Contents preface (VII) introduction 1—37



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Fig. 17.26 Spillway aerators
The aerators are relatively cheap and have proved successful in preventing cavitation. These devices produce a local pressure drop which causes the flow to suck air. The compressibility of the air-water mixture reduces considerably the collapsing pressure of vapour bubbles and thus protects the concrete surface from cavitation erosion.
EXERCISES


  1. Explain the functions of different components of a spillway and suitability of different types of spillway for different site conditions and other requirements.




  1. Which types of gate are generally used for spillway crests ?




  1. Why is the cavitation erosion of a spillway surface caused ? What are the possible measures to prevent or reduce it ?




  1. Determine the profile of an overfall spillway with vertical upstream face using the following data:

Spillway discharge = 1600 m3/s Effective length of spillway = 100.00 m Coefficient of discharge for spillway = 2.0 Spillway crest elevation = 205.00 m River bed elevation = 100.00 m


If the tail-water depth for discharge of 1600 m3/s is 10.5 m, design a suitable stilling basin and sketch it.



SPILLWAYS

599




  1. A flood comes into a reservoir at zero hour. The ordinates of the flood hydrograph are as follows:




Time in hours

0

1

2

3

4

5

6

7

Discharge (m3/s)

0

100

200

300

400

500

400

300

The area elevation curve is also linear and is given by the following ordinates:




Height above spillway crest (m)

0

1

2

3

4




Area (105 sq.m)







7

8

9

10

11




The discharge formula for the spillway is Q = 300 H3/2 where, Q is in m3 /s and H is in metres. Find the reservoir level at 4 hours after the start of the flood assuming that water is up to the crest level at zero hour.

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