Ensemble Flood Forecast Maps: A Case Study in the Delaware River near Philadelphia

Track 3A: Forecasting Droughts and Floods
Thursday, October 12, 2017, 3:40 pm – 5:10 pm

Michael Gomez
Graduate Student
Department of Civil and Environmental Engineering
Penn State University

In this study, we investigate the ability to generate and enhance flood forecast maps through weather ensembles and statistically postprocessed water surface elevations (WSELs). To generate the maps, a 1-D hydraulic model is coupled to a regional hydrological ensemble prediction system (RHEPS). The coupled framework is applied to the Delaware River near Philadelphia. Flood forecast maps are generated at lead times from 0 to 7 days, over the period 2008-2013. To verify the forecast maps, 4 sets of maps are generated: i)observed, ii)deterministic, iii)raw ensembles, and iv)postprocessed ensembles. The observed maps are generated by forcing the hydraulic model with streamflow observations, while deterministic and raw ensemble maps are generated using deterministic and ensemble hydrological forecasts, respectively. Postprocessed forecast maps use a statistical method to correct biases in the raw WSEL forecasts at individual cross-sections. Overall, we find that using ensemble hydrological forecasts as forcing improves the quality, over deterministic forecasts, of the medium-range flood forecast maps. Postprocessing the WSELs further enhances the quality of the raw ensemble flood maps.