Contents preface (VII) introduction 1—37

(a) Plan
Reservoir water surface
			Water surface profile A
	Overflow crest
Subcritical	Subcritical flow
depth
			Control
		Water surface profile ‘B’
			section
Subcritical		Floor profile ‘A’
	depth	Supercritical
		flow

Side channel trough floor profile
Discharge channel floor profile ‘B’

(b) Side channel profile
Reservoir water surface

Overflow	Water surface ‘A’

crest

Subcritical flow
Water surface ‘B’
Supercritical flow
(c) Side channel cross-section (X-X)

Fig. 17.10 Side-channel spillway

SPILLWAYS

579

Because of spatial flow conditions, the depth of flow in the side-channel trough would never be the same at different sections. For any short reach of channel ∆x, the change in water surface ∆y can be determined by either of the following equations (4):

Q1 (v1

+ v2 ) L

v2 (Q2 − Q1) O

∆y =

M^( ^v2

− v1) +

P

(17.7)

g (Q1

Q1

+ Q2 ) N

Q

Q2 ( v1

+ v2 ) L

v1 ( Q2 − Q1) O

∆y =

M^( ^v2

− v1) +

P

(17.8)

g ( Q1

Q2

+ Q2 ) N

Q

Here, Q₁ and v₁ are the values of discharge and velocity, respectively, at the beginning of the reach, and Q₂ and v₂ are those values at the end of the reach. For free flow conditions, the behaviour of a side-channel spillway is similar to that of an overflow spillway and is dependent on the profile of the weir crest. For larger discharges, however, the flow over the crest may be submerged and the flow conditions will then be governed by the conditions in the side-channel trough. A side-channel spillway is an ideal choice: (i) for earth or rockfill dams in narrow canyons and for situations where direct overflow is not permissible, (ii) where the space required for a chute spillway of adequate crest length is not available, or (iii) when a long overflow crest is required in order to limit the surcharge head for the design inflow flood. Because of the turbulences and vibrations inherent in side channel flow, a side channel spillway is generally not considered except when a strong foundation (such as rock foundation) exists.
Design of side channel trough involves computation of water surface profile starting from the control section (rectangular in shape and at which the critical depth, velocity and the velocity head are known for given discharge) to the upstream end of the side channel trough. The trough is usually trapezoidal with side slopes of 0.5H:1V and relatively flat bed slope that would provide larger depth and smaller velocities to insure better intermixing of flows in the initial reach of the trough. A cross-section with minimum width – depth ratio will result in the best hydraulic performance (5). However, minimum bed width (say, about 3 m) is required to avoid construction difficulties due to confined working space. There would be a transition between the downstream end of the trough and the control section. The head loss in transition (to include losses due to contraction and friction, and also losses due to diffusion of flows in the trough) is assumed to be equal to 0.2 times the difference in velocity heads between the ends of the transition. The flow characteristics (depth and velocity or velocity head) at the downstream end of the trough (i.e., upstream end of the transition) are obtained by solving Bernoulli’s equation. The equation will have to be solved by trial and error. For this, assume a suitable value of the depth of flow at the upstream end of the transition and the corresponding velocity head. If these values satisfy the Bernoulli’s equation (applied for the two end sections of the transition), one has obtained the flow characteristics at the upstream end of the transition. Otherwise, one has to assume another trial value and repeat the computations till the Bernoulli’s equation is satisfied. Thereafter, the water surface profile along the side channel trough can be determined using either Eq. (17.7) or Eq. (17.8). The channel profile and the water surface profiles are, then, plotted relative to the crest (of the control structure) and the reservoir water level. The maximum submergence at the upstream end of the trough that can be tolerated is about two-third the head over the control structure. If the maximum water surface level in the side channel trough results in submergence more than the permissible value, the end of the side channel trough will have to be lowered.

580 IRRIGATION AND WATER RESOURCES ENGINEERING
The control structure of a side channel spillway generally consists of an ogee crest which is designed by the method described in section 17.4.2. Flow in the discharge channel downstream from the control will be the same as that in an ordinary channel or chute spillway.
17.4.4. Chute Spillway
In a chute (or trough) spillway, the spillway discharge flows in an open channel (named as ‘chute’ or ‘trough’) right from the reservoir to the downstream river. The open channel can be located either along the abutment of the dam or through a saddle, (Fig. 17.11). The channel bed should always be kept in excavation and its side slope must be designed to be stable with sufficient margin of safety. As far as possible, bends in the channel should be avoided. If it becomes necessary to provide a bend, it should be gentle. The spillway control structure can be an overflow crest, or a gated orifice or some other suitable control device. The control device is usually placed normal or nearly normal to the axis of the chute. The simplest form of a chute spillway is an open channel with a straight centre line and constant width. However, often the axis of either the entrance channel or the discharge channel is curved to suit the topography of the site. The flow condition varies from subcritical upstream of the controlling crest to critical at the crest and supercritical in the discharge channel. The chute spillway is ideally suited with earthfill dams because of: (i) simplicity of their design and construction, (ii) their adaptability to all types of foundation ranging from solid rock to soft clay, and (iii) overall economy usually obtained by the use of large amounts of spillway excavation for the construction of embankment. The chute spillway is also suitable for concrete dams constructed in narrow valleys across a river whose bed is erodible for which the ogee spillway becomes unsuitable.

480

Datum
L-Section

				Dam	axis
520
	500
			480
		460
					500
					480
				River
					460
				480
				500
	520
540
560

Plan

Fig. 17.11 Chute spillway

SPILLWAYS