A Fundamental Study of Sedimentation (Part 3) : Deposition and Movement of Particles

Abstract

When sedimentary particles are in motion, the larger particles cause greater friction against the bottom bed, while, these larger particles are influenced by greater velocity along the vertical distribution of the current velocity., Accordingly, the inference is that the larger particles are not always stable in the currents., In order to test this inference, the writer made a theoretical and experimental study of the relations between the velocity distribution of bottom currents and "proper critical velocity Vp" for traction (cf., Part 1 & 2), and obtained the following results : (1) Under changing current velocities, the size of the most stable particle is about 1., 5 -5mm in diameter (the writer names this size "the first stable granularity dc₁"); but if the depth of current water is too shallow for the particle size, "second stable granularity dc₂", which is larger in size and more stable, appears according to the depth., (2) When the current velocity is increased, the particles which first move are both the smallest and the largest one compared with dc₁, and after that, with increasing current velocity the difference between the sizes of these two decreases; the particle of dc₁ is the last move., If the current velocity is decreased, the order of deposition of these particles is reversed; but in this case it must be noticed that this velocity is less than Vp., (3) The velocity of a particle moved by traction is expressed as follows : if the particle size is fixed, v_t∽V; and if the current velocity is fixed, v_t∽dⁿ=d^2∼1/2, where v_t is the velocity of a particle moved by traction, V is the current velocity, d is a particle size, and n is a certain numerical constant depends on Reynolds number (cf., Part 2)(4) Critical granularity of saltation ds is expressed as follows ; de ∽V⁴ (5) Under a constant current velocity, the particle sizes which have the same moving velocity are generally of two sorts or more; in this case the type of movement of the small particles is saltation, and the other is rolling or slipping., These above-mentioned facts have geological significance as the factors to determe the size distribution of particles of sediments.,