Quantifying Precipitation, Streamflow, and Floodplain Forecasting Skills during Extreme Weather Events in Brays Bayou, Houston, Texas

Abstract:

Integrated forecasts systems for precipitation, streamflow, and floodplain inundation are of critical importance in mitigating devastating flooding caused by extreme precipitation events, which are likely to occur with greater frequency and intensity. However, the streamflow and inundation forecasting skills associated with uncertainties in Quantitative Precipitation Forecasts (QPFs) requires a better understanding of how skill propagates through the system. In this study, a set of QPFs developed by the National Weather Service (NWS) were used to drive a flood modeling system with off-line coupling of a physically-based distributed hydrological model, the Distributed Hydrological Soil and Vegetation Model (DHSVM), and a hydrodynamic model (Two-dimensional Runoff Inundation Toolkit for Operational Needs, TRITON). This flood modeling system was used to produce the forecasts of streamflow, and inundated area during three major flood events in the Brays Bayou watershed (Houston, Texas), with an emphasis on Hurricane Harvey. Then, the forecasting skills for the QPFs, streamflows, and inundated area were quantified under multiple lead times (6 ~ 72hr). Results showed that: 1) QPF skills for more intense and sustained events such as hurricanes (e.g., Harvey) and tropical storms are higher than for shorter less intense events (e.g., Memorial Day flood). 2) The decreasing trends in forecast skills of QPF with increasing leading time generally propagate through streamflow forecast. 3) The streamflow and inundated area forecasting skills for Hurricane Harvey were evaluated under a number of different lead times. The results indicate that the decision by the West Gulf River Forecast Center (WGRFC) to extend the QPF duration from 12 hours, under normal circumstances, to 72 hours, for extreme events, increased the lead time and forecast skill. Skillful forecasts with long lead-times significantly benefit the preparedness and planning efforts of Emergency Managers for hurricane-level events like Harvey.