Forebay function and surge tank types

Function of forebay

A forebay is a small storage basin at the end of a headrace canal (free-flow), from which the penstock starts. Its functions are:

1. To provide a transition from open-channel flow to pressure flow in the penstock.
2. To act as a small balancing reservoir: supplies extra water when load suddenly increases and stores water (or spills it) when load decreases.
3. To absorb water-hammer/surge at the penstock entrance (free water surface) for the canal side.
4. To provide submergence above the penstock inlet to prevent vortex and air entry.
5. To act as a final settling basin and house trash racks, gates and a spillway for excess water.

Types of surge tank

A surge tank is an open-topped tank/shaft at the junction of a pressure tunnel and penstock that absorbs pressure rise and supplies water during load changes.

1. Simple surge tank: a vertical shaft of uniform section connected directly to the tunnel with an unrestricted opening. Simple and effective, but large size and slow damping of oscillations.
2. Restricted orifice (throttled) surge tank: a simple tank with a restricted orifice at its entry. The orifice loss quickly retards the tunnel flow, so the tank can be smaller and oscillations damp faster, but more water hammer passes into the tunnel.
3. Differential surge tank (Johnson type): a narrow central riser with ports opening into an outer large tank. The riser responds quickly while the outer tank provides storage; combines advantages of simple and restricted types.
4. Gallery (chamber) type surge tank: a narrow shaft with expansion chambers (galleries) at top and bottom—upper chamber limits up-surge, lower one prevents emptying in down-surge. Economical in rock (common in Nepal).
5. Inclined surge tank: shaft laid along a slope; acts like a larger area tank for a given height; suited to hillside.
6. Air-cushion (closed) surge chamber: an underground chamber with compressed air; water level fluctuations compress the air (used where topography does not allow an open shaft).

 Simple     Restricted   Differential   Gallery
  |  |        |  |       |  |  |  |     ==|  |==
  |  |        |  |       |  |  |  |       |  |
  |  |        |  |       |  |__|  |       |  |
 _|  |_      _|_ |_     _|___  ___|_    ==|  |==
 tunnel     orifice     riser+ports    chambers