Contents preface (VII) introduction 1—37

17.4.3. SIDE-CHANNEL SPILLWAY
The control weir of a side-channel spillway (Fig. 17.10) is located alongside and approximately parallel to the upstream portion of the spillway discharge channel which itself may be either an open channel, a closed conduit, or an inclined tunnel. The spillway discharge flows over the weir crest and falls into a narrow trough (i.e., upstream of the discharge channel) and takes an approximately 90° -turn before continuing into the spillway discharge channel. The control structure, in plan, may be straight, curved, semi-circular or U-shaped. The overflow section may be broad-crested instead of ogee-shaped.
One should note that when the flow in the side channel trough is subcritical, the incoming flow from the control structure (i.e., the overflow crest) will not cause high transverse velocities because of the low drop due to relatively higher depth of flow in the trough. This would effect good diffusion and intermingling of the incoming flow with the trough water due to relatively low velocities of both the incoming flow and the flow in the trough. Therefore, there would be comparatively smooth flow in the side channel trough. However, when the flow in the side channel trough is supercritical, the flow velocities in the trough would be high and the depth of flow small, causing the incoming flow to have a relatively higher drop. Therefore, the intermixing of the high-energy transverse flow with the trough stream will be rough and turbulent producing violent wave action causing vibrations. Therefore, the flow in the side channel trough should be maintained at subcritical condition for good hydraulic performance.
Moreover, the amount of excavation would also increase for larger bed widths. While the flow in the side channel trough should preferably be subcritical, the flow in the discharge channel is supercritical and a control section downstream of the trough is provided by either constricting the channel width or raising the channel bottom.

578 IRRIGATION AND WATER RESOURCES ENGINEERING

Dam
axis

River

400
420
440	460	480
X	500
			520
		X