Contents preface (VII) introduction 1—37

10.4. TYPES OF CANAL FALL
Canal falls are generally one of the following types:
(i) Canal falls which nearly maintain the normal depth-discharge relationship: Notch falls—trapezoidal or rectangular in shape—are of this type. The rectangular notch or low weir is not able to maintain accurately the normal depth-discharge relation-ship, but is economical and more suitable for discharge measurement. In a trapezoidal notch fall, a number of trapezoidal notches are made in a high breast wall across the channel. This arrangement provides an opening for flow right up to the bed level and thus eliminates silting in the channel upstream of the fall. The shape of the trapezoidal notch is decided on the basis of full supply and half supply conditions (1).
(ii) Canal falls which nearly maintain a fixed water surface level in the upstream chan-nel: When either a subsidiary channel takes off upstream of a fall, or the fall is combined with a hydro-electric plant, it is desirable that the water surface in the parent channel be maintained at a fixed level as far as possible. Siphon falls and high-crested weir falls fulfil this requirement. However, siphon falls, although very efficient, are too expensive and, hence, used only as siphon spillways in dams and not used as canal falls. High-crested weir falls are usually not flumed so as to keep the discharge per metre length of fall, q, small. The smaller the discharge intensity, the smaller is the head required and, hence, the water level upstream of the fall can be maintained at a relatively fixed level to a considerable extent. A smaller value of q also makes energy dissipation easier. Such falls are, therefore, relatively cheaper.

CANAL REGULATION STRUCTURES

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Generally, the length of a fall is limited to the width of the channel but, can be increased by providing an expansion followed by contraction in the channel. How-ever, this type of provision would increase the construction cost of the fall. Depend-ing upon the type of the weir crest, whether broad or narrow, and the flow condition, whether free or submerged, one can use these falls as metering devices after suitable calibration.

A raised-crest fall with vertical impact was first used on the Sarda canal system in UP. The amount of the drop at any fall in this canal system does not exceed 1.80 m. A large number of smaller falls were necessitated on this system because of the stratum of pure sand lying below the thin stratum of clay sand. Therefore, the depth of excavation for channel construction had to be kept low so as to keep the seepage losses to the minimum.
(iii) Canal falls which permit variation of water level upstream of the fall: The necessity of such falls arise when the subsidiary channel upstream of the fall has to be fed with minimum supply level in the parent channel. Such falls consist of either trapezoidal or rectangular notches. But, trapezoidal notches are relatively expensive and render the operation of regulators, such as stop logs and vertical strips, more difficult. In general, falls of this category, therefore, consist of rectangular notches combined with one of the following three types of regulators:
(a) Sluice gate—Raising or lowering the gate helps in controlling the upstream level.
(b) Horizontal stop logs inserted into grooves—Their removal or insertion causes the required change in the upstream level.
(c) Vertical strips (or needles)—These change the effective width (i.e., the width of opening) of the channel and do not cause silting.
(iv) In addition to the above main types of falls, there may be falls designed to meet specific requirement. Cylindrical falls (also known as well falls), pipe falls, chutes, etc. are falls designed for specific requirements. For example, a cylindrical fall (Fig. 10.4) is suitable for low discharge and high drop whereas chutes or rapids may have to be constructed if the canal is to carry timber logs as well.
Side pitching

U/S FSL
U/S BED

Side pitching
Earth filling
D/S FSL
D/S BED
Slope 1 in 10