Morphological Development of Meandering Rivers Due to Changing Discharge Regimes

Abstract

This PhD thesis investigates the effects of changing discharge regimes on the morphological development of meandering rivers. The reduction of discharge causes bed morphology and planform changes in alluvial rivers with erodible banks, and restoring the discharge may alter the existing morphology, leading to a new (so far unpredictable) river course. The present thesis explores the capability of the open-source software Delft3D to simulate the adaption of both planform dynamics and bed topography to changed discharge regimes. A specific reach in the Dhaleshwari River (Bangladesh) is taken as a case study as it provides relevant data for model validation. Both two-dimensional (2D) and three-dimensional (3D) modelling approaches were applied. The 3D model, which was calibrated against measured 3D flow data, was used for predicting bed level changes over a one-year period. The simulated morphological changes showed a certain degree of resemblance with the available field data, but the required computational time prevented further analyses. Therefore, a 2D model, in which the parameterization of the 3D flow effect was validated against curved flume data, was used to simulate the morphological development for different discharge scenarios over a 10-year period. The results of the simulations revealed that the 2D model could predict scour depth, bank erosion, and riffle-pool sequences under both constant and varying discharge scenarios. However, the prediction of channel bankfull width showed some deficiencies. The simulations with varying discharge demonstrated a more realistic prediction of the meander planform than the simulations with constant discharge. The conclusion from this research is that a 2D modelling approach, in combination with a time-varying discharge, can be used to simulate the natural dynamics of meandering rivers, both in terms of the development of bed topography and channel planform. The results further revealed that a discharge magnitude of about 90% of the bankfull discharge represents the dominant discharge, and that the meander wavelength increases with the discharge magnitude. The results of the simulation for time-varying discharge records revealed that larger floods favor enlargement of meander wavelength and smaller floods favor shortening of meander wavelength during the first 50% of the simulation period. However, after the first 5-year period, the meander wavelength becomes nearly unresponsive to altered high- and low- floods.