2 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
One important point should be recognised: complete
standardisation of civil works is not possible due to the
variation in site conditions. Instead, standard approaches to
design are used, providing methods and criteria that enable
a design to be adapted to conditions at a site.
Safe, accurate and economic design are fundamental in civil
engineering but, because of site variations, a practical
understanding of design is also crucial. Failures in civil
engineering do not usually occur through an error in
calculations but because of a seemingly minor event or
circumstance which did not seem important. A thorough
understanding of which elements are critical is required.
1.3 Components of micro-hydro schemes
Although no two micro-hydro sites are similar, all of them
require specific common components of different
dimensions to convey the stream water to the power
generation units and back into the stream. These components
are shown in Figure 1.1. The civil components are briefly
discussed below:
HEADWORKS
Structures at the start of the scheme are collectively called the
headworks. In micro-hydro schemes, the headworks always
include the diversion weir, intake and gravel trap. A spillway
and a settling basin are also usually at the headworks.
DIVERSION WEIR
A diversion weir is a low structure (small dam) placed across
the river which diverts some of the river flow into the
hydropower scheme. The weir can be of permanent, semi-
permanent or temporary nature.
INTAKE
This is at the riverbank upstream from the diversion weir
where water is initially drawn into a conduit (canal or a pipe).
Usually a flow control structure and a coarse trashrack are
incorporated at the intake.
INTAKE CANAL
Generally the gravel trap is sited away from the intake at
some downstream location to protect it from flood and to
provide sufficient flushing head. If such an arrangement is
made in the project layout, then an intake canal will convey
the flow that enters in the intake to the gravel trap. Whenever
possible, provision for a spillway should be made immediately
downstream of the intake so that excess flow during the flood
season can be spilt back into the river. This will improve the
settling efficiency of the gravel trap as only the desired flow
reaches this structure. However, care should be taken to site
the spillway such that flood flow will not re-enter to the
waterways system via the spill way instead of spilling the
excess water. Though the intake canal is steeper than the
headrace canal, sometimes both types of canals are referred
to as "headrace canal."
GRAVEL TRAP
This is a basin (pond) close to the intake where gravel and
other coarse materials are trapped and then removed. In the
absence of this structure gravel can settle along the gentler
section of the headrace or in the settling basin.
SETTLING BASIN
This is also a basin where sand and other fine suspended
particles present in the river water are settled and then
removed. If allowed to enter the penstock, such particles would
abrade the penstock pipe and the turbine and hence shorten
their operational lives.
HEADRACE
This is a canal or a pipe that conveys the water from the headworks
to the forebay structure. The headrace alignment is usually on
even to gently sloping ground; a headrace pipe is generally not
subject to significant hydraulic pressure. Sometimes the canal
stretch from the intake structure to the gravel trap is also referred
to as the ‘Intake Canal”. This section of the canal is generally
steeper than the headrace canal downstream as it needs to convey
the gravels along with discharge from the intake to the gravel
trap. Similarly, the canal stretch from the gravel trap to the
settling basin is also referred to as the “Approach Canal”. This
section of the canal is also steeper than the headrace downstream
as it needs to convey sediments along with the discharge.
However, the slope of the approach canal can be lower than that
of the intake canal, i.e., a less sloped area is required to convey
suspended sediments than gravels. In these guidelines, the
entire canal (or pipe) stretch from the intake to the forebay is
referred to as the headrace.
FOREBAY
This is a tank at the entrance to the penstock pipe. The forebay
tank allows for flow transition from open channel to pressure
flow, maintains submergence depth for the penstock pipe to
avoid vortex formation and provides storage when there are
flow fluctuations in the turbine. It can also serve as a final
settling basin. In fact, sometimes the settling basin and the
forebay structures are combined together. An overflow spillway
should always be provided in the forebay structure to allow
spilling of the entire flow in case of emergency plant closure
and excess flow in case of excessive load fluctuations.
SPILLWAYS AND ESCAPES
Spillways are openings in headrace canals that divert excess
flow and only allow the design flow downstream. Note that
some literature may use the terms spill weir or overflow to
refer to the spillway. Escapes are similar in structure but their
function is to divert flows from the headrace canals in case
the downstream sections get blocked (in case of a landslide).