When a screen is buried in the ground, such as well screens or infiltration galleries, the surrounding pack provides distribution so that flows are nearly uniform over the entire screen surface. A screen in open water does not have a ‘surrounding pack’ and thus must be engineered to balance the flow velocities.
A long, small diameter screen may have the same screen area and therefore the same average velocity, but nearly all the flow occurs at the outlet end, with very little (or none) of the flow happening at the closed end. The high flow velocities at the outlet end are much higher than those allowed for fish protection and suck debris onto the screen and clog it much more quickly. This causes the high velocities to migrate along the screen until they reach the end, having clogged the entire screen.
