Quantifying uncertainty in large-eddy simulation results of a natural river flow*

High-fidelity numerical modeling of rivers requires many assumptions related to implementing the environmental heterogeneity of the channel boundaries and flow dynamics which introduce uncertainty into the model results. Specifically, uncertainty in the stream discharge, channel roughness and inclusion of, vegetation can influence the distribution of flow in a river. To address the effect of trees on the flow field, we have employed a vegetation model to remove momentum from the flow using a depth dependent drag coefficient which is also a source of uncertainty. For determining the combined uncertainty of the results, we use repeated large-scale eddy simulations over a range of discharges, roughness and vegetation parameters on a reach of the American River, California. Using the polynomial chaos expansion and Monte Carlo sampling techniques, we express the uncertainty as confidence intervals in the spanwise flow velocities, velocity profiles and bed shear stresses in the river. Sobol indices have been determined to provide an estimate of the relative influence of each unknown input parameter.