CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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the wetted area for a given cross-section of the canal, seepage
can be calculated using data from the table. Example 2.1
illustrates this method.
2.6 Other considerations
2.6.1 FLOOD RISK
In site investigation, the concern is for the selection of the
best option for the design of the scheme. Therefore knowledge
of flood levels is important at the two extremes of the
micro-hydro scheme, the intake and the powerhouse, or at
other parts of the scheme that may be vulnerable to flood
damage from the river.
Flood levels may be predicted by hydrological calculation from
available data to give the 20 year or 50 year flood level, or by
consulting local people. Ideally, both methods should be used
to give a reliable estimate. Always allow a margin of error so
that a rare flood event is allowed for, and think carefully about
how the floods will affect each of the proposed layouts for the
project. The location of the power-house higher on a slope will
reduce the available head and therefore have an important
impact on the capacity and the economics of the project.
2.6.2 CROSS DRAINAGE
Sometimes because of the nature of the topography, the
headrace canal and/or the penstock alignments will need to
cross gullies and small streams. Note that dry slopes are more
stable than saturated slopes. Surface water can be diverted
by constructing various types of cross drainage works. For
example, catch drains can be constructed uphill from the
micro-hydro alignment to divert the surface runoff. Catch
drains are small channels that divert surface runoff (thus
catch it) and divert it into nearby gullies or natural drainage.
Another example of cross drainage works is the use of a
superpassage. This is a covered headrace canal arrangement
such that the surface runoff flows over it whereas the design
flow is safely conveyed in the canal.
2.6.3 WATER RIGHTS
Sometimes there can be regarding the water usage conflict
between the proposed micro-hydro scheme and other prior
uses of the source stream. For example, if there is an irrigation
scheme downstream of the proposed micro-hydro intake that
may receive less water (once the micro-hydro plant is
commissioned), there will be conflicts. Such water rights issues
should be resolved before implementing the micro-hydro
scheme.
It should be noted that irrigation and micro-hydro can be
co-ordinated if an agreement with all concerned parties is
reached in the initial stage. This is because irrigation water
is not required throughout the year and therefore water can
be used for power production at other times. This may result
in less or even no power available during peak irrigation period.
However, if the electricity users are also owners of the irrigated
land, they can prioritise their needs, such as by irrigating in
the afternoons and nights and producing power during
mornings and evenings.
2.6.4 LAND OWNERSHIP AND LAND USE
The surveyor should note down the issues concerning land
use and ownership. If the alignment traverses through a
farmer’s paddy field, the land may have to be bought by
the project.
Another example is that an open channel headrace may be
technically feasible but the designer may have to choose a
buried pipe if the headrace alignment is along cultivated
land. Similarly, sediment flushing and spillway flows need
to be safely diverted away from cultivated land. It is
important to note down land owners whose land will be
used for structures, so that agreements such as lease
arrangements can be negotiated. These factors will affect
the design of the scheme.
2.6.5 HIGH ALTITUDE SITES
These guidelines are generally applicable to micro-hydropower
in Nepal, but some particular measures need to be taken for
high altitude sites to avoid ill-efffects from freezing
temperatures. To avoid frost damage to concrete and masonry,
the following measures are necessary:
Keep the water to cement ratio as low as possible, preferably
not more than 0.50.
Avoid aggregate with a large maximum size, or a large
proportion of flat particles.
Use a water reducing air entraining agent (plasticiser).
Ensure good compaction.
Do not build while night temperatures are below freezing.
Surfaces must be prevented from drying out for at least
three weeks if the ambient temperature is on average 5°C
or less.
To avoid ice damage to canals and structures, the water face
of walls should be smooth concrete or plastered masonry, and
inclined at approximately 1:1. The expanding ice can then
rise between the walls, instead of pushing the walls apart.
Headrace canals should be designed for a minimum velocity
of 0.6 m/s. Even though the surface may freeze, water will
flow under the ice.
The top of trashracks should be below any expected ice level,
to avoid ice forming around the trashrack bars. Timber
trashrack bars may be less liable to icing than steel bars.
The foundation level of structures should be below the depth
of ground freezing. This is likely to be about one metre depth.