Flood energy dissipation in anabranching channels

Abstract

AbstractThis study examines the character of developing anabranched channel networks on the River Wear, north England, using metre‐scale aerial LiDAR. DSM‐DTM interpretation reveals a well‐developed vegetation structure and a locally diverse terrain, dominated by an interlinked channel network split by low‐elevation depositional areas with the gross morphology of the reach resembling that of a strongly active meandering/wandering channel suggesting that an anabranching network may develop within systems that were initially active meandering and wandering, evolving in line with floodplain vegetative succession. Utilization of the LiDAR DEM in the hydrological component of the CAESAR‐Lisflood (version 1.4) morphodynamic model has generated local hydraulic variable estimates through the anabranched reaches for a range of flows. These data clearly demonstrate how elevated flows are transferred out of the primary channel and distributed along the interconnected secondary channel network, creating a diverse set of hydraulic environments. Areas between the channels rapidly become inundated as flows increase, dissipating flow energy. Shear stress estimates throughout the study site reveal a generally reduced ability to mobilize sediments and erode channel margins, in comparison with a single‐thread reach immediately downstream. Anabranched secondary channels appear to operate in disequilibrium and act predominantly as aggradational zones, although with some evidence of scour at channel bifurcation and confluence points. It would appear that the topographic character of anabranching sites efficiently manages flood flow energy, activating secondary channels and low‐elevation areas to distribute flood flows. These findings contrast with the hydraulic data from an adjacent single‐thread reach, characterized by flood flows concentrated in‐channel creating a high erosive potential. We propose that anabranching rivers could play an important role in natural flood and sediment management in many U.K. river systems.