Contents preface (VII) introduction 1—37

Fig. 10.4 Cylindrical fall
Canal falls can, alternatively, be divided on the basis of their capability to measure discharge. Accordingly, they may be either meter falls or non-meter falls.
10.5. CISTERN ELEMENT

As a result of the flow passing over a fall, the potential energy of the flow gets converted into kinetic energy. This excess kinetic energy, if not dissipated properly, will result in undesirable

356 IRRIGATION AND WATER RESOURCES ENGINEERING
scour of the bed and sides of the downstream channel. Hence, provision of means to dissipate the surplus kinetic energy is essential in all types of canal falls and is provided in a portion of the canal fall known as cistern element.
The cistern element or, simply, cistern, Fig. 10.5, located at the downstream of the crest of a fall structure, forms an important part of any canal fall. The cistern element is defined as that portion of the fall structure in which the surplus energy of the water leaving the crest is dissipated and the subsequent turmoil stilled, before the water passes into the lower level channel (1). The cistern element includes glacis, if any, devices for ensuring the formation of hydraulic jump and deflecting the residual high velocity jets, roughening devices, and the pool of water in which hydraulic impact takes place. In other words, the cistern element includes the complete structure from the downstream end of the crest to the upstream end of the lower channel.

U/S TEL

U/S FSL _E

Crest	HL	H2

D/S TEL

U/S BED

h3	D/S FSL
	D/S BED

x
L_C

Fig. 10.5 Cistern element (vertical impact cistern)
Most of the cisterns employ hydraulic impact of the supercritical stream of the falling water with the subcritical stream of the lower channel for the dissipation of surplus energy. Depending upon the type of impact, the cisterns are divided into four categories discussed in the following paragraphs.