Page 283 - Water Engineering Hydraulics, Distribution and Treatment

8.8 Raw Water Intake Structures and Raw Water Pumping Wells
f. Design flow (maximum daily demand) = 37.85 MLD.
g. Design water velocity through the gate = 0.1524 m/s (maximum; the Ten-States Standards).
h. One of the two square gates can be closed for screen repair or replacement.
Solution (SI System):
Design flow Q = 37.85 MLD = 0.438 m /s.
Selected velocity for design v = 0.09144 m/s (below 0.1524 m/s, okay).
Total area of two gates A = Q/v = (0.438 m /s)/(0.09144 m/s) = 4.79 m .
Select two equal-size square gates, L = W = (4.79/2)
Select the next larger standard-size gate from a manufacturer’s catalog.
Length L = width W = 1.6764 m. 3 3 0.5 = 1.5476 m. 2 261
Check velocity when both gates are open:
v = 0.438∕[(1.6764 × 1.6764) × 2] = 0.078 m∕s (less than 0.1524 m∕s, okay).
Check velocity when only one gate is open:
v = 0.438∕[(1.6764 × 1.6764)] = 0.1558 m∕s(closeto0.1524 m∕s, okay).
Elevation of the top of the gate = 70.83 m (msl).
Elevation of the bottom of the gate = 69.16 m (msl).
The above are the final design results for the two gates. The following are the tentative design data for the concrete intake
structure that will hold the two gates. The following tentative design data require (a) a hydraulic stability analysis in accordance with
Example 8.13 and (b) a reinforced concrete structural analysis which is beyond the scope of this water engineering class.
Outside dimensions of the raw water concrete intake structure = 8.5 m × 8.5 m (tentative design).
Inside dimensions of the raw water concrete intake structure = 6.5 m × 6.5 m (tentative design).
Height of the raw water concrete intake structure = 87 m – 60 m = 17 m (tentative design).
Base-reinforced concrete slab selected to sit at the reservoir bottom = 14 m × 14 m × 2.5 m (thickness) (tentative design).

EXAMPLE 8.13 HYDRAULIC STABILITY ANALYSIS OF RAW WATER INTAKE STRUCTURE
Conduct a hydraulic stability analysis for the raw water RC intake structure tentatively designed in Example 8.12. The intake structure
displaces a large amount of water. The weight of the intake structure must be greater than the weight of water displaced in order to
ensure hydraulic stability. In case the tentatively designed RC intake structure (from Example 8.12) is concluded to be hydraulically
unstable, please suggest how to make corrections. The following engineering conditions are assumed:
a. Outside dimensions of the raw water concrete intake structure = 8.5 m × 8.5 m; inside dimensions of the raw water concrete
intake structure = 6.5 m × 6.5 m; height of the raw water concrete intake structure = 87m–60m = 27 m; base-reinforced
concrete slab selected = 14 m × 14 m × 2.5 m (thickness)
b. Elevation of the intake structure top (maximum) = 87 m (msl = mean sea level); elevation of maximum reservoir water level =
82 m (msl); elevation of normal reservoir water level = 81 m (msl); elevation of minimum reservoir water level = 80m(msl);
elevation of the intake structure bottom (planned) = 60 m (msl)
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c. Weight of reinforced concrete = 2,308 kg/m ; weight of water = 1,000 kg/m 3
d. Required safety factor = 1.5–2.0

Solution (SI System):
The worst condition for the hydraulic stability of an intake structure will occur when (a) the two gates are closed, (b) the intake
structure is empty, and (c) the reservoir is at its maximum water level of 82 m elevation.
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Although the outside volume of the intake structure is (8.5 m)(8.5 m)(87 – 60)m + (14 m)(14 m)(2.5 m) = 2,440.75 m , actual
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volume of water displaced by the intake structure is (8.5 m)(8.5 m)(82 – 60)m + (14 m)(14 m)(2.5 m) = 2,079.5 m .
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Total weight of water displaced = (1,000 kg/m )(2,079.5 m ) = 2,079,500 kg.