Contents preface (VII) introduction 1—37

10.5.1. Vertical-impact Cisterns
In these cisterns (Fig. 10.5) there is an impact of a stream of water falling freely. The path of such a stream is, obviously, parabolic. This type of cistern is very efficient for the dissipation of surplus energy when the drop is sufficient so that the falling stream becomes almost vertical. The dimensions of the cistern should be such that it serves the purpose of stilling and combing out the residual eddies and disturbances. The length L_c and depth x of the cistern can be determined only by empirical expressions given by different investigators. Some of these are as follows (1):

Dyas’ formula	x =	H	h 1/ 3
			L 3
Glass’ formula	x + h = 1.85 E^1/2H	1/3
	3		L
	Lc = 5	(x + h3)
Etchevery formula	Lc= 3	EHL

_{x =} ^Lc

6
(10.1)

(10.2)
(10.3)
(10.4)
(10.5)

CANAL REGULATION STRUCTURES				357
UPIRI formula	x =	1	(EHL )^2 /3	(10.6)
4
		L = 5	EHL	(10.7)
		c

The symbols used in the above relations have been explained in Fig. 10.5 and their values are in metres.
In most of the vertical-impact cisterns, roughening is not provided. Instead, a short length of cistern is provided for stilling and such a provision has been found to be satisfactory. To prevent the falling nappe from adhering to the masonry face of the fall, aeration of the nappe is necessary and is provided by aeration pipes embedded in the wing walls just downstream of the crest. The exit from the cistern should be smooth so that the flow is streamlined before it enters the downstream channel.
Considerations such as combining the fall with a bridge may require contracting the width of the channel at the site of the fall. This increases the discharge per unit width and also the surplus energy to be dissipated. The design of an efficient cistern may then be difficult and expensive. In such situations, one may have to design some other type of cistern even if the available drop is, otherwise, large enough and suitable for a vertical-impact cistern.
10.5.2. Horizontal-impact Cisterns
In this type of cistern (Fig. 10.6), water, after passing over the crest, flows on a glacis whose reverse curve at the downstream end turns the inclined supercritical flow to horizontal supercritical flow before it strikes the subcritical flow of the downstream channel resulting in the formation of a hydraulic jump. However, the position of a hydraulic jump on a horizontal floor is very sensitive to the variations in the depth or velocity of the downstream flow. As a result, surging (movement of the jump) is very likely to occur on the horizontal floor of this type of cistern. In the absence of a perfect jump, the energy dissipation may not be proper. Therefore, it is usual to depress the cistern downstream of the jump location until the depth in the cistern increases by about 25 per cent of the tail-water depth (1). Although this provision is generally adequate for energy dissipation, it should, however, be noted that when the jump forms on sloping floor, the impact is no longer horizontal. Inglis has, instead, suggested a low baffle for holding the jump on a horizontal floor (1).

U/S TEL
U/S FSL		D/S TEL
	E	D/S FSL
U/S Bed

Fig. 10.6 Horizontal-impact cistern
For known discharge intensity q and the drop H_L, the total energy downstream of the jump E₂ can be calculated. The level of the cistern bed is then fixed at 1.25 E₂ below the downstream total energy line. Thus, the level of the bed of the cistern is independent of the level of the downstream channel. The length of the cistern is usually kept equal to 5 to 6 times E₂ to keep the jump within the cistern. Roughening devices, if required, may be provided starting from a section which is at a distance of half the height of the jump from the toe of the jump.

358 IRRIGATION AND WATER RESOURCES ENGINEERING
The design for such a cistern becomes more complicated when the supercritical jet is to be splayed. In such situations, the discharge intensity will be greater in the central part of the channel resulting in a ‘bowed’ jump. The expansion, therefore, must be very gradual. Although the horizontal-impact is an efficient energy dissipator (so long as the hydraulic jump continues to form), this type of provision requires expensive devices to hold the jump. Hence, provision of this type of cistern is usually made for dam spillways rather than for canal falls.