The geomorphic impact of large in-stream wood on incisional avulsions

© 2017 Dr. Justin StoutMany anabranching rivers depend on the development and maintenance of avulsions. The avulsion, or formation of a new channel across the floodplain of such rivers,is integral in the construction of the floodplain stratigraphy. The load of large in-stream wood in a river channel interacts with the routing of flow and sediment in the channel and has received much attention over the last three decades, but the geomorphic role of wood in avulsions has not been quantified. This thesis investigates the relative role of large in-stream wood on the avulsion process. A detailed wood census was combined with LiDAR data and strategic stratigraphic coring on the King River (northeast Victoria, Australia). This multi-tiered approach was used to address four key areas: First, the overall length of each evolutionary stage of the avulsion process was calculated using floodplain sediment cores and Optically Stimulated Luminescence (OSL) dating techniques. The length of each stage was necessary to estimate how long the load of wood was in contact with the river channel. Second, to describe the spatial distribution of wood loads within the river a statistical analysis resulted in the novel use of the Weibull distribution. This provided a way to describe how both the wood load and its distribution may alter over time. Third, the delivery and removal processes of wood to the channel was modelled using a wood budget. Fourth, the wood delivery model was combined with a channel evolution model developed to simulate the avulsion process and the delivery of wood to the channel. Results show it takes an average of 4920 years for the complete avulsion process to pass through all five evolutionary stages. The development and completion of the avulsion (Stages 1-2) takes 1380 years, after which it takes a further 3540 years for the channel to become inefficient and develop a new avulsion (Stages 3-5). Given a healthy riparian forest, the accumulation of a steady state natural load of wood requires approximately 270 years, but this is dependent on the balance between wood delivery and removal processes within the channel, and the evolutionary stage of the avulsion. Throughout the entire avulsion process, wood is a dominant hydraulic inefficiency factor 21 % of the time. During avulsion development, wood in the channel provides approximately 50 % of the hydraulic roughness compared to the bed and bends. Wood may also act as a potential trigger of hydraulic inefficiency for 10% of the time towards the end of the avulsion lifecycle. This thesis advances previous models of anabranching. For each stage of development processes have been described, stratigraphic markers identified, and dated. This has allowed, for the first time in Australia, the sequence of anabranch development to be dated. The role of wood in anabranch development has now been quantified, and a model set up that can be adapted by others. The results can now inform management decisions on passive vs. active wood recovery. The role of in-stream wood, riparian and floodplain forests cannot be ignored in the process of river channel avulsion