/or discharge by capturing sediments, pollutants, and nutrients through the retention and slow release of stormwater improving waterway health through restoring or preserving the natural hydrological regime of catchments through treatment and reuse technologies improving the aesthetic and the connection with water for urban dwellers promoting a significant degree of water-related self-sufficiency within urban settings by optimizing the use of water sources to minimise potable, storm, and waste water inflows and outflows through the incorporation into urban design of localised water storage counteracting the 'urban heat island effect' through the use of water and vegetation, assisting in replenishing groundwater. techniques the use of water-efficient appliances to reduce potable water use greywater reuse as an alternate source of water to conserve potable supplies stormwater harvesting, rather than rapid conveyance, of stormwater reuse, storage, and infiltration of stormwater, instead of drainage system augmentation use of vegetation for stormwater filtering purposes water efficient landscaping to reduce potable water consumption protection of water-related environmental, recreational, and cultural values by minimising the ecological footprint of a project associated with providing supply, wastewater, and stormwater services localised wastewater treatment, and reuse systems to reduce potable water consumption, and minimise environmentally harmful wastewater discharges provision of stormwater or other recycled urban waters (in all cases subject to appropriate controls) to provide environmental water requirements for modified watercourses flexible institutional arrangements to cope with increased uncertainty and variability in climate a focus on longer-term planning including in related domains such as water demand management a diverse portfolio of water sources, supported by both centralised and decentralised water infrastructure. common wsud practices common wsud practices used in australia are discussed below. usually, a combination of these elements are used to meet urban water cycle management objectives. bioretention systems bioretention systems involve the treatment of water by vegetation prior to filtration of sediment and other solids through prescribed media. vegetation provides biological uptake of nitrogen, phosphorus, and other soluble or fine particulate contaminants. bioretention systems offer a smaller footprint than other similar measures (e.g. constructed wetlands) and are commonly used to filter and treat runoff prior to it reaching street drains. use on larger scales can be complicated and hence other devices may be more appropriate. bioretention systems comprise bioretention swales (also referred to as grassed swales and drainage channels) and bioretention basins. bioretention swales bioretention swales, similar to buffer strips and swales, are placed within the base of a swale that is generally located in the median strip of divided roads. they provide both stormwater treatment and are. a bioretention system can be installed in part of a swale, or along the full length of a swale, depending on treatment requirements. the runoff water usually goes through a fine media filter and proceeds downward where it is collected via a perforated pipe leading to downstream waterways or storages. vegetation growing in the filter media can prevent erosion and, unlike infiltration systems, bioretention swales are suited for a wide range of soil conditions. bioretention basins parking lot that drains to a small bioretention basin. bioretention basins provide similar flow control and water quality treatment functions to bioretention swales but do not have a conveyance function. in addition to the filtration and biological uptake functions of bioretention systems, basins also provide extended detention of stormwater to maximise runoff treatment during small to medium flow events. the term raingarden is also used to describe such systems but usually refers to smaller, individual lot-scale bioretention basins. bioretention basins have the advantage of being applicable at a range of scales and shapes and therefore have flexibility in their location within developments. like other bioretention systems, they are often located along streets at regular intervals to treat runoff prior to entry into the drainage system. alternatively, larger basins can provide treatment for larger areas, such as at the outfalls of a drainage system. a wide range of vegetation can be used within a bioretention basin, allowing them to be well integrated into the surrounding landscape design. vegetation species that tolerate periodic inundation should be selected. bioretention basins are however, sensitive to any materials that may clog the filter media. basins are often used in conjunction with gross pollutant traps (gpts or litter traps, include widely used trash racks), and coarser sediment basins, which capture litter and other gross solids to reduce the potential for damage to the vegetation or filter media surface. infiltration trenches and systems infiltration trenches are shallow excavated structures filled with permeable materials such as gravel or rock to create an underground reservoir. they are designed to hold stormwater runoff within a subsurface trench and gradually release it into the surrounding soil and groundwater systems. although they are generally not designed as a treatment measure but can provide some level of treatment by retaining pollutants and sediments. runoff volumes and peak discharges from impervious areas are reduced by capturing and infiltrating flows. due to their primary function of being the discharge of treated stormwater, infiltration systems are generally positioned as the final element in a wsud system. infiltration trenches should not be located on steep slopes or unstable areas. a layer of geotextile fabric is often used to line the trench in order to prevent the soil from migrating into the rock or gravel fill. infiltration systems are dependent on the local soil characteristics and are generally best suited to soils with good infiltrative capacity, such as sandy-loam soils, with deep groundwater. in areas of low permeability soils, such as clay, a perforated pipe may be placed within the gravel. regular maintenance is crucial to ensure that the system does not clog with sediments and that the desired infiltration rate is maintained. this includes checking and maintaining the pre-treatment by periodic inspections and cleaning of clogged material. sand filters sand filters are a variation of the infiltration trench principle and operate in a way similar to bioretention systems. stormwater is passed through them for treatment prior to discharge to the downstream stormwater system. sand filters are very useful in treating runoff from confined hard surfaces such as car parks and from heavily urbanised and built-up areas. they usually do not support vegetation owing to the filtration media (sand) not retaining sufficient moisture and because they are usually installed underground. the filter usually consists of a sedimentation chamber as pre-treatment device to remove litter, debris, gross pollutants, and medium-sized sediments; a weir; followed by a sand layer that filters sediments, finer particulates, and dissolved pollutants. the filtered water is collected by perforated underdrain pipes in a similar manner as in bioretention systems. systems may also have an overflow chamber. the sedimentation chamber can have permanent water or can be designed to be drained with weep holes between storm events. permanent water storage however, can risk anaerobic conditions that can lead to the release of pollutants (e.g. phosphorus). the design process should consider the provision of detention storage to yield a high hydrologic effectiveness, and discharge control by proper sizing of the perforated underdrain and overflow path. regular maintenance is required to prevent crust forming. porous paving porous paving (or pervious paving) is an alternative to conventional impermeable pavement and allows infiltration of runoff water to the soil or to a dedicated water storage reservoir below it in reasonably flat areas such as car parks, driveways, and lightly used roads, it decreases the volume and velocity of stormwater runoff and can improve water quality by removing contaminants through filtering, interception, and biological treatment. porous pavements can have several forms and are either monolithic