Contents preface (VII) introduction 1—37

Table 12.1 Tortuosity of the river Ganga in different reaches (3)

Reach	Valley length	Talweg length	Tortuosity
	(km)	(km)	(%)
Balawalli to Garhmukteshwar	104.6	117.5	12
Garhmukteshwar to Rajghat	59.5	67.6	14
Rajghat to Kanpur	281.6	313.8	11
Kanpur to Allahabad	117.0	217.3	23
Allahabad to Varanasi	137.8	209.2	51
Varanasi to Sara	684.0	869.0	27
Sara to the Bay of Bengal	297.7	321.9	8

It should be noted that the meandering pattern of alluvial rivers is commonly encountered in alluvial stream. River training methods are generally adopted for meandering rivers. It may also be noted that the meander pattern is not stationary and moves slowly in the downstream direction.

12.3.3. Cutoffs
Cutoffs can be defined (3) as a process by which an alluvial river flowing along curves or bends abandons a particular bend and establishes its main flow along a comparatively straighter and shorter channel. During the development of meanders, there is always a lateral movement of the meanders due to their gradual lengthening. Increased frictional losses and bank resistance tend to stop this lateral movement. When the bend and the bank resistance become too large for continued stretching of the loop, the flow finds it easier to cut across the neck than to flow along the loop (Fig. 12.5). This results in a cutoff. Cutoff is, thus, a natural way of counter-balancing the effect of the ever-increasing length of a river course due to the development of meanders. Usually, a river has shallow side channels within the neck of the meander loop. These side channels may either be part of the main channel of an earlier river course or are formed by floods spilling over the banks of the river channel. Cutoffs can develop along these shallow side channels. Alternatively, cutoff may be artificially induced for some other purpose.

RIVERS AND RIVER TRAINING METHODS

413

Permanent banks

Caving

Caving
Shallow side channel

_Bar Low sand island
Low sand island

Bar
Transition

Deep

High
caving

bank

Permanent banks

Fig. 12.5 A river bend
The rapidity with which a cutoff channel develops depends on local conditions. For example, the cutoff on Chenab near Shershaha took one year but the great Golbethan bend of the Ganga downstream of Hardinge Bridge began to cutoff in 1911, and the cutoff developed only after five years (7).
Whenever a river succeeds in establishing a cutoff, there follows a period of non-equilibrium for long distances upstream and downstream of the newly-formed channel. Banks start caving in and new channels are formed while some other channels get silted up. Only after a couple of floods, the equilibrium is, once again, established.
Sometimes it is advantageous to make a controlled artificial cutoff to avoid the chaotic or non-equilibrium conditions when a natural cutoff develops. An artificial cutoff reduces flood levels and flood periods. Artificial cutoffs have been used to shorten the travel distance and increased ease of manoeuvring of boats along the bend during navigation. In such situations, use of training measures like groynes and revetment on banks usually becomes necessary to prevent bank erosion and arrest the natural tendency of the river to meander.
For inducting an artificial cutoff, a suitable pilot cut (or pilot channel) of small cross-section is initially made so as to carry 8 to 10 per cent of the flood discharge (5). The pilot channel is then allowed to develop by itself and sometimes such gradual development is assisted by dredging. Pickles (8) has made the following recommendations for design and execution of artificial cutoffs:

414 IRRIGATION AND WATER RESOURCES ENGINEERING
(i) The pilot channel should be tangential to the main direction of river flow ap-proaching and leaving the cutoff.
(ii) The pilot channel is usually made on a mild curve, the curvature being less than the dominant curvature of the river itself.
(iii) Entrance to the pilot channel is made bell-mouthed. Such transition at the exit is considered unnecessary because the cut develops first at the lower end and works progressively upstream.
(iv) The cut, when unlikely to develop because of either coarseness of the material or low shear stress, should be excavated to average river cross-section.
(v) The width of the pilot cut is unimportant as the cut ultimately widens due to scouring. Hence, in practice, the width is determined by consideration of the type and size of the dredging equipment used.
(vi) When a series of cutoffs is to be made, the work should progress from the down-stream to upstream.