Critical hydro-geomorphological characteristics related to lateral erosion–deposition difference within the multilevel bifurcation system of the Yangtze Estuary, China

Abstract

Swings of ebb flow axes among branching channels alter the lateral hydrodynamics within bifurcating estuaries, causing the channels to undergo erosion–deposition transitions. This study examines the three-order bifurcation system of the Yangtze Estuary through integrated analysis of observed water–sediment and terrain data from 1950 to 2022 and simulations by a shallow flow model based on Delft 3D. The model is shown to perform well in hindcasting the flow behavior in the Yangtze Estuary and in identifying critical runoff discharges at which the ebb flow axis migrates between the north and south branching channels of each-order bifurcation under corresponding tidal ranges. These critical runoff discharges occur at mutation points where ebb partition ratios of the north/south branching channels increase/decrease abruptly with gradually increasing runoff. By applying linear regression to critical runoff discharges and corresponding tidal ranges at each bifurcation order, the value domain divides into two subareas corresponding to the position of the ebb flow axis in north and south branching channels. We find the multiyear average duration days of ebb-flow-axis location to be effective as an indicator of the mean and spread of erosion–deposition in the branching channels and detect the critical values at erosion–deposition transitions. Other influence factors, including peak river discharge, sediment flux, offshore dynamics, and local engineering projects, also impact on erosion–deposition in the branching channels. The ranked orders of critical runoff discharges and critical duration days among the three-order bifurcations are interpreted, and these critical indexes are linked to the water excavating force, indicating their reliability and effectiveness. Seasonal flattening of runoff discharge, controlled by large cascade reservoirs in the upper Yangtze, has greatly shortened and lengthened location duration of ebb flow axis in the north and south branching channels at each-order bifurcation, and is likely to maintain their respective deposition and erosion behaviors in the future.