Q. Briefly explain about Artificial cut offs?
Alignment of a cut off is chosen so that approach and exit ends remain more or less tangential to the river course. A cut off can be made either by open excavation or by means of a dredger. Since considerable amount of work is usually involved below low water level making a cut becomes easier with a dredger.
Normally only a small pilot cut of about 10 per cent capacity of the channel discharge is made where the river cut off is desired. The river is relied upon to open out the pilot cut and develop it once the water starts flowing. When arc to chord ratio is about 5 or more, development of a cut off may be expected to be rapid. A deep and narrow cut may be preferred to a shallow and wide section, since side erosion is easier and faster than bed scour.
Velocities in deeper cut are also more. In designing an artificial pilot cut, its progressive development and final adjustment of the river are required to be visualised. When cut off is made across the neck of the bend, length is shortened so that with the available fall, slope steepens increasing the velocities. These increased velocities open out the cut rapidly and the river gets diverted on the new cut off channel. With increased velocities, bed scour and degradation occurs upstream of the cut off. The scoured material is deposited on the downstream side and finally equilibrium bed and water surface slopes are established which can be a little steeper than the original. Thus in effect river attains a somewhat steeper slope and generates higher velocities. Bank protection is, therefore, usually needed at and in the vicinity of a cut off to prevent side erosion. Otherwise new bends may form in the wake of a cut off as was experienced in the Mississippi River. Superimposition of the expected longitudinal section with cut off over the existing section permits a rough assessment of possible degradation and aggradation. Such an attempt should be made to estimate the final bed and water levels and lengths likely to be affected by the cut off.