Divers preparing for North Fork gate swap

Divers prepare for a dive at North Fork Reservoir to inspect hydraulic equipment. The divers found unexpected corrosion problems that could necessitate draining the reservoir to make repairs (photo courtesy of Jord Gertson).

A scuba-diving crew is finalizing plans to spend two to four days diving in North Fork Reservoir to replace a damaged slide gate that controls the reservoir’s water releases.

Upper Arkansas Water Conservancy District Hydrologist Jord Gertson said he expects four divers, and they are “tentatively scheduled to go up next week” to perform the work.

The District decided to hire divers to perform the work to avoid draining the reservoir, which remains a possibility if the divers are unable to complete the work underwater.*

The mountain lake sits at the headwaters of the North Fork of the South Arkansas River, and about half the water in the 300-acre-foot reservoir belongs to the City of Salida. The other half belongs to the Upper Ark District.

As previously reported here, Gertson discovered the damage to the gate while testing a telemetry system that enables the gate to be operated remotely.

A rehab project 10 years ago upgraded the reservoir infrastructure with a hydraulic stainless steel slide gate and new down-gradient piping with a backup valve. Gertson said that, before the divers arrive, he will test the backup valve by closing it and opening the slide gate.

If leaks or seepage are observed, the reservoir will need to be drained to complete the work, he said, because leaks in the backup system could adversely affect dam integrity. “We don’t anticipate any problems, but it’s important to do a safety check prior to getting the divers out there to do the work.”

Gertson said he expects the divers will need one day to remove the old gate and one day to install the new gate. Once the new gate is installed, the hydraulic lines will need to be purged of air and water. He is planning two additional days for testing and troubleshooting.

The excessive corrosion on the old gate has been attributed to galvanic corrosion, which can happen when electrical current passes through two different metals that are in contact with one another.

To prevent the issue from recurring, Gertson said the gate has cathodic protection. He said the old gate will be inspected to see if it can repaired and kept as a backup,

* In the event the reservoir must be drained, the water will be retained downstream, most likely in Pueblo Reservoir.

Joe Stone