A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering of the Asian Institute of Technology, Bangkok, Thailand.

Thesis (M.Eng.) - Asian Institute of Technology, 1972

The effect of gradation on the critical shear stress was investigated in this study. Three sets of sands from the bed of Chao Phraya river were used as the bed material. The median diameters and the measure of gradations of these sands were as follows: Sample (1) d 50 = 0.52 mm, v = 1.80; Sample (2) d5o = 0. 76 mm, a = 2.02, and Sample (3) d50 = 0.83 mm, v = 2. 064. Data obtained by DARANANDANA, Niwat (1961, Ph.D. Dissertation, Colorado), CHANG, J.J. (AIT Thesis No. 312), KHUHAPINANT, O.A. (AIT Thesis No.157), TALUKDAR, K.H. (AIT Thesis No.373) and SIMON, D.B. and RICHARDSON, E.V. (Studies of flow in alluvial channels, basic data for flume experiments, CER61EVR31) were also used in this study. There were altogether 16 series of runs. Each series of run was conducted with different bed material sand. The critical shear stress was determined by Straub's, Shield's, the extrapolation and the visual observation methods. The consistency of each method was discussed and the effect of gradation was analyzed. By dimensional analysis it was proved that {u0373} c = f( v, W_V ) in which w is also a function of d 50. The equation giving relationship of the critical shear stress, the measure of gradation and the d 16 (16 percent is finer by weight) is proposed. It was found that the gradation has insignificant effect on the critical shear stress.