The strange case of First Creek - If the flood doesn't fit the curve, should the curve fit the flood?

Abstract

First Creek has its source at Mount Lofty, and discharges through the eastern suburbs of Adelaide. Floodplain mapping has been carried out for the catchment, but the derivation of design hydrographs has been complicated by the behaviour of the rural upper catchment. The upper catchment lies within a conservation park, and is largely unaltered from its natural state. Flood frequency analysis carried out for the study identified one outlier in 27 years of record, where a large flow occurred two weeks after a major bushfire affected the majority of the catchment. This outlier was discounted, and the design hydrographs produced. However during the final preparation of the flood maps, a second flood occurred that was several times the magnitude of the previously estimated 100 year Average Recurrence Interval Flood, from a rainfall event of approximately 10 to 20 years ARI for the critical duration. A review of catchment condition following the flood event revealed that failure of Wilson's Bog (a valley-fill deposit) might have contributed to the high flood peak. While the impact of such failures is transient, there is a risk that similar events could happen in the future. The flood frequency used for mapping thus had to take into account the potential for large flood peaks resulting from changes in catchment behaviour due to fire and stream geomorphic processes. The derivation of predicted flows for low probability events in this catchment has implications for many small catchments in Australia.