The Disconnected Sediment Conveyor Belt: Patterns of Longitudinal and Lateral Erosion and Deposition During a Catastrophic Flood in the Lockyer Valley, South East Queensland, Australia

The sediment (dis)connectivity concept is the water-mediated transfer of sediment between different compartments of a catchment sediment cascade involving four possible dimensions or linkages (longitudinal, lateral, vertical and temporal). Quantifying the strength of these linkages within and between compartments provides a means to understand the internal sediment flux dynamics of a catchment. The aims of this paper are to examine (1) the dynamics of longitudinal and lateral (dis)connectivity by quantifying patterns of erosion and deposition that occurred during a catastrophic flood, and (2) how the patterns of connectivity can be changed through management actions that better utilise floodplain sediment storages. Multi-temporal LiDAR and air photos are used to quantify volumetric change with respect to geomorphic settings and units. The results show that over the length of the trunk stream, the high-magnitude event was net depositional with high longitudinal sediment disconnectivity. At the reach scale, an alternating pattern of high and low longitudinal connectivity associated with contraction and expansion zones was evident. The efficiency of sediment transfer from the uppermost compartment to the most downstream compartment decreased exponentially, while the strength of lateral connectivity increased for each expansion reach. Modelling results show that increasing channel boundary roughness along expansion reaches with riparian revegetation can increase the frequency of lateral connectivity and floodplain sediment storage, thereby decreasing reach-to-reach connectivity and reducing end-of-catchment sediment delivery. This contrasts with the current trend of building levees along the bank tops of expansion reaches, which decrease lateral connectivity and increase reach-to-reach connectivity.