Minnegang Creek Flood Study Report - Wollongong City Council

The catchment was divided into 64 sub-catchments. Each sub-catchment within theRAFTS model was split into a pervious area and an impervious area. The imperviousarea of the sub-catchment was determined by measuring the area of roads anddeveloped area within each sub-catchment. The roads were considered to be 95%impervious and developed areas (largely med ium density residential) to be 40%impervious.4.3.2 Hydraulic roughnessThe hydraulic roughness of each sub-catchment is represented by a Manning’s nvalue. Based on recommendations outlined in the RAFTS User Manual (WPSoftware, 1994), the following roughness values have been adopted for use in themodel:• 0.015 - impervious areas (typical of asphalt or rough concrete surface)• 0.025 - pervious areas (typical of short grass)4.3.3 Rainfall lossesRainfall losses for the catchment were entered into the RAFTS model us ing theinitial/continuing loss method. This requires that an initial loss be estimated tosimulate the initial wetting of the catchment when no runoff is generated. A constantcontinuing loss accounts for infiltration once the initial loss process is com pleted. Inadopting the rainfall loss values for the Minnegang Creek catchment, considerationwas given to the catchment topography, underlying soil types, recommended valuesgiven in Australian Rainfall and Runoff (AR&R) (Institution of Engineers, 1987), theRAFTS User Manual (WP Software, 1994) and previous experience in theWollongong area.Losses were varied between the minimum and maximum values shown in Table 4-1 todetermine the sensitivity of the model to rainfall losses. It was found that the flowswere only affected to a minor amount (