1. Catastrophic floods in the Austin area
Raymond Slade, Jr., Registered Professional Hydrologist
“Floods are merely a hazard… mankind is the disaster”
Gilbert White

2. If you Google “Texas flood” You likely will get Stevie Ray Vaughan playing the song “Texas Flood”

3. Definitions
Flood peaks are measured by discharge (i.e, gallons per second or cubic feet per second) and can be expressed by a recurrence probability. For example, a peak having a 1-percent chance of being equaled or exceeded in any given year is often called a 100-year flood.
A 25-year peak has a 4 percent chance of being equalled or exceeded in a given year (1/25 = 0.04 = 4 percent).
The author believes that insufficient data exist in the area to document peak discharge values for floods exceeding the 100-year flood.

4. 1893: Austin dam built (McDonald Lake)
Austin floods
1893: Austin dam built (McDonald Lake)
Austin’s dam was deemed as the largest masonry dam in the World

5. 1900: Dam destroyed by flood
1912: Rebuilt

6. 1915: Flood damaged dam
Floods in 1935, 1936, and 1938 crippled the
dam further
1940: Rebuilt again

7. 1915 flood over Congress Avenue

8. Colorado River at Austin, 1935

9. Colorado River at Austin, 1936
A peak in 1938 also inundated the Congress Avenue bridge
Buchanan Dam was built after the 1938 flood—Congress Ave. hasn’t been inundated since

10. Austin floods in 1915, 1921 and other years have claimed many dozens of victims
Waller Creek at 6th Street after 1915 flood. Reported drownings range from people

11. Austin, May 24-25,1981- Shoal Creek Flood
Flooded area in blue

12. Flooded area superimposed on aerial photo of part of Shoal Creek basin
Austin, Texas, 1981 Memorial Day flood: 13 people drowned, $36 million damages
The peak on Shoal Creek was deemed as a 100-year flood—the greatest rain from this storm occurred west of Austin. At the time, at least 7,000 families were known to live within 100-year flood plains in Austin—most did not know.
Flooded area in blue
Shoal Creek near 12th Street
Press release after flood
Flooded area superimposed on aerial photo of part of Shoal Creek basin

13. Halloween 2013 floods in Austin
Onion Creek—5 drownings and 1200 homes inundated
William Cannon Drive at South Pleasant Valley Road

14. Streamflow data (and other water-resource data) collected Nationwide by USGS
Streamflow data used as basis for floodplain maps and flood warning systems.
About 540 active streamflow gages and about 500 discontinued gages in Texas
Active gage
Discontinued gage

15. Annual maximum flood peak discharges
1921 peak comparable to 1869 peak, 38 feet
Oct 31, 2013
40.13 feet
1941
2001
36.5 feet
Highest yearly flood peaks range several orders of magnitude. Also it is not unusual for many years to pass without a large flood.
1998
32.4 feet
31.2 feet
30.5 feet

16. Big peaks can be clustered in time
1 big peak in 31 years
Then 5 big peaks in 6 years

17. Texas topography and principal sources of moisture
Warm moist air from the Gulf or Pacific often meets cool air from the north and/or is cooled by lifting along the elevated Balcones Escarpment. The cooler moist air condenses and can create large storms.
Texas topography and principal sources of moisture

18. Selected major storms along the Balcones Escarpment, 1913-80
Many storms since 1980 not shown here. These “rain bombs” can produce flood peaks much greater than 100-year peak

19. Recent large storms in or near Austin, 1980-2015

20. 1921 storm and flood Thrall rainfall - 23. 4 inches in 6 hrs 38. 2 in
1921 storm and flood Thrall rainfall inches in 6 hrs 38.2 in. in 24 hrs in Thrall 215 drownings statewide--deadliest flood in Texas history. 87 people drowned in and near Taylor and 93 in Williamson County.
Austin was on the edge of storm—19 inches in 1 day
Taylor, Texas

21. From “Flash Flood Alley”

22. World record rainfall rates for various time durations
Texas cities along Balcones escarpment represented by boxes, Other Texas cities represented by triangles

23. The documentary film “Flash Flood Alley” discusses catastrophic flooding along the Balcones escarpment

24. Trailer for “Flash Flood Alley”

25.
Hyperlink to USGS report documenting major and catastrophic storms and floods in Texas

26. Arrows indicate passage of time
Begin
here

27. After 1981 flood, City of Austin enacted stricter floodplain development rules and installed flood-warning system
Most of Austin streams are small—flood peaks often occur within minutes after heaviest rain, thus an Austin flood-warning system usually provides minimal warning time.
“The floodplain” referenced above probably represents the 100-year floodplain—much more land is in the floodplain for larger floods.

28. Example: 3. 3 means that a flood peak discharge 3
Example: 3.3 means that a flood peak discharge 3.3 times greater than the 100-year peak occurred here

29. Flood Plain Problems Why do people and property flood
Texas leads the Nation in drownings and flood damages
Driving across flooded low-water crossings
Many floods exceed 100-year peak discharge
Low flood plains adopted by some communities (economic incentives)
Ignorance of or disbelief of floodplains
Floodplain encroachment increases with duration after last big flood
Lack of streamflow gages and early-warning flood systems
Cars and other large objects wash into streams and reduce bridge
and culvert openings, causing flood peaks to be higher
Increases in urbanization cause increases in floodplain elevations
Some development allowed in flood plains (i.e., variances)
Fallible or non-existent early-warning flood prediction models
Some flood plains incorrect - lack of data
Dams provide false security

30. Why do many people reside in flood plains
Many want to live proximate to streams and lakes thus market demand deems water-front property as expensive real estate.
Who profits:
Land owners profit from selling such expensive property
Home developers and building contractors profit from building on expensive
water-front property
Real estate companies profit from selling such expensive property
Banks and mortgage companies profit from loans on expensive property
Insurance companies profit from selling flood and related insurance
Cities and counties receive substantial funding from taxing expensive water-front property
Building contractors and home improvement stores profit from repairing and
rebuilding flooded housing and property.
Who pays:
Renter or housing owner after flood
Taxpayers when governmental agencies remove housing and purchase floodplain
land

31. Summary
Many dozens of rain bombs have occurred in the Central Texas area the past 100 years. None have been centered in Austin, which might be “overdue” for such a storm. These storms have produced peak discharges up to about 4 times greater than the 100-year peak.
Several thousand families live in mapped 100-year flood plains in Austin-many more live in unmapped flood plains along small streams.
Many thousands of more Austin families live in floodplains for a flood peak from rain bombs.