Heterogeneous Channel Routing Strategy for the National Water Model

Abstract:

The National Water Model (NWM) is a continental scale hydrologic modeling framework that provides forecast stream flow. Channel routing is a key component of the NWM and affects its ability to forecast flood depth, duration, and extent. The current operational NWM applies Muskingum-Cunge hydrologic routing, which is effective at moving water, but in situations where backwater effects are important, or in the case of complex channel and flood plain interactions, its accuracy can suffer. As discharge is the dependent variable in hydrologic routing, estimation of stage requires a rating curve. The domain of the NWM inherently covers a broad range of hydraulic conditions in the rivers and streams over the entire continental United States (CONUS). Thus, deploying computationally efficient hydraulic routing approaches is crucial to maintain the ability of the NWM to provide short-term river stage forecasts. To accommodate the balance between accuracy and computational efficiency, we developed and evaluated a heterogeneous routing method wherein the model internally transitions among the wave approximations. The approach evaluates the balance between friction, inertia and pressure and strategically triggers the appropriate approximation of the de St-Venant equations. This strategy strikes a balance among the primary performance metrics of operational and forecast models, namely, computational efficiency, accuracy, and minimization of computational instabilities. In this analysis, we present assessment of the performance in terms of accuracy and computational expense of the wave approximations. Results drawn from several examples ranging in size and flow conditions to highlight the potential benefits offered by the heterogeneous routing approach. Preliminary results indicate that the diffusive wave algorithm is computationally comparable to the Muskingum-Cunge routing technique but superior in terms of accuracy. The results also show that the dynamic-wave is needed only in cases where flood depths far exceed normal depths and when the flood hydrographs are “flashy.” The presentation ends with a discussion regarding plans for implantation of the heterogenous routing scheme in the National Water Model.