Author(s): Jonathan Nelson; Yasuyuki Shimizu; Richard Mcdonald
Keywords: Bedforms; Rivers; Morphodynamics
Abstract: We present an approximate technique for incorporating bedform computations from relatively small-scale high-resolution models in larger-scale flow and morphologic evolution codes suitable for use in long river reaches. The method integrates a two-dimensional streamwise/vertical (2DV) formulation for bedform modeling with a two-dimensional planform (2DH) river reach model. By incorporating the roughness predictions and form drag corrections from the 2DV model run along some average streamlines of the 2DH model, it is possible to incorporate relatively complex bedform responses into river-scale morphologic evolution. This method explicitly includes the initiation of bedforms, the washout of bedforms at high flows, and hysteresis effects, all of which are difficult to treat using typical empirical approaches. To illustrate the technique, an example computation for the Kootenai River in Idaho, USA is described. Comparing model results to field measurements shows the critical importance of including bedform dynamics for predicting the flow and sedimenttransport field correctly; it also demonstrates that these effects can drastically affect largerscale morphologic evolution, especially with regard to the rate of evolution of bar features. Until it is computationally feasible to use very high-resolution computational flow and sediment-transport models in rivers over scales of practical significance, this hybrid approach can be successfully used to improve predictive capability in flows with complex bedform behavior.