56 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Photo 4.14 Jhankre mini-hydro penstock crossing, Dolakha,
Nepal.
Photo 4.15 Jhankre mini-hydro headrace crossing Ghandruk, Nepal.
siphons are pipes that are buried across the gully. They traverse
down to the lowest point of the gully and then come up at the
other side (hence the name inverted siphon). As long as there
is sufficient head and the pipe is below the hydraulic grade
line, the flow can be conveyed through such siphons. A flush
out valve must be incorporated at the low point of the siphon
(since sediment can be deposited at the low point). Such
siphons can also reduce costs of crossings as well as risks due
possible damages from floods. Sufficient protection against
scour and damages due to rolling boulders should be provided
for inverted siphons. This can be achieved by encasing the
pipe in concrete (generally nominally reinforced) along with
stone pitching on the top of the pipe as shown in Figure 4.8.
Inverted siphons across larger streams will also require some
bank protection structures based on site conditions. A typical
section of an inverted siphon under a stream or a gulley is
shown in Figure 4.8.
4.6 Headrace pipe
4.6.1 GENERAL
Pipes may be required along the headrace alignment where slopes
are unstable and where landslides may occur. Although masonry
and concrete canals can minimise seepage induced landslides,
they are rigid structures and in the event of slope failures, such
canals can be swept away. These canals will also crack if there are
small slope movements. Where soil instability problems are
expected, flexible pipes may be an appropriate solution provided
that the required pipe length is not too long (see Box 4.6). Another
case for the use of flexible pipes is when the entire hillside is
slowly sliding (i.e. mass movement is occurring) and part of the
headrace alignment needs to traverse it.
In Nepal HDPE pipes are often used to address the above
problems. These pipes are flexible enough to accommodate
some ground movement and can be joined by heat welding,
which is described in Box 4.7. HDPE pipes should be buried
to protect them from sunlight, cattle and vandalism.
The reason why PVC pipes have not been used for headrace in
Nepal is because although they are easy to join (with a PVC
cement solution), they are also very rigid. Therefore, they
cannot accommodate ground movement.
Appendix B includes data on standard pipe sizes available in
Nepal.
4.6.2 DESIGN CRITERIA
The design criteria for headrace pipes are similar to those of
headrace canals. Specifically, the design should address the
following issues:
The pipe diameter should be such that for the ground slope
of the alignment, it should be able to convey the design
flow. If there is a possibility of flood flows entering into the
pipe, make provision for spilling such excess flows.
The inlet to each section of headrace pipe should be
protected with a trashrack, so that debris does not get in
and block the pipe. The spacing of the trashrack bars should
be no more than one third of the pipe diameter, and the
velocity through the trashrack should not exceed 1 m/s.
Where a section of headrace pipe ends in an unlined canal,
a masonry transition structure is recommended, to avoid
scour by the high velocity flow.
Headrace pipes are efficient when they are flowing full,
but if the head on the pipe exceeds the rated pipe head (i.e.
allowable head on the pipe) break pressure tanks need to be
provided. Such tanks dissipate the head over the pipe and
avoid the need to use a higher pipe rating. However, in
practice, repeated use of break pressure tanks has sometimes
induced cyclic surge (i.e. periodic change in head and hence
the flow). Another option in such cases is to select a larger
pipe diameter such that open flow condition prevails. Break
pressure tanks should be provided with lockable covers, so