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.
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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
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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.
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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
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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.
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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.
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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.
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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
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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
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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)
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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:
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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.
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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,
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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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
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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.
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Page 5-49