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t \ I 2. May 15, 1970 3. September 1952 4. 1929 and 5. 1913 Large floods are known to have occurred on the Blanco River in the vicinity of San Marcos on: ~ September 1921 ~ 1929 ~ September 1952 ~ October 1953 ~ May 1970 Major flooding (listed in priority order) occurred on Purgatory and Willow Springs Creeks on: ~ September 10, 1921 (highestknown) ~ May 15, 1970 (second highest known) ~ 1955 ~ 1959 ~ 1970 I As described in the February 18, 1998, Hays County and Incorporated Areas Flood Insurance Study, most of the major rainfall events are the result of frontal-type storms. Major flooding can be also be produced by intense rainfall from severe localized thunderstorms, which typically occur with the greatest frequency during the spring and summer months, but are capable of occurring throughout the year. Major floods have occurred in 1869, 1921, 1929, 1970, 1972, 1974, 1981, 1991, and 1998. The floods of 1869, 1921, and 1998 were all extremely large, well in excess of the 1 percent-chance flooding event. Until 1998, the most disastrous flood of recent years occurred on May 15, 1970. The total storm rainfall for this flood, over a 24-hour period, varied from 6 inches in the upper portion of the watershed to 13 inches recorded at the City of San Marcos. The resulting flood was estimated to have a recurrence interval of 84 years. Approximately 1,850 acres of floodplain were inundated; approximately half was urban area. Two children drowned; however, if the flood had occurred during the night, the loss of life could have been greatly multiplied. According to a National Weather Service (NWS) report, the October 17-22, 1998 flood was caused by a slow moving cold front in combination with deep tropical moisture from the Gulf of Mexico and Eastern Atlantic Ocean. The resulting rainfall produced Floods-of-Record for much of the Hill Country and southeast Texas rivers, including the San Marcos River. The damages, as reported by the National Climactic Data Center, exceeded 67 million dollars. Less significant floods have also occurred on several occasions in the last decade, in 1994, 1997, and 1999-2002. These events resulted in additional accumulated damages of over three million dollars. I The drastic changes in elevation across the county yield very steep channel slopes, which cause rapid runoff and high peak discharges. These factors present a danger due to high velocity stream flows, most pronounced in the Blanco River. Of the part of the river within the county, two-thirds of its length flows greater than ] 0 feet per second, and the remaining portion flows Hays counl~', Texas Miligalion Plan 56