Functions and layout of forebay

A forebay is a small pond or tank at the end of a headrace canal (free-flow system), just upstream of the penstock inlet. It connects the open canal to the pressure pipe.

Functions of forebay

1. Transition from free surface flow in the canal to pressure flow in the penstock.
2. Temporary storage: supplies extra water when the load increases suddenly and stores water when load drops, until the canal flow adjusts.
3. Spills excess water safely through a side spillway on load rejection.
4. Final settling of fine sediment and removal of floating debris with a trash rack.
5. Provides enough submergence over the penstock entrance to prevent vortex and air entry.
6. Acts as a small surge tank for the canal system.

Layout

Main components:

• Inlet transition from the canal, widening gradually.
• Forebay tank: volume usually enough for about 2–3 minutes (some use 30–120 s for small schemes) of design flow.
• Trash rack in front of the penstock mouth, inclined about 60–75°.
• Penstock intake with bellmouth, gate and air vent behind it.
• Spillway/overflow weir and spill channel to the river.
• Flushing (scour) gate at the low point for sediment.
• Minimum submergence of penstock top: $S = cV\sqrt{D}$ (Gordon, c ≈ 0.54 symmetric, 0.72 asymmetric approach).

 PLAN
 headrace          forebay tank
 canal   +------------------------------+
 ======> | transition |      trash rack |  penstock
 ======> |  /         |  pond      ||| |==========>
         | /          |            ||| | gate, vent
         +-----+-------------+-------+--+
               |   spillway  | flush gate
               v  to river   v

 SECTION
 canal WL ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 =====\                       |rack|
       \                      | \  |_____
        \_____________________|__\_| penstock
          sediment --> flush       ===========