Boots in the Water: Assistance
During and After Hurricane Harvey

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By Andy Yung, P.E., CFM

Hurricane Harvey made landfall as a Category 4 storm on the Texas coastal city of Rockport at about 10 p.m. on August 25, 2017. Over the next five days, it moved inland and stalled over the Houston Metropolitan Area, causing unprecedented flooding and devastation. In the midst of this catastrophe, the Water Resources Engineering Group at Walter P Moore had the opportunity to serve the community, initially by responding to calls from federal, state, and local agencies requesting technical information support during the storm, and later through post-Harvey analysis and recovery.

 

During the Storm

On Sunday, August 27 around 2 p.m., while Houstonians were battening down at home watching the disaster play out on the local news, Walter P Moore received a call from University of Texas at Arlington (UTA) and the U.S. Army Corps of Engineers (USACE) with a request to assist the Texas Department of Emergency Management (TDEM). The TDEM needed help to rapidly project maximum flood extents and potential inundation areas along some of Harris County’s creeks and bayous to figure out where to deploy personnel and organize emergency response. Within a few hours, the team had assembled online and started putting together a work plan based on their knowledge of Harris County’s watersheds and models.

With the USACE directly managing forecasting efforts immediately downstream of the Barker and Addicks Reservoirs, the Walter P Moore team analyzed seven additional stream systems within Harris County. This was achieved by using rainfall provided by UTA, available hydrologic and hydraulic models, and GIS software and data to produce inundation maps. Because the storm was continually evolving over the following five days, our team updated the inundation maps daily based on measured and forecasted rainfall supplied by UTA. Because Walter P Moore’s Houston office was closed during the event, the Water Resources Group worked from both The Woodlands office and remotely — a few while surrounded by water themselves.

Working side-by-side with the Harris County Flood Control District (HCFCD) who was actively trying to convey information to the public, and using our inundation maps, the team began to evaluate how much of Harris County was underwater at any given point. We ran rough calculations and plotted a map, which helped us inform HCFCD that about 70% of the county had been flooded.

Immediately following the rainfall, the Water Resources Group spent Labor Day weekend assisting the HCFCD in collecting high water marks at various locations across the county. Early post-flood data collection was crucial to HCFCD for gaining accurate measurements, and the widespread nature of the flooding meant that they needed extra manpower to get it done quickly. Teaming up with spotters and surveyors, our engineers helped HCFCD to identify, measure, and categorize high water marks at 588 locations across the county. These high water marks will be 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.

Long-term Recovery

Although Harvey has passed, the work has only just begun. Starting about mid-week after the storm, Walter P Moore staff was contacted by the HCFCD to assist in developing information that would be useful in communicating the magnitude of the flooding event to 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. This resulted in a staggering statistic — Harvey was approximated as a 50,000-year storm event over a four-day period at a single location. Given the short period of record resulting in a high degree of uncertainty with this statistic, however, we re-evaluated the rainfall in terms of the Probable Maximum Precipitation (PMP) and found that rainfall across the county on average was about 80% of the PMP. This means that, on average, the rainfall experienced during Harvey reached about 80% of the maximum rainfall that can physically occur, based on our current understanding.

Having proven both our technical acumen and willingness to help our local community in the months since Harvey, we have been working closely with multiple agencies to provide studies and analyses to further identify flood event parameters, improve our existing methods and models, and help inform future floodplain management strategies.