The mechanical screens include the rotary (drum) type or travelling
screens with water jets or stationary screens (with duplicate numbers for
maintenance purposes) with louvre guides. One of the recent trends
(Odgaard, Cherian and Elder, 1987) in screen design in the turbine intake
is to suspend a screen in the intake water passage to direct fish toward and
into a gate well (Fig. 9.35), for subsequent collection and release down-
stream of the dam.
Electrical screens consist of live electrodes, vertically suspended
along a ground conductor, which generate electrical pulses across the flow
thus forming a barrier (electrical screen) to stop and divert the fish into
safer passages for their onward (downstream) migration.
Recent technology suggests a ‘wall’ (sound barrier) that emanates
from a Bio Acoustic Fish Fence (BAFF) consisting of a perforated pipe
down which compressed air is pumped. Every few metres, pneumatic
sound transducers vibrate to produce low frequency sound waves which
become trapped in the air bubbling up, out of the pipe, creating a ‘wall’ of
sound to the surface of water. Thompsett (2000) suggests that diverting
fish with sound barrier systems (acoustic screens) ahead of the turbine
installation are generally less expensive, require little maintenance and
reduce loss of head in comparison to traditional steel/mechanical screen
diversion systems.
Protection of downstream migrating fish, particularly eel at hydro-
electric power stations could also include turbine management (Hadder-
ingh and Bruijs, 2003) and special turbine design.
FISH PASSES 415
Fig. 9.35 Fish screen at hydropower intake