8.6.2. Longitudinal Section of Irrigation Channels 8.6.2.1. Longitudinal Slope An irrigation channel flowing in an alluvium can be designed by any of the methods discussed in Secs. 8.4 and 8.5. The slope of the channel would thus be known. However, it is convenient to choose a slope close to Lacey’s regime slope [Eq. (8.35)] for the first trial design. This may need modification subsequently. The slope of an irrigation channel is governed by the country slope too. If the ground slope is more than Lacey’s regime slope (or the slope satisfying the critical velocity ratio and bed width-depth ratio, or that calculated on the basis of other methods), then the slope of the channel bed is kept as the required slope (i.e., the calculated one) and extra fall in the ground level is compensated for by providing vertical falls in the channel. On the other hand, if the required bed slope is more than the available ground slope, the maximum allowed by the ground slope is provided. In such cases, it is advisable to prevent the entry of coarse sediment at the head of the channel (see Example 8.7). Usually, ground slope is steeper than the desired bed slope and falls are invariably provided along the channels. The slope of reservoir-fed channels, carrying water with practically no sediment, may be determined by the consideration of the maximum permissible velocity.
For convenience, the slope is kept in multiples of 2.5 cm/km. The advantage of such values is that the drop in the bed level for every 200 m would be 0.5 cm which can be easily read on a levelling staff.
