Grassed swales for urban storm drainage
Sammanfattning: This thesis investigates the environmental, economic and functional aspects of using grassed swales for the conveyance and treatment of stormwater. A grassed swale is a shallow, grass-lined channel. The environmental aspects relate to the transport and retention of stormwater pollutants in grassed swales and the utilisation of natural resources for the construction and operation of different stormwater transport systems (e.g. grassed swales). The economic aspects considered are the costs needed to construct and operate stormwater transport systems. The investigated functional aspects of grassed swales relate to conveyance and infiltration. Two full-scale studies were performed: 1) a study of sediment removal in a 70-m long grassed swale (Gammelstad, Luleå) and 2) a study of the transport and retention of suspended solids and heavy metals in a 110-m roadside grassed swale (Södra Hamnleden, Luleå). Particle trapping and flow conditions in different grassed swales were analysed using a standardised runoff event simulation procedure where water and road sediment were mixed in a 1-m3 tank and then pumped into a swale. The utilisation of resources of a pipe system and a swale system was analysed. It was suggested that the use of physical resources (in terms of exergy) could be one indicator on environmental sustainability and that a cost analysis would measure how society (human beings) valued these resources. Grassed swales with fully developed turfs and mild bottom slopes (<7%) provide high flow resistance. Grassed swales require lower amounts of natural resources, in terms of exergy, than a comparable stormwater pipe system. A grassed swale is more cost-efficient than a pipe system in areas with low land prices and good topsoil. The variations in pollutant removal efficiency of a grassed swale could be explained by the variations in influent pollutant concentrations. In general, grassed swales may be regarded as facilities that even out the peaks in pollutant loads without being capable of producing consistent high removal rates. Low to moderate removal efficiencies could be expected for heavy metals, especially metals in solution (i.e. the dissolved phase). Pollutant load reductions in grassed swales may be considerable over an extended time period with several successive rain events. This thesis shows that there are exponential relationships between grassed swale sediment removal potential and mean hydraulic detention time. Furthermore, surface loading or specific swale area (i.e. the ratio between swale area and contributing impervious drainage area) might be used as design parameters when constructing grassed swales for pollution control.
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