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Catastrophic floods and flow declines in the Upper Colorado basin, Texas
- 1. Catastrophic floods and flow declines in the Upper Colorado River Lake Travis near Austin World’s largest multi-arch dam Lake Buchanan Raymond Slade, Jr. Certified Professional Hydrologist
- 2. The Oasis on Lake Travis diving?
- 3. Colorado R. basin Austin 42,000 sq. mi. drainage 5th largest in Texas but larger than 15 states serves 36 million people The Colorado River basin
- 4. Colorado basin Austin Lower Colorado basin excluded Lake Buchanan Lake Travis Lake Buchanan 64% of basin
- 5. Contents Floods Extremes Trends Most of the data and informaiton herein are from Slade, R.M., Jr., 2020, Runoff Inflow Volumes to the Highland Lakes in Central Texas: Temporal Trends in Volumes and Relations between Volumes and Selected Climatic Indices: Texas Water Journal, vol. 11 No. 1 https://twj-ojs-tdl.tdl.org/twj/index.php/twj/article/view/7025
- 6. Arrows indicate passage of time Flood planning Floods flood panic concern awareness apathy People often have extended panic
- 7. Colorado River at Austin, 1935 “Floods are merely a hazard… mankind is the disaster” Gilbert White
- 8. Remnants of 1935 flood Iron Works BBQ restaurant Downtown Austin
- 9. Colorado River at Austin, 1936 A 1938 peak also inundated the Congress Avenue bridge Buchanan Dam was built after the 1938 flood—Congress Ave. hasn’t flooded since
- 10. Typical storms in the Colorado basin • During a storm its location, size, intensity, direction and speed can quickly change. • Storms often retain their character but tend to move east following many Texas streams.
- 11. Causes for large floods • The Colorado has a large watershed but individual storms never cover the entire basin. • Storms in the Southwest can be intense but are limited to a few counties at most. • Many of the storms travel in a similar direction as the river, at speeds of about 15 to 20 miles per hour. • This speed coincides with the approximate flood-wave velocity of the river. • Such storms often cause local tributaries to peak at the same time as the river, thus causing an increase in the main-channel crest.
- 12. Storms moving down a basin often cause local tributaries to peak at the same time as the river, thus causing an increase in the main-channel crest. Basin boundary Affect of moving storms on peaks The video shows the leading and trailing edges of runoff. Dispersion of runoff is not shown.
- 13. Extreme wet: Colorado River At Wharton, 1935 Extremes
- 14. Extreme dry Inflow to Buchanan has been no flow on 43 days
- 15. Range in inflow to Lake Buchanan 1957 2011 1957 2011 Avg Year Annual flow (ft3/s) % of mean 2011 69 7% 1957 4340 450%
- 16. 1 big peak in 31 years Extremes in flood peaks 5 big peaks in 6 years 98% decrease The gage is near Lake Buchanan 3
- 17. Variation and decline in Lake Buchanan inflow 59% reduction Spence Reservoir Ivie Reservoir From Slade, 2020, TWJ report inflow Basin regulation Extremes and trends
- 18. Lake Buchanan North 88% 82% 39% Reservoirs impacting Lake Buchanan inflow Impoundment dates 1963 1934 1991 Limited data Reduction in flow due to reservoirs Major releases from these reservoirs 3-5% of time Basin boundaries 69% of basin regulated by 3 reservoirs However, most of the reduction is due to ponds in the reservoir basins
- 19. Pond density By 2013, the area exceeded 165,000 acres 1 In 1940, ponds in the basin had a combined surface area of less than 40,000 acres
- 20. Water budget major components Inflow rainfall Outflow runoff evapotranspiration and seepage to groundwater (ET) overland flow stream channels impoundments withdrawals
- 21. Water-budget analysis rainfall Year Much of the increased evapotranspiration would have otherwise been lost downstream as ET in the channel before reaching Lake Buchanan. runoff represents about 2% of rainfall Inflow Outflow 12%
- 22. Affect of ponds on Buchanan inflow • Basin rainfall increased 12% • Lake Buchanan inflow reduced 59% • 19 reservoirs and 70,000 ponds in basin • > 81 percent of ET from ponds • Ponds exceeding 200 acre foot permitted: creation subject to contested public hearings • Most ponds have less than permitted volume--built without scrutiny, permitting, or regulation • 17,000 to 70,000 ponds from 1940 to 2013 • Pond ET exceeds inflow by 215% • Basin area extensively developing--ponds will increase • Buchanan inflow continue to decline