or modular. monolithic structures consist of a single continuous porous medium such as porous concrete or porous pavement (asphalt) while modular structures include porous pavers individual paving blocks that are constructed so that there is a gap in between each paver. commercial products that are available are for example, pavements made from special asphalt or concrete containing minimal materials, concrete grid pavements, and concrete ceramic or plastic modular pavements. porous pavements are usually laid on a very porous material (sand or gravel), underlain by a layer of geotextile material. maintenance activities vary depending on the type of porous pavement. generally, inspections and removal of sediment and debris should be undertaken. modulate pavers can also be lifted, backwashed, and replaced when blockages occurs. generally porous pavement is not suited for areas with heavy traffic loads. particulates in stormwater can clog pores in the material. sedimentation basins sediment basin installed on a construction site. sedimentation basins (otherwise known as sediment basins) are used to remove (by settling) coarse to medium-sized sediments and to regulate water flows and are often the first element in a wsud treatment system. they operate through temporary stormwater retention and reduction of flow velocities to promote settling of sediments out of the water column. they are important as a pretreatment to ensure downstream elements are not overloaded or smothered with coarse sediments. sedimentation basins can take various forms and can be used as permanent systems integrated into an urban design or temporary measures to control sediment discharge during construction activities. they are often designed as an inlet pond to a bioretention basin or constructed wetland. sedimentation basins are generally most effective at removing coarser sediments (125 μm and larger) and are typically designed to remove 70 to 90% of such sediments. they can be designed to drain during periods without rainfall and then fill during runoff events or to have a permanent pool. in flow events greater than their designed discharge, a secondary spillway directs water to a bypass channel or conveyance system, preventing the resuspension of sediments previously trapped in the basin. constructed wetlands further information: constructed wetlands constructed wetlands are designed to remove stormwater pollutants associated with fine to colloidal particles and dissolved contaminants. these shallow, extensively vegetated water bodies use enhanced sedimentation, fine filtration, and biological uptake to remove these pollutants. they usually comprise three zones: an inlet zone (sedimentation basin) to remove coarse sediments; a macrophyte zone, a heavily vegetated area to remove fine particulates and uptake of soluble pollutants; and a high flow bypass channel to protect the macrophyte zone. the macrophyte zone generally includes a marsh zone as well as an open water zone and has an extended depth of 0.25 to 0.5m with specialist plant species and a retention time of 48 to 72 hours. constructed wetlands can also provide a flow control function by rising during rainfall and then slowly releasing the stored flows. constructed wetlands will improve the runoff water quality depending on the wetland processes. the key treatment mechanism of wetlands are physical (trapping suspended solids and adsorbed pollutants), biological and chemical uptake (trapping dissolved pollutants, chemical adsorption of pollutants), and pollutant transformation (more stable sediment fixation, microbial processes, uv disinfection). the design of constructed wetlands requires careful consideration to avoid common problems such as accumulation of litter, oil, and scum in sections of the wetland, infestation of weeds, mosquito problems or algal blooms. constructed wetlands can require a large amount of land area and are unsuitable for steep terrain. high costs of the area and of vegetation establishment can be deterrents to the use of constructed wetlands as a wsud measure. guidelines for developers (such as the urban stormwater: best practice environmental management guidelines in victoria) require the design to retain particles of 125μm and smaller with very high efficiency and to reduce typical pollutants (such as phosphorus and nitrogen) by at least 45%. in addition to stormwater treatment, the design criteria for constructed wetlands also include enhanced aesthetic and recreational values, and habitat provision. the maintenance of constructed wetlands usually includes the removal of sediments and litter from the inlet zone, as well as weed control and occasional macrophyte harvesting to maintain a vigorous vegetation cover. swales and buffer strips two swales for a housing development. the foreground one is under construction while the background one is established. swales and buffer strips are used to convey stormwater in lieu of pipes and provide a buffer strip between receiving waters (e.g. creek or wetland) and impervious areas of a catchment. overland flows and mild slopes slowly convey water downstream and promote an even distribution of flow. buffer areas provide treatment through sedimentation and interaction with vegetation. swales can be incorporated in urban designs along streets or parklands and add to the aesthetic character of an area. typical swales are created with longitudinal slopes between 1% and 4% in order to maintain flow capacity without creating high velocities, potential erosion of the bioretention or swale surface and safety hazard. in steeper areas check banks along swales or dense vegetation can help to distribute flows evenly across swales and slow velocities. milder-sloped swales may have issues with water-logging and stagnant ponding, in which case underdrains can be employed to alleviate problems. if the swale is to be vegetated, vegetation must be capable of withstanding design flows and be of sufficient density to provide good filtration). ideally, vegetation height should be above treatment flow water levels. if runoff enters directly into a swale, perpendicular to the main flow direction, the edge of the swale acts as a buffer and provides pre-treatment for the water entering the swale. ponds and lakes ponds and lakes are artificial bodies of open water that are usually created by constructing a dam wall with a weir outlet structure. similar to constructed wetlands, they can be used to treat runoff by providing extended detention and allowing sedimentation, absorption of nutrients, and uv disinfection to occur. in addition, they provide an aesthetic quality for recreation, wildlife habitat, and valuable storage of water that can potentially be reused for e.g. irrigation. often, artificial ponds and lakes also form part of a flood detention system. aquatic vegetation plays an important role for the water quality in artificial lakes and ponds in respect of maintaining and regulating the oxygen and nutrient levels. due to a water depth greater than 1.5m, emergent macrophytes are usually restricted to the margins but submerged plants may occur in the open water zone. fringing vegetation can be useful in reducing bank erosion. ponds are normally not used as stand-alone wsud measure but are often combined with sediment basins or constructed wetlands as pretreatments. in many cases, however, lakes and ponds have been designed as aesthetic features but suffer from poor health which can be caused by lack of appropriate inflows sustaining lake water levels, the poor water quality of inflows and high organic carbon loads, infrequent flushing of the lake (too long residence time), and/or inappropriate mixing (stratification) leading to low levels of dissolved oxygen. bluegreen algae caused by poor water quality and high nutrient levels can be a major threat to the health of lakes. to ensure the long-term sustainability of lakes and ponds, key issues that should be considered in their design include catchment hydrology and water level, and layout of the pond/lake (oriented to dominant winds to facilitate mixing. hydraulic structures (inlet and outlet zones) should be designed to ensure adequate pre-treatment and prevent large nutrient 'spikes' landscape design, using appropriate plant species and planting density are also necessary. high costs of the planned pond/lake area and of vegetation establishment as well as frequent maintenance requirements can be deterrents to use of ponds and lakes as wsud measures. the maintenance of pond and lake systems is important to minimize the risk of poor health. the inlet zone usually requires weed, plant, debris, and litter removal with occasional replanting. in some cases, an artificial turn over of the lake might be necessary. rainwater tanks rainwater tanks (see also rainwater harvesting) are designed to conserve potable water by harvesting rain and stormwater to partially meet domestic water demands (e.g. during drought periods). in addition, rainwater tanks can reduce stormwater runoff volumes and stormwater pollutants from reaching downstream waterways. they can be used effectively in domestic households as a potential wsud element. rain and stormwater from rooftops of buildings can be collected and accessed specifically for purposes such as toilet flushing, laundry, garden watering, and car washing. buffer tanks allow rain water collected from hard surfaces to seep into the site helps maintain the aquifer and ground water levels. in australia, there are no quantitative performance targets for rainwater tanks, such as size of tank or targeted reductions in potable water demand, in policies or guidelines. the various guidelines provided by state governments however, do advise that rain water tanks be designed to provide a reliable source of water to supplement mains water supply, and maintain appropriate water quality. the use of rainwater tanks should consider issues such as supply and demand, water quality, stormwater benefits (volume is reduced), cost, available space, maintenance, size, shape, and material of the tank. rainwater tanks must also be installed in accordance with plumbing and drainage standards. an advised suitable configuration may include a water filter or first flush diversion, a mains water top-up supply (dual supply system), maintenance drain, a pump (pressure system), and an on-site retention provision. potential water quality issues include atmospheric pollution, bird, and possum droppings, insects e.g. mosquito larvae, roofing material, paints, and detergents. as part of maintenance, an annual flush out (to remove built-up sludge and debris) and regular visual inspections should be carried out. aquifer storage and recovery (asr) aquifer storage and recovery (asr) (also referred to as managed aquifer recharge) aims to enhance water recharge to underground aquifers through gravity feed or pumping. it can be an alternative to large surface storages with water being pumped up again from below the surface in dry periods. potential water sources for an asr system can be stormwater or treated wastewater. the following components can usually be found in an asr system that harvests stormwater : a diversion structure for a stream or drain a treatment system for storm water prior to injection as well as for recovered water a wetland, detention pond, dam or tank, as a temporary storage measure a spill or overflow structure a well for the water injection and a well for the recovery of the water systems (including sampling ports) to monitor water levels and water quality. the possible aquifer types suitable for an asr system include fractured unconfined rock and confined sand and gravel. detailed geological investigations are necessary to establish the feasibility of an asr scheme. the potential low cost of asr compared to subsurface storage can be attractive. the design process should consider the protection of groundwater quality, and recovered water quality for its intended use. aquifers and aquitards need also be protected from damaged by depletion or high pressures. impacts of the harvesting point on downstream areas also require consideration. careful planning is required regarding aquifer selection, treatment, injection, the recovery process, and maintenance and monitoring. policy, planning, and legislation in australia, due to the constitutional division of power between the australian commonwealth and the states, there is no national legislative requirement for urban water cycle management. the national water initiative (nwi), agreed upon by federal, state, and territory governments in 2004 and 2006, provides a national plan to improve water management across the country. it provides clear intent to "create water sensitive australian cities" and encourages the adoption of wsud approaches. national guidelines have also been released in accordance with nwi clause 92(ii) to provide guidance on the evaluation of wsud initiatives. at the state level, planning and environmental legislation broadly promotes ecologically sustainable development, but to varying degrees have only limited requirements for wsud. state planning policies variously provide more specific standards for adoption of wsud practices in particular circumstances. at the local government level, regional water resource management strategies supported by regional and/or local catchment-scale integrated water cycle management plans and/or stormwater management plans provide the strategic context for wsud. local government environment plans may place regulatory requirements on developments to implement wsud. as regulatory authority over stormwater runoff is