The effect of climate change on catchment flows is currently the subject of much debate. Although rainfall events are predicted to become more intense, the effect of other hydrological drivers, such as soil moisture, evaporation, and infiltration are less well defined. For the coastal hazard from flooding to be increased, the catchment flows must also occur simultaneously with the peak tide and storm-surge levels. For the Barwon River, analysis indicates no correlation between flood flows and storm surge in the long-term data record (Swan et. al., 2010). For most catchments along the Bellarine Peninsula, short duration storms (in the order of up to 6 hours) are likely to be the storms that will produce the highest run off. These are generally thunderstorm type events that are not associated with high storm-surge levels.

Catchment flows have been assessed for the Barwon River, upstream of Lake Connewarre. Two inflow scenarios have been assessed (Table 4-5). One is a constant flow calculated as the 99th percentile daily average flow in the Barwon River based on four years of measured flows. This flow, which is 87 m ³/s, was applied for all sea-level rise events. The second inflow was the 10% AEP flood event, which was timed to coincide with the peak 1% AEP storm-tide level in the Barwon Estuary for the 0.0 m, 0.8 m and 1.4 m SLR scenarios. The probability of the 10% AEP catchment flow case occurring at the peak of a storm surge is extremely low; hence the high daily-average flow case has been the focus of the modelling, with the 10% AEP catchment flow case assessed for sensitivity. Assessment of the high daily-average flow case with the 1% AEP storm-tide inundation event also allows the implications of the storm-tide inundation to be assessed independently. The results are presented to give an indication of the possible maximum impact should this very rare combination of events occur. The magnitude of the 10% AEP catchment flow event is 670 m ³/s and the adopted hydrograph shape (based on the January 2011 flood event) is shown in Figure 4-2. For other locations, the effects of catchment flows are less significant, and therefore have not been incorporated

Table 4-5     Study modelling scenarios + catchment flows for Barwon hydrodynamic models

Figure 4-2     Barwon River inflow - 10% AEP catchment flow (arbitrary dates)