Contents preface (VII) introduction 1—37

Fig. 17.5
Example 17.1 A flood enters a reservoir at 0 hours. The ordinates of the inflow hydrograph are as follows:

Hour	0	1	2	3	4	5	6	7
Discharge, Q(m^3/s)	0	100	200	300	400	500	400	300

The area of the waterspread increases linearly from 0.7 × 10⁶ m² at the spillway crest level to 1.1 × 10⁶ m² at 4 m above the crest level. The effective length of the spillway is 150 m and the coefficient C in the discharge equation Q = C L H^3/2 is 2.0 in S.I. units. Using a single time step from 0 to 4 hours and assuming that water is at the crest level at the beginning of the flood, find the reservoir level at 4 hours.
Solution: Let reservoir level be H (m) above the spillway crest at 4 hours from the beginning of the flood. Therefore, Eq. (17.2) reduces to

0 + 400

× (4

× 3600)

−

0

+ 300 H ^3 /2

× (4 × 3600)

2

2

L

F

11. − 07.I

O

6

6

6

H

= M07. ×

10

+ (07.

× 10

+ G

J

× 10

H P

2

N

H

4 K

Q

5 H^2 + 216 H^3/2 + 70 H = 288

∴

H = 0.9925 m

Example 17.2 The pond upstream of a power house may be approximated as a rectangular channel of width 80 m and length 2.0 km. The inflow as well as outflow into the pond at the beginning was 100 m³/s. The inflow increases gradually to 200 m³/s in two hours. Assuming that the outflow is through a sluice gate and discharge through which is expressed

as Q = 80 H (in which, H is the head of water above the sill of the sluice gate in the pond),

570 IRRIGATION AND WATER RESOURCES ENGINEERING
determine the head, H₁ of water above the sill of the sluice gate at the beginning of the increased inflow and at two hours since the beginning.
Solution: Let H be the head of water at the beginning of the increased inflow. Using Q = 80 H ,

	100 = 80	H1
	H1 = 1.5625 m
Using Eq. (17.2),
100 +	200	× (2	× 3600) −	100	+ 80	^H2	× (2	× 3600)
2			2

= 80 × 2000 (H₂ – 1.5625)
in which, H₂ is the head of water at 2 hours since the beginning of the increased flow. Thus,

80 H2 + 144	H2 = 485
∴	H2 = 2.96 m

17.3. COMPONENTS OF SPILLWAY
The main components of a spillway are: (i) control structure, (ii) conveyance structure, (iii) terminal structure, and (iv) entrance and exit channels.
17.3.1. Control Structure
The control structure of a spillway regulates and controls the outflow from the reservoir. It is usually located at the upstream end of the spillway and consists of some form of orifice or overflow crest. In some cases, however, the control may be at the downstream end. For example, in a ‘morning glory’ spillway, the downstream tunnel rather than the crest of orifice controls the flow at higher heads. In plan, the outflow crest can be straight, curved, U-shaped, semicircular, or circular. The crest can be sharp, broad, ogee-shaped, or of some other cross-section. Similarly, orifice can have different shapes and may be placed in a horizontal, vertical, or inclined position. Orifices too can be sharp-edged, round-edged or bellmouth-shaped.
17.3.2. Conveyance Structure
The outflow released through the control structure is usually conveyed to the downstream river channel through a discharge channel or waterway. Free fall spillways, however, do not require any such conveyance structure. The conveyance structure can be the downstream face of the dam (if the spillway has been constructed in the main body of the dam), or an open channel excavated along the ground surface of one of the abutments, or an underground tunnel excavated through one of the abutments. The conveyance structure too can have a variety of cross-section depending upon the geologic and topographic characteristics of the site and hydraulic requirements.
17.3.3. Terminal Structure
When water flows from the reservoir level to the downstream river level, the static energy is converted into kinetic energy which, if not properly dissipated, may cause enough scour, near the toe of the dam, that can damage the dam, spillway, and other structures. Therefore, suitable stilling basins at the downstream end of the spillway are usually provided so that the excess

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571

kinetic energy is dissipated, and the discharge into the river does not result in objectionable scour. The excess kinetic energy can be dissipated by a hydraulic jump basin or a roller bucket or some other suitable energy dissipator or absorber. In some cases, however, the overflowing water may be delivered directly to the stream if the stream bed consists of erosion-resistant bed rock. The incoming jet should always be projected some distance downstream from the end of the structure by means of such structures as flip buckets or cantilevered extensions.

17.3.4. Entrance and Exit Channel
Entrance channel conveys water from the reservoir to the control structure while the exit channel conveys flow from the terminal structure to the stream channel downstream of the dam. Entrance and exit channels are, however, not required because such spillways draw water directly from the reservoir and discharge it directly into the stream channel as, for example, in case of the overflow spillway in a concrete dam. In spillways which are placed along the abutments or located near saddles or ridges, entrance and exit channels would be needed.

17.4. TYPES OF SPILLWAY
Spillway is usually referred to as controlled or uncontrolled depending on whether spillway gates for controlling the flow have been provided or not. A free or uncontrolled spillway automatically releases water whenever the reservoir level rises above the overflow crest level. The main advantage of an uncontrolled spillway is that it does not require constant attendance and operation of the regulating devices by an operator. Besides, there are no problems related to the maintenance and repair of the devices. If it is not possible to provide a sufficiently long uncontrolled spillway crest or obtain a large enough surcharge head to meet the requirements of spillway capacity, one has to provide regulating gates. Such gates enable release of water, if required, even when the reservoir level is below the normal reservoir water surface level. Most common types of spillway are as follows:
(i) Free overfall (straight drop) spillway, (ii) Ogee (overflow) spillway,

(iii) Side-channel spillway,

(iv) Chute (or open channel or trough) spillway, (v) Shaft (or morning glory) spillway,

(vi) Siphon spillway,

(vii) Cascade spillway, and (viii) Tunnel (conduit) spillway.