GBRA looks at funding options to replace dam gates

When the Lake Wood H-5 Dam failed – under non-flood conditions – it became evident to Guadalupe-Blanco River Authority personnel some serious considerations had to be made, concerning all of the Guadalupe Valley Hydroelectric System dams.

All 15 dam gate systems at the six lakes where GVHS operates a hydroelectric dam are at least 85 years old. The dam gates were all installed in the late 1920s and early 1930s; but the H-5 hydroelectric dam at Lake Wood is priority.

According to Guadalupe-Blanco River Authority General Manager Kevin Patteson, Freese and Nichols Engineers were brought in back in April of 2016 to evaluate what the best replacement option is for Lake Wood and the 85-year-old bear trap dam gate that failed earlier that same year, since the gate is non-repairable.

According to Patteson, the engineering firm has narrowed replacement options down to two.

Option 1: The Bear Trap Dam Gate

Bear trap dam gates consist of two leaves extending across the opening to be closed and are hinged to the sill of the dam on their outer edge to form a triangle when in use and lie fiat on the sill of the dam when open; one leaf, when open, overlaps the others. The space within the triangle is filled with water. On drawing out this water through suitable openings, the dam fills, and on admitting water beneath the leaves, when the dam is open. the leaves are raised slowly into position. Thus the dam is operated by the force of the water.

Option 2: The Obermeyer Dam Gate

An Obermeyer dam gate is a metal structure that is supported by rubber bladders at the base. The bladders are protected by the structure to minimize the risk of something puncturing the bladder. The Obermeyer gate is a patented bottom hinged spillway gate that can maintain accurate automatic water level control even under power failure conditions, has a modular design that simplifies installation and maintenance, is supported for its entire width by an inflatable air bladder which has simple foundation requirements, minimal maintenance requirements and is a cost effective, efficient gate structure.

Patteson said while the historical look of the bear trap gate would be lost, a new-style dam gate is the preferred gate after consideration, because of foreseeable future maintenance costs and easy operation. According to Patteson, the bladder on the Obermeyer gate has a useful life of 20 years, and is a lot easier to replace than having to replace the whole dam gate. The operation of the new-style gates can be set to automatic, with the dam gate opening and closing by maintaining air levels.

GBRA and the engineers are still looking into preventative maintenance measures to be taken, because of the increased amount of debris that travels the Guadalupe river. Patteson said to avoid bladder compromise by debris a model dame would have to be built and studied.

The next issue GBRA faces in the H-5 project is funding. The cost to replace the failed gate would be between $3 million and $5 million. Patteson said if the GBRA is going to have to go through the process of setting up a work site and bringing in cranes and crews, it would be in the authority's best interest to replace both gates, for a cost of $6 million to $10 million.

"We do not want to potentially have a situation where we put in one gate and the other one fails," Patteson said. "So, ideally – if this is the way we go – then we want to do them both at the same time."

Replacement of the older-style bear trap gate would come at a similar cost.

Patteson said funding options include the possibility of a low-interest loan from the Texas Water Development Board, state funding – which is unlikely – and possibly federal grant money.

The Texas Water Development Board had been to Lake Wood, to evaluate the gate and see if the project would qualify for the TWDB loan.

Patteson said once funding is found, the timeline for repairs and replacement could be anywhere between nine months and two years, following the implementation of a dewatering system, to completely empty the lake bed, and remove ground water at the site, as well as pouring of concrete to install the new gate/gates.