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.

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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