Hydrologic-Hydraulic Modeling of Fluvial Sediment Transport During a Storm Event in a Highly Managed Watershed

Abstract:

Sediment movement along the main stem of a watershed is strongly affected by the sediment supply and the channel morphology. Anthropogenic interventions tend to alter the hydraulic conveyance and consequently modify the sediment regime of the main stem. This connection between channel hydraulics and sediment transport is often overlooked in hydrologic models where simplified methods are used for flow and sediment routing. In this study, we adopt a hydrologic-hydraulic modeling approach to quantify the fluvial sediment transport along the main stem of a watershed during a storm event. The hydrologic model CUENCAS is implemented to estimate the sub-hourly hydrographs of the major tributaries of the watershed. The simulated hydrographs are used as boundary conditions for the depth-averaged two-dimensional hydraulic model FESMWS to simulate the propagation of the flood wave along the main stem. The corresponding sub-hourly, unsteady non-equilibrium sediment transport along the main stem is also simulated with FESWMS. This procedure is applied to a highly managed agricultural watershed of Iowa. The study site has a catchment area of 70 Km² with soils that are silty clay loams. The land-use is mostly row crop, but in the past decade a large portion of the watershed was converted to native prairie. The main stem is a meandering stream with a length of 15 Km and ten major tributaries contribute to its flow. Several sections of the main stem have been heavily channelized and straightened since the 1930s. Different grain size distributions and sediment boundary conditions are investigated to discern the effects of land-use changes and channelization on the sediment regime along the main stem. The simulations are able to capture the typical hysteresis between flow and sediment transport. The results indicate that the in-stream sediment transport rate is in general higher in the channelized sections and depends, to a certain extent, on the degree of straightening.