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.”