EXPERIMENTAL STUDY ON FLOW RESISTANCE IN ALLUVIAL CHANNEL

Abstract

Flow velocity of alluvial channel is an area of intense research due to its complexity of nature. The dependability of velocity on multiple factors viz., bed characteristics, discharge, channel topography (slope), flow depth etc., makes it more difficult to estimate. Velocity is directly related to flow resistance. Both Manning's and Chezy's equations which are being used worldwide for velocity calculations, gives a single value (mean velocity): and do not generate depth wise variation in velocity. Moreover, these equations have been derived for rigid beds only. In alluvial channel flow, sediment movement at bed as well as in suspension has significant influence on velocity variation. Validity of log-law distribution of velocity in vertical section is studied in this thesis and comparison of their numerical coefficients for different localized bed condition is made. It is found that numerical coefficient values depend on relative roughness (Rr) and Hari's Number (Ph). A simple relationship between flow & fluid characteristics and velocity of flow is proposed to be established, which could be used for study of vertical movement of sediment in alluvial channels. Accordingly, a non-linear empirical equation is developed with a satisfactory, coefficient of regression of 0.786. In this equation, velocity is related to relative depth, flow depth, shear velocity, discharge and representative bed size. Study of shear velocity which depends on turbulence motion needs an: in depth knowledge of turbulence strength and sweep-ejection motion. As, per literature, sweep-ejection is the main contributor to Reynolds's stress. Analysis of results of experiments shows 'that.Reynolds's stress is evenly distributed for sandy bed. As such, in sandy bed condition the-calculation of shear velocity parameter may be taken as the function of hydraulic radius and slope for whole depth. In the case where the bed representative size is 'increased, distribution of Reynolds's stress gets randomness. The shear velocity in the coarser bed case- is not only the function. of hydraulic' radius and slope but also it varies significantly with depth Quadrant distribution of velocity gives the variation in contribution of velocity to generate shear. Percentage distribution of first and fourth quadrant look like a wine cup which is wider in the case, of- sandy" bed and gets' narrower as the representative bed size is increased. It is also an indication of increasing randomness throughout the depth as representative bed size increased.