Post Hurricane Harvey Assessment

Science Center Objects

In the immediate aftermath of Hurricane Harvey related flooding, the USGS Texas Water Science Center and the Federal Emergency Management Agency (FEMA) initiated a cooperative study to evaluate the magnitude of the flood, determine the probability of occurrence, and map the extent of the flood in Texas.

On August 25, 2017, Hurricane Harvey made landfall at Rockport, Texas. Although Hurricane Harvey was classified as a category 4 hurricane when it made landfall, Harvey's speed slowed considerably and Harvey was reclassified as a tropical storm on August 26. Harvey produced epic amounts of rainfall which exceeded 60 inches in some locationsHistoric flooding occurred in Texas 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.

The data collection methods, flood-peak magnitudes, and flood-inundation products generated by the USGS in support of the FEMA response-and-recovery operations following the August and September 2017 flood event in southeastern Texas caused by rainfall from Hurricane Harvey are described in USGS Scientific Investigations Report 2018-5070. This report includes

  1. a description of the atmospheric conditions and the temporal and spatial patterns of rainfall that triggered the flooding and a description of the flood and its effects,
  2. a description of the flagging and surveying of high-water marks (HWMs),
  3. analysis of flood-peak magnitudes and their statistical probabilities at selected locations, and
  4. geographic information system (GIS) analysis of HWM locations and elevations to produce flood-inundation maps (areal extent of the maximum depth of flooding)

for five heavily flooded major river basins primarily in Texas, six smaller coastal basins, and coastal areas in Texas from Port Aransas to Matagorda Bay.

 

STREAMFLOW DATA

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.

 

HIGH-WATER MARKS

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.

USGS FIELD CREWS COLLECTED 2,123 HIGH-WATER MARKS IN 22 COUNTIES IN SOUTHEAST TEXAS AND THREE PARISHES ACROSS SOUTHWEST LOUISIANA.

 

FLOOD-INUNDATION MAPS

Nineteen 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.

The flood-inundation maps created for six 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.

 

before
after
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Addicks Reservoir, Houston TX. Before image (8/29/2017) shows Hurricane Harvey flooding. Reservoir level was 108.8 feet at the time of the picture; the peak was 109.09 feet on 8/30. After image (4/6/2018) is eight months after the flooding; reservoir level was 69.45 ft.

USGS 08073000 - Addicks Reservoir near Addicks, TX