Prioritising the placement of riparian vegetation to reduce flood risk and end-of-catchment sediment yields: Important considerations in hydrologically-variable regions

In perennial stream settings, there is abundant literature confirming that riparian vegetation affects flood hydrology by attenuating the flood wave, enhancing deposition and reducing bank erosion. In contrast, relatively little is known about the effectiveness of riparian vegetation during floods in hydrologically-variable regions. The dominant channel form in these settings is often referred to as a ‘macrochannel’ or compound channel-in-channel which displays multiple inundation surfaces where it is often difficult to identify the active channel bank and bank top. This study uses the inundation pattern of recent flood events in the Lockyer Valley of South East Queensland (SEQ), Australia to present a framework which specifically considers the interaction between inundation frequency and trapping potential on a range of inundation surfaces. Using hydrological modelling and a consistent definition of floodplains and within-channel features, it outlines five key priority areas for the placement of riparian vegetation to alleviate common flood problems within the catchment. The highest priority for the placement of riparian vegetation to ameliorate the effects of small-moderate floods is on within-channel benches. For out-of-macrochannel flows, riparian vegetation is most effective on genetic floodplains which occupy the largest spatial extent within the valley. In particular, it identifies the need for, and benefits of, revegetation in spill out zones (SOZ) which occur where upstream channel capacity is larger and flow is funnelled at high velocity onto the floodplain downstream. This study highlights the importance of understanding the key geomorphic processes occurring within a catchment and developing effective catchment management plans to suit these conditions.