Contents preface (VII) introduction 1—37

12.2.13. Meandering Rivers
On account of the slight asymmetry of flow in alluvial rivers, there is a tendency for such rivers to vary their plan-forms into bends which eventually result in a meandering pattern (Fig. 12.2). The term meandering has been derived from the Great Menderes river in Turkey which follows a winding or intricate course (Fig. 12.2). Rivers having such meandering patterns are known as meandering rivers which, in plan, comprise a series of bends of alternate curvature. The successive curves are connected through straight reaches of the river called ‘crossing’. Meandering increases the length of river and decreases its slope.

Concave bank

Convex bank

Meander wave length

^ML

Point
bar

(a) Definition sketch

B

M

Meanderbeltwidth

	Erosion
Crossing	Point bar
	Crossing
	Erosion
(b) Pattern of erosion and deposition

0	1	2
	Scale (1000 FT)

(c) Reach of the Büyük (Great) Menderes river, Turkey

Fig. 12.2. Meandering channel

410 IRRIGATION AND WATER RESOURCES ENGINEERING
12.2.14. Braided River
When a river flows in two or more channels around alluvial islands, it is called a braided river (Fig. 12.3). The braided patterns in a river develop after local deposition of coarser material which cannot be transported under prevailing conditions of flow and which subsequently grows into an island consisting of coarse as well as fine material.



Fig. 12.3 Typical braided reach of a river
12.3. BEHAVIOUR OF RIVERS
The behaviour of a river is mainly affected by the characteristics of the sediment-laden water flowing in the river. The available energy of the flow is utilised in transporting the sediment load as well as in overcoming the resistance due to the viscous action and the roughness of bed and sides. On account of the interdependence of the factors affecting the flows, there is an inherent tendency of these rivers to attain equilibrium. As such, whenever the equilibrium of a river is disturbed by man-made structures or natural causes, the river tends to attain a new equilibrium condition by scouring the bed or by depositing the sediment on the bed or by changing its own plan-form. These changes can be either local or extended over a long reach. The behaviour of a river can, therefore, result in the variation of the shape of the river cross-section and/or its plan-form. Aggradation, degradation, scour and deposition of sediment around bends, and meandering are a few examples of such changes.
12.3.1. Bends
With slight asymmetry in flow, an alluvial river tends to develop bends which are characterised by scour and erosion of sediment on the concave (i.e., outer) bank and deposition of sediment on the convex (i.e., inner) bank. Because of curved flow lines around the bend, the flow is subjected to centrifugal forces and, hence, there is a transverse slope of the water surface due to the superelevation of the water surface at the concave bank. As a result, the bottom water (moving with relatively smaller velocity) moves from the concave bank to the convex bank and also carries with it the bed material and deposits it near the convex bank. To replace this bottom water, water dives in from the top at the concave bank and flows along the bottom carrying sand and silt to the convex bank where it is deposited. This secondary motion is primarily responsible for the erosion of the sediment on the concave bank and the deposition of the sediment on the convex bank. The depth of flow in a river at the bend thus becomes deeper at the concave bank (Fig. 12.4).