Fish, Ice, and Wedge-Wire Screen Water Intakes

Abstract

The U.S. Clean Water Act requires that the Environmental Protection Agency ensures that the location, design, construction, and capacity of water intakes for thermal power plants reflect the best technology available to reduce entrainment mortalities of fish and invertebrates during their early stages of life. Cylindrical wedge-wire screens are an efficient technology for protecting fish and invertebrates at water intakes, as they draw water at low flow velocities through a fine mesh. Although substantially reducing fish and invertebrate mortality, wedge-wire screens run significant risks of blockage by ice. This paper discusses how the characteristics of wedge-wire screens that enhance their ability to reduce mortalities also make them susceptible to blockage by frazil and anchor ice, and presents observations and field data regarding ice formation on a segment of wedge-wire screen placed in a small river. Ice formation on this screen involved initial frazil adhesion with subsequent metamorphism of the frazil disks through in situ ice growth. This resulted in accumulations of porous, relatively strong ice consisting of individual large, irregularly shaped ice crystals. The ice that formed on the wedge-wire screen was identical to anchor ice found on the surrounding riverbed. These findings indicate that ice blockage of wedge-wire screens typically comprises frazil deposition followed by in situ growth of frazil crystals, often resulting in a relatively impermeable, hard-to-remove ice accumulations on wedge-wire screens.