August 17, 2007 Flash Flood In Hermosa, SD

Located in North Central Custer County

Summary of the Weather Event that Caused the Flooding:

At 6:00 pm August 17, 2007, an intense thunderstorm developed along the eastern foothills of the Black Hills in central Custer County, South Dakota. Southeasterly low-level winds fed moist air into the storms. These thunderstorms remained nearly stationary for at least 2 hours!

Around 7:00 pm National Weather Service (NWS) Doppler Radar in New Underwood, SD had estimated rainfall of 1 to 3 inches along Battle Creek from 7 miles west of Hermosa to Hermosa. At 7:09 pm, the NWS Rapid City staff issued a Flash Flood Warning for central Pennington County and northeastern Custer County. Another 3 inches of rain was expected through 8:00 pm. The rain continued through the evening, with most of the rain falling before 9:00 pm. Approximately 100 square miles were estimated to have received at least two inches of rainfall, while radar estimates indicated approximately 50 square miles received 4 to 7 inches of rain. An automated raingage in Hermosa reported 0.86 inches in 15 minutes between 7:30 pm and 7:45 pm. A rain gage in Ghost Canyon reported 5.33 inches and a gage in Hermosa reported 5.5 inches of rainfall.

Doppler radar image below shows storm total precipitation. The red area around Hermosa represents 5 to 7 inches of rain.

Storm Total Precipitation August 17, 2007
Storm Total Precipitation Color Scale

Preliminary River Stage Data for Battle Creek at Hermosa:

Please note all observations are preliminary and are courtesy of the US Geological Survey (USGS) who owns and maintains the station. A higher crest may have occurred sometime between 8:45 pm and 9:15 pm, but the river gage only makes measurements every 15 minutes.

Battle Creek 1S Hermosa Hydrograph

Flooding and Damage Reports:

The large amount of precipitation from this storm in such a short period of time caused flash flooding across north central Custer County. The first report of flash flooding occurred at 7:20 pm in Custer and 5 miles west of Hermosa, as water covered Highway 40 and portions of Custer. Between 8:00 and 8:15 pm, Battle Creek overflowed its banks around Hermosa and covered Highway 40 in several locations within 5 miles of Hermosa. Between 7:30 pm and 9:00 pm, the creek rose over 12 feet. Around 9:00 pm law enforcement reported water flowing over Highway 79 where it crosses Battle Creek. Between 8:00 pm and 9:00 pm, water backed up along the west side of a railroad embankment in the southeastern portion of Hermosa. Flooding was reported along the southern portions of 2nd and 3rd streets, the eastern portion of Whitney Street and the eastern portion of Manning Street. At approximately 9:00 pm the railroad embankment failed, causing flood waters to sweep 3 houses from their foundations and flooding about 30 other homes in a subdivision on the southeast side of Hermosa. According to the Rapid City Journal, the storm damaged 60 to 70 homes around the Hermosa area. At least a half mile section of railroad tracks and the embankment as well as several roads and vehicles were damaged. Total damage estimates are yet to be determined.

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