Post-urban water prediction modelling gnangara mound, Western Australia

Urban drainage design has traditionally been based on monitoring of the pre-development groundwater regime alone. However, the increase in groundwater levels due to urban development has often not been taken into account in the design process. Drainage design based on the traditional approach has resulted in significant inundation problems in some residential developments, with drainage retrofitting being logistically and socially difficult and very expensive. RPS applied an approach to control groundwater levels and set finished lot levels for the highly constrained Whiteman Edge development on the Gnangara Mound. The approach was based on the prediction of postdevelopment groundwater levels which was undertaken using detailed computer generated groundwater flow models. The approach included (1) assessment of the regional pre-development hydrological regime including monitoring of groundwater levels and assessing soil types; (2) construction of a regional groundwater model that is calibrated to pre-development measured groundwater levels, seasonal fluctuations and long-term trends; (3) manipulation of the model to incorporate proposed fill material and increased rainfall recharge rates of the urban development. The model outputs indicated the site would be inundated without groundwater control provisions; and (4) the model outputs were used to design appropriate subsurface drain spacing and pipe sizing (via the Hazen-Williams Equation) to ensure adequate groundwater clearance from finished lot levels. Although the model has a site focus, its regional extent means that it can be readily manipulated for other investigations in the study area.