From Equation 2.140
1/ 3
1/ 3
4.65
q
2
2
=
yc =
= 1.30 m
g
9.81
Then
3
3
Hmin = 2 yc = 2 1.30 m = 1.95 m
and
∆Zmax = 3.17-1.95 = 1.22 m
If the structure is built to a crest elevation 1.22 m above the bed, critical flow will occur at the dam
for a flow of 141.6 cms and the dam will cause no backwater. At lower discharges the check dam
will cause backwater. However, discharges larger than 141.6 cms at the check dam will not cause
backwater. Local scour downstream of the check dam should be evaluated (see HEC-23).
How much backwater will the dam cause for a flow of 28.37 m3/s if the normal depth for this
discharge is 1.52 m and the dam height is 1.22 m?
Upstream of the dam the discharge for foot of width (q) is,
Q
28.32
= 0.93 m 3 / s / m
q =
=
W
30.48
The average velocity (V) is
q
0.93
V =
=
= 0.61 m / sec
y
1.52
At the check dam the flow is critical so from Equation 2.140
1/ 3
1/ 3
q2
0.93 2
yc =
=
= 0.45 m
g
9.81
and from Equation 2.144
3
3
yc =
=
0.45 m = 0.67 m
Hmin
2
2
Assuming no energy loss, the specific head upstream of the dam is
H = Hmin + ∆Z = 0.67 + 1.22 = 1.89 m
2.78
Previous Page
