That is, the dam can be built to a height of ∆Zmax which decreases the specific head at the dam to
Hmin. From Section 2.6.3 and Equation 2.147
3
Hmin = 2 yc
From Equation 2.140
1/ 3
1/ 3
q2
50
2
yc =
=
= 4.27 ft
g
32.2
Then
3
3
yc =
=
4.27 ft = 6.40 ft
Hmin
2
2
and
∆Zmax = 10.39 - 6.40 = 4.0 ft
If the structure is built to a crest elevation 4.0 ft above the bed, critical flow will occur at the dam for
a flow of 5,000 cfs and larger and the dam will cause no backwater. At lower discharges the at the
check dam will cause backwater. However, discharges larger than 5,000 cfs check dam will not
cause backwater. Local scour downstream of the check dam should be evaluated. HEC-23 has
am problem for determining this scour.
How much backwater will the dam cause for a flow of 1000 cfs if the normal depth for this
discharge is 5 ft and the dam height is 4.0 ft?
Upstream of the dam the discharge for foot of width (q) is,
Q
1,000
q =
=
= 10 cfs / ft
W
100
The average velocity (V) is
q
10
V =
=
= 2.00 ft / sec
y
5
At the check dam the flow is critical so from Equation 2.142
1/ 3
1/ 3
q
10 2
2
yc =
=
= 1.46 ft
g
32.2
2.88
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