Hurricane Harvey was the most significant rainfall event in U.S. history since records began in the 1880s. Hurricane Harvey made landfall near Rockport, Texas, on August 25, 2017, as a Category 4 hurricane with wind gusts exceeding 150 miles per hour.
As Harvey moved inland, the forward motion of the storm slowed down and produced tremendous rainfall amounts over southeastern Texas, with 8-day rainfall amounts exceeding 60 inches in some locations, which is about 15 inches more than average annual amounts of rainfall for eastern Texas and the Texas coast.
Historic flooding occurred as a result of the widespread, heavy rainfall. Wind and flood damages were estimated to be $125 billion, and the storm resulted in at least 68 direct fatalities.
In the immediate aftermath of Harvey, the USGS and FEMA initiated a study to evaluate the magnitude of riverine flooding, determine the probability of future occurrence, and map the extent of the flooding in Texas.
USGS field crews collected 2,123 high-water marks in 22 counties in southeast Texas and three parishes across southwest Louisiana.
High-water marks are the evidence of the highest water levels during a flood and provide valuable data for understanding flood events. The best high-water marks are formed from small seeds or floating debris carried by floodwaters that adhere to smooth surfaces or lodge in tree bark to form a distinct line. High-water marks were surveyed to determine the water-surface elevation.
During August and September 2017, USGS personnel made more than 280 streamflow measurements in the study area. The maximum streamflow value recorded at a streamflow-gaging station at a given time is the peak streamflow. The August and September 2017 peak streamflows were used in this study.
Seventy-four USGS streamflow-gaging stations were analyzed to determine the flood frequency at each site. These gaging stations were chosen because they had at least 15 years of data with no large gaps in the data. New record peak streamflows were recorded at 40 of the 74 USGS streamflow-gaging stations. Peak streamflows ranked second or third highest measured at another 25 USGS streamflow-gaging stations.
Peak streamflow data from USGS streamflow-gaging stations were used along with high-water marks measured after Hurricane Harvey to create 19 inundation maps to document the areal extent of the maximum depth of the flooding.
Hear more from USGS Hydrologist, Kara Watson, about this study and the impact by clicking the arrow in the box below:
Detailed flood information, along with the 19 flood-inundation maps, is available in USGS Scientific Investigations Report 2018-5070. Digital data created as part of this report is available from the USGS data repository.
Nineteen riverine flood-inundation maps in 11 river and coastal basins were created by using GIS for areas near rivers that flooded as a result of Harvey in southeastern Texas and southwestern Louisiana. The flood-inundation maps estimate the areal extent of the maximum depth of flooding that corresponds to the HWMs identified and surveyed by USGS field crews following the flood event. Flood inundation maps for the Houston metro area are being developed by Harris County Flood Control District.
The flood-inundation maps created for six of the 19 heavily flooded river basins, including the Lower Brazos, Lower Neches, Pine Island Bayou, Sabine, San Jacinto and San Bernard, as well as the coastal areas of Corpus Christi, Port Aransas, and Matagorda Bay are available below.
Images below are thumbnails of the fullsize maps. Click on the thumbnail to view the entire map.