Towards Predicting Street-Level Inundation: using Operational Forecast Modeling Techniques during 2011 Hurricane Irene in Hampton Roads, VA

Storm surge-induced coastal inundation poses numerous personal, commercial, industrial, and sociopolitical challenges for society. Flooding can be caused by the combination of storm surge and river-induced inland flooding in many locations throughout the coastal plain. The cross-disciplinary nature of the hydrodynamics involved (hydraulics, oceanography, and hydrology), coupled with the complexity of the atmospheric forcing, makes a numerical model the best approach for a comprehensive study of the dynamics of coastal inundation.

This study builds upon the lessons learned from forecast modeling experiences during 2011 Hurricane Irene in Tidewater Virginia, to ascertain the most effective way to approach predicting street-level inundation. During the storm event, an open-source large-scale ocean model, SCHISM, was provided atmospheric forcing from the National Oceanic and Atmospheric Administration’s Global Forecast System, updated every 6 hours to simulate 9 separate 30-hour simulations, which were provided to emergency managers and the National Weather Service in Wakefield, VA.

To accurately predict street-level inundation, water elevations at key points near the mouths of vulnerable tributaries can be used to drive a separate street-level high-resolution sub-grid model (UnTRIM) to simulate localized flooding events on the scale of 5-meter resolution. To this end, high-resolution Digital Elevation Models including building and roadway infrastructure were developed from Lidar-derived topography for the Hampton Roads Region of Virginia, and used to accurately predict flooding in low-lying areas of the Cities of Norfolk, Portsmouth, and Chesapeake along the Elizabeth and Lafayette Rivers, the City of Virginia Beach along the Lynnhaven River, and along Hampton, York, and Poquoson along the Back River.

Tropical storm surge flood heights were validated via temporal comparison with water level observations from NOAA, the USGS, and NASA aggregated to an average R²=0.84. Spatial extent of flooding was evaluated using USGS data retrieved from high water marks and from rapid deployment overland water level gauges during Hurricane Irene to reveal favorable agreement with the model’s inundation predictions.