The effect of upstream sediment supply on flood risk

Fluvial hydrodynamics and morphodynamics are closely linked in alluvial rivers. Flood risk conditions under future predictions generally consider only changes in hydrological forcing, while the combination of altered hydrology and sediment supply is often neglected. Furthermore, the long term feedback between hydrodynamics and morphodynamics that can result in changes in channel carrying capacity, and therefore flood risk, are also omitted from most flood modeling efforts. In this study, we examine the combined effects of altered hydrology, changes in sediment supply, and feedback between hydro- and morphodynamics on flood risk in a lowland river with high sediment supply. Particularly, we use a depth-averaged hydro- and morphodynamic model (Delft3D Flexible Mesh) to simulate flood risk in the lower 50 km of the Nooksack River in northwestern Washington State. We use field observations to initialize and calibrate model hydro- and morphodynamics. The implications of combined changes in hydrology and sediment supply are evaluated by performing decadal scale simulations that simultaneously vary upstream water and sediment discharge. Three hydrologic scenarios are tested (dry, normal, wet) as well as three sediment supply scenarios (low, medium, high), resulting in nine morphological states that are evaluated using a design flood. Findings from this study connect changes in upstream sediment supply and hydrology with changes in channel morphology and the resultant flood risk.