CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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5. Gravel trap, settling
basin and forebay
5.1 Overview
5.1.1 THE SEDIMENT PROBLEM
Most rivers carry a substantial quantity of sediment in the
form of gravel, sand or finer material depending on the river
characteristics, geology of the catchment area and the dis-
charge. Steeper rivers such as those that originate from the
Himalayas carry cobbles and even move large boulders during
annual floods. Intakes are located and designed to limit the
amount of sediment entering the micro-hydro system, but
such sediment cannot be entirely eliminated. Intakes can
only prevent boulders and cobbles from entering into the
system and minimise the influx of gravel and finer sediment.
Large particles can block the headrace and reduce its capacity.
Suspended sediment can cause severe wear on the turbine
runner, seals and bearings, since the flow velocity at the runner
is high. Such wear causes a reduction in the efficiency of the
turbine and eventually leads to its complete failure. In either
case, maintenance is necessary, requiring high expenditure
in terms of replaced parts, man-hours and in loss of power
production. There are abundant examples of turbine runners
completely destroyed within a few years after installation at
micro-hydro plants that lacked settling basins. The rate of
wear of turbine parts due to sediment abrasion is governed
by the following factors:
Concentration of suspended particles
Hardness of particles i Size of particles
Shape of particles
Resistance of turbine runner
Turbine head
It is not necessary to exclude all sediment at the settling
basin. This is virtually impossible and would not be economi-
cally viable, especially for micro-hydro schemes. A small
concentration of fine sediment is often permissible as will be
discussed later. The design should be such that the size and
concentration of sediment passing the settling basin are
within acceptable limits.
5.1.2 FUNCTION OF THE STRUCTURES
Gravel traps, as the name denotes are designed to trap gravel
that enters the intake along with the diverted flow. If a river
only carries fine sediment and not gravel (even during floods),
then this structure is not required. However, most mountain
rivers in Nepal carry gravel, especially during floods. In the
absence of a gravel trap, gravel will settle along the gentler
sections of the headrace or in the settling basin, where it is
difficult to flush out.
A settling basin is a basin whose function is to settle the
suspended particles present in the diverted river flow. Since
rivers are never free from sediment, all micro-hydro schemes
should have a settling basin. For small schemes, this may
simply be a widened section of the canal. The flushing mecha-
nism may be rudimentary, which is acceptable provided that
damaging sediment does not reach the turbine.
A forebay is a tank located at the end of the headrace and the
beginning of the penstock pipe. It is a structure that allows for
the transition from open channel to pressure flow condi-tions.
The water level at the forebay determines the operational
head of the micro-hydro scheme.
5.1.3 LOCATION OF THE STRUCTURES
Whenever possible the gravel trap, settling basin and forebay
should be combined. This minimises the construction cost.
Sometimes, either the gravel trap and the settling basin or
the settling basin and the forebay are combined, but the topo-
graphic conditions are rarely appropriate to be able to combine
all three structures. The Jhankre mini-hydro is a rare example
where it was possible to combine all three structures as
described in Box 5.1. Selection of an appropriate settling basin
site is governed by the following criteria:
The location should be such that it is possible to flush the
sediment and spill excess flow from the basin without
causing erosion problems or damage to other structures.
There must be sufficient head to flush the sediment and
drain the basin.
The settling basin should be located as close to the
headworks as possible, especially if it is separate from the
forebay. The earlier the sediment is removed the less the
maintenance of the headrace. Furthermore, the headrace
alignment downstream of the settling basin can be gentler
(hence less loss in the available head) since the flow will be
sediment free. A location close to the intake allows easy
discharge of sediment back to the river. From an operational
viewpoint, it will also be easier for the operator/helper to
combine work at the intake, such as cleaning the coarse
trashrack, and flushing of the settling basin.
There needs to be adequate space to construct this structure
as designed. Note that it can be a relatively wide and long
structure. Therefore, locating this structure on fairly level
ground minimises the excavation costs. The forebay is
located immediately uphill of the transition area where
the ground profile changes from level to steep. The
following additional factors should be considered before
deciding whether a site is suitable for a forebay:
It should be possible to spill the entire design flow from the