The three main causes of delay, injury, and loss of fish at water intakes are entrainment, impingement, and predation. Entrainment occurs when the fish is pulled into the diversion and passes into a canal or turbine. Impingement is where a fish comes in contact with a screen, a trashrack, or debris at the intake. This causes bruising, descaling, and other injuries. Impingement, if prolonged, repeated, or occurs at high velocities also causes direct mortality.
Predation also occurs.
A. Positive Barriers Positive barrier screen systems and criteria for their design have been developed, tested, and proved to minimize harm caused at diversions. Screens with small openings and good seals are designed to work with hydraulic conditions at the site, providing low velocities normal to the screen face and sufficient sweeping velocities to move fish past the screen. The positive screen and fish bypass systems are designed to minimize predation, and to reduce mortality, stress, and delay from the point of diversion, through the bypass facility, and back the river. The main drawback to positive barrier screens is cost. At diversions of several hundred cubic feet per second or greater, the low velocity requirement and structural complexity can drive the cost for fish protection and the associated civil works over a million dollars.