Fishways for low head dams using labyrinth weirs

The purpose of this research was to investigate the differences between the use of labyrinth-type and straight weirs in a modified pool and weir fishway for low head dams. The literature review provided information on what parameters need to be considered in designing a fishway. It also provided information on the results of model study for a fishway at the Little Falls Dam on the Potomac River, which incorporated the use of labyrinth-type weirs.;The model study was conducted at Utah State University Utah Water Research Laboratory in Logan, Utah. A 1:8 scale physical hydraulic model of an ogee crest dam with a modified pool and weir fishway was constructed and tested in the laboratory under various flow rates. Two series of tests were conducted. In the first series of tests the bed of the fishway had 1 on 7 slope, and the elevation differences between the tops of weirs were greater than 1/2 foot. In the second series of tests the slope of the fishway was 1 on 10, and the elevation differences between the tops of weirs were fixed at 1/2 foot. In both series of tests labyrinth-type weirs were compared with straight weirs.;Results showed that when labyrinth-type weirs were used, the average velocities along certain flow paths in the fishway were generally 30 percent lower than when straight weirs were used. The shape of the labyrinth-type weirs causes the flow to either diverge or converge as it passes over it. The diverging of the flow reduces the velocity of the fluid.