Sedimentology zyxwvuts (1977) zyxwvu 24, 507-527 zyxwv Threshold of sediment motion under unidirectional currents M. C. MILLER*, I. N. McCAVEt undP. D. KOMAR* *School zyxwvuts of Oceanography, Oregon State Univ., Corvallis, Oregon, USA, und tSchool of Environmental Sciences, Univ. Eust Angliu, Norwich, England ABSTRACT Carefully selected data for the threshold of sediment movement under uni- directional flow conditions have been utilized to re-examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of uni- granular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced. This selected data is used to develop a modified Shields-type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain-Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids. The often used empirical curves of the friction velocity u,, the velocity 100 cm above the bed ul,,,,, the bottom stress zyxwv T, and Shields’ relative stress zyxw Bt, all versus the grain diameter zyxwvutsr D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are uloo = 122.6 Do.2B for D < 0.2 cm uloo = 160 .O for D > 0.2 crn where the velocity uloo measured 100 cm above the sediment bed is given in cmjsec and the grain diameter D is in cm. The limitations on any of the threshold relationships are severe. These limi- tations should be properly understood so that the empirical curves and relationships are not improperly employed. INTRODUCTION As the fluid velocity or stress it exerts increases over a bed of sediment, a situation is eventually reached such that any additional increase will cause particles to be moved from the bed and be transported. The conditions slightly less than those necessary 507