Safe Buffer Distances for Offstream Earth Dams

Abstract

On the Darling Downs in Queensland there is a proliferation of offstream water supply storages (known locally as ring tanks), used mainly for irrigation of broad-acre crops. Many ring tanks are sited close to public roads for many reasons; the sealed roads provide all-weather access to irrigation areas and particularly waterharvesting pumps but also provide a hard catchment in close proximity to the storage thus improving the frequency of harvestable overland flows. There are many ring tanks in the district located within 50m of public roads. The question immediately arises of the potential safety hazard to road users in the event of dam failure. In this paper the NWS FLDWAV model was applied to simulate the dam break flood wave for a typical ring tank using a planar surface of varying slope and roughness. The model output was then analysed with regard to vehicle stability to determine a safe buffer distance between the ring tank and road. It was found that the flood wave would produce a subcritical flow on outlet slopes typical of the area for which vehicle stability would be controlled by buoyancy rather than inertia. The impact of the dam break wave on stability would be reduced by the hydraulic effect of the elevated roadway and through the maintenance of a smooth outlet. A minimum buffer distance of250m is recommended for smooth, mild outlet slopes in which the flow is impeded by an elevated roadway, increasing to up to 400m depending on the final breach width.