70 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
forebay without causing erosion or instability problems.
Ideally if this structure can be located close to a gully, it
may be possible to safely divert the spillway flows into it.
Similar to the settling basin there needs to be adequate
space to construct this structure as designed. However, the
forebay is usually smaller in size.
5.2 Gravel trap
A gravel trap is recommended for all micro-hydro schemes in
Nepal. In the absence of a gravel trap, the settling basin must
be close to the intake and able to flush the gravel that enters
the basin. Gravel traps differ from settling basins in that they
handle coarse material that enters near the bed, rather than
suspended material that needs to be settled. The main design
principle for a gravel trap is that the velocity through it should
be less than required to move the smallest size of gravel to be
removed. The largest size allowed to enter into the intake can
be controlled by the spacing of the coarse trashrack bars. In
general gravel traps should settle particles larger than 2 mm
diameter. Smaller sized particles will be settled and removed
in the settling basin. The following criteria should be used for
the design of the gravel trap:
To be able to trap particles down to 2 mm diameter, the
velocity in the gravel trap should be limited to 0.6 m/s.
If the gravel trap is hopper shaped, the floor slopes should
be about 30° (1:1.7). Such an arrangement will facilitate
easy flushing of gravel. If it is not possible to construct such
a shape, the floor should slope towards the flushing end,
with a longitudinal slope of 2-5%.
The length of the gravel trap should be at least three times
the width of the headrace canal or 2 m, whichever is larger.
With this fixed length and a velocity of 0.6 m/s, the required
width of the trap can now be determined. Note that this is
a general rule of thumb, but if a significant bed load can
enter the intake, then a longer length may be required.
Since studies regarding the movement of gravel in rivers
are rare (rarer than sediment studies), it is usually difficult
to estimate the storage required in a gravel trap. Note that
the storage must be provided below the normal flow depth.
To minimise blockage of the headrace or damage due to
abrasion in the headrace, gravel traps should be located as
close to the intake as possible.
Gravel traps can be emptied via flushing gates or by lifting
stoplogs (i.e. wooden planks). Since gravel enters the intake
only during high flows, incorporating stoplogs is generally
more convenient and economic.
The Galkot gravel trap is shown in Photograph 5.1 and in
Drawing 420/04/2C02 of Appendix C. Although a gravel trap,
this structure has also been designed as a primary settling
basin. This is because the headrace canal is long (1.1 km) and
if significant sediment load can be trapped in the gravel trap,
the maintenance requirement will be less far downstream in
the headrace canal. Furthermore, once the sediment is removed,
the headrace canal slopes can be gentler as discussed in Chapter
4. Since this is a combined structure, the calculations are
presented after the discussion on settling basins.
Note that as can be seen in Drawing 420/04/2A01 (Appendix
C), the Galkot gravel trap is located 35 m down-stream from
the intake. This is because the initial length of the intake
was felt to be vulnerable to flood damage. For the same reason
the coarse trashrack is placed at the end of the gravel trap.
In the Galkot micro-hydro scheme, significant gravel load is
not expected for the following reasons:
The diversion weir is of a temporary nature and does not
extend throughout the river width.
The intake is located on the outside of a bend.
Photo 5.1 Galkot gravel trap
5.3 Settling basin
5.3.1 DESIGN CRITERIA
Suspended sediment that is not settled in the gravel trap is
trapped in the settling basin. The basic principle of settling is
that the greater the basin surface area and the lower the
through velocity, the smaller the particles that can settle. A
settling basin has a significantly larger cross sectional area
than the headrace canal and therefore the flow velocity is
lower which allows the settling of the suspended materials. A
settling basin must satisfy the following three criteria:
Settling capacity
The length and width of the basin must be large enough to
allow a large percentage of the fine sediment to fall out of
suspension and be deposited on the bed. The sediment concen-
tration passing the basin should be within acceptable limits.
The geometry of the inlet, the width of the basin and any
curvature must be such as to cause minimum turbulence,
which might impair the efficiency.