Powells Creek Flood Study
4.
HYDROLOGIC MODELLING
4.1.
Sub-catchment Definition
The total catchment represented by the current DRAINS model is 9.14 km2. This area has been represented by 781 sub-catchments (Figure 10) giving an average sub-catchment size of approximately 1.17 hectares. The sub-catchment delineation ensures that where hydraulic controls exist that these are accounted for and able to be appropriately incorporated into hydraulic routing. The pit and pipe network is shown on Figure 11. The drainage system defined in the model comprises: •
•
•
4.2.
1457 pipes. 1593 inlet pits. 487 junction pits.
Impervious Surface Area
Runoff from connected impervious surfaces such as roads, gutters, roofs, or concrete surfaces occurs significantly faster than from vegetated surfaces. This results in a faster concentration of flow within the downstream area of the catchment and increased peak flow in some situations. It is therefore necessary to estimate the proportion of the catchment area that is covered by impervious surfaces. DRAINS categorises these surface areas as either: •
Paved areas (impervious areas directly connected to the drainage system).
•
Supplementary areas (impervious areas not directly connected to the drainage system; instead, connected to the drainage system via the pervious areas) and Grassed areas (pervious areas).
•
Within the Powells Creek catchment, the impervious value was determined using the Table 14 and the land types within each sub catchment. The proportion of pervious area and remaining impervious area was defined as: • For sub catchments with imperviousness below 25% (typically parks), the pervious area is defined as 70% of the non-impervious area and the remaining impervious area is defined as 30% of the non-impervious area. •
For sub catchments with imperviousness above 25% (typically residential properties), the pervious area is defined as 30% of the non-impervious area and the remaining impervious area is define as 70% of the non-impervious area.
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