Hurricane Harvey Emergency Response

During the flood event in the Houston metro area following Hurricane Harvey's landfall, Walter P Moore's Water Resources Engineering Group provided technical support to local, state, and federal agencies as they developed emergency response and communication plans for the public.

At 2:00 p.m. on Sunday, August 27, 2017, Walter P Moore was contacted by the U.S. Army Corps of Engineers (USACE) with a request to assist the Texas Department of Emergency Management (TDEM) in projecting maximum flood extents along some of Harris County's creeks and bayous. Within a few hours, we put together a work plan based on our knowledge of Harris County's watersheds and models. The approach was discussed with the USACE and the University of Texas at Arlington (UTA) and was found to be agreeable.

Because the Houston office of Walter P Moore was closed during the event, the Water Resources Group worked from The Woodlands office and remotely to develop maximum flood extent maps along seven major streams in Harris County. This was achieved by using rainfall provided by the UTA, available hydrologic and hydraulic models, and GIS software and data to produce the maps. Over the next five days, each map was updated daily to reflect new rainfall estimates and compared with real-time gauge data for quality control.

About mid-week, Walter P Moore was contacted by the Harris County Flood Control District (HCFCD) to assist in developing information that would be useful in communicating the magnitude of the flooding event to the public. As a part of this effort, we helped define the magnitude of the rainfall from Harvey across Harris County. To do this, our staff modified a proven, but fairly new, statistical method to arrive at a conclusion. When it appeared that we were looking at what could be referred to as a 50,000-year storm event, we realized such a number seemed difficult to verify given the limited amount of data in the record. From here, we re-evaluated the rainfall in terms of the "Probable Maximum Precipitation," or PMP. In layman's terms, the PMP is theoretically the most amount of rain that the atmosphere can produce. Upon examination, we found that Harris County averaged about 80% of the PMP across the entire 1,777-square-mile area. One location was approximately 95% of the PMP for the most intense four-day period.

Additional efforts on behalf of HCFCD included estimating how much of the county was covered in floodwaters, not just from bayou and creek flooding, but also from overland flooding. Our estimates from a "back-of-the-envelope" calculation which attempted to balance the volume of water found that about 80% of the County was subject to flooding during the storm.

Lastly, over the Labor Day weekend, Walter P Moore assisted the HCFCD in collecting high water marks at various locations across the County. These high water marks were used to document the impacts of Hurricane Harvey, to compare with other historical flood events, and to calibrate future hydrologic and hydraulic analyses, as needed.