Fusegates Increase Dam Capacity Safely Via Failsafe Flood Release
The reliability of conventional dam gates can be questionable. As many as 30 percent of gated dam failures are due to gates not opening because of mechanical problems, loss of power or operator error. These problems have led many dam builders to incorporate uncontrolled spillways, gaining safety at the expense of potential storage capacity.
McClure dam, sited outside Santa Fe, N.M., was built in 1926. In 1995, a stability survey showed that full supply level could safely be raised by two meters, and a flood wave study found that the installation of fusegates would not cause any significant impact in river flow downstream of the dam. Just prior to the spring floods, eight steel fusegates were installed, increasing live reservoir storage capacity by 22 million cubic feet and spillway discharge capacity by 8 percent with no change in the maximum reservoir level.
The fusegates were provided by French company Hydroplus, which developed the system. Fusegates combine the advantages of ungated and gated spillways to provide a reliable method for releasing floodwater from storage dams. The system uses the natural forces produced by a rising water level beyond a predetermined height. Neither operator intervention nor any form of electrical or mechanical power is needed. These gates have been installed on dams in South Africa, England, France, the United States, India and Malaysia.
A fusegate unit has a skirt or retaining wall, a horizontal base chamber and an intake well or stack connected to the chamber. Each fusegate sits on a concrete base and is held in place by gravity and abutment blocks, which prevent sliding in the downstream direction. The upstream side of the chamber is sealed. Drain holes on the downstream side of the chamber dispose of any seepage. When several units are disposed side by side on top of a spillway, their abutted skirts form a watertight retaining wall that allows relief of small to moderate floods by letting water flow over the retaining wall crest.
If the level of upstream water continues to rise it will eventually top the lip of one inlet stack leading to the chamber of a fusegate. When the inlet stack flow rate exceeds the capacity of the drain holes, water pressure builds up in the chamber causing the fusegate to pivot around the abutments and roll downstream. This opens a gap in the retaining wall, increasing the capacity of the spillway. If the water level continues to rise, it will reach the top of a second inlet stack, initiating the tipping of a second fusegate. Units that tip must be replaced.