USING TIME-LAPSE LIDAR TO QUANTIFY RIVER BEND EVOLUTION ON THE MEANDERING COASTAL TRINITY RIVER, TEXAS, USA

Time-lapse airborne LiDAR on the coastal Trinity River in east Texas, USA, shows profound downstream variability in point-bar growth and cut-bank erosion as a result of a historically large flood. The difference map generated from two surveys separated by 4 years covers 55 river bends (65 river kilometers) and captures the downstream transition from quasi-uniform flow into backwater flow. In the upstream portion of the survey, reach-averaged deposition is sub-equal to erosion despite more localized variability between the two due to the occurrence of bend cutoffs. In the backwater zone, cut-bank erosion consistently outpaces deposition on point-bars with minor associated variability. The sub-aerial point-bar area decreases by more than an order of magnitude within the survey (59,000 m² upstream to 5,700 m² in the backwater zone) before point-bars become entirely and continuously sub-aqueous. Channel width, measured from scroll-bar to outer bank, increases for the majority of bends. However, in the quasi-uniform portion of the survey the widening is often due to outward migration of the scroll-bar. The combination of net erosion and channel widening within the backwater zone implies that during large sustained floods, bank pull is the mechanism that drives channel migration in this area. Farther upstream, both the inner and the outer bank move in concert to facilitate overall channel migration. Patterns of deposition and erosion within a single bend suggest that during flood, channel migration is dominated by downstream bend translation with little associated bend deformation. Results from this study help to deepen the understanding of coastal river system morphodynamics by connecting changes in the geomorphology to changes in river bend kinematics, hydraulics, and sediment transport.