CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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Photo 5.7 Ghandruk settling basin
Example 5.1 Galkot gravel trap
The Galkot gravel trap/primary settling basin is designed for a
flow of 4551/s. Note that the gross head of the scheme (h) is 20
m.
As gravel trap only, the minimum dimension should be as fol-
lows:
Cross sectional area required for V = 0.6 m/s to trap particle size
down to 2 mm.
A = Q / V = 0.455 / 0.6
= 0.758 m2
With a flow depth = 0.5 m:
Width B = 0.758/0.5
= 1.52 m, say 1.5 m,
Using L = 3 x headrace canal width (B’ = 0.6 m)
= 1.8 m. Use 2.0 m minimum length.
Therefore if this structure were designed as a gravel trap only,
the dimensions would have been as follows:
B = 1.5m
L = 2.0 m
As a settling basin, the dimensions required are as follows:
Since 10 m < h < 100 m, the scheme is classified as medium
head. Recall that the turbine is a crossflow type.
Therefore, dlimit = 0.3 mm.
With a mean river temperature of 15 °C during the high flow
season, from Figure 5.2, the fall velocity w = 0.037 m/s for
dlimit = 0.3 mm
LB = 2Q/w = 2 x 0.455/0.037=24.6 m2
Set B = 2.5 m
L = 24.6/2.5 = 9.8 m. Set L= 10.0 m
L/B = 4
Inlet profile: due to space constraints set á = 26°,
Linlet = B-B’ / 2tan(á )
= 2.5 - 0.6 / (2 tan 26°)
= 1.94 m. Set Linlet = 2.0 m
Check required depth of settling zone, y: Maximum horizontal
velocity
V = 0.44 d limit
= 0.44 0.3
= 0.24 m/s
Y = Q /BV =0.445 / 2.5 x 0.24 = 0.76m
Sediment storage requirement:
Assume sediment concentration, C = 2 kg/m3
Flushing frequency, T = 8 hours was chosen for this scheme
(but 12 hours is recommended). 8 hours = 28,800 s
Sdensity
Sload
Vsediment
= 2600 kg / m2 pfactor = 0.5
= Q xT x C
= 0.455 x 28,800 x 2
= 26,208 kg
= Sload / Sdensity xSfactor
= 26,208 / (2600 x 0.5) =20.16 m3
Actual basin area = LB = 10 x 2.5 = 25 m2
Required storage depth, Ystorage = 20.16/25
=0.81 m
Required depth of basin
= Freeboard + y + Ystorage
= 0.3 + 0.76 + 0.81
= 1.87 m
Therefore, as can be seen in Drawing 420/04/2C02 in Appen-
dix C, the actual internal dimensions of the basin are as fol-
lows:
Lsettling
Bsettling
= 10.0 m
= 2.5 m