Flume Study of Silt Transportation and Deposition

Abstract

A small recirculating laboratory flume was used to study the transportation and deposition of coarse silt (D₅₀ = 0.045 mm) under conditions of approximate transport equilibrium. Nineteen runs were made with a range of Froude numbers between 0.07 and 1.23 and with total sediment concentrations as high as 393.000 mg/1. Depths of flow (as much as 10 cm or one half of channel width) were adjusted by means of a sidewall correction for subsequent calculations of Reynolds number, Froude number, and stream power.

Ripple bed forms in the lower regime of flow developed at Froude numbers less than 0.75 approximately. Plane bed and standing wave or antidune bed forms developed at Froude numbers in excess of 1.0. Flat symmetrical ripples, irregular low ripples, and plane bed forms characterized the transition between the lower and upper regimes of flow. The transition occurred at a stream power value of approximately 2 g.cm.^-1'.s^-1. There was, however, some overlap in the stability fields of the several types of bed forms.

Lenticular, tabular, and irregular units of crosslamination were deposited, in addition to parallel (plane) lamination, in the lower regime of flow. Parallel (plane) lamination, undulóse lamination, and thin units of cross-lamination were deposited in the transitional regime. The upper regime of flow was characterized primarily by parallel and undulose lamination. The cross-laminated units deposited by ripple migration in the lower regime of flow had an average length of about 0.7 x the ripple wave length and an average maximum thickness of 0.8 x ripple height. The silt behaved essentially as a cohesionless material.