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Record Rainfall

Record Rainfall

Figure 1: Section of a map showing rainfall totals in southeastern Texas and southwestern Louisiana from Aug. 25 through Sept. 1, 2017, resulting from Hurricane Harvey. Red areas received more than 45 inches of rain. Image: USGS

Post-Harvey report provides inundation maps and flood details for the largest rainfall event recorded in U.S. history.

Nineteen inundation maps and detailed flood information from Hurricane Harvey are now available from the U.S. Geological Survey (USGS), in cooperation with the Federal Emergency Management Agency (FEMA). Hurricane Harvey was the most significant rainfall event in U.S. history, both in scope and peak rainfall amounts, since records began in the 1880s.

Hurricane Harvey’s widespread eight-day rainfall, which started on Aug. 25, 2017, exceeded 60 inches in some locations (see Figure 1), which is about 15 inches more than average annual amounts of rainfall for eastern Texas and the Texas coast. The second largest rainfall event recorded in continental U.S. history was during Tropical Storm Amelia in 1978, which left Texas Hill Country with 48 inches of rain. Not only were rainfall totals exceptional during Hurricane Harvey, the area affected was also larger than previous events.

In the immediate aftermath of Harvey, the USGS and FEMA initiated a study to evaluate the magnitude of 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. Although parts of central Louisiana experienced Harvey-related flooding, the report only documents the extent of flooding in southwest Louisiana along the Sabine River. High-water mark data, along with flood flow information from USGS streamgages, were used to create 19 inundation maps to document the areal extent and depth of the flooding.

FEMA requested time-perishable high-water marks, updated water-level records, and Harvey inundation maps, which are key materials that will be used by state and local resource managers. The data and records will assist officials in updating building codes, planning evacuation routes, creating floodplain management ordinances, providing environmental assessments, and planning other community efforts to become more flood resilient.

Record streamflow was measured at 40 USGS streamgages in Texas that have been in operation at least 15 years. At two streamgage locations, scientists determined that the percent chance for flooding of this magnitude to happen in any given year was 0.2 percent — i.e., a 500-year flood. Thirty other USGS streamgages experienced flooding at levels with a 1 percent chance of occurring each year — a 100-year flood.

The USGS conducts research on the physical and statistical characteristics of flooding, estimating the probability of flooding at locations around the United States. FEMA uses this information to revise their Flood Insurance Rate Maps. These maps help identify areas most likely to experience flooding in any given year.

The USGS produced 19 maps for six heavily flooded river basins, to include 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.

The full report is available at https://pubs.er.usgs.gov/publication/sir20185070.

Information provided by the U.S. Geological Survey (www.usgs.gov).