News

Signing marks rebirth for Medina Dam

By William Hoover
Anvil Herald Correspondent

Last Tuesday, Feb. 23, Bexar County Commissioners agreed to contribute $3 million toward the $10 million improvement of Medina Dam owned by Bexar-Medina-Atascosa Water Control and Improvement District No. 1. BMA Business Manager Ed Berger said Bexar County was the last of three entities needing to sign on before dam repairs could begin.

Residents living downstream of Medina Dam will be able to rest easier now that the BMA has obtained funding to reinforce the 164-foot dam.

Possible instability in the 98-year old dam was first brought to light in July 2002 by Bexar County Judge Nelson Wolff, who went on television to warn downstream residents to evacuate if the record setting rains continued. The spillway is 12 feet below the top of the structure when the reservoir is full, but water had risen to within a foot of the top of the dam when Wolff made his comments, based on information from Department of Public Safety officials in Austin.

BMA will contribute $3 million and the Texas Water Development Board will contribute another $4 million toward the project. Bexar County’s $3 million will be used for design, construction and project management.

Landowner and BMA chairman Will Carter has worked for the last eight years trying to secure funding for the dam.

“It’s been a long, hard road,” said Carter about securing funding to make repairs to the concrete dam built in 1912. “We have been working with Bexar County since August 2008.”

In 2008, Medina County Commissioners agreed to fund a portion of the dam project through Bexar County’s $500 million regional flood control program.

The water district’s business manager said residents should thank State Rep. Tracy King, who introduced the bill providing the $4 million grant from the state.

“He worked hard for his constituents and for the water district,” said Berger of the District 80 representative. “He was a real factor in getting us funding. He realized his constituents were in harm’s way and he stepped up to help us.”

The side abutments of the dam are the main safety concern, according to Carter “The abutments, which have roughly one-third the mass of concrete the dam does, anchor the main dam into the bedrock,” he said. “The abutting sides are where most dams are weakest.

“The force of water on both sides of the dam, if the dam overflowed (by several feet), could lift the dam from its base,” said Carter. “The side abutments of the dam don’t have any relief wells. The main dam does. It actually has wells that go into the bedrock to allow the (water) pressure to escape and that keeps the dam stable.”

Carter said repairs will consist of anchoring the sides of the dam to the limestone canyon walls with extra suspension cables and adding concrete to the back side of the dam.

“The biggest bang for the buck was with the 32 cable repairs,” said Carter. “They are massive cables roughly 15 inches in circumference.”

There will be 21 cables on the spillway side of the dam and 11 cables on the other side of the dam. Cables will be connected to the canyon wall with anchors drilled then grouted into the bedrock. The cables will then be connected to anchors drilled into the dam, securing the structure to the canyon wall.

The repairs will reinforce the dam so it can withstand an unprecedented rain event and remain in place even if the lake fills and water flows over the top of the dam by eleven feet. “It is called a Probable Maximum Flood Event, which is 36 inches of rain in 24 hours,” said Carter. “And the PMF event would also have to occur when the lake is full to overtop the dam. Medina Lake is not a constant level lake, so we often have plenty of extra capacity.”

“The dam is stable in our normal conditions, which includes extremes of dry and wet weather,” said Carter. “In 2002, however, the water came within two feet of overtopping, which was a new record. We are going to be pouring concrete aprons on the dry side of the dam to protect the foundation. The concrete covering will prevent rock from being plucked out by the velocity of the water falling over the top of the dam. We will pour a lot of concrete at the base of the dam to ensure undercutting won’t be a factor.”

The benefits of the reservoir’s impounded water, the number of residents below the dam and other downstream assets are of incalculable economic value, which Carter believes figured into the decision to move forward.

“I think, finally, we convinced the state to realize they needed to do the right thing,” he said. “They also realized Medina Dam affects a lot more than the BMA and farmers who irrigate.”

The Edwards Aquifer receives more acre-feet of recharge water from Medina Lake than BMA uses for irrigation water for farmers in the district, according to the BMA president.

“The EAA receives around 60,000 acre-feet of water a year via the recharge features in Medina Lake and Diversion Lake,” said Carter. “Medina Dam and Diversion Dam provide a major source of recharge for the aquifer. I just don’t understand their thinking. The big brother who looks over their shoulder is SAWS. You would think they would want to protect the aquifer’s water supply but they didn’t. If Medina Dam failed, it’s not only the destruction of assets and the loss of life, it’s the loss of water supply for the region. If the dam failed, close to one-third of the drinking water for San Antonio would be gone. It is crazy that neither SAWS nor the EAA would agree to contribute to the repairs in some form or fashion, but they didn’t give one dime to the project.”

The water in Medina Lake plays a vital role in the state’s water strategy because it recharges the Trinity and Edwards Aquifers and is one of the area’s few sources of surface water.

Berger said BMA attempted to obtain a grant from the state to stabilize and reinforce the dam, but they were required to enter an interlocal agreement with all local water agencies.

“The interlocal agreement is between the TWDB, Bexar County, Bexar Metropolitan Water District, Edwards Aquifer Authority and the San Antonio River Authority” said Berger. “When the legislature awarded the grant, it required EAA to be a party of the interlocal agreement. They are a party to the agreement even though they are not contributing money. SARA will administer the grant.

Berger credited Judge Wolff for backing up his 2002 remarks by having commissioners help fund the dam’s repairs.

“He came through,” said the BMA business manager. “He put his money where his mouth was. He was the first to commit $3 million from Bexar County.”

Bexar County would receive the brunt of the water from a maximum flood event, according to Berger, who said 156.9 square miles of land could be flooded if the dam were to fail without the repairs.

“Medina County would have 21.5 square miles of land flooded if the dam failed. Wilson County would have 37.5 square miles covered. Karnes County, 47 square miles,” he said, noting 50.2 square miles of Bexar County would be covered. “Obviously, as the water comes down out of the hills, the land flattens out so the water would just spread at a shallower depth. The projection is the water would go all the way to Goliad County.”

The main communities which would be impacted by a failure of Medina Dam include Rio Medina, Castroville, La Coste, Macdona and others downstream, according to the BMA’s engineering report. The Toyota Truck plant and two SAWS sewage plants would also be inundated.

Berger said the lake level would be unaffected by the planned year-long repairs to the downstream side of the dam.

“I’m hoping we can get started on work within the next three or four months,” said Berger of the project to be bid in April. “A project of this size requires specialized contractors. We will have our first construction meeting on March 23. By this time next year, I’m hoping the repairs are complete.

“I’ve been involved with the repair of the dam since I took this job five years ago, so it’s nice to finally see the light of day.”

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