Flood Mitigation
Seepage of floodwaters into the underlying or adjacent alluvial sediments can reduce the impact of a major flood event by mitigating peak flood levels. Many inland rivers in Australia have very low gradients with very wide and extensive floodplains. This results in floods with very slow travel times that can inundate large areas, providing the opportunity of significant infiltration and aquifer recharge. For example, the Diamantina River in southwestern Queensland has a floodplain that varies between 5 to 60 km in width, resulting in transmission losses amounting to 70-98% of flow for floods discharging up to 2300 GL (Costelloe et al, 2003). In smaller ephemeral streams, transmission losses can totally consume flood flows over relatively short distances downstream. A study of a storm event in a small (30 km2) semi-arid catchment in western New South Wales found that stream flow only reached 7.6km downstream (Dunkerley and Brown, 1999). Seepage into aquifers as well as evaporative losses and infilling of in-stream storages effectively reduces the magnitude of the flood as it progresses downstream.
Engineering works such as installation of levees are designed to limit the area of inundation resulting from a flood event. However, these works can also reduce the extent of floodplain seepage. As well as providing a mechanism for reducing flood peaks, this seepage can also have major implications for ecosystems by providing opportunities for high biological productivity in the floodplain. Also, episodic flooding is a major recharge mechanism for aquifers in the arid zone. Hence, there are opportunities for engineering works to divert flood flows to identified parts of the floodplain for flood mitigation as well as to help fulfil ecosystem requirements or for groundwater replenishment.
References
Costelloe JF, Grayson RB, Argent RM, McMahon TA, 2003. Modelling the flow regime of an arid zone floodplain river, Diamantina River, Australia. Environmental Modelling and Software 18, 693-703
Dunkerley D, Brown K, 1999. Flow behaviour, suspended sediment transport and transmission losses in a small (sub-bank-full) flow event in an Australian desert stream. Hydrological Processes 13, 1577-1588.