Water

Our Water: Twin Buttes special report continues

Part two of three

San Angelo, TX - The Twin Buttes Reservoir, located in Tom Green County west of the city of San Angelo, is a reservoir like none other in the United States. Its unique backstory and physical characteristics have earned it a place in text books concerning dam construction and techniques.

It started as most of the dam building projects in the 1950-60s — a long history of droughts and history-changing flood events led to the construction of thousands of public works projects across the nation by authorities. The O.C. Fisher Reservoir, officially impounded in 1952, and the Twin Buttes Reservoir, started in 1960 and finished in 1963, completed official projects of this time period in the Concho Valley. The reservoirs are important to the area for impoundment of water to use for the purposes of irrigation, industrial, flood control, recreational, and municipal use.

A dam like no other

Twin Buttes is unique in the reservoir world in that two separate pools of water permanently separated by hilly terrain were impounded by the same dam. No other single dam in the United States holds back two separate river basins. A 250-foot wide equalization channel, another feature not seen elsewhere, was constructed to join the South Concho River and the Middle Concho and Spring Creek drainage areas to equalize the surface area of the reservoir.

The Twin Buttes dam is the longest dam built of its type of construction. The earthfill structure is 8.2 miles long at the 30-foot wide top (crest) of the dam, and rises to 130 feet high at the Middle Concho River. The volume of the dam is 21.442 million cubic yards of material.

The many purposes of Twin Buttes - including major flood protection

The Concho Valley region is "historically known for intermittent droughts and floods, and Twin Buttes Reservoir provides protection against both weather extremes. The San Angelo Project supplies water for municipal and industrial use as well as agricultural irrigation, and also provide flood control," explains a memo from the United States Bureau of Reclamation (USBR).

The memo continues: "A conservation storage capacity of 127,850 acre feet (41.6 billion gallons) is designated for joint use of irrigation, and municipal and industrial supply; and 454,370 acre-feet (148 billion gallons) of capacity are reserved for flood control. This flood storage, combined with spillway and outlet works capacities, is sufficient to protect against an inflow design flood having a peak of 725,000 cubic feet per second."

Along with the impressive flood control aspect, the reservoir can be used for irrigational purposes. The reservoir empties into Lake Nasworthy, which then feeds an irrigation distribution system comprised of a main canal with siphons, laterals and culverts under the canal and bridges over it.

The canal travels northeast past San Angelo into irrigation lands. The concrete-lined canal is 15.9 miles long with a capacity of 165 cubic feet per second — with the lateral distribution channels, the system totals 39 miles of irrigation capability.

Completed in '63, but not turned over until '73 — thanks, drought

Though completed in 1963, construction status continued through 1965, when the San Angelo Water Supply Corporation (SAWSC) assumed operation of project storage facilities while the USBR maintained control of the rest of the facilities.

The USBR transferred the reservoir to standby conditions on New Year’s Day in 1966 due to drought in the region which hindered complete operation of all facilities. With the end of the drought in 1971, the USBR transferred complete control to the SAWSC for operation and maintenance (O&M). The following year operation of irrigation facilities was transferred to the Tom Green County Water and Improvement District No. 1 (TGCWC & ID) with the first deliveries of project water made on March 6, 1972. On January 1, 1973, the USBR turned over all O&M responsibilities to TGCWC & ID.

And then it leaked... and leaked... and leaked...

As soon as the dam started to collect water, foundation seepage issues literally surfaced. In 1964, a year after construction was complete, was when the foundation seepage was first noted.

In 1974, as the reservoir rose to conservation level for the first time since construction due to heavy rains runoff, significant and rapid expansion of the downstream water table was noticed.

In the hangar complex of Mathis Field, water began surfacing and forming in pools 4,000 feet from the dam. The water table had risen to within two feet of the surface at the airport.

Seepage was detected along the side of the mid valley ridge of the dam embankment. One estimate put the water leaking from the dam at near 22 cubic feet per second. Some land areas were becoming so saturated as to be inaccessible. Landowners sued the USBR for damages caused by the leaking. The court documents gave testimony that the leakage from the dam had risen to serious proportions.

A Threat to Life

As the reservoir level rose, seepage rates increased to nearly 60 cubic feet per second or more. Piezometric levels downstream indicated that the uplift pressure on the dam might result in dam failure if the reservoir rose much further. In a bulletin from the USBR, what a catestrophic dam failure might mean to the area was spelled out:

"A Threat to Life Study determined that the total population at risk impacted by a dam failure by embankment instability and piping is about 22,600 people, and the estimated loss of life is about 2,850 people. Property damage to San Angelo and the surrounding area would exceed $660 million. Failure of Twin Buttes Dam would eliminate the primary municipal water supply for San Angelo and expose the area to frequent flooding and irrigation losses." — Water Operation and Maintenance Bulletin No. 184, June 1998, USBR

In 1991, a reservoir restriction of EL 1930 was placed on Twin Buttes Reservoir until remedial measures to address the seepage deficiency had occurred. The restriction allowed a safe passage of up to the 50-year flood event.

Stopping the leak

Several dam modifications were attempted to control the problem, either by controlling the uncemeted gravel under the dam or collecting the seepage in a controlled manner.

Starting in 1976, grouting of the seeping areas was attempted three times. That effort lead to the realization that the seepage was coming from an area of the foundation that had not been sealed with a cutoff trench. Because of caving during drilling of grout holes in areas of loose uncemented sands and gravel, safety officials discontinued the grouting program. Also, higher water levels showed that grouting remained ineffectual in stopping the seepage.

In another attempt at a solution, 61 relief wells in two areas along the downstream toe of the dam were installed in 1984. However, the wells were not intended to control the seepage beyond the downstream toe. A drainage system was also installed to protect the area from becoming saturated and inaccessible. Nevertheless, seepage continued.

The grouting, relief wells, and cutoff trenches were all installed, but none provided a satisfactory solution to the problem.

Dam safety alternatives investigated

Structural and nonstructural alternatives were considered to mitigate the dam's seepage by a 1994 USBR study.

Nonstructural alternatives: reservoir restrictions, early warning systems, and no action, were deemed unacceptable because of inadequate protection of the public or loss of public benefits.

Structural alternatives considered included three concepts:

  • “Cutting off” the seepage—a cutoff wall, an upstream blanket, and grouting.
  • "Control" seepage by providing drainage features — seepage berm, deep drainage trench, sand and wick drains, and relief wells.
  • "Elimination" — by breaching the dam.

Information gathered from studies and previous efforts raised concerns over piping of the foundation materials, channelized seepage paths, and variability of the foundation material. The concerns were significant enough that the alternatives were considered to have an unacceptably high risk of not
adequately mitigating the problems and, therefore, were not technically adequate alternatives for correcting the seepage deficiency.

A special wall for a special problem

The permanent solution to the elusive seepage problem proposed and accepted was the need for a cutoff wall along the troublesome area of the dam.

A cutoff wall is defined as "A wall of impervious material usually of concrete, asphaltic concrete, or steel sheet piling constructed in the foundation and abutments to reduce seepage beneath and adjacent to the dam." — Federal Guidelines for Dam Safety, U.S. Department of Homeland Security, Federal Emergency Management Agency

The scope of the Twin Buttes dam would require an enormous cutoff wall: over 4 miles long, up to 100 feet deep embedded into at least 2.5 feet of the underlying bedrock. The 1.4 million square feet surface of this wall makes it the largest such wall constructed in the United States.

Several types of material for the wall were considered. For some common fill, the size of the wall to meet safety standards and prevent a blowout would require a wall up to 12' in width. This was beyond the practical capability of construction equipment of the day due to the depths and expected difficulty of excavation.

A rare solution and fairly new material, Soil-Cement-Bentonite, backfill was settled upon. Given the unique properties of the material and the requirements to prevent blowout, a 2.5' width was determined to be necessary. Under strict requirements, quality control methods and construction oversight, the $39.3 million-dollar wall was started in July 1996 after the reservoir water level was lowered to EL 1920, which allowed over half of the wall to be constructed in the dry, while still allowing public use of the lake. Coffer dams were installed for the rest of the construction process.

The cutoff wall was completed in 1999, and this Twin Buttes Dam Safety Modification so far has met the safety and performance goals of its design. The dam no longer leaks, the piezometric pressures under the dam have decreased, and the dam has been released of all restrictions regarding its use.

The Twin Buttes Reservior — this one-of-a-kind, unique gem of the Concho Valley, is now able to perform at its full design specifications with safety.

Twin Buttes today

The Bureau of Reclamation, Texas Parks and Wildlife (TPWD) and the City of San Angelo agreed in 2016 to place the 13,000 acres surrounding the reservoir into the TPWD Annual Public Hunt. This allows TPWD to manage hunting, increasing both the management of Wildlife and opportunities for recreation.

A permit is now required to access the public recreation areas and can be obtained from any merchant that sells hunting and fishing licenses.

The reservoir has also produced a Toyota ShareLunker largemouth bass exceeding 13 pounds. Angler Austin Terry of San Angelo caught the 13.40 pound, 25 ¾ inch Legacy Class lunker on a Santone spinnerbait ½ ounce red river special in less than 4 feet of water at the lake March 14, 2018.
 



SOURCES, FOR FURTHER READING

Out About Texas
http://www.outabouttexas.com/LocSrv?locId=128093&name=Twin%20Buttes%20Reservoir

Water Data For Texas
https://waterdatafortexas.org/reservoirs/individual/twin-buttes

Texas Water Development Board
http://www.twdb.texas.gov/surfacewater/rivers/reservoirs/twin_buttes/index.asp

Texas State Historical Association
https://tshaonline.org/handbook/online/articles/rot10

US Bureau of Reclaimation
https://www.usbr.gov/projects/index.php?id=307
https://www.usbr.gov/projects/index.php?id=429

Flood control parameters of Twin Buttes Reservoir
https://www.law.cornell.edu/cfr/text/33/208.22

USBR: Twin Buttes Documents
https://search.usa.gov/search?affiliate=Reclamation&query=twin+buttes


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