George awoke to the ringing of his cellular telephone. An automated voice announced that the water level at Eagle Canyon Dam had dropped below the low alarm level. He was puzzled. The spring snowmelt runoff had begun. If anything, the water should be at flood level. George dressed and began the one-hour drive to the dam. Halfway there, his cell phone rang again. This time, the voice advised that the water level had exceeded the high flood level. George was now thoroughly alarmed.
Upon arriving at the dam at dawn, George saw water flowing over the crest. A massive logjam was blocking the automated spillway gate from opening.
George could see that a dam washout at the left abutment was possible, so he called the owner of a camp facility below the dam to evacuate all campers and notified the local police. Fortunately, the head pond contained only a small volume of water, so a washout would not cause major downstream flooding.
George then turned his attention to minimizing the developing damage. The trashrack cleaning machine, a converted backhoe, was parked upstream on a now-inundated alluvial terrace. There was a chance the terrace would be washed out through the breach and the backhoe lost in the canyon below the dam. George attached a steel rope from the winch on a bulldozer and hauled the backhoe to higher ground.
There was nothing else he could do. The sluice gate was thoroughly jammed closed, with logs resting on top of the gate and threatening to dislodge the sluice hoist mechanism.
With water flowing over the dam, erosion was transporting sand and gravel into the intake and through the turbine-generating units at the 15-mw plant. Excessively high guide bearing temperatures confirmed that the turbines were being damaged. But George decided to keep the units running, to keep from increasing the flow of water over the dam.
Within hours, water was overtopping the dam by about 1 meter. Although the dam remained intact, erosion was seriously undermining a retaining wall at the right abutment. Maintenance staff were brought in to maintain a watch until the flood receded and personnel could use a mobile crane to open the spillway gate and dewater the forebay.
Once the problem was under control, George concentrated on determining what had caused the situation. The run-of-river facility was built in a steep-sided canyon, with a concrete dam containing an overflow weir, one radial sluice gate, and the power intake built into the left bank. The dam site was in a rainforest with very large trees, with logging in some areas of the watershed. George’s company had purchased the facility about a year previously.
Just before the flood event, the radial sluice gate was fully open, passing the spring flood, and the remote-controlled power plant was operating at about 40 percent capacity. Apparently, a massive logjam had formed upstream in the canyon and blocked the flow. The dropping water level caused the automated radial sluice gate to close. Eventually, the force of water building up against the logjam must have caused it to release, and a large flood wave of water and log debris rushed down the canyon onto the dam.
A month later, the flood flow had receded sufficiently to allow inspection of the damage. The project owner installed a temporary cofferdam, allowing generation to commence a few days later. Fourteen months were needed to assess repair options, rebuild the dam, and increase spill capacity by adding another gate and stoplog sluice.
There are several lessons here:
- The new owners had not undertaken a thorough review to ascertain that the dam’s design met their standards. The hydro plant was originally family-owned, with a 500-kw micro-hydro unit. An entrepreneur bought the plant, upgraded the facility, and increased capacity to 15 mw. But the entrepreneur left the original spillway intact, with a capacity only capable of passing the 1/10 (one in ten years) flood. After the failure, the dam owner more than doubled the spill capacity to pass the 1/1,000 (one in 1,000 years) flood flow.
- With a small dam in a steep canyon, subjected to a large volume of debris during floods, it is wise to maintain constant surveillance at site during the flood season.
- In a rainforest, there is the constant danger of large logs damaging small hydro structures, particularly during floods. Logs tend to lodge on streambanks in jams, and these grow in volume in the absence of large floods to carry the debris downstream. When a large flood does occur, the resulting morass of debris can easily overwhelm spillways.
This occurred during the 1996 Saguenay 1/5,000 (one in 5,000 years) flood in Québec, Canada. To quantify this problem, a large Canadian utility recently undertook a helicopter survey of the watershed upstream of a major dam and discovered an estimated 24,000 cubic meters of logjams in about 30 kilometers of river bed. The jams appeared stable under a normal flood but would be transported downstream during a major flood event.
— By James L. Gordon, B.Sc., hydropower consultant. Mr. Gordon may be contacted at (1) 514-695-2884; E-mail: firstname.lastname@example.org.