elevation of the water surface at the bridge, cause deposition of bed material, protect the abutments
and piers from long term degradation and contraction scour and increase the elevation of the be for
local scour. There is to be no increase in water surface elevation upstream of the bridge at the
design discharge. The energy losses are expected to be small so they can be neglected.
At the bridge, the design flood discharge is 5000 cfs. The river is 100 ft wide and has a uniform
flow depth of 10 ft for the design discharge.
What is the maximum height of the structure that will not cause backwater at the bridge at
the design discharge?
The unit discharge (q) in the river at design flood discharge is:
Q
5,000
q =
=
= 50 cfs / ft
W
100
The average velocity (V) is
Q
5,000
V =
=
= 5.00 ft / sec
A
1,000
The specific head (H) is
2
2
H = V + y = 5
+ 10 = 10.39 ft
2g
64.4
As a first approximation assume no energy loss in the bridge reach. Then at the check dam, the
elevation of the total energy line is 10.39 ft above the bed (Figure 2.44). At the dam:
Hmin + ∆Zmax = 10.39 ft
Figure 2.44. Sketch of backwater curve over check dam.
2.87
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