Abstract

An experimental flow-channel was used to investigate the rapid deposition of sand and gravel mixtures downstream of a negative step. Hydraulic processes give rise to the quasi-simultaneous accumulation of openwork gravels, matrix-filled contact framework and dilated framework gravels. The resultant structure is analogous to a bar-front in gravel-bed streams. Flow conditions were non-uniform but steady in time, whereas open and closed gravel fabrics are usually interpreted as representing deposition from unsteady flows—for example a varying discharge regime.

Consideration of the percentage of sand in the deposits indicated that, in terms of packing, the sediments may be considered as a simple binary mixture with partial framework dilation occurring with sand percentages in excess of c. 32%. Despite dilation, for sand percentages less than 50%, structural stability of the foreset gravels was maintained so that the bar-front exhibited a distinct brinkpoint, and a planar slip-face (at the angle of initial yield) which made an angular contact with the base of the flume. For sand percentages in excess of 80%, individual gravel clasts were completely supported by sand. The original structural stability was lost so that the bar-front, eroded by reverse currents, assumed a tangential contact with the flume base below the angle of initial yield. The brinkpoint was rounded and indistinct. In addition to altering the bar-front slope, increasing the quantity of sand in the bed sediments had a major influence on the dip of gravel particles on the foresets.

In all experimental runs a surface sand-seal (with openwork gravels beneath) developed upstream of the brinkpoint. The mechanics of sand-seal formation require further investigation. However, an overpassing mechanism was noted to be very important in sorting gravel particles rolling over the brinkpoint. A published mathematical model describing the overpassing mechanism was found to be appropriate to the present experimental observations.

First Page Preview

First page PDF preview
You do not currently have access to this article.