Water Wise farmers build earth dams: Part 8
Connecting an earth wall and a weir, the enlargement of a dam and waterholes I
n this last article of our series on dam building, we wrap up the last few, but very important points that you should consider when building an earth dam. Connecting an earth wall and a weir at the side or in the centre poses a few technical challenges. Table 1 provides the discharge through rectangular weirs per metre length for a free overflow. These figures are only valid for free-falling water, where the water on the downstream side of the wall is not higher than 0,75 from the weir’s top.
h = Flow depth in metre (m) q
=
Q
=
L
Discharge in cubic metres per second per metre length (m³/s/m)
v = Velocity of water in metre per second (m/s)
Example: A weir is 6 metres long and must be designed to accommodate a flood peak (Q) of 5 m³/s. What is the depth of flow through the spillway?
Table 1 Flow depth (m)
0,3
0,6
0,9
1,2
1,5
Discharge q (m³/s/m)
0,295
0,831
1,53
2,38
3,29
Velocity v (m/s)
0,984
1,385
1,7
1,985
2,19
22
Q 5 q = - = - = 0,831 m³/s/m length L 6 From Table 1, if q = 0,831 m³/s/metre length, the corresponding value of h = 0,6 m. High overflow over weirs will cause erosion on soft rock such as shale, limestone, and soft sandstone. In such cases, a concrete apron and/or a water cushion will have to be made (Figure 1).
ProAgri BNZ 10
