with variable times according to a specific installation and its operating
procedures. Examples of Borland lift passes (Aitken, Dickerson and
Menzies, 1966) are in both Scotland and Ireland.
(d) Fish traps and transportation
At high-head structures, fish ladders are neither economical nor practical.
Instead, tramways or cableways are used. The installation consists of
hoppers into which fish swim; they are then transported to the reservoir by
tramway or cableway. Fish may also be unloaded into a tank truck (filled
with aerated and refrigerated water) which conveys them to either the
reservoir or a hatchery where they are carefully unloaded.
(e) Fish-barrier dams
These are low-head weirs (with an electrical field if necessary) which stop
the upstream migration of fish and induce them to swim into a fish ladder
or hopper situated downstream of the barrier (with a proper entrance to
attract fish) from where they are transported by tank trucks.
9.3.3 Downstream fish-passing facilities
The seaward-migrating fish are normally protected from spillways and
turbine intakes by screens (Aitken, Dickerson and Menzies, 1966) which
divert or deflect them (Fig. 9.35) into safer bypass arrangements. The veloc-
ity of approach to screens is generally kept at around 0.5 m s^1 , whereas for
louvre diverters it is around 1 m s^1 to maintain the required turbulence
levels. Fish screens are usually of either mechanical or electrical type.
414 DIVERSION WORKS
Fig. 9.34 Borland fish lift
