STUDIES ON THE CHARACTERISTICS OF MEANDERING STREAMS

Abstract

Though it has been known for a long time that alluvial streams tend to meander, no satisfactory theory for the cause of meandering is available at present. Further a relaticn for the geometry of meanders in terms of flow, fluid and sediment characteristics is also not well established. Some theoretical work has been carried out concerning instability leading to meander formation in alluvial streams. However, there is no experimental work reported on the response of a straight channel to oscillatory disturbances introduced near the bed at its upstream end. Further, information on resistance and sediment transport characteristics in curved channels is limited. Hence the present study was taken up with the . following objectives In mind: (a) Experimental investigation on the effect of arti ficial disturbances near the bed on the formation of alternate bars and pools as well as on bed forms in straight alluvial channels; (b) the analysis of field and laboratory data for evolving a predictor for river channel patterns and the geometry of the plan forms; and (c) the analysis of available field and laboratory data for understanding the resistance and sediment transport characteristics in curved channels. The experimental investigation was carried out in a recirculatory rectangular channel of 0.3 m width with mobile bed and rigid banks, A thin metallic horizontal strip of 1 cm width wis attached to a vertical rod and suspended near the bed, making a prefixed angle with the centre line of the flume. The rod was connected to a suitable driving mechanism and the plate was oscillated at desired frequencies. Equilibrium runs were conducted with Froude numbers ranging from 0.42 to 0.806. A flat bed with a high sediment transport rate was obtained at a Fr of 0.806 when no disturbances were introduced at the upstream end. Alternate bars and pools formed at the same Froude number on introduction of disturbances at frequency of 1.83 Hz or more. The process was found to be accelerated with the increase in frequency. On stopping the disturbances, the existing pattern of bars and pools at the upstream end induced alternate bar forma tion further downstream and the entire channel was found to be covered with such pattern. The bed was found to be covered with dunes for Froude numbers varying from 0.4-2 to 0.708 under the undisturbed conditions. The average steepness of dunes defined is the ratio of dune height h to dune spacing I., has been found to decrease with increase in frequency of disturbance. The average dune spacing I*** to depth D ratio is found correlated to f D/Ubyalinear relation. Here f is the frequency of disturbance and U is the average velocity. Alternate "oars did not form due to the introduction of double harmonic disturbances at Froude number Fr equal to or less than 0.708. For the study on meander characteristics a large amount of field and laboratory data have been used. The field data on meandering, braided and straight channels include data mainly from India, Australia, U.S.A and Canada. The existing criteria for predicting the river channel patterns were checked with the help of the above data and found tu be unsatisfactory. A new criterion involving dimensionless shear stress c« and B S/D has been developed which gives satisfactory results. Here B is the width of channel and S Is the slope. The meander length Kr and width of meander belt M-g have been found to be better correlated to the discharge than to some other dimensionless parameters. The change in resistance coefficient and sediment transport rate on account of channel curvature in alluvi il channels has been found to be functions of central deflection anglea , the ratio of radius of bend to the width of the channel rc/B, Reynolds number (Re = 4-UR/v), the ratio of width to depth B/D and the Froude number Fr =U/VgR Here R is the hydraulic radius and v is the kinematic viscosity. Regression analysis shows that the increase ( or decrease) of resistance in curved alluvial channels (over that of the straight channel) is greatly affected by the Froude number and central deflection anglea . It is also found that Re has limited effect on the change in resistance of curved alluvial channels. When regression analysis was carried out for the change in sediment transport rate in curved alluvial channels, it was found that the parameters having maximum influence on this change are a and r /B. The regression analysis also shows that unlike in the case of resistance, Re and Fr both affect the change in sediment transport rate. Another important result is that with increase in B/D for a curved channel, the sediment transport rate increases and the resistance decreases