Table 13: Validation of 1% AEP Event Unit Flow Rates

8.3 Model System Validation

8.3.1 Unit Flow Rate Estimates

Comparison of study area unit flow rates estimates (also known as the specific yield) for the 1 % AEP event, with known values from the Sydney metropolitan area has been undertaken as a means of verification of the flood modelling system. The unit flow rate refers to the peak flow generated per unit area and has units of m3/s per hectare.

Calculations of unit flow rate area affected by the following factors:

• Catchment slope: a steeper catchment will tend to produce a higher rate of runoff; • Catchment roughness: a smooth catchment will tend to produce a higher rate of runoff than a vegetated catchment; • Catchment size: a larger catchment will tend to produce lower specific yield. Accordingly, catchment areas of up to 20 hectares only are recommended and were selected for the analysis (see Table 13); and • Flow obstructions: catchments with defined flow paths (such as roads in urban areas) will produce higher rates of runoff than catchments where flow is obstructed (such as flow which meanders between buildings).

Across the Sydney Metropolitan area, typical unit flow rates range between 0.3 to 0.6 m3/s per hectare for the 1% AEP event (ARR 1987), depending on the individual catchment characteristics.

Comparable catchments within Sydney include:

• Hawthorne Canal (Inner West Council): This catchment in similarly dense and urban. A unit flow rate of 0.5 m3/s per hectare was calculated for the 1 % AEP; and • North Sydney (North Sydney Council): North Sydney is a dense, urban catchment with an average slope of 10% with flow mainly confined to roadways. An average unit flow rate of 0.52 m3/s per hectare was calculated for the 1% AEP event.

In comparison, Exile Bay has an average slope of 4% and many upstream flow paths meander through areas of residential development and as such unit flow rate was expected to be relatively low. Four locations were selected and the unit flow rates were calculated (see Table 13). The 1% AEP (ARR 1987) peak flood depths and the location of the four sample areas are shown in Figure 19 .

Table 13: Validation of 1% AEP Event Unit Flow Rates

ID Area (ha) Peak Flow (m3/s)

1 10.6 3.6 2 7.3 2.5 Unit Flow Rate (m3/s per hectare) 0.34 0.34

3 9.5 4.0 4 14.9 5.5 0.42 0.37 Average 0.37

It was found that the calculated unit flow rates, presented in Table 13, align with the calculated flow rates in similar Sydney Metropolitan areas and the specific catchment characteristics. As such, the flood modelling system is producing robust design flow estimates.

The 1% AEP 2 hour event (ARR 1987) was specifically derived for model validation purposes to ensure continuity between comparisons at other Sydney Metropolitan catchments.

8.4 Summary

The model system built to represent flooding in the Exile Bay catchment has been demonstrated to recreate historical flood behaviour (based on eight community observations distributed throughout the study area).

Further, the model’s suitability for design flood estimation was also assessed, by comparison of 1% AEP estimate, against unit flow rates with those achieved for other similar catchments in the Sydney Metropolitan area. This verification established that model estimates are reasonable.

Overall, it was determined that the model was suitable for design flood estimation.