34 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
Figure 3.12 A plum concrete permanent weir proposal for Ghami micro-hydro scheme. The 1:3 slope allows rolling boulders to travel
downstream through the weir and the RCC blanket helps to strengthen the weir surface against abrasion due to rolling boulders.
Photo 3.9 Bottom intake of Theme micro-hydro scheme, Nepal .
Photo 3.10 Bottom intake with the grill removed (Jhong)
Example 3.2 Head over weir calculation
A broad crested weir has been placed across a river for a
micro-hydro intake as shown in Figure 3.12. The weir
height is 0.5 m and the length 5 m. How high should the
flood protection wall be for a 20 year return flood of 11
m3/s?
Cw = 1.6 for broad crested weir :
Figure 3.12 Broad crested weir
( )hovertop =
Q
Cw x Lweir
0.667
Note that Cw is 1.6 for broad crested weir
( )hovertop =
11
0.667
= 1.24 cm
1.6 x 5
Height of flood protection walls from river bed level = 0.5 m
m + hovertop + 0.3 m (allow 300 mm of freeboard) = 2.04 m.