Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 63

An alluvial canal is said to be 'in regime' or in dynamic stability when,
over some suitably long period, its depth, width and slope stabilize to
average or equilibrium values (Raudkivi, 1990). There may be seasonal
deposition or erosion in the canal but the overall canal geometry in one
water year remains unchanged. This can only occur when the sediment
input to the canal matches the average sediment transporting capacity,
sediment deposition during periods of high sediment input being balanced
by periods of scour when the sediment input is low.
The regime method considers the three main features of a canal, namely
the perimeter of the open canal, the amount of water and the sediment flow-
ing in it, as a whole. It attempts to derive the most crucial characteristics
of a stable (non-silting and non-scouring) canal primarily on the basis of
field and laboratory studies that investigated the interaction of the above-
mentioned factors (Naimed, 1990). The regime equations are based on
past and present observations and experiences and they present a long-
term average rather than an instantaneously variable state. Therefore, the
regime method tries to express the natural tendency of canals that convey
sediment within alluvial boundaries, in order to seek a dynamic stability.
Canals are described as in regime if they do not change over a period
of one or more typical water years. Within this period, scour and depo-
sition are allowed to occur as long as they do not interfere with canal
operations. Since the observed canals withdraw varying amounts of water
and sediment from different rivers and since sediment excluders may be
in operation at some of the head works, the regime theory can only pro-
vide some approximate average design values. Nevertheless, the adequate
experience obtained from the design and operation of these canals give
some proficient guidance for the design of stable channels with erodible
banks and sediment transport. However, the applicability of this method
can be challenged in the case of highly time-dependent operational regimes
as practised in many irrigation systems at present (Bruk, 1986).
Another limitation of the regime theory is that it assumes that the
discharge is the only factor determining the wetted perimeter while the
fact is that canals with less stable banks tend to be wider than those with
strong banks for the same discharge.
Regime methods were developed at the end of the 19th century to aid
the design of major irrigation systems on the Indian sub-continent. The
development started with Kennedy (1895) followed by Lindley (1919),
Lacey (1930), Blench (1957, 1970), and Simons and Albertson (1963).
A critical analysis of these methods is given by Stevens and Nordin
(1987) and they are well explained in the topics by Shen (1976), Chang
(1988), Raudkivi (1990) and Simons and Senturk (1992).
One of the most popular regime methods is the set of equations of
Lacey (1930). The equations were based on data from three canal systems
on the Indian sub-continent. They specify the cross section and slope of
regime canals based on the incoming discharge and a representative bed