Flood of August 20, 1979, on a tributary of Chickamauga Creek, near Gallipolis, Ohio

Abstract

An intense summer rain estimated at 4 to 5 inches in 2.5 hours caused flooding on a Chickamauga Creek tributary 4.5 miles northwest of Gallipolis, Ohio. Computation of a peak discharge of 1,350 cubic feet per second from 3.03 square miles of drainage area (446 cubic feet per second per square mile) indicates a greater-than-100-year flood. INTRODUCTION An intense summer thunder storm in southeast Ohio during the early morning hours of August 20, 1979, caused severe flooding on several small streams near Gallipolis. Rainfall from this storm in the vicinity of the peak discharge measurement site, a culvert on State Route 160, 4.5 miles northwest of Gallipolis, was reported to be 4 to 5 inches in 2.5 hours. The resulting flood-peak discharge was computed as 1,350 ft 3/s from Chickamauga Creek tributary (drainage area 3.03 mi2 ). The flood was investigated by the U.S. Geological Survey as part of a cooperative program with the Ohio Department of Transportation. One of the objectives of the program is to collect data on high intensity, infrequent hydrologic events. HYDRAULIC ANALYSIS Peak discharge was determined to be 1,350 ft 3/s using indirect methods at a 14-foot 3-inch by 8-foot 8-inch multiplate pipe arch culvert, #GAL 160-0149, which carries the flow of the tributary under State Route 160. The upstream (approach) water surface elevation was determined to be 588.9 feet, National Geodetic Vertical Datum of 1929 (NGVD of 1929), and the downstream (tailwater) elevation 580.8 feet (NGVD of 1929). Elevation of the low point in State Route 160 in the vicinity of the culvert is 589.9 feet (NGVD of 1929), indicating no flow over the road during this flood peak. The measured peak discharge (1,350 ft. 3/s) was outflow from the culvert. Because of considerable storage above the highway embankment, the inflow discharge was larger. A profile of the 100-year frequency flood for this site, published in a flood hazard report by the U.S. Soil Conservation Service (1979), shows a lower water surface elevation upstream from the culvert than that for the August 20, 1979 flood peak. Flood peaks at nearby gaging stations; on Raccoon Creek at Adamsville, drainage area 585 miles 2 , (discharge 3,780 ft 3/s), 7 miles west of the measurement site; and Shade River near Chester, drainage area 156 miles 2 , (discharge 1,610 ft 3/s), 27 miles northeast of the site were less than the 2-year frequency floods. HYDROLOGY The tributary is ungaged, and there are no rainfall recorders in its basin. So, to indicate the severity of the storm and the height of the resulting flood peak, observed and statistical data are presented. Precipitation Precipitation recorded at National Weather Service (NWS) rain gages show that the storm was local. These data are as follows: Time Distance NWS of Rainfall, in inches from site Direction Station reading Aug.19 Aug.20 Aug.21 (in miles) from site Gallipolis 6 p.m. 0.02 3.76 0.25 Jackson 8 a.m. 1.51 0.03 0.81 Athens 7 a.m. 0.75 1.00 Ironton 7 a.m. 0.20 0.64 1.02 4 24 33 33 SE