Contents preface (VII) introduction 1—37

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.
   

2. 
   Which types of gate are generally used for spillway crests ?
   

3. 
   Why is the cavitation erosion of a spillway surface caused ? What are the possible measures to prevent or reduce it ?
   

4. 
   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

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5. 
   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 (m^3/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 (10^5 sq.m)			7	8	9	10	11

The discharge formula for the spillway is Q = 300 H^3/2 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.