Effects of Unsteady River Discharge on Deltaic Morphodynamics

Morphological evolution of river deltas is subject to hydrodynamic forcings such as river discharges, waves and tides. Among those forcings, rivers, regardless of their scales and geographic locations, are characterized with natural and human-induced variability in their discharges. While previous studies have demonstrated the effects of both inter- and intra-annual variabilities of unsteady river discharge on delta morphological evolution, the specific effects of intra-annual unsteadiness on the morphological evolution of river deltas has remained hitherto elusive. To address this issue, numerical experiments with simplified unsteady discharge scenarios with recurrent intra-annual variability were performed in Delft3D and compared with that assuming constant bank-full discharge. The progradation of the entire delta planform and the evolution of the mouth bar representing the regional and local effects, respectively, were explored. The results showed that the overall trends of the delta progradation and the ultimate delta area created were found to be similar between the unsteady discharge scenarios and their corresponding constant discharge scenarios, while the nuances of intermittent zig-zag variation in natural delta area were well reproduced by unsteady discharge scenarios. In addition, predictions on decadal-scale delta progradation suggested the potential existence of a tipping point in the delta area growth trajectory beyond which the delta area declines during periods of low discharge. For the evolution of mouth bar at local scale, numerical simulations suggested the existence of three regimes for mouth bar formation, namely, stable, ephemeral, and absent, which were dictated by the relative wave strengths during both periods of high and low flows. Suppression of mouth bar formation further led to the inhibition of deltaic distributary networks. The results have important implications for delta protection and restoration measures, such as the water-sediment regulation scheme in the Yellow River and artificial channel diversions in the Mississippi Delta.