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Fish ladder position and necessity

Short answer

A fish ladder is a stepped channel placed along the divide wall or bank, allowing fish to migrate past the barrage. It is necessary because weirs/barrages block spawning migration; the ladder with baffles provides low velocity (<2–3 m/s) and resting pools, preserving fish populations and complying with environmental regulations.

Showing the position of a fish ladder, write its necessity in headworks.

A fish ladder is a sloping channel with a series of baffles or steps and pools, built along the divide wall (on the weir side or the river bank side) as shown in the plan below.

 Left  <------ river flow ------>
 bank  ________________________________
  |    | weir bays |DW| sluices |..| head
  |    |           |  | pocket     | reg
  |    |  fish ladder  <-- along DW |
  |____|__________________________|

Necessity

  • A weir or barrage across a river blocks the migration of fish, which swim upstream to spawn (e.g. mahseer and snow trout in Nepali rivers). The ladder allows them to pass through the structure.
  • Fish need a velocity they can handle, usually below 2-3 m/s; the baffles and pools reduce the velocity and give them resting places.
  • It keeps the aquatic ecology and fish population of the river, which provides livelihood and is required in environmental clearance (EIA) of the headworks.
  • The ladder is usually 1:10 to 1:15 in slope, with entrance at the lowest point below the weir and exit at the pond level.

✓ Answer checked by Claude Opus 5.5 (high effort)

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