The water carried by the power canals or conveyance channel is distributed to various penstocks leading to the turbines through the forebay, also known as the head pond. This pond is created through the excavation and the use of masonry wall, retaining structures or the embankment. The transition from the conveyance channel to the forebay should be accomplished through gradual variation. As the surface area of the forebay is much larger than the conveyance channel thus the flow velocity will be greatly reduced and sediment particles if escaped from the de-sander may also settle in this pond. Water is temporarily stored in the forebay during the event of a rejection of load by the turbine and there is withdrawal from it when the load is increased. Thus, forebay also acts as a sort of regulating reservoir. Additionally, it also minimizes the hydraulic jump hammering action in the penstock pipe due to the sudden closer of the valve.
In cases where the pants are located at the base of the dam, the reservoir itself acts as a forebay.
The basic things to be considered during the Design procedure of the forebay:
- Firstly, the economical diameter has to be sorted and fixed.
- Then design discharge to the turbine shall be determined.
- Remember the size of the forebay should be fixed, such that the flow velocity in front of the intake should be in the range of 0.5 m/s 1.0 m/s.
- Below the intake sill, flushing gallery should be provided for the removal of accumulated sediments.
- For the smooth entry of flow during the start of the turbine as well as for minimum submergence depth, about minimum of 2-3 minutes duration (turbine startup time) of designed discharge-volume is allocated at forebay as the reserve storage of water.
- Spillway is necessary to spill the excess water (in case of the rejection of the flow discharge from the turbine). It is provided at the level of the reserve storage surface.
- Sufficient freeboard shall be strictly provided.
- Lastly, remember that it is very necessary that minimum submergence depth (s) should be maintained even in the case of the minimum flow discharge.