Ord 1990-037 - Laserfiche WebLink

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The cllmate of the region is generally warm and subhumid. During the winters, occasional cold periods of short duration result from the rapid moving of cold high pressure air masses from the northwest. Data obtained for this area from the National Oceanic and Atmospheric Administration at Austin, Texas, shows an average annual rainfall of 33.34 inches during the past 40 years. The extreme values of 51.30 inches occurring in 1957 and 11.42 inches occurring i.n 1954. The mean relative humidity is 67 percent. 0 Temperatures range from a mean maximum of 96 degrees Fahrenheit ( F) 0 in July to a mean minimum of 40 F in January. Generally, the soils of the area are well drained, shallow, stony clays with moderately slow to slow permeability. The major land use in the watershed is rangeland. The plant community in the Edwards Plateau is predominantly an oak-juniper association. 2.3 Principal Flood Problems Generally, the major storms experienced in the vicinity are produced by heavy rainfall from frontal-type storms, which may occur at any time during the year. Major flooding produced by these intense rainfalls usually occur in the spring and summer months. San Marcos has a long history of catastrophic floods. Major floods have occurred in 1921,1929,1970,1972, 1974, and 1981. The most disastrous flood in 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 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. If the flood had occurred during the night, the loss of life could have been greatly multiplied. With present conditions, approximately 890 residential properties, 80 commercial establishments, and 20 public buildings would receive floodwater damage by a flood equivalent to the 1970 event. Hinor flooding occurs annually. Rapid runoff and high peak discharges result from steep topography and relatively steep stream gradients. 2.4 Flood Protection Measures Two floodwater retarding structures have been built on Sink Creek. Also, two floodwater retarding structures were built on Purgatory Creek. One additional floodwater retarding structure is planned for Sink Creek. The city has relocated some residents and businesses along Willow Springs Creek. Some channel improvements on Purgatory Creek and Willow Springs Creek have been made. The city has installed an electronic flood warning system on Purgatory Creek and has also installed staff gages along Sink Creek as part of a flood warning system. 5