In June 2012, a 50-ton piece of concrete broke free and fell into the intake at Horse Mesa Dam on Apache Lake. The resulting damage forced a complete shutdown of hydroelectric Unit #4, resulting in a loss of 97,000 kW in the middle of the peak power generation season. Normally, repairing the now missing vane would simply mean forming and placing concrete, but lowering the lake level to make the repairs would eliminate boat access and significantly impact the local economy.
Instead, dam owner Salt River Project decided to undertake complicated underwater repairs. Four divers worked in two-man teams, living and working in pressurized conditions due to the 160-foot depth. In addition to the difficult working conditions, there were no as-built drawings of the intake vane. We used a remote operated vehicle to gather sonar data, which was translated into a three-dimensional image of the intake. But even with this new data, exact measurements could not be made, leaving a great deal of uncertainty regarding the final size of the replacement vane.
With an aggressive schedule to restore generating capacity, we developed a design that was flexible enough to accommodate project uncertainties while remaining manageable for the underwater team. The replacement vane was built in 15 prefabricated steel components, totaling more than 100,000 pounds. Each piece was individually lowered and fit into place, making construction more manageable. The pieces were fastened together with more than 4,000 pounds of bolts, nuts, washers, and concrete fasteners. Rather than circular bolt holes, slotted holes allowed the final size of the vane to be adjusted to fit the uncertain dimensions.