Simulation of unsteady flow in the Roanoke River from near Oak City to Williamston, North Carolina

Abstract

A one-dimensional, unsteady-flow model was calibrated, validated, and applied to a 30.4-mile reach of the Roanoke River between State Highway 42-11 bridge near Oak City (river mile 67.0) and U.S. Highway 17-13 bridge (river mile 36.6) at Williamston, North Carolina. The model was calibrated and validated for water levels ranging from 5.62 to 16.44 feet at river mile 67.0, and for flows ranging from about 2,000 to 12,000 cubic feet per second. For model calibration, the mean absolute difference between 22 measured and simulated flows was 3.4 percent. The mean absolute difference between nine measured and simulated flows obtained in the validation process was 3.3 percent. The sensitivity of model results to small changes in computational time step, momentum coefficient, numerical scheme weighting factors, resistance coefficients, and boundary values was evaluated. The model, which was calibrated at a time step of 15 minutes, was unstable at a computational time step of 30 minutes, but results were insensitive to changes in the momentum coefficient and the numerical scheme weighting factors. Results were somewhat sensitive to small changes in the resistance coefficients and boundary values. The model was used to compute daily mean flows at river miles 67.0, 59.2, and 36.6 for water years 1988-90. Flows were calculated for the range of conditions for which the model was calibrated and validated. Simulated monthly mean flows at river mile 67.0 were within 5 percent of flows measured at Roanoke Rapids (river mile 137.0), after adjustment for inflow from the intervening drainage area between river miles 137.0 and 67.0, for months having mean flows less than about 14,000 cubic feet per second.