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PRINCIPLES OF RIVER TRAINING
Two approximate theories, the theory of tractive force for bed load, and regime theory for suspended load are available for application to problems of river training.
1. Tractive Force Theory
Tractive force approach is essentially lan tractive force for the incipient conditions of bed movement. The alluvial rivers carry sediments in suspension as well as bed load. The capacity of river channel is designed to cater for the discharge and sediment load by the following formula:
Qs =0.17/m3/4 r2 BR2 S2
Also Q=1/n BR5/ 3 S1/ 2 (Manning's formula)
Qs = silt bed charge (cumecs), m = dia. Of sand (mm), r = specific weight,
2. Regime Flow Theory
In incoherent alluvium, Lacey’s regime theo dimensions, although there is a marked divergence when applied to large rivers due to omission
of an important parameter, viz., quantity of silt in suspension are important where silt charge in the flow predominates.
METHODS OF RIVER TRAINING
Planning and design of river training works is done by empirical methods and on the intuitive judgments of engineers. The method of river training applicable depends mainly on the type of river and sediment load, as under. Under certain cases, where river is very unstable, training is almost impossible and the situation has to be met as it arises.
1. Stable rivers:
Stable rivers are characterized by stability of alignment, slopes and regime as they mould their beds to carry into sea almost all the silt brought down by them. They are amenable to river training to attain ultimate stability with the aid of training works such as spurs and guide banks.
2. Aggrading rives:
Aggrading rivers have inherent instability and are not equally amenable to river training. Stability cannot be imposed, e.g., bank protection works may either be destroyed by severe erosion or get bur.
3. Degrading rivers:
Degrading rivers too have inherent instability and not easily amenable to river training. Stability cannot be imposed, e.g. training works may be destroyed by undermining due to bed scour. Gradient control by constructing dams and weirs is a prerequisite to attempting any other modifications in the river. Local training works may be possible if their repercussions on river regime upstream and downstream are not drastic.
Model studies for details refer ‚Hydrologyandwater resources enginee is a very useful tool in the design of river training works. Generally mobile bed models are constructed. When a river has a predominant bed load movement, its simulation, both qualitatively and quantitatively, is possible is possible in the model. However, similitude of the suspended load in models is not possible. The conformity between hydraulic models and prototype is subject to limitations.
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