A different type of dam

Dams have come a long way since they were first constructed in the 4th millennium BCE.

For example, the oldest known dam in the world is a masonry and earthen embankment at Jawa in the Black Desert of modern Jordan, according to the Encyclopedia Britannica.

Jumping ahead to the 20th century,  the Brazos River Authority built the Morris Sheppard Dam with the aid of the Works Progress Program. Construction began in 1938 on the dam that would soon create Possum Kingdom Lake and was completed in 1941, the same year Orson Welles’ epic Citizen Kane was released.

Morris Sheppard Dam 

At 2,700 feet long and 190 feet high, the dam in Palo Pinto County consists of nine crest "roof weir" type gates. Notably, it is the only BRA-owned dam with this type of gate structure, and these gates testify to the ingenuity of past engineers.

Like the other two dams that create the BRA reservoirs, the structure is a buttress dam, with a watertight wall facing the lake and buttresses on the downstream side. Morris Sheppard’s gates are in the middle, with a wing of buttresses on either side.

One unique aspect of the Morris Sheppard Dam is that it is operated manually. While modern technology has advanced significantly, operating these gates manually feels like stepping back in time, offering a tangible connection to the history of dam engineering. Each steel roof weir-type gate, also known as a bear-trap gate, is about 74 feet long and 13 feet high.

These gates operate manually, using pressure from lake water to open and close them, just as they did when the dam was originally built.

The gates are designed to "trap" water, hence the name "bear-trap." As the gate is raised, it creates a unique water-flow pattern that resembles the jaws of a bear trap. When a gate release is expected, BRA staff begin the process. The gate chamber fills with water, and the water's uplift pressure causes the upstream and downstream leaves to float.

Once the floating status is secured, a BRA employee, tethered above a 130-foot drop, must physically climb onto the gate to unlatch a series of locking bars that secure the gate in the closed position when not in use. The gates are accessed via a catwalk that runs the length of the gates.

At this stage, the gate is “floated” and unlocked.

When it’s time to open a gate, water is drained from the gate chamber, and the gate collapses onto itself, allowing water from the lake to flow over the spillway gate, through the flip bucket, and down the river.

Because of this design, each gate can only be operated fully open or fully closed. When each gate is open, the release can be between 9,000 and 10,000 cubic feet per second per gate when the lake is full. Due to their reliance on a manual system, these gates are rarely used in new dams.

There are three primary methods for releasing water at the Morris Sheppard Dam:

1. Low-Flow Gates: Three gates used for basic operations and smaller releases.
2. Ring Jet Valves: 78-inch and 42-inch valves for intermediate releases.
3. Bear-Trap Roof-Weir Spillway Gates: Nine specialized spillway gates, each capable of releasing approximately 9,000 cubic feet per second.

Tainter gates

The BRA’s other two dams have Tainter gates, or radial gates, which are arc-shaped radial arm floodgates.

The BRA built the DeCordova Bend Dam in 1969 to form the water supply reservoir Lake Granbury. The dam is 2,200 feet long and 84 feet high and has 16 Tainter-style gates, which, unlike those at Possum Kingdom Lake, can be operated electronically via remote computer and opened incrementally to release varying amounts of water beneath the gate.

Electric motors allow the Tainter gates to open rapidly—typically within 5 to 10 minutes—making release changes a quick process. They feature a concave design that draws water from the bottom of the lake rather than the top surface.

Similarly, Lake Limestone was formed by the construction of the Sterling C. Robertson Dam, an earthen and concrete structure. The dam is 72 feet tall, 11,395 feet long, made of earth and concrete, and includes a 3,000-foot emergency spillway. The concrete central section includes buttresses and five Tainter gates, each measuring 40 feet by 28 feet. These radial gates pivot around a center-point trunnion swivel in the back, lifting upward via hoist cable mechanisms attached to the downstream side. They can be set to any height and are generally simple to operate.

As with the Lake Granbury dam, these gates release water from below, and each may be opened remotely by computer, incrementally allowing varying amounts of lake water to pass.

Water passing through this dam flows down the Navasota River, joining the Brazos at its confluence in Grimes and Washington counties.

From ancient beginnings to modern marvels, dams continue to play a vital role in water management and safety. The Brazos River Authority was the first agency in the country to be tasked with managing the water resources of an entire river basin. Its three reservoirs now serve as part of an 11-reservoir system, ensuring families, businesses, and industry get the water they need across the Brazos River Basin.
 

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