VELOCITY DISTRIBUTION AND SEDIMENT TRANSPORT IN RIGID-BED OPEN CHANNELS

Abstract

lined channels are used in irrigation and hydropower systems with the object of minimising seepa^ losses and conserving the available head. The sediment transporting capacity of such channels should be greater than or equal to the rate at which sediment is entering the channel. The presence of material in suspension affects the resistance coefficient and velocity distribution in the vertical. In the present study, experiments were carried out to predict the critical condition of incipient deposition of transported material in rigid boundary channels of various shapes. The variation of resistance coefficient and velocity distribution law for sediment-laden flows in smooth and rough bed channels have been investigated. Experiments were conducted in a recirculatory flume of 0.40 m width and 17m length. The first experiment in each series was carried out using clear water. Then the sediment was mixed in recirculatory system and experiments were conducted with increasing concentrations until material was deposited on the bed. Smooth as well as artificially roughened rectangular channels as well as trapezoidal and semicircular channels with smooth boundaries were used. The data collected include the average concentration of transported sediment and the velocity profile in the vertical. tge 11 For a few runs the sediment concentration profiles in the vertical were also measured. Three uniform sands of sizes 0.147mm, 0.106 mm and 0.082 mm and coal of size 0.164 mm were used as sediment. The Reynolds number of flow ranged from 7x104 to 3x10 , Eroude number from 0.55 to 1.3 and average concentration of transported material from 35 ppm to 6562 ppm by volume. These data were analysed along with data from other sources. The limiting concentration relations given by Rossinsky and Kuzumin, Novak and Falluri, Pullaiah, Oja, Itakura and Kishi and Ranga Raju were examined with the data obtained in the present study. Pullaiah' s relation for variation of Karman constant and additive constant in single logarithmic law of velocity distribution was verified. The velocity distribution laws for flows with suspended sediment due to Song and Yang, Itakura and Kishi and Coleman were also verified. Alternative analysis was also carried out for determination of limiting concentration, resistance coefficient and velocity distribution in sedimentladen flows. The principal findings of the present study are as follows: (i) The limiting Concentration of transported sediment Ill qS2,5 D 2 is uniquely related to c x (**—" ) and vft(T} increases with increase in its value. (2) The friction factor in sediment-laden flows is less than the value for clear water flows when C~co 0 © -—^ is less than 1200 and greater than it when -——- US Ub is greater than 1200. The ratio of friction factors for flows with suspended sediment and for clear water 0 co flow (in case of •••Q~ > 1200) varies with 0 US -70^- XLm (3) A single logarithmic lav/ does not describe the velocity distribution in sediment laden flows. The velocity distribution law for flows with suspended sediment in smooth rigid bed channels of various shapes Is given as u - U y ' Au — 2^2. = 5.75. log __ m — *c o *c The term —is a function of Cljco /uZ,. u*c ° C (4) In sediment laden flows over large scattered roughnesses placed on the bed, the velocity distribution law Is given as u - U y ' Au AuR —2^- = 5.75 log — - — 4 — u*c - 0 u*c u*c in which Au^/u^is the wake region velocity augment ay tion function and is zero for - > 1.0. The term h % y —— is a function of r- . u* iv (5) A predictor for maximum velocity in open channels has been obtained. Knowing the average velocity over the cross-section, the friction factor and aspect ratio, the maximum velocity (i.e. velocity at the water surface) can be found from a plot U I . ~ max 0 ra- 0f . .SJf-u versus -—. U D Y o