80 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Photo 5.9 A dry stone masonry forebay showing the connection
with the penstock
Chialsa mini-hydro scheme is based on such conventional
flushing system as can be seen in Photograph 5.6.
5.4 Forebay
5.4.1 GENERAL
The forebay of a micro-hydro scheme serves the following
functions:
It allows for the transition from open channel to pressure
flow conditions.
It regulates the flow into the penstock, particularly through
the release of excess water into a spillway.
It releases the surge pressure as the wave travels out of the
penstock pipe.
It can also serve as a secondary/final settling basin and
trap some particles that enter the headrace downstream of
the settling basin.
Although very rare in micro-hydro schemes, the forebay
can also provide water storage for use during peak power
demand period as discussed in Box 5.2.
Structurally, the forebay tank is similar to the settling basin
except that the outlet transition is replaced by a trashrack and
the entrance into the penstock pipe.
5.4.2 PIPE LEVEL
The forebay allows for the transition from open channel to
pipe flow by providing adequate submergence for the penstock
pipe. As discussed in Chapter 4, if the submergence head is
not sufficient, the pipe will draw in air and be unable to
convey the design flow. Similarly, recall from Chapter 4 that
the minimum submergence head required for the penstock
pipe is as follows: hs>1.5V2 / 2g where: hs is the submergence
head, and V is the velocity in the penstock.
5.4.3 DESIGN OF A FOREBAY
If the length of the headrace canal between the settling basin
and the forebay is long, then sediment can enter the canal,
such as when debris falls from uphill of the headrace alignment.
Similarly if an earth canal (or stone masonry in mud mortar)
is constructed between the settling basin and the forebay,
sometimes high velocity in the canal, such as during the
monsoon, can cause erosion and carry sediment to the forebay.
In such cases the forebay should also be designed to serve as
a secondary settling basin and the design method used for
sizing a settling basin should be used. However, a lower
sediment concentration (say C = 0.5 kg/m3) can be used since
only particles that have escaped the settling basin or those
that have been eroded from the headrace canal are expected
in the forebay. If the headrace upstream of the forebay consists
of HDPE pipe or of cement masonry canal and the settling
basin is functioning well, there will not be any need for
secondary settling. However, as a precaution, some storage
depth below the pipe invert should be allowed for. A depth of
300 mm or equal to the pipe diameter, whichever is larger is
recommended for this purpose.
5.4.4 FOREBAY SIZE
The minimum size of the forebay should be such that a person
can get in and clean it. The minimum clear width required
for this is 1 m. Even if the sediment load is not expected in
the forebay, it may sometimes reach this structure such as
when the settling basin is filled in quickly during the monsoon
or there is a small landslide. If a person can get into the
forebay and clean it occasionally or during the annual
maintenance period, limited sediment accumulation will not
be a problem. A storage depth below the invert of the penstock
should be provided for this, as can be seen in Figure 5.8.
If possible, the forebay should also be sized such that 15
seconds of the design flow can be safely stored in the tank
above the minimum pipe submergence level. This is more
important if the scheme consists of a headrace pipe instead
of a canal. There can be small transient surges in the headrace
pipe which result in uneven flow. The 15 second storage
capacity helps to balance such uneven flows.
5.4.5 TRASHRACK