University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Great Plains Research: A Journal of Natural and Social Sciences Great Plains Studies, Center for Spring 2007 The October 1998 Flood of the Upper Guadalupe River System, Central Texas Richard A. Earl Texas State University - San Marcos Follow this and additional works at: https://digitalcommons.unl.edu/greatplainsresearch Part of the Other International and Area Studies Commons Earl, Richard A., "The October 1998 Flood of the Upper Guadalupe River System, Central Texas" (2007). Great Plains Research: A Journal of Natural and Social Sciences. 886. https://digitalcommons.unl.edu/greatplainsresearch/886 This Article is brought to you for free and open access by the Great Plains Studies, Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Great Plains Research: A Journal of Natural and Social Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Great Plains Research 17 (Spring 2007):3-16 © 2007 Copyright by the Center for Great Plains Studies, University of Nebraska-Lincoln THE OCTOBER 1998 FLOOD OF THE UPPER GUADALUPE RIVER SYSTEM, CENTRAL TEXAS Richard A. Earl Department of Geography Texas State University San Marcos, TX 78666 [email protected] ABSTRACT-The October 1998 flood on the upper Guadalupe River system was produced by a 24-hour pre­ cipitation amount of 483 mm at one station, over 380 mm at several other stations, and up to 590 mm over five days, precipitation amounts greater than the 100-year storm as prescribed in Weather Bureau Technical Papers 40 (1961) and 49 (1964). This study uses slope-area discharge estimates and published discharge and precipitation data to analyze flow characteristics of the three major branches of the Guadalupe River on the Edwards Plateau. The main channel of the Guadalupe has a single large flood-control structure at Canyon Dam and five flood dams on the tributary Comal River. On the upper San Marcos River there are five detention dams that regulate 80% of its drainage. The Blanco River, which has no structural controls, generated a peak discharge of 2,970 m 3/s from a 1,067 km2 basin. Downstream of Canyon Dam, the Guadalupe River generated a peak discharge greater than 3 2 3,000 m /s from an area of 223 km . The ev~nt exceeded the capacity of both the Comal River and San Marcos flood-control projects and produced spills that inundated areas greater than the 100-year floodplain defined by the Federal Emergency Management Agency. Key Words: Guadalupe River, hydrometeorology, structural flood control, Texas floods INTRODUCTION The October 1998 flood was a remarkable event in the and Persky 1999), the U.S. Natural Resource Conserva­ history of the Guadalupe River system. Not only were the tion Service (USNRCS 1999), the Lower Colorado River flood discharges generated by the storm a stark reminder Authority (LCRA 1999), and the National Weather Service of the flood potential of the region, but the effectiveness (NWS 1999), as well as work by researchers at Texas State of headwaters flood-control structures during this event University (Earl and Wood 2002; Earl and Dixon 2005). provided useful lessons on the ability of such structures to reduce downstream flood damage even when the storm UPPER GUADALUPE RIVER BASIN exceeds their project design. Statewide, but mostly con­ fined to the Guadalupe and San Antonio river systems, The focus of this paper is the Guadalupe River system the flood caused over $1 billion in property dam,ages and upstream and west of the Ba1cones Escarpment. This resulted in 31 deaths (USNCDC 2006c). region is on the Edwards Plateau landform section of the This paper describes the October 1998 flood on the Great Plains landform province. East of the escarpment, upper Guadalupe River system on the Edwards Plateau, the area known as the Blackland Prairie is a portion of the upstream of its confluence with the San Marcos River Atlantic and Gulf Coastal Plain (Fenneman 1931, 1938). (Fig. 1) and analyzes this flood event in the context of the By this definition, the 5,577 km2 study area includes the regional flood hazard, effects of land use on the runoff upper Guadalupe River, the Blanco River, and the San that was generated, and the effectiveness of flood-control Marcos River and its headward extension known as Sink measures. This paper complements published reports of Creek. the flood by the U.S. Geological Survey (USGS) (Slade The climate of the study area is transitional between the humid SUbtropical characteristic of east Texas and the subtropical semiarid of south Texas. When mapped at Manuscript received for review, July 2006; accepted for publication, the state scale, the area is "subtropical subhumid," which October 2006. manifests itself by having the droughts characteristic of 3 4 Great Plains Research Vol. 17 No.1, 2007 ~ I .. USGS stream gauge • City \vith precipitation data o__ -= 10 __ -= 20 ___ 40C:::::=:=::J 60___ 80 Kilomelers Figure 1. The study area showing the San Marcos, Blanco, and upper Guadalupe rivers, precipitation stations, and stream gauges used in this analysis. regions farther west and having the floods characteristic of TABLE 1 coastal east Texas (Larkin and Bomar 1983; Bomar 1995). THIRTY-YEAR (1971-2000) CLIMATE SUMMARY Long ago, for the classic volume Fluvial Processes in FOR BLANCO, TEXAS Geomorphology, the region was mapped as one of the two Record* most flood-prone regions of the United States (Leopold Mean Mean of daily temperature precipitation precipitation et al. 1964, fig. 3-16). This flood hazard manifests itself (OC) (mm) (mm) because a variety of storm types can produce floods here. January 8.1 45 82 (1968) The region has a bimodal precipitation regime (Table 1) February 10.4 53 118 (1961) that reflects two of the more significant storm types. In March 14.5 67 80 (1992) late spring, cold fronts often produce vigorous squall April 18.2 69 206 (1949) lines when they encounter "juicy" maritime tropical air May 22.4 115 207 (1958) that has been drawn in from the Gulf of Mexico. In late June 25.9 106 130 (1997) summer and early fall, tropical storms and their remnants July 27.7 51 117 (1903) enter the region and sometimes stall over the Ba1cones August 27.6 60 120 (1942) Escarpment with devastating results (Slade 1986). Oc­ September 24.6 83 444 (1952) casionally the region is hit by a series of tropical easterly October 19.4 106 175 (1959) waves that produce prodigious amounts of precipitation November 13.5 68 161 (2001) over many days such as happened in June-July 2002 when December 9.2 60 105 (1913) an area of lO,770 km2 received greater than 5lO mm over Total for 18.4 883 444 (1952) lO days (USGS 2002). Cold-cored upper-level lows and year even stationary fronts can produce flooding if high pres­ Source: USNCDC 2006a. sure over the eastern Gulf of Mexico pumps maritime * Period of record since 1895. © 2007 Center for Great Plains Studies, University of Nebraska-Lincoln The October 1998 Flood of the Upper Guadalupe River System, Central Texas • Richard A. Earl 5 TABLE 2 FIVE LARGEST FLOODS ON STREAMS OF THE UPPER GUADALUPE RIVER SYSTEM Guadalupe River Guadalupe River at San Marcos River Blanco River at Blanco River at at Spring Branch New Braunfels at San Marcos Wimberley Kyle USGS gauge 08167500 08168500 0817000/ 08171000 08171300 number 08170500* Drainage area 3,406 3,932 247 919 1,067 (km21 Years 1923-present 1869-present 1921-present 1925-present 1929-present Largest (m3/s) 4,540 >4,540 2,700 3,200 3,340 August 1978 July 1869 September 1921 May 1929 May 1929 Second largest 3,430 >4,540 2,200 3,060 3,260 (m3/s) July 1932 December 1913 May 1970 November 2001 September 1952 Third largest 3,290 4,020 1,700 2,730 2,970 (m3/s) June 1997 October 1998 June 1981 May 1958 October 1998 Fourth largest 3,230 2,860 1,300 2,690 2,780 (m3/s) June 1935 June 1935 May 1958 September 1952 May 1958 Fifth largest 2,680 2,700 1,300 2,510 2,470 (m3/s) July 2002 July 1932 October 1998 October 1998 November 2001 Sources: Earl and Wood 2002; Earl and Dixon 2005; USGS 2006. * Gauge moved in 1994. tropical air into the region at low levels, and low pressure Sink Creek basin with ephemeral flow. Downstream of over northwestern Mexico pumps in mid-level moisture San Marcos Springs, the upper San Marcos River receives (Bomar 1995). Most streams in the region have had major drainage from Purgatory Creek and Willow Creek. The floods in every month except January and February when Blanco River, which begins in Kendall County approxi­ high pressure over the continental interior blocks major mately 120 km west of San Marcos, is mostly sustained intrusions of maritime tropical air masses (Table 2). by incremental groundwater contributions throughout its For the Guadalupe River, the study area is west of In­ course, but does receive springflow from Jacobs Well that terstate Highway 1-35 in New Braunfels and includes the feeds Cypress Creek, which flows into the Blanco River spring-fed Carnal River and its ephemeral tributaries. The in Wimberley. Guadalupe River begins 200 km west of New Braunfels in Gauging of these streams dates back to the early Kerr County, where it is mostly fed by groundwater seep­ 1900s. There are six gauges on the upper Guadalupe River age. Both the San Marcos and Carnal rivers begin at large and one on the Carnal River. The stream-gauging history springs draining the Edwards Aquifer.