Salmon and trout are the main migratory fish. The smolts (young
salmon) travel to the ocean 1–2 years after spawning in fresh water, while
the kelts (fully grown salmon) return to their spawning grounds after
spending 1–3 years in the sea. The design criteria and layout of the facili-
ties, largely depending on the type of fish and whether they are migrating
downstream or upstream, are best established by monitoring existing
structures and by model testing.
Fish-passing facilities may be divided into two groups:
- facilities for upstream migrant fish, consisting of fish ladders, fish
locks (lifts), tramways, and arrangements for trapping and trucking
the fish; - special facilities for a safe passage for the migrating smolts, which
consist of the arrangements to collect the migrating fish in the
forebay (headwater) at fine-mesh screens (fixed or moving types)
and directing them to safe bypass systems through which they are
transferred downstream.
Fish passes (for both upstream and downstream migratory fish) must
be designed so that the fish are able to find the entrance to the system pro-
vided for their passage and swim through without undue effort and
unusual risk of injury.
9.3.2 Upstream fish-passing facilities(a) Fish ladder (pool and traverse fish pass)
This is an artificial upstream fish passage, most commonly used for heads
up to 20 m consisting of (1) a fish entrance, (2) a fish ladder proper and (3)
a fish exit. Sometimes an auxiliary (additional) water supply is also pro-
vided to attract fish to the entrance.
The fish ladder proper consists of a series of traverses (cross-walls)
and pools circumventing an obstruction (such as a weir or dam) for the fish
to migrate to the head waters in easy stages. This is achieved by creating a
series of drops of around 300 mm–450 mm between pools (Fig. 9.33) on a
gradient of around 1 in 8 to 1 in 15 (for high heads). Rest pools of a larger
size (normally twice the size of an ordinary pool) are also provided after
every 5–6 pools.
The actual arrangement of pools and traverses is chosen according to a
particular obstruction; a low level weir or dam may need a fish pass of
shallow gradient (corresponding to the surrounding gradient of the land)
whereas a tightly folded pass may be necessary in case of a high weir or dam.
The fish pass is designed to take a fixed proportion of the flow over
the main weir or spillway. This is normally achieved by siting the invert