Long-term, process-based morphodynamic modeling of a fluvio-deltaic system, part I: The role of river discharge

Tidal basin and delta morphodynamics have been broadly examined by numerical modeling efforts, but large scale estuarine morphodynamics under combined river and tidal forcing have been less examined. This study aims to explore morphodynamic development of a river- and tide-forced fluvio-deltaic system using a 2DH process-based numerical model (Delft3D). The model domain describes a 560. km long, convergent inner basin attached to a >50. km wide outer sea basin allowing for delta formation. Long-term (millennial) morphodynamic simulation is aided by a morphological factor approach which captures the feedback mechanisms between water motion, sediment movement, and morphology. The morphodynamic sensitivities to river discharge magnitude and its seasonal variations are systematically examined. Model results show that a fluvio-deltaic system develops with meandering channels and alternating sand bars in the estuarine part, and distributary channels with elongated sand bars in the delta. Sensitivity analysis shows that river discharge plays a profound role in the morphodynamic behavior by supplying sediment and enhancing ebb-directed residual transport. An intermediate river discharge can be defined which leads to a deeper estuarine basin. A river discharge smaller than the intermediate threshold limits ebb flow magnitude and erosive capacity. A river discharge larger than the threshold supplies much more sediment to the estuary so that the basin becomes shallower with larger meanders, more pronounced ebb tidal channels, and a larger delta. Based on our simulations we can differentiate tide-dominance from river-dominance in estuary classifications by ebb to flood ratios of flow and sediment fluxes. Overall, this modeling study extends understanding of morphodynamics of a comprehensive fluvio-deltaic system, thus allowing for a better management of estuaries and deltas.