1D River Hydraulic Model for Operational Flood Forecasting in the Tidal Potomac: Evaluation for Freshwater, Tidal, and Wind-Driven Events

The National Oceanic and Atmospheric Administration (NOAA) is working toward improving water forecasts in the river-estuary transition zone. One operationally viable method is to extend one-dimensional (1D) hydraulic models downstream well into the tidal estuarine environment. Recent advances in NOAA's National Weather Service (NWS) operational software infrastructure have made this method easier to implement. This paper evaluates the strengths and limitations of an unsteady implementation for the tidal Potomac River. The model was calibrated and validated to propagate tidal signals upstream for a wide range of freshwater events. Focusing on water-level gauges near Washington, DC, root-mean-squared error for tide simulation at the Washington Waterfront gauge was 0.05m (0.16ft) with a phase error of approximately 2h. For historic flood events, simulated peak water-level error varied from -0.40 m (-1.34 ft) to 0.40m (1.34ft) with a mean absolute error of 0.25m (0.82ft) at the Wisconsin Avenue gauge. While the NWS implemented model shows considerable potential to improve upon an existing empirical forecast technique for freshwater, tidal, and freshwater-tidal events, it did not adequately model wind-driven events because it does not include an explicit wind forcing term. Therefore, to further understand the influence of wind on Potomac water levels, both a 1D and a two-dimensional (2D) model were developed for Hurricane Isabel. With storm-specific calibration, wind-driven simulations from the models improved peak water-level simulation for Hurricane Isabel over the NWS model. The NWS model without wind forcing missed the Hurricane Isabel peak at the Washington Waterfront by 0.66m, while the 1D and 2D models came within 0.08 and 0.01m, respectively. Little improvement was seen over the NWS model in prediction of peak time. While the use of dynamically coupled 2D or three-dimensional hydraulics models for operational river forecasting may be the wave of the future, the value of loosely coupled, 1D river hydraulic models to efficiently provide water-level forecasts in should not be discounted.