Flood - TCRFC.org
Transcription
Flood - TCRFC.org
Section 5. Flooding Contents Why Floods Are a Threat..............................................................................................................................5-1 Hazard Profile ................................................................................................................................................5-7 History of Flooding......................................................................................................................................5-11 Location of Hazardous Areas.....................................................................................................................5-31 NFIP Program Participation.......................................................................................................................5-34 People and Property at Risk........................................................................................................................5-39 Potential Damages and Losses ...................................................................................................................5-45 Why Floods Are a Threat Unique Geographic and Atmospheric Conditions According to American Hazardscapes: The Regionalization of Hazards and Disasters published by the National Academy Press, Texas, because of its size and location, consistently outranks other states in deaths and damage from floods. Texas is second in casualties and damage from hurricanes and tropical storms. The State’s vulnerability is the result of several factors: its Gulf of Mexico coastline; its proximity to the Pacific Ocean off the west coast of Mexico; its geographical location near the Rocky Mountains of Colorado and Arizona and the high-altitude jet stream; and its nearness to the unique West Texas “dry line,” a shifting, invisible atmospheric separation of dry desert air from the moist Gulf air. These factors create a breeding ground for the big storms of spring and fall that spawn tornadoes and suck up Gulf or Pacific moisture that feed the heavy rains that cause flash flooding. All these geographic factors cause Texas to experience extensive, annual storms. Figure 5-1 shows the State’s vulnerability to damaging storms. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-1 Characteristics of Flooding in the Basin Flooding is the most frequent and deadly disaster facing the Lower Colorado River Basin. Eightyfour percent of Presidential Disaster Declarations for Basin Counties have been for flooding. Of weather-related fatalities in a seven county area surrounding greater Austin between 1973 and 2001, over half were from floods. Figure 5-1. Weather Fatalities, 1973-2001, Greater Austin Seven-County Area1 Figure 5-2. Texas Sources of Moisture 1 National Weather Service Page 5-2 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Flash Flooding Flooding can take many forms in the Basin. Most flood deaths are due to flash floods. The National Weather Service characterizes the Hill County watershed of the Lower Colorado River as “Flash Flood Alley,” saying it is one of the regions in the United States at greatest risk for flash floods. Several factors contribute to giving it this dubious distinction. These factors include its proximity to the moisture-laden Gulf atmosphere; its record rainfall intensity and duration; its thin, easily saturated soils; location in and proximity to the uneven terrain of the Hill Country; and increasing urbanization that reduces the land’s natural ability to absorb water and causes rainfall to rush off buildings and pavement. The Colorado River also has a large drainage area. The Hill Country watersheds of the Colorado River and its tributaries cover several thousand square miles. That means the Highland Lakes can receive rain and runoff from storms that occur hundreds of miles away. These factors can transform heavy rains into walls of fast-moving water with great destructive potential. Figure 5-3. Potential Dam Overtopping Most flash flooding is caused by slow-moving thunderstorms, by thunderstorms repeatedly moving over the same area, or by heavy rains from hurricanes and tropical storms. Flash floods can occur within a few minutes or after hours of excessive rainfall. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of thirty feet or more. Flash flood-producing rains can also trigger catastrophic mudslides. Often there is no warning that flash floods are coming. Hill Country flash floods devastated the river basin in the early days and are a major reason why the Lower Colorado River Authority located Mansfield Dam and Lake Travis—the flood control components of the Highland Lake chain – upstream of Austin. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-3 Flash flooding poses a deadly danger to residents of the Lower Colorado River Basin. A number of roads run through low-lying areas that are prone to sudden and frequent flooding during heavy rains. Motorists often attempt to drive through barricaded or flooded roadways. It takes only 18-to24-inches of water moving across a roadway to carry away most vehicles. Floating cars easily get swept downstream, making rescues difficult and dangerous. Seventy-seven percent of flood and flash-flood fatalities in the Greater Austin area are vehicle related. Figure 5-4. Flood/Flash Flood Fatalities, 1973-2001, Greater Austin Seven County Area2 Riverine Flooding Riverine flooding is a natural and inevitable part of the city’s life. It is the overbank flooding of rivers and streams, typically resulting from large-scale weather systems that generate prolonged rainfall over a wide geographic area. Some river floods occur seasonally when winter or spring rainfalls fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding. Figure 5-5 shows historic floods that have endangered Central Texas. 2 National Weather Service Page 5-4 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Figure 5-5. Floods Endanger Central Texans Record Rainfalls Amounts in Inches from Single Rain Events The rapid and unexpected riverine flooding downstream from a possible dam failure can cause loss of life and significant property damage. Dam failure is addressed in Section 15 of this Plan. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-5 Urban Flooding Urban flooding occurs as land is converted from fields or woodlands to roads, buildings and parking lots and when the natural land loses its ability to absorb rainfall. Urbanization changes the natural hydrologic systems of a basin, increasing runoff two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while highway underpasses and underground parking garages can become death traps as they fill with water. Urban flooding in Houston after Tropical Storm Allison. El Niño Phenomenon Flooding can occur in cycles. The El Niño phenomenon – the cyclical disruption of the oceanatmosphere system in the tropical Pacific Ocean – has important consequences around the globe and here in Texas. The presence of El Niño is indicated by unusually warm water in the eastern Pacific Ocean, altering wind and ocean currents. El Niño generally brings cooler winters and wetter than normal conditions to Austin. In 1997-1998, El Niño increased surface temperatures in the Eastern equatorial Pacific Ocean by 5-to-7-degrees Fahrenheit warmer than normal, thus contributing to the 1998 flooding in Central Texas. Tropical Flooding Hurricanes and tropical storms also bring floods. Between 1900 and 2000, thirty-seven hurricanes made landfall in Texas. Four were a Category 4 on the Saffir-Simpson scale, ten were Category 3, nine were Category 2 and twelve were Category 1. Texas was hit by sixteen hurricanes and tropical storms from 1975 to 1998 and, after a period of relative quiet, is overdue for another big hurricane. Section 6 of this plan addresses hurricane threats. Page 5-6 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. The Basin is not immune to the death and destruction that tropical systems can bring. Indeed, almost 60 percent of deaths in the U.S. from tropical cyclones have been from inland, freshwater flooding. Hazard Profile Major flooding and flash flooding events can have a substantial severity of impact. They can cause multiple deaths, completely shut down facilities for thirty days or more, and cause more than fifty percent of affected properties to be destroyed or suffer major damage. The frequency of occurrence of flooding is highly likely, with an event probable in the next year. The annual probability of observing a 100-year flood is one-percent. The annual probability of observing a 500-year flood event is 0.2 percent. Flooding occurs in seasonal patterns. Thunderstorms form when warm, moist air collides with cooler, drier air. Since these masses tend to come together during the transition from summer to winter, most thunderstorms and resulting flooding occur during the spring (April, May and June) and fall (October, November, and December). Managing Floodwaters Of the six Highland Lakes, only Lake Travis is designed to store and hold floodwaters. Mansfield Dam, which forms Lake Travis, creates a storage area called a “flood pool” to contain runoff from Hill County storms that would otherwise inundate Austin and downstream communities. Lake Travis is in its flood pool whenever it is above its full elevation of 681 feel above mean sea level (msl). The flood pool extends to 714 feet msl, which is also the elevation of the spillway at Mansfield Dam. Lakes Travis and Buchanan serve as the two major water storage reservoirs in the Highland Lake chain. Unlike Travis, Buchanan has no flood pool – it has very little additional capacity when it is at full elevation. And the smaller “pass through” lakes (Inks, LBJ, Marble Falls and Austin) have no extra capacity. When heavy rains and floods occur in the Hill Country, LCRA’s goal is to capture the runoff in Lakes Travis and Buchanan. Rains and floods are the only way these lakes can be refilled. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-7 Figure 5-6. The Highland Lakes and Dams, Lower Colorado River Authority While all of the Highland Lakes dams except Inks have floodgates, LCRA rarely uses the gates to manage floodwaters. Many times, LCRA passes storm runoff through the hydroelectric power stations at the dams, gaining additional benefit by using the water to generate electricity. LCRA Page 5-8 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. opens floodgates only when it needs to pass through more water than the hydroelectric power stations can accommodate. Figure 5-7. Estimated Flow Times for Floodwaters, Lower Colorado River Authority Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-9 The LCRA operates Lake Travis and Mansfield Dam during floods under guidelines developed by LCRA and the U.S. Army Corps of Engineers. The guidelines determine how much water LCRA may release, depending on the amount of water in the Lake Travis storage pool, as well as downstream conditions. In all but the most catastrophic flood scenarios, the goal is to keep the downstream river at or below flood stage by monitoring “control points” at Austin, Bastrop and Columbus. Based on the guidelines, Figure 5-8 shows key elevations at Lake Travis during flood events. Figure 5-8. Key Elevations for Lake Travis (mean sea level) Page 5-10 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Thanks to the Highland Lakes, Austin and other downstream communities along the Colorado River have not experienced the frequency of devastating floods that were typical of the 19th and early 20th centuries. The Highland Lakes dams can minimize or prevent many potential floods, but they cannot prevent all floods. If storms pour more water into a lake than the dam can release, the lake will rise. And the dams have no control over storms or floods that occur downstream. Figure 5-9. Flood Hazard Profile Summary This Lower Colorado River Basin is still at risk for floods, mainly from heavy rains that fall downstream of Mansfield Dam, beyond the reach of the dam’s flood control capabilities. Downstream of Austin, major floods occur less frequently than before the Highland Lakes were created. When they occur, they may take hours or days to reach their peak, as opposed to the sudden flash flooding that can occur in the Hill Country. Even so, the floods can be widespread and cause significant damage, such as the October 1998 flood that inundated several areas along the river, including parts of Wharton. History of Flooding Major Flood Events More than 80 flood events have been recorded in the lower Colorado River basin since the 1800s. These events range from isolated floods that affected local areas to basin-wide floods spawned by unusually heavy rainfalls. Here are some of the major events: Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-11 Page 5-12 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-13 Page 5-14 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-15 Reported flood events in the Lower Colorado River Basin since 1950 are listed below. Table 5-1. Reported Flood Events, by County, 1950 to 2002, Lower Colorado River Basin, National Oceanic and Atmospheric Administration Lower Basin Event Year Date Deaths Comments Colorado County Major flood 1985 11 Nov to 12 Nov 0 Flood Flash flood/flood Flash flood Flash flood Flash flood Flash flood Flash flood River flooding Flash flood Flash flood Flash flood Flash flood Flash flood 1993 1994 1995 1997 1997 1997 1997 1998 1998 1998 1998 1998 2001 5 May 18 Oct 18 Dec 20 Feb 12 Mar 6 Jun 13 Oct 17 Oct 18 Oct 12 Nov 13 Nov 14 Nov 9 Sep 0 0 0 0 0 0 0 1 0 0 0 0 0 Intense rains of 5-21 in. caused widespread flooding in a 10-county area bordering and west of Houston. Garwood in Colorado County recorded 21 in. The most severely flooded areas were in Colorado County from south of Eagle Lake to the communities of Garwood and Lissie. Colorado Colorado Colorado, Eagle Lake Countywide Garwood Eagle Lake North Portion Countywide Countywide Countywide Countywide West Portion Matagorda County Major flood 1921 Major flood 1922 Page 5-16 The tropical storm entered the Gulf Coast between Houston and Corpus Christi June 22. Max. recorded 22 Jun to 0 rainfall was 10.0 in. at Matagorda in Matagorda County. 23 Jun The storm moved north and caused flooding in the Red and Sulphur Rivers. The largest amounts of rain fell on Matagorda and 15 Sep to Brazoria Counties. The largest amount recorded in the Unknown 17 Sep three day period was 11.84 in. in the city of Matagorda in Matagorda County. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood River flooding River flooding Flash flood Flash flood Flash flood Flash flood Major Flood 1993 1994 1995 1996 1996 1997 1998 1998 1998 2001 2001 2002 2002 2003 15 Mar 18 Oct 17 Dec 21 Sep 24 Oct 9 May 10 Sep 17 Oct 12 Nov 31 Aug 1 Sep 6 Sep 10 Sep 15 Jul 0 0 0 0 0 0 0 1 0 0 0 0 0 0 Palacios Matagorda Bay City Countywide Matagorda County Wadsworth Countywide Northwest Portion Sargent, Bay City Bay City Hurricane Claudette 1994 1996 1997 1997 1997 1998 1998 1998 1998 1998 1998 2001 2001 2002 2002 2002 18 Oct 29 Dec 4 Apr 11 Apr 13 Oct 6 Oct 17 Oct 18 Oct 12 Nov 13 Nov 14 Nov 30 Aug 1 Sep 13 Jul 15 Jul 7 Sep 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 Wharton Lane City El Campo Countywide Countywide Wharton Countywide Countywide Countywide Countywide Countywide Countywide West Portion Hungerford Countywide Wharton Wharton County Flash flood, flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Middle Basin Event Year Date Deaths Comments Bastrop County Major flood 1926 Major flood 1932 20 Apr to Heavy amounts of rain caused a large flood on Plum Unknown 24 Apr Creek in the Guadalupe River basin. 03 Jan to Unknown The largest amounts of rainfall occurred in Anderson, Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-17 Event Year Date Deaths Comments 06 Jan Major flood 1940 Flooding Flash flood Flash flood Flash flood Flash flood Flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 1994 1994 1997 1997 1997 1997 1998 2000 2000 2000 2000 2001 2001 2002 2002 Houston, and Smith Counties. The largest amount recorded was just over 5 in. on the 4th in Anderson County. Max. recorded rainfall at Engle in Fayette County was 22.7 in. during the 2-day period June 29-30, with 17.5 in. 8:00 p.m. June 29 to 8:00 a.m. June 30. The heaviest 2 day rain at Smithville in Bastrop County was 20.40 in. June 28 Jun to 29-30, with 16 in. 7:00 p.m. June 29 to 10:00 a.m. June 30. Unknown 30 Jun This record rainfall caused destructive floods along lower parts of the Colorado and Guadalupe Rivers and along upper parts of the Lavaca River and its creek tributaries. Two people drowned on the Colorado River, and seven lives were lost on the Lavaca River at Hallettsville. 16 Oct 0 Bastrop 6 Nov 0 Bastrop 25 Apr 0 Countywide 6 Jun 0 Countywide 21 Jun 0 Countywide 22 Jun 0 Countywide 17 Oct 0 Countywide 2 May 0 Elgin 2 Nov 0 North Portion 3 Nov 0 Countywide 23 Nov 0 West Portion 6 May 0 West Portion 15 Nov 0 Northwest Portion 8 Apr 1 Southeast Portion 2 Jul 0 South Portion Fayette County Major flood 1936 Major flood 1981 Flooding Flash flood Flash flood 1994 1996 1997 Page 5-18 The storm began at 7:00 a.m. May 22 on the Gulf Coast. 22 May to Unknown Max. recorded storm rainfall was 14.9 in. at La Grange in 28 May Fayette County. Flood-producing rains extended along the coastline from Corpus Christi to Port O’Connor and straight northward from these two points for about 120 mi inland. Six storm 30 Oct to Unknown centers in south-central Texas had 6-13 in. of rain. Max. 31 Oct recorded rainfall was 13.20 in. at La Grange in Fayette County. 16 Oct 26 Jun 25 Apr 0 0 0 Fayette Schulenburg Countywide Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood 1997 1997 1997 1998 1998 1998 1998 1999 1999 2000 2000 2001 2002 2002 6 Jun 10 Oct 13 Oct 5 Jun 17 Oct 12 Nov 14 Nov 1 Jan 28 May 3 Nov 5 Nov 16 Nov 2 Jul 16 Jul 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Countywide Countywide Countywide Southeast Portion Countywide South Portion South Portion Countywide Countywide Countywide Countywide County South Portion South Portion Catastrophic Flood 1869 03 Jul Major flood 1894 29 Apr to 01 May Major flood 1900 05 Apr to 08 Apr Catastrophic flood 1913 01 Dec to 06 Dec Major flood 1919 18 Jul to 23 Jul Major flood 1925 12 Oct to 17 Oct Travis County The greatest rain known in Austin (at least until 1921) Unknown began July 3 and lasted about 64 hours. The towns of Webberville and Bastrop were inundated. A narrow band of 5-6 inch rain from vicinity of Bandera, Unknown Kendall, Blanco, and Travis Counties to Lamar County caused minor flooding. Substantial rainfall from the Rio Grande to the High Plains damaged the Colorado, Brazos, and Guadalupe 23 River basins. McDonald Dam on the Colorado River in Austin was destroyed. Rainfall the last 10 days of November averaged 4.21 in., Guadalupe River basin; 3.74 in., Colorado River basin; 3.53 in., Brazos River basin; 2.98 in., Trinity River basin; and 4.05 in., San Antonio River basin. These rains laid the foundation for floods greater than any known at that 177 time. Rains for first few days of December were more or less continuous but were heaviest Dec. 2-4. Rainfall Dec. 1-6 averaged 4.78 in., Guadalupe River basin; 3.95 in., Colorado River basin; 5.37 in., Brazos River basin; 5.30 in., Trinity River basin; and 2.94 in., San Antonio River basin. About 85 percent of the rain fell Dec. 2-4. Excessive and damaging local rains occurred in portions 0 of Cooke County on the 19th, Travis County on the 21st, and DeWitt County on the 23rd. The largest amounts of rainfall occurred in Washington, Austin, Brazos, and Travis Counties. 6.68 in. on the 12th Unknown in Washington County followed by 6.95 in. falling in a 24 hour period in Austin, Travis County on the 13th. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-19 Event Year Date Major flood 1958 16 Jun to 18 Jun Major flood 1961 17 Jun to 18 Jun Major flood 1974 23 Nov Major flood 1975 23 May Catastrophic flood 1981 24 May to 25 May Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood 1994 1994 1994 1995 1995 1995 1996 1996 1996 1997 1997 1997 1997 1997 1997 1997 1997 1997 1997 1997 1998 1998 1999 1999 2000 7 Oct 16 Dec 22 Dec 29 May 30 May 29 Jun 24 Aug 18 Sep 28 Oct 4 Apr 25 Apr 26 Apr 23 May 27 May 6 Jun 8 Jun 17 Jun 22 Jun 30 Jul 20 Dec 21 Feb 17 Oct 21 Jun 10 Jul 9 Jun Page 5-20 Deaths Comments Flooding in the Devils River basin on the headwaters of the Nueces and Guadalupe Rivers and on certain tributaries to the Colorado River above Lake Travis produced Unknown peak discharges at several streamflow-gaging stations. The heaviest rainfall amounts reported were 6-10 in.; however, there were a few unofficial reports of 16-20 in. Flash flooding in Austin during the night June 17-18 0 resulted from intense rainfall. The greatest recorded rainfall was 6.86 in. Heavy rainfall in Travis County caused flooding to claim 13 13 lives and caused $1 million in property damages. Heavy rainfall caused 4 drowning deaths and about $5 4 million in property damages. A short-duration, intense rainfall caused the worst flooding since 1935 on many of the small watersheds in 13 and around Austin. The rainfall began at 9:30 p.m. May 24 and ended shortly before midnight May 25. Some locations had more than 10 in. of rain during 4 hours. 0 Travis 0 Eastern Portion 0 Travis 0 Countywide 0 Countywide 0 Northwest Portion 0 Austin 0 Northwest Part 0 0 Countywide 0 Countywide 0 Countywide 0 Countywide 1 Austin 0 Countywide 1 Countywide 0 Austin 0 Countywide 0 Austin 1 Countywide 0 Countywide 1 Countywide 0 Countywide 0 North Portion 0 Southwest Portion Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 2000 2000 2000 2001 2001 2001 2001 2 Nov 3 Nov 23 Nov 6 May 20 May 26 Aug 31 Aug 0 0 0 0 0 0 0 Countywide Countywide East Portion Major flood 2001 15 Nov 0 Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 2002 2002 2002 2002 2002 2002 2002 2002 1 Jul 2 Jul 2 Jul 3 Jul 5 Jul 10 Aug 7 Sep 8 Sep 0 1 0 0 0 0 0 0 Countywide Countywide Austin The storm of November 15, 2001 began with mild to moderate rainfall over the night of November 14th and through the morning of November 15th. At about 3:30 p.m. on November 15th, a large thunderstorm cell began to approach the Hays - Travis County line, with tornadoes and intense rainfall. The tornadoes touched at various locations along the I-35 corridor starting near Buda and the Ben White Blvd – I-35 interchange and continuing north and east through Travis County. The heaviest rainfall began about 3:30 p.m. over south central Austin in the middle of the Slaughter Creek basin and upper South Boggy Creek basin. The intense thunderstorm cells continued to track rapidly and generally north-northeast over Barton Creek at Loop 360, West Bouldin Creek, Johnson Creek, and Shoal Creek. The area of the most intense precipitation decreased as it tracked further north over the upper Little Walnut and Walnut Creek watersheds. Although the storm gradually decreased in intensity over the next six hours, it continued to produce intense rainfall. Widespread rainfall totals typically ranged from five to eight inches, with individual reports of ten inches and more. Much of this rain fell within about six hours. Generally, the storm intensities and flood levels were higher on the south and west side of Austin. Rainfall intensities exceeded the estimated 100-year rainfall rates in some locations and caused widespread but isolated flood damage where the drainage capacity of streets and storm drains was exceeded by localized rainfall. West Portion Countywide Countywide West Portion Countywide Countywide Countywide Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-21 Event Year Date Deaths Comments Williamson County Catastrophic flood 1913 01 Dec to 06 Dec 177 Catastrophic flood 1921 08 Sep to 10 Sep 224+ Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 1994 1995 1995 1995 1996 1996 1996 1997 1997 1997 1997 1997 1998 1998 1999 1999 1999 1999 1999 1999 2000 2000 2001 8 Sep 29 May 28 Jun 1 Aug 31 Aug 18 Sep 19 Sep 4 Apr 25 Apr 8 Jun 22 Jun 20 Dec 4 Jul 17 Oct 10 May 11 May 13 Jun 21 Jun 10 Jul 12 Jul 1 May 23 Nov 6 May 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Page 5-22 Rainfall for the last 10 days of November averaged 4.21 in., Guadalupe River basin; 3.74 in., Colorado River basin; 3.53 in., Brazos River basin; 2.98 in., Trinity River basin; and 4.05 in., San Antonio River basin. These rains laid the foundation for floods greater than any known at that time. Rains for first few days of December were more or less continuous but were heaviest Dec. 2-4. Rainfall Dec. 1-6 averaged 4.78 in., Guadalupe River basin; 3.95 in., Colorado River basin; 5.37 in., Brazos River basin; 5.30 in., Trinity River basin; and 2.94 in., San Antonio River basin. About 85 percent of the rain fell Dec. 2-4. Heavy rainfall over a large area in Central Texas Sept. 810 produced peak discharges at several streamflow-gaging stations. Taylor in Williamson County recorded 23.98 in. during 35 hours, with 23.11 in. during 24 hours. Bucket surveys determined that some areas of Taylor had 30 in. of rain during about 15 hours. Countywide Taylor Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide East Portion Georgetown Countywide Countywide Countywide Countywide South Portion East Portion Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 2001 2001 2001 2002 2002 2002 2002 2002 1 Jul 26 Aug 15 Nov 2 Jul 3 Jul 13 Jul 7 Sep 8 Sep 0 0 2 0 0 0 0 0 East Portion South Portion Countywide Countywide Countywide Northwest Portion Countywide Countywide Date Deaths Comments Upper Basin Event Year Blanco County Major flood 1894 Catastrophic Flood 1952 Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 1995 1995 1997 1997 1997 1997 1997 1997 1997 1997 1997 1998 1998 1998 1998 2000 2000 2001 A narrow band of 5-6 inch rain from vicinity of 29 Apr to Unknown Bandera, Kendall, Blanco, and Travis Counties to 1 May Lamar County caused minor flooding. Two to 26 in. of rain fell on a 25,000-mi square area that formed a 100-mi-wide belt extending from Corpus Christi northwestward for 250 mi. Storm 9 Sept to 5 totals of 20-26 in. were concentrated in a small area 11 Sept in Blanco and Kendall Counties. Hye in Blanco County recorded 23.55 in. during 48 hours, with 20.70 in. during one 24-hour period. 29 May 0 Countywide 29 Jun 0 Countywide 4 Apr 0 25 Apr 0 Countywide 19 May 0 Countywide 27 May 0 Countywide 6 Jun 0 Countywide 8 Jun 0 Countywide 9 Jun 0 Countywide 21 Jun 0 Countywide 22 Jun 0 Countywide 16 Mar 0 Countywide 30 Mar 0 Countywide 4 Jul 0 Countywide 17 Oct 0 Countywide 2 Nov 0 South Portion 3 Nov 0 North Portion 31 Aug 0 South Portion Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-23 Event Year Date Deaths Comments Flash flood Flood Flash flood Flash flood Flash flood Flash flood 2001 2001 2002 2002 2002 2002 5 Sep 15 Nov 30 Jun 2 Jul 3 Jul 5 Jul 0 1 0 0 0 0 Countywide Countywide Countywide South Portion Countywide Countywide Brown County Major flood 1956 29 Apr to 01 May 0 Major flood 1990 25 Apr 0 Flash flood Flash flooding Flash flooding Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood 1991 1994 1995 1995 1996 1996 1997 1997 1997 1997 1998 1998 1999 1999 2000 2000 2000 2000 2000 2001 2001 2002 2002 2002 2003 2003 20 Dec 11 May 4 Apr 17 Sep 4 Jun 15 Sep 6 Jun 10 Jun 22 Jun 8 Oct 26 May 4 Jul 26 Apr 9 May 11 Apr 15 Jun 23 Oct 29 Oct 3 Nov 22 Apr 13 Aug 3 Jul 4 Jul 6 Jul 6 Jun 9 Oct 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Page 5-24 Rainfall began about 8:00 a.m. Apr. 29 and continued, with varying intensity, for 30 hours. A gage in the upper end of the watershed recorded 8.3 in. during 2.5 hours and 3.1 in. during 30 minutes. This gage recorded 10.09 in. of rain during the storm. As much as 16 in. of rain fell during a 24-hour period. Brownwood Airport measured 16.05 in. Flooding was the worst in the Brownwood area since 1954. Brownwood May Brownwood Brownwood Brownwood Brownwood Brookesmith Brownwood Brownwood Zephyr Brownwood Brownwood Brownwood Brownwood Brownwood Brownwood East Central Portion Brownwood Countywide Brownwood Brownwood North Portion West Portion Countywide 4.84 inches total, 2.2 inches in 30 minutes 5.35 inches total Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments 1995 1995 1996 1996 1996 1997 1997 1997 1997 1997 1998 1998 1998 1999 1999 2000 2000 2001 2001 2002 2002 2002 2002 2002 2002 2002 2002 29 May 28 Jun 20 Sep 27 Oct 28 Oct 25 Apr 6 Jun 8 Jun 22 Jun 20 Dec 16 Mar 4 Jul 17 Oct 10 May 28 May 3 Nov 5 Nov 17 Apr 15 Nov 30 Jun 2 Jul 3 Jul 4 Jul 5 Jul 13 Jul 7 Sep 8 Sep 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Countywide Northwest Portion East Portion Countywide South Portion South Portion Countywide Countywide Countywide North Portion Countywide Countywide 1997 1997 1998 2000 20 Feb 2 Mar 16 Mar 5 Nov 0 0 0 0 Lampasas, Lometa Lampasas Countywide Kempner Burnet County Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Lampasas County Flash flood Flash flood Flash flood Flash flood Llano County Flooding of the Llano River in the City of Llano Event Year Date Deaths Flood 1935 14 June Unknown Comments Maximum Flow Gage Height 380,000 41.50 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-25 Event Year Date Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood 1940 1941 1942 1943 1944 1945 1946 1947 1948 1948 1949 1949 1949 1950 1951 1952 1952 1952 1952 1953 1953 1955 1955 1955 1956 1957 1957 1957 1957 1957 1957 1957 1957 1958 1958 1959 1959 1959 1960 1961 1961 1962 20 June 27 Apr 23 Aug 5 June 25 May 18 Jan 17 May 18 Jan 25 June 6 July 24 Feb 26 Feb 25 Apr 16 May 25 May 21 April 10 Sept 18 Sept 30 Dec 12 May 4 Oct 19 May 16 June 25 Sept 31 Aug 23 Apr 25 Apr 29 Apr 13 May 17 May 27 May 1 June 14 Oct 23 Feb 3 May 24 June 26 June 4 Oct 6 Aug 16 June 19 June 2 June Page 5-26 Deaths Comments 28,200 26,700 23, 400 50,600 10,100 8,500 18,200 8,500 108,000 30,700 9,160 13,700 14,600 7,770 13,900 11,600 232,000 11,500 9,720 16,500 3,460 72,000 8,600 37,000 1,850 30,200 29,700 10,200 21,600 11,800 47,200 20,100 83,700 25,800 13,400 12,400 35,600 103,000 53,600 12,000 57,600 1,700 12.90 12.64 11.22 16.92 8.24 7.61 10.47 7.55 22.90 9.78 7.49 9.27 9.01 32,60 9.66 10.02 11.88 7.09 19.00 9.49 14.66 5.82 14.30 14.22 9.54 12.34 9.92 16.39 11.36 21.78 13.39 9.58 9.29 14.06 22.28 17.70 9.35 18.87 4.48 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood 1963 1964 1964 1964 1965 1965 1965 1966 1966 1966 1967 1967 1968 1968 1968 1968 1969 1969 1969 1969 1970 1970 1971 1971 1971 1971 1971 1973 1973 1974 1974 1974 1974 1974 1974 1975 1975 1975 1975 1975 1975 1976 16 Sept 22 Sept 24 Sept 27 Sept 16 May 9 Feb 18 May 25 Apr 2 May 10 Sept 20 May 20 July 20 Jan 11 Mar 10 Apr 11 May 7 May 5 Oct 12 Oct 29 Oct 6 Mar 16 May 26 July 2 Aug 12 Aug 4 Oct 20 Oct 1 Aug 13 Oct 9 May 13 Aug 29 Aug 18 Sept 22 Sept 31 Oct 24 May 2 Feb 10 May 11 May 14 May 20 May 4 July Deaths Comments 1,810 11,300 67,200 23,800 24,800 11,300 13,000 15,400 10,300 9,080 27,400 25,800 44,400 12,000 7,800 16,900 4,520 154,000 9,080 15,400 30,100 13,200 12,500 18,600 28,000 7,750 24,500 11,400 154,000 43,000 9,000 95,600 20,100 10,700 8,530 19,400 7,540 10,800 10,800 11,700 10,600 61,500 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. 4.58 9.32 20.10 12.46 12.66 9.28 9.82 9.97 8.59 8.20 12.56 12.35 16.22 9.30 7.80 10.79 6.22 27.02 8.30 10.14 13.83 9.68 9.48 11.58 14.14 7.78 13.37 9.11 26.98 16.06 9.10 21.78 12.41 9.68 7.78 10.92 7.40 8.58 8.57 8.86 8.50 17.92 Page 5-27 Event Year Date Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood Flood 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1981 1981 1981 1981 1983 1984 1984 1988 16 Apr 11 May 3 Aug 20 Sept 1 June 5 June 8 Sept 30 Sept 18 April 4 June 16 June 17 June 18 June 14 Oct 6 June 28 July 31 Dec 11 July Deaths Comments 67,500 40,000 139,000 20,900 25,800 7,590 210,000 13,100 10,500 16,200 8,200 22,900 32,900 116,000 5,970 490 120,000 42,000 18.66 14.84 25.61 11.25 12.31 7.42 31.11 10.35 9.28 10.39 8.36 12.43 14.24 23.79 7.58 3.29 23.80 16.00 Other Flooding in Llano County Event Year Date Major flood 1995 29 May Flash flood Flood Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flood Flash flood Flood 1996 1997 1997 1997 1997 1998 1998 1998 1999 1999 1999 1999 1999 2000 2000 2000 2000 28 Oct 20 Feb 25 Apr 22 Jun 24 Jun 6 Jan 16 Mar 4 Jul 18 Mar 10 May 11 May 28 May 12 Jul 1 May 24 Oct 3 Nov 4 Nov Page 5-28 Deaths 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Comments Rainfall depths up to 8 in. caused flooding in parts of Williamson, Mason, Llano, and Gillespie Counties. The largest flood damages occurred in Sandy Creek and Lake LBJ. Countywide Countywide Countywide Countywide Countywide Llano Countywide Countywide Countywide Countywide Countywide Countywide North Portion East Portion West Portion Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Event Year Date Deaths Comments Flash flood Flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood 2000 2000 2001 2001 2002 2002 2002 2002 2002 5 Nov 6 Nov 27 Mar 15 Nov 13 Apr 2 Jul 4 Jul 7 Sep 8 Sep 0 0 0 1 0 0 0 0 0 Countywide Southeast Portion Countywide West Portion Countywide East Portion Countywide Countywide Mason County Rainfall depths up to 8 in. caused flooding in parts of Williamson, Mason, Llano, and Gillespie Counties. The largest flood damages occurred in Sandy Creek and Lake LBJ. Major flood 1995 29 May Unknown Flood Flash flood Flash flood Flood Flood Flood Flash flood 1996 1996 1997 1997 1997 1997 1998 19 Sep 28 Oct 20 Feb 9 Mar 21 Jun 22 Jun 25 Feb 0 0 0 0 0 0 0 Flash flood 2000 4 Jun 0 Flash flood Flash flood Flood Flash flood 2000 2000 2001 2002 23 Oct 3 Nov 15 Nov 5 Jul 0 1 0 0 Countywide 1996 1996 1997 1997 1997 1997 15 Jul 29 Aug 20 Feb 20 Feb 9 Mar 22 Jun 0 0 0 0 0 0 Goldthwaite Priddy Goldthwaite Priddy, Goldthwaite Democrat, Goldthwaite Goldthwaite 1996 1996 1997 1997 7 Jun 15 Jul 20 Feb 30 May 0 0 0 0 San Saba San Saba San Saba Cherokee Mason Mason Mason Mason Countywide Countywide Countywide Mills County Flash flood Flash flood Flash flood Flash flood Flash flood Flash flood San Saba County Flash flood Flash flood Flood Flash flood Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-29 Event Year Date Deaths Comments Flood Flood Flood Flash flood Flash flood Flash flood Flood Flood Flood Flash flood Flash flood Flash flood 1997 1997 1997 1999 2000 2000 2000 2000 2001 2002 2002 2002 6 Jun 21 Jun 22 Jun 11 May 23 Oct 3 Nov 4 Nov 6 Nov 15 Nov 3 Jul 5 Jul 14 Jul 0 0 0 0 0 0 0 0 0 0 0 0 Richland Springs Cherokee San Saba Northwest Portion Countywide Countywide Northeast Portion Countywide Historic elevations at the Highland Lakes Dams are shown in Figure 5-10 below. Figure 5-10. Historic Elevations at Highland Lakes Dams, Lower Colorado River Authority Dam Buchanan Normal Operating Range 1018.0 (May through October) 100-Yr. Flood Level Top of Dam Historic High 1,021.0 1,025.35 1,021.4 Dec. 1991 901.7 922.0 902.8 July 1938 Dam Photo 1020.35 (Nov. through April) Inks Page 5-30 886.9 to 887.7 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Normal Operating Range 100-Yr. Flood Level Alvin Wirtz 824.4 to 825.0 Max Starcke Dam Mansfield Tom Miller Top of Dam Historic High 827.9 838.0 836.2 Sept. 1952 736.2 to 737.0 753.0 738.0 756.3 Sept. 1952 At or below 681.0 716.0 750.0 710.4 Dec. 1991 491.8 to 492.8 493.0 519.0 495.2 May 1981 Dam Photo Location of Hazardous Areas Flood-hazard areas are determined using statistical analyses of records of riverflow, storm tides, and rainfall; information obtained through consultation with communities; floodplain topographic surveys; and hydrological and hydraulic analyses. FEMA’s Flood Insurance Rate Maps (FIRMs) identify areas subject to flood hazard. These include Special Flood Hazard Areas, which are defined as areas that will be inundated by a flood event having a one-percent chance of being equaled or exceeded in any given year. The one-percent-annual-chance flood is also referred to as the base Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-31 flood or 100-year flood. Moderate flood-hazard areas are also shown on the FIRM, and are the areas between the limits of the base flood and the two-tenths-percent-annual-chance (or 500-year) flood. Figure 5-11 is a map showing the approximate 100- and 500-year floodplains in the Lower Colorado River Basin, according to the available data on FEMA Q3 Data. Figure 5-11. Riverine Flooding Potential as depicted on Q3 Flood Maps, Lower Colorado River Basin Page 5-32 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Coastal Flooding As shown in Figure 5-12, only Matagorda County is at risk from coastal flooding from hurricane surge in the Lower Colorado River Basin. Figure 5-12. Coastal Flooding Potential in the Lower Colorado River Basin, by Category of Hurricane Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-33 NFIP Program Participation Policies and Losses All but two of the TCRFC jurisdictions participate in the National Flood Insurance Program (NFIP), thus making available flood insurance to their residents. As of this date, Goldthwaite, and Bertram do not participate in the NFIP. Flood insurance offered through the NFIP is the best way for home and business owners to protect themselves financially against the ravages of flooding. There are currently over 10, 000 flood insurance policies in the force in participating TCRFC jurisdictions with over $1.2 billion in property covered. There have been 3,271 losses to date with $29.4 million in claims payments. Table 5-2. National Flood Insurance Program, Policies and Losses September 30, 2002 County Blanco Brown Burnet Lampasas Llano Mason Mills San Saba Page 5-34 Community Blanco County Johnson City Brownwood Burnet County Cottonwood Shores Granite Shoals Highland Haven Marble Falls Meadowlakes Lampasas County Kempner Lampasas Lometa Llano County Sunrise Beach Village Mason County Mills County Goldthwaite San Saba County San Saba Total Policies Coverage Total in effect in Losses thousands Dollars Paid, Historical 28 4 226 405 9 115 10 124 91 17 $5,289 $461 $37,031 $65,455 $1,616 $17,253 $1,342 $18,669 $17,788 $1,341 3 1 223 66 4 10 0 36 8 6 $39,644 0 $4,302,424 $677,467 $172,208 $35,463 0 $761,575 $253,656 $78,376 14 $1,015 2 $3,627 789 137 6 2 -19 38 $140,180 $21,185 $245 $50 -$1,191 $1,558 161 13 0 0 -2 2 $1,577,875 $25,230 0 0 -$21,548 $1,727 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. County Bastrop Fayette Travis Williamson Colorado Matagorda Wharton Community Bastrop County Bastrop Elgin Smithville Fayette County Fayette County W.C. Carmine Flatonia La Grange Austin Lago Vista Lakeway Mustang Ridge Village of the Hills West Lake Hills Williamson County Colorado County Columbus Eagle Lake Matagorda County Bay City Palacios Wharton County Wharton TOTAL Policies in effect 152 25 2 24 42 4 Total Coverage Total Losses in thousands $21,318 43 $4,232 3 $510 1 $3,426 14 $5,135 3 $669 18 Dollars Paid, Historical $941,409 $10,813 0 $303,614 $126,988 $666,458 26 5,006 103 42 $2,557 $604,001 $20,965 $9,855 11 1,517 23 6 $360,063 $16,864,262 $325,384 $41,567 28 450 106 94 24 1,538 962 467 646 411 $7,153 $72,694 $12,844 $11,182 $3,395 $170,037 $127,882 $55,726 $77,066 $39,088 8 87 10 3 4 840 433 71 85 87 $184,440 $750,405 $115,119 $1,621 $183,341 $4,745,095 $1,519,048 $118,702 $1,697,270 $468,865 10,211 $1,252,534 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. 3271 $29,447,739 Page 5-35 Market Penetration Despite the benefits of flood insurance, the National Flood Insurance Program estimates that less than 45 percent of structures in the Basin that are located in Special Flood Hazard Areas (the 100year floodplain) have insurance contracts in force. Unfortunately, it is often after a flood that many people discover that their homeowner or business property insurance policies do not cover flood damage. Approximately 25 percent of all flood damages occur in low risk zones, commonly describes as “outside the mapped flood zone.” The present market penetration is shown in the Table 5-3. In determining market penetration, the estimated number of structures in the Special Flood Hazard Areas are as reported by local community officials. “Contracts in force” are used by the NFIP in determining sales penetration, rather than “policies in force.” One “contract” is placed on an entire condominium building with 50 units, each of which may have separate policies in force. Page 5-36 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Table 5-3. NFIP Sales Penetration, September 30, 2002 County Blanco Brown Burnet Lampasas Llano Mason Mills San Saba Bastrop Fayette Travis Williamson Community Blanco County Johnson City Brownwood Burnet County Cottonwood Shores Granite Shoals Highland Haven Marble Falls Meadowlakes Lampasas County Kempner Lampasas Lometa Llano County Sunrise Beach Village Mason County Mills County Goldthwaite San Saba County San Saba Bastrop County Bastrop Elgin Smithville Fayette County Fayette Co. WC&IDMonument Hill Carmine Flatonia La Grange Austin Lago Vista Lakeway Mustang Ridge Village of the Hills West Lake Hills Williamson County Estimated SFHA Total 0 32 372 254 10 12 50 87 20 0 Contracts in Force 2 1 76 304 7 84 6 97 30 10 Percent Penetration 0 3.13 20.43 119.69 70.00 700.00 12.00 111.49 150.00 0 0 7 0 818 50 0 587 106 0 71.76 212.00 0 860 173 150 200 64 13 0 18 10 29 77 10 1 9 24 1 1.16 16.76 51.33 5.00 1.56 69.23 0 5.56 71 6,370 126 0 12 3,078 81 13 16.9 48.32 64.29 0 90 1,503 12 195 13.33 12.97 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-37 Colorado Matagorda Wharton Colorado County Columbus Eagle Lake Matagorda County Bay City Palacios Wharton County Wharton TOTAL 25 715 95 41 1,300 900 1,646 260 16,325 49 66 14 1,106 412 110 378 286 7,290 196.00 9.23 14.74 2,697.60 31.69 12.22 22.96 110.00 44.66% Repetitive Losses According to the National Flood Insurance Program, as of August 2003, there are 21properties in the Basin on the Federal Emergency Management Agency’s list of Target Repetitive Loss Properties. A repetitive loss property is defined as a property that is currently insured through the NFIP that has experienced two or more losses of $1,000 or more in any rolling 10-year period since 1978. Properties on the Target Repetitive Loss list are those that have experienced 2 losses within a ten year period that exceed the value of the structure; 3 losses within the life of the structure that exceed the value of the structure; or 4 or more losses. Together, these properties have experienced 77 losses for cumulative losses of $2.2 million. They are expected to incur damages in the future of almost $ 1.3 million. Table 5-4 shows the target repetitive loss properties in the Basin, including the number of losses, the cumulative losses, average annual insured losses, and projected future damages in dollars for the target repetitive loss properties in each jurisdiction. Page 5-38 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Table 5-4. Target Repetitive Loss Properties, National Flood Insurance Program, August 2003 Cumulative Number Number NFIP Losses of Target County of Losses Community (in dollars) Propertie s Brown Brownwood 2 7 $362,972 Burnet Burnet County 1 2 50,278 Llano Llano County 1 2 19,445 Travis Austin 4 16 716,120 Travis County 2 4 365,175 Matagorda Matagorda County 6 31 816,457 Bay City 4 17 288,052 Wharton El Campo 1 4 39,609 TOTAL 21 77 $2,217,217 Average Annual Insured Losses (in dollars) $18,646 2,558 1,326 36,100 19,339 34,190 11,525 1,501 $104,024 Projected Future Damages (in dollars) $231,374 31,745 16,459 447,972 239,980 424,262 143,022 18,623 $1,255,640 People and Property at Risk There are two principal sources of information used in this Plan to assess the risks of flooding in the Lower Colorado River Basin. The first source of information is results from The Lower Colorado River Basin Study, being conducted by the U.S. Army Corps of Engineers. The second source of information is the HAZUS-MH model utilizing FEMA Q3 Flood Zones to delineate the 100- and 500-year flood boundaries. Results of the Lower Colorado River Basin Study The Lower Colorado River Basin Study is being conducted in response to requests for assistance from the City of Austin and the Lower Colorado River Authority. The LCRA acts as the cost sharing sponsor for the study with the Corps of Engineers, on behalf of other local entities that to date include the cities of Austin, Wharton, and Sunset Valley, and Travis County. The Lower Colorado River Basin Study is being conducted in three phases. Phase 1 is identifying existing conditions regarding flood damages to residential, commercial, industrial and public structures; environmental conditions and opportunities for enhancement of natural ecosystems; and recreational needs within the basin. The Phase 1 study area is the main stem of the Colorado River. Under Phase 2, a series of interim feasibility studies will be conducted focusing on known flood Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-39 problem areas. Under Phase 3, the U.S. Army Corps of Engineers will develop and evaluate alternatives for implementing solutions to water resource-related problems. A number of alternatives for reducing the risk or magnitude of flood damages will be evaluated. The Corps of Engineers will then examine the consequence of each alternative, including the economic and environmental costs and benefits, and work with the LCRA and communities in the basin, through the Coalition, to decide on the most effective and practical course of action. Phase 1 of the Lower Colorado River Feasibility Study, conducted by Halff Associates, Inc. for the Texas Colorado River Floodplain Coalition and the Fort Worth District Engineer, was completed in July 2002. This hydrologic and hydraulic study used an in-depth, basin-wide approach for modeling, simulating and computing frequency-based rainfall, runoff, reservoir elevations and stream flood elevations along the entire Colorado River Corridor, including in the city of Austin. The Feasibility Study focused on riverine flooding only and did not examine all flooding sources in Austin. The study involved a hydrologic and hydraulic analysis utilizing a thorough and in-depth, basin-wide approach to modeling, simulating, and computing frequency-based rainfall, runoff, reservoir elevations, and stream flood elevations along the entire Colorado River corridor. The analytic tools and engineering analyses prepared for the Corps study include the most comprehensive and detailed examination of flooding issues in the Colorado Basin to date, utilizing extensive detailed topographic mapping along the river corridor, state-of-the art Geographic Information System (GIS) and statistically sound hydrologic modeling tools. The Corps of Engineers Study data is believed to be the most definitive. The Corps of Engineers computed profiles for eight frequency storm events. For those areas with previously identified floodplains, the study provided both lower and higher water surface elevations depending on the area. A summary of 100-year frequency peak flood elevations at selected locations is shown in Table 5-5. As you will note, there are significant differences between current flood elevations on the Colorado River and FEMA Elevations identified on FEMA Flood Maps. Page 5-40 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Table 5-5. Summary of 100-Year Peak Flood Elevations Location on the Colorado River Lake Buchanan Inks Lake Lake LBJ Lake Marble Falls Lake Travis Lake Austin Town Lake Austin Gauge Upstream of U.S. 183 Bastrop Gauge at Loop 150 Columbus Gauge at U.S. 90 Wharton Gauge at U.S. 59 (Bus) Current Study Computed 100-year Elevation (Feet NAVD88) 1021.0 901.7 828.1 754.3 722.0 492.8 438.6 437.0 352.2 190.2 102.4 FEMA 100Year Elevation (Feet NAVD88) 1021.2 901.9 828.1 753.2 716.2 493.3 439.8 435.3 353.9 194.1 103.3 Difference Current – FEMA (Feet) -0.2 -0.2 0.0 +1.1 +5.8 -0.5 -1.2 +1.7 -1.7 -1.9 -0.9 Figure 5-13 shows the number of structures at four flood levels in Lake Travis, comparing the FEMA Flood Insurance Rate Map data with the current Corps of Engineers study findings. For example, there are 2,698 structures on Lake Travis that are in the 100-year floodplain according to the current Corps of Engineers study. However, only 2,205 structures are shown in the 100-year floodplain according to FEMA Flood Insurance Rate Maps. Thus, an additional 673 structures are at risk of a 100-year flood based upon the new data. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-41 Figure 5-13. Lake Travis Structures at Four Flood Levels, A Comparison of FEMA Flood Insurance Rate Maps and Current U.S. Army Corps of Engineers’ findings. According to Halff Associates, Inc., several reasons explain why the flood elevations have changed along the Colorado River. An additional 25 years of historical flood and rainfall records have been collected since the previous flood studies of the mid- to late 1970’s. There is also a greater degree of accuracy in the flood models used in today’s studies. Based on the computed flood elevations for the Corps of Engineers study, the total 100-year floodplain for the Colorado River, from the mouth to the Red Bluff gauge, is about 449 square miles or 287,000 acres. The Corp of Engineers estimates a total of over 29,000 structures in the Lower Page 5-42 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Colorado River Basin in the 100- and 500-year floodplain. A breakdown of these structures, by location, is provided in Figure 5-14. Figure 5-14. Structures Affected by Large Floods in the Lower Colorado River Basin, U.S. Army Corps of Engineers, January 2003. HAZUS-MH Model Results HAZUS-MH utilized FEMA Q3 Flood Zones to delineate the 100-year and 500-year flood boundaries. Because of limitations on time and budget, input to the HAZUS model was limited and approximate methods were used. Thus, the Army Corps of Engineers Study results are believed to be more accurate than the approximate methods used in the HAZUS-MH model. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-43 The HAZUS-MH model shows the estimated number of people at risk from riverine (in both the 100- and 500-year floodplain) and coastal flooding. Almost 73,000 people live where there is a potential for damage to property and loss of life in the Lower Colorado River Basin. These social loss estimates are based on FEMA Q3 flood data, which is not available in five of the Counties. TABLE 5-6. Social Loss Estimates: People at Risk from Riverine and Coastal Flooding in the Lower Colorado River Basin, Using the HAZUS-MH Model County Bastrop Blanco Brown Burnet Colorado Fayette Lampasas Llano Mason Matagorda Mills San Saba Travis Wharton Williamson TOTAL Page 5-44 Riverine Flooding 4,565 288 Not available 5,266 3,434 2,770 Not available 2,705 322 11,573 Not available Not available 52,243 20,459 Not available 72,702 Coastal Flooding 0 0 0 0 0 0 0 0 0 20,000 0 0 0 0 0 20,000 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Potential Damages and Losses Results of the Lower Colorado River Basin Study The Army Corps of Engineers Study conducted flood damage analyses to quantify single event and average annualized flood damages within the Colorado River Basin. Damages were computed for structural property as well as motor vehicles and agricultural lands. Future increases in flood damages resulting from additional development within the watershed, manifesting itself either as an increase in precipitation run-off and increased flood depths, and/or an increase in the number of structures and other property at risk, are not anticipated or accounted for in their analysis of flood damages. The theoretical computation of structural flood damages is relatively straightforward. lt is based on the depth of flooding for various flood events (exceedence probabilities), and a relationship between the depth of flooding and the estimated damages based on a percentage of the structure and content, or vehicle value. Damages to the various structures, accumulated by frequency, produce a frequency- damage function. An integration process using this frequency-damage data calculates estimates of expected annual damages. This involves aggregating the multiplication of the mean damage between each pair of flood events by the difference in exceedence probabilities. This is then repeated for the range of flood events in each damage category. The computation of the agricultural flood damages for the basin was based on assigning all agricultural acreage into six general categories: fruits and nuts, field crops, pasture, hay, rice, and other. Agricultural damages due to flooding for each acre of each crop are computed by adding four elements: • The cumulative direct production or annual variable costs incurred prior to flooding • The net value of the crop affected by the flood event • Depreciated value of perennial crops lost as a direct result of flooding • The land clean-up and rehabilitation resulting from flooding Table 5-7 provides the structural and vehicular flood damage assessment for each county along the main stem of the Lower Colorado River. Approximately 12,400 structures and 3,765 vehicles would be impacted by a 100-year (1-percent chance) flood along the main stem of the Colorado River. Total damages from that flood would exceed $377 million in personal property (structures, contents and vehicle) losses. The expected annual damages total $25,186,000. The Lake Travis reach would experience the greatest economic impact of a 100-year flood and also has the highest annual expected damages. A 100-year flood event along the Wharton reach would impact more structures than any other location along the main stem of the Colorado River. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-45 TABLE 5-7. Structural and Vehicular Flood Damages, Results from the U.S. Army Corps of Engineers Lower Colorado River Basin Study, August 2003 County Reach Llano Lake Buchanan Inks Lake Lake LBJ Llano Llano Llano Total Burnet Above Burnet Upper Burnet Lake Buchanan Inks Lake Lake LBJ Lake Marble Falls Lower Burnet Burnet Burnet Burnet Burnet Burnet Burnet Burnet Total Travis Travis Travis Travis Travis Lake Travis Lake Austin Town Lake Lower Austin Lower Travis Travis Total Bastrop/Colorado/Fayette Upper Bastrop Bastrop/Colorado/Fayette Bastrop Bastrop/Colorado/Fayette Middle Bastrop Bastrop/Colorado/Fayette Smithville Bastrop/Colorado/Fayette Upper Fayette Bastrop/Colorado/Fayette LaGrange Page 5-46 No. of Structures Affected by the 1% Flood No. of Vehicles Affected by the 1% Flood Damages from 1% Flood (in millions of dollars) Expected Annual Damages (millions of dollars) 52 105 1,108 1,265 1 40 448 489 $0.04 1.95 30.30 32.34 $0.08 0.15 2.10 2.34 4 7 3 6 0.10 0.20 0.00 0.01 32 129 462 6 48 175 0.05 3.50 13.00 0.06 0.22 1.10 333 182 1,149 1,698 88 9 50 187 2,032 197 103 538 1168 50 2 14 87 1,321 16.10 5.20 38.19 175.40 7.85 1.20 6.10 1.70 192.20 0.78 0.28 2.46 8.80 2.00 1.10 0.65 0.21 12.76 77 127 47 94 0.48 3.00 0.05 0.24 110 36 68 15 2.10 0.63 0.16 0.06 112 379 51 319 2.40 18.30 0.20 0.88 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. County Reach No. of Structures Affected by the 1% Flood No. of Vehicles Affected by the 1% Flood 144 437 427 79 1,928 20 206 201 9 1,030 1.50 2.20 5.30 0.11 36.15 0.11 0.24 0.39 0.01 2.33 224 742 3,818 0 28 30 1.45 7.30 51.20 0.11 0.62 2.96 506 5,290 10 68 2.40 62.33 0.10 3.79 179 557 736 65 254 319 4.30 11.80 16.15 0.35 1.10 1.50 12,400 3,765 $377.36 million $25.186 million Bastrop/Colorado/Fayette Lower Fayette Bastrop/Colorado/Fayette Columbus Bastrop/Colorado/Fayette Eagle Lake Bastrop/Colorado/Fayette Garwood Total Wharton Upper Wharton Wharton Glen Flora Wharton Wharton Wharton Lower Wharton Wharton Total Matagorda Upper Matagorda Matagorda Matagorda Matagorda Total TOTAL BASIN Damages from 1% Flood (in millions of dollars) Expected Annual Damages (millions of dollars) Over 365,000 agriculturally based acres are included in the Corps of Engineers Study area. An estimated 162,000 acres are considered as total cropland that is actively managed by farmers and ranchers for the production of crops and forage. Of these 162,000 acres, an estimated 85,000 acres are harvested annually, either for hay or for crop production. The production of crops such as pecans, cotton, soybeans, sorghum, and rice is expected to be produced on almost 69,000 acres of the total harvested 85,000 acres within the study area. Over $18 million annually is produced from crops in the Lower Colorado River Basin, with Wharton County accounting for over $10.5 million alone. The counties in the southern most part of the Basin would incur the majority of the agriculturally related flood damages. Table 5-8 provides a breakdown of agricultural flood damages within the Basin for various frequency storms. Average annual agricultural damages from flooding in the Lower Colorado River Basin are estimated at $1.36 million. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-47 Table 5-8. Agricultural Flood Damages in the Lower Colorado River Basin, by Flood Frequency Flood Event County 500-Year 100-Year 50-Year 25-Year 10-Year 5-Year 2-Year Llano San Saba Lampasas Burnet Travis Bastrop Fayette Colorado Wharton Matagorda 51,800 125,600 171,000 231,300 883,500 1,330,100 2,106,800 2,254,100 7,922,000 3,504,800 22,200 80,400 113,200 160,400 453,400 548,700 1,260,700 1,776,500 4,918,800 1,554,000 14,300 61,700 87,800 119,900 425,200 519,100 1,049,300 1,578,100 3,795,300 993,200 14,200 61,700 87,800 117,200 418,000 509,100 1,028,800 1,198,800 1,638,300 974,100 8,200 34,100 42,200 51,700 122,600 290,600 541,900 953,600 570,800 471,600 6,200 27,200 19,000 35,400 82,900 159,800 216,900 582,200 193,400 338,300 5,300 22,100 13,300 20,200 44,200 84,900 115,300 107,600 83,200 156,700 $18,581,000 $10,888,300 $8,643,900 $6,047,800 $3,087,200 $1,661,200 $652,700 TOTAL Table 5-9 below shows an estimate of the dollar losses that would result from a single event 100flood, for selected occupancy classes. These data are from the Lower Colorado River Basin Study of the U.S. Army Corps of Engineers. Table 5-9. Structural Damage from a 100-Year Flood Event for Selected Occupancy Classes (in thousands of dollars) County Bastrop Burnet Bastrop/Fayette/Colorado Matagorda Travis Wharton Page 5-48 Residential $24,853 $28,228 $20,644 $10,362 $56,738 $42,067 Public 0 $ 229 $ 667 $1,052 $1,796 $5,659 Residential Commercial Outbuildings $ 642 $2,901 $2,983 $ 955 $3,811 $3,802 $1,954 $ 493 $5,617 $3,388 $4,482 $2,278 Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. HAZUS-MH Model Results HAZUS-MH Model results estimate that the Basin faces over $36 million in annualized losses from flooding. Table 5-10 shows the estimated annualized losses and annualized loss ratios that would occur as a result of a 100-year flood, by County. The annualized loss ratio is calculated as annualized losses divided by the total exposure at risk and is more appropriate to compare the relative risk among counties. Table 5-10. Annualized Losses and Annualized Loss Ratios from Flooding in the Lower Colorado River Basin, using HAZUS-MH County Bastrop Blanco Brown Burnet Colorado Fayette Lampasas Llano Mason Matagorda Mills San Saba Travis Wharton Williamson TOTAL Annualized Loss [x$1,000] Residential 775 181 507 679 404 500 149 496 86 660 23 149 24,513 695 953 30,722 Commercial 262 27 318 25 17 11 62 13 6 66 1 8 4,705 36 111 5,666 People at Risk 4,565 288 747 5,266 3,434 2,770 147 2,705 322 11,573 65 124 52,243 20,459 792 105,500 Loss Ratio (%) 0.231 0.680 0.106 0.106 0.096 0.297 0.149 0.068 0.198 0.394 0.118 0.117 0.047 0.377 0.130 -- IMPACT ON LCRA FACILITIES LCRA facilities (described starting on page 3-34) are also at risk from this hazard. However, no estimate is currently available of potential damages and losses to those facilities. Creating a Disaster-Resistant Lower Colorado River Basin Protected Proprietary Information Developed By H20 Partners, Inc. Page 5-49