78 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Depth of basin = 1.9 m
Linlet = 2.0 m
Loutlet= 1.25 m
Since the headrace canal is long, a secondary settling basin has been incorporated upstream of the forebay as can be seen in
Drawing 420/04/3C01 in Appendix C. As can be seen in this drawing, the settling basin has the following dimensions:
Lsettling = 7.8 m
Bsettling = 2.4 m
Depth of basin = 1.7 m (av.)
Linlet = 4.4 m including the weir length
Loutlet = 0 m (outlet into forebay)
Check settling basin efficiency in settling particle diameters of 0.3 mm, i.e., selected dlimit
Recall Vetter's equation
η=
1
−
e
−⎢⎡
⎣
w*As
Q
⎤
⎥
⎦
Where for the above design: w = 0.037 m/s for dlimit = 0.3 mm and As the surface area of settling basin = 25 m2.
η = 1 − eOr:
−⎣⎡⎢
0.037*25
0.455
⎤
⎥⎦
=
0.87
or
the
settling
basin
has
87%
efficiency
in
settling
particle
size
of
0.3
mm.
Note that the efficiency
is close to the desired value of 90% and thus the basin dimensions derived from the first method is in agreement with Vetter's
equation.
Note the following:
Qdesign = 4211/s in the secondary settling basin/forebay. Recall from Chapter 3 that the flow required for power production was
4211/s and the headrace canal was designed for 4551/s to provide extra irrigation water.)
As discussed earlier, a weir has been incorporated at the inlet zone to ensure an even distribution of flow. Since the settling basin
is combined with the forebay, the outlet zone has been omitted. Also notice the submerged weir at the end of the basin which
controls the velocity through the basin and provides sediment storage depth. Since the gravel trap is expected to settle most of the
sediment, there has been a compromise in the ratio of L/B (3.25) in the settling basin: in this case the weir and parallel sides
upstream of the settling zone give a good flow distribution across the basin.
As discussed earlier, note the flushing arrangement where sediment is flushed by removing the vertical pipe. The sizing of this pipe
is as follows:
Since there are a number of spillways along the headrace canal (upstream of the settling basin/forebay), flood flow is not expected.
Therefore, the criterion is to ensure that the flush pipe is able to divert both the incoming design flow and the water volume in the
basin, thus emptying it during flushing
Check the flush pipe diameter using the following two equations:
Note that as can be seen in the Galkot drawing (420/04/3C01) hbasin = 1.3 m and hflush=1.7 m