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).

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

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