Routine dam inspection saved Possum Kingdom Lake

We’ve all seen it at one time or another. The image of a catastrophic dam failure causing a wall of water to rage over the countryside. Of course, the image originated in one of the numerous disaster movies produced by Hollywood. But, 30 years ago, that Hollywood-style image might have become a reality without the quick thinking of Brazos River Authority staff and engineers. A routine five-year inspection found that Possum Kingdom Lake’s Morris Sheppard Dam had begun to shift. Standing nearly 19 stories high and one-half mile long, a section of the concrete structure had moved 4.5 inches downstream.

Constructed from 1938 – 1941, Possum Kingdom Lake was the first reservoir built by the Brazos River Reclamation and Conservation District, now the Brazos River Authority. The water supply and hydropower reservoir was the result of more than a decade of planning efforts to tame a river that historically claimed hundreds of lives with flash flooding and made water supply efforts difficult at best due to periodic extreme drought conditions. The Morris Sheppard Dam was, at the time, the tallest flat slab buttress dam in the United States and was classified as a high hazard structure due to the many farms, ranches and cities downstream.

On a blustery December day in 1986, BRA staff and members of the Federal Energy Regulatory Commission (FERC) performed the routine five-year inspection required for all FERC-regulated hydroelectric dams. The group was startled when the review found that a portion of the dam had moved about 4.5 inches downstream, requiring immediate repairs to prevent failure of the dam.

The BRA began releasing water to relieve pressure on the dam structure. More than 187,000 acre-feet, one-third of PK’s capacity, was safely released downstream so that repairs could begin. For the next two years, the reservoir level would sit 15 feet below normal while repairs moved forward.

As efforts to discover the cause of the dam’s movement progressed, engineers determined the shift was caused by hydrostatic pressure in the dam’s foundation, basically causing a section of the dam to float. Workers began repairs by drilling 8-foot holes through each of the 24 concrete buttresses inside the hollow-core dam, removing 18,000 cubic yards of mud and debris and replacing it with 66,000 cubic yards of crushed rock that would later be anchored with concrete ballast to increase the dam’s weight.

A series of 146 relief wells were drilled inside the dam’s bays. Working under adverse conditions complicated by naturally occurring hydrogen sulfide gas, the engineers discovered a 1 to 3 inch wide crack in an upstream transition beam, which served as a channel for water to flow from the reservoir into the dam’s foundation. Before this flaw was repaired, water flowed through the crack at a rate of about 550 gallons per minute. When work was complete, leakage was reduced to less than one gallon per minute.

The emergency repairs were finished in March 1989, averting the potential crisis and allowing normal operations to resume.

Though the repairs took more than two years to complete, the process was considered to be a textbook crisis response and remediation. The BRA’s efforts were recognized in a 1990 nomination for the Outstanding Engineering Achievement Award by the Fort Worth Chapter of the Society of Professional Engineers. “The Morris Sheppard Dam project is an outstanding example of how a dam that was under serious threat of failure was saved through an in-depth investigation and repair program,” said a report filed by the Texas Society of Professional Engineers.

While those heroic efforts in the 1980s are still worth remembering and celebrating today, the work to ensure the integrity of the Possum Kingdom dam and the dams at the organization’s two additional reservoirs, Lakes Granbury and Limestone, remains a continuous process for the BRA.

“Every five years, we hire an outside engineering firm to conduct an inspection of each dam,” said Steve Vaughan, emergency safety compliance manager for the BRA. “They look for signs of stress or failure and file a report based on their findings. In addition, our staff routinely inspects the dams throughout the year. They are trained to detect signs of stress or failure and report any concerns to engineering.”

The BRA submits regular reports to the Texas Commission on Environmental Quality, Vaughn said. “They review and analyze the reports and conduct their own inspections to ensure the dams are being operated in as safe an environment as possible.”

The BRA also has site security plans that law enforcement officers at each implement work to ensure the security of the dams, Vaughn said.

The Federal Emergency Management Association notes that there are more than 87,000 dams in the United States, and about one-third of these pose a high or significant risk to lives and property if those dams were to fail.

“Dams are a critical part of our nation’s infrastructure, and all Americans enjoy the benefits they provide, including flood protection, water supply, hydropower, irrigation and recreation,” FEMA reports. “However, our dams are aging, and many are deteriorating, while downstream and upstream populations are increasing. Everyone has a role to play in creating a future where all dams are safer – including dam owners, engineers, community planners/leaders, and federal and state regulators.”

Monitoring the dams involves a detailed process at each of the BRA reservoirs.

“On a daily basis, all employees watch for any change in the embankment, such as cracks along the crest, any sign of movement or sloughing of the slopes, or excessive or unusual water downstream,” said Davy Moore, Lake Limestone area project manager. “There are documented inspections of the entire dam conducted on a weekly, monthly, quarterly, annual, and five-year basis. There are also documented inspections performed after an unusually high volume release, or after an unusual occurrence such as a nearby earthquake. During these inspections, mechanical and electrical functions are tested, and visual inspections document the integrity of the physical structure.”

Moore said keeping people safe is always the top priority. “People should be aware that dams are regulated by state and federal agencies and are operated and maintained with public safety in mind,” he said. “Emergency Action Plans exist for most dams and are regularly updated and reviewed, including table top exercises where different scenarios are practiced to ensure proper response to any unusual event by owners of dams and emergency responders.”

Jeff Posvar, P.E., senior project engineer for the BRA, said each of the agency’s three dams have different checklists to ensure they are safe. In addition to reservoir staff inspections, he personally visits each dam once a year for an engineering inspection, although the Texas Commission on Environmental Quality only requires inspections every five years. Other detailed inspections are done on site every month. But beyond that, standards are in place as a matter of routine, he said, and detailed plans have been made so BRA staff, working with state and local officials, know how to deal with nearly every situation that may arise.

“We’ve been very vigilant, and the TCEQ has noted good things about our facilities,” Posvar said. “Our employees are doing the right things, and they know what to do whenever they observe something – or don’t see something. Our staff goes above and beyond the basic requirements set by the TCEQ.”

Posvar said the Water Services Department plays a vital role in keeping the dams operating efficiently and safely.

“Water Services does a great job of coordinating everything,” he said. “When a storm comes in at 3 a.m., they’re not lying in bed. They’ve planned things out before the storm and are monitoring the situation throughout the night. The staff is coordinating things at all hours of the day and night, and they do a tremendous job. That’s an element that the public isn’t always aware of. They are the ones making sure everything happens like a well-oiled machine.”

Posvar said detailed plans help staff know what to do each step of the way – even if communications break down.

“Everybody is available at a moment’s notice, and that’s key,” he said. “But the plans even let people know what to do if they can’t reach someone. There is a very detailed process in place to help keep everyone safe.”

Keeping our dams safe is a team effort and the focus of daily operations for BRA staff and engineers.