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Rapid deposition of sand and gravel mixtures downstream of a negative step: the role of matrix-infilling and particle-overpassing in the process of bar-front accretion

Freshwater Biological Association, The Ferry House, Ambleside, Cumbria LA22 OLP, UK

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.

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