Landcom Book 4 Maintenance - WSUD

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Water Sensitive Urban Design 3 | WSUD Elements Examples of bioretention systems in streetscapes 3.1 Bioretention Basins Bioretention systems can provide a degree of flow attenuation and hence reduce the volume and frequency of runoff delivered to downstream waterways. Bioretention systems are vegetated filter systems designed to allow water to pool temporarily before percolating through the filter media. The filter media controls the flowrate of water through the system, as well as providing a growing media for the plants. The filtered water is directed via perforated pipes to the existing stormwater system, natural waterways or a detention basin for reuse. Bioretention systems can provide a degree of flow attenuation and hence reduce the volume and frequency of runoff delivered to downstream waterways (ARQ, 2006). Critical to the performance of a bioretention system is the filter media and vegetation. The filter media needs to provide a hydraulic conductivity that ensures sufficient contact time is available for pollutants to be taken up by biofilms. Vegetation root systems provide the surfaces for the epiphytic biofilms that take up dissolved pollutants. Vegetation is also critical in maintaining the porosity of the soil media of the bioretention system. Bioretention systems can be integrated into open space areas and streetscapes (for example, parking stations and traffic calming devices). The systems can take the form of a basin or a swale. A basin is typically employed for relatively flat areas. Swales provide a stormwater treatment and flow conveyance role and are suitable for long linear sites with a grade ranging between one and four percent. For some sites, a combined bioretention basin and a conventional swale may be required. 8 Book 4 | MAINTENANCE
Water Sensitive Urban Design Examples of constructed wetlands 3.2 Constructed Wetlands and Ponds The deep open water bodies typical of ponds provide larger detention volumes as compared to constructed wetlands. Constructed wetlands use enhanced sedimentation, fine filtration and biological uptake processes to remove pollutants from stormwater. The wetland processes are engaged by slowly passing runoff through heavily vegetated areas where plants filter sediments and pollutants from the water. Biofilms that grow on the plants can absorb nutrients and other associated contaminants. Ponds also provide physical, biological and chemical mechanisms for pollutant uptake. The main physical difference between a pond and a constructed wetland is the ratio of surface area to volume and the coverage by vegetation (ARQ, 2006). Typically ponds have a depth exceeding 1.5 meters as compared to constructed wetlands, which have an average depth in the macrophyte zone of 0.3 metres. Furthermore, a regular water level fluctuation regime is promoted in a constructed wetland through the design of the outlet structures. Combined, the high surface area to volume ratio and the promotion of regular water level fluctuation through the design of the outlet structure in a constructed wetland, supports a densely vegetated system and hence a greater biological uptake of pollutants as compared to a pond. The deep open water bodies typical of ponds provide larger detention volumes as compared to constructed wetlands. Higher detention times promote improved sedimentation; however the risk of large open water bodies is short circuiting, redox potential conditions and elevated levels of nutrients. These processes can have a reverse effect on stormwater treatment causing release of pollutants from the sediment into the water. High nutrient concentrations can also lead to nuisance macrophyte growth or algal blooms that can be toxic and aesthetically unpleasant. For these reasons, ponds are commonly designed in combination with constructed wetlands, providing polishing of stormwater quality, attenuation of flows and protection of downstream waterways, and storage for reuse applications. Book 4 | MAINTENANCE 9
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Level 2, 330 Church Street Parramat
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Landcom Book 4 Maintenance - WSUD

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