To determine the depth of flow upstream of the dam (y) solve the specific head equation (Equation

2.137)

q2

0.93 2

H=

+y=

= 1.89 m

2gy 2

19.62 y 2

or

0.93 2

3

2

y - 1.89 y +

=0

19.62

The solution is

y = 1.87 m

As the normal depth is only 1.52 m, the backwater is

∆y = 1.87 - 1.52 = 0.35 m

That is, the depth upstream of the dam is increased 0.35 m by the 1.22 m high dam when the flow

is 28.32 cms.

Calculate the correction factors α and β for a cross-section given the discharge measurement

during the peak flood event for the year. From Table 2.8, the following values are obtained:

Q

=

5,370 cfs

1,485 ft2

A

=

W

=

163 ft

21,070 ft4/sec2

=

Vi∆Qi

85,500 ft5/sec3

Vi 2∆Qi

=

coefficient G of 3.27. If the value of D65 is used for ks, then for yo = 12.8 ft (the maximum depth)

y0

12.8

(304.8) ≅ 8,700

=

0.45

ks

and for yo = 1.1 ft (the smallest non-zero depth)

y0

1.1

(304.8) ≅ 750

=

0.45

ks

2.79

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