Experimental marine suspension currents, competency and capacity

Abstract

In a circular flume (diameter 4 m) suspension currents were generated with various kinds and concentrations of clay (lutum) and sand. Starting from maximum velocity (230 or 300 cm/sec) samples were drawn off at successively lower velocities at three levels. The amount in suspension coarser than 33 microns was measured and for a number of representative cases grain size analyses of these sands were made. There is an increase in median and amount from the higher to lower levels. At curent velocity of 48 cm/sec particles smaller than 38 microns are not concentrated towards the bottom. At higher velocities there is even concentration upwards in some cases for grains smaller than 150 microns. The results demonstrate that the concepts of competency and capacity are strongly interrelated, the grain size distribution of the load in suspension depending on the original charge placed in the flume. The deposits tended to show log-normal distribution, the steepness of the cumulative curve depending on the composition of the suspension. All grain sizes in suspension were involved in the deposit forming. This was not due to current fluctuations and only partially to entrapment of fines between coarser grains. The coarser the fraction the more is abandoned relatively. For this reason the remaining suspension does show a decreasing median. The lutum greatly increases the carrying power, especially for finer grain sizes, the strongly flocculated Wadden Sea lutum having the greatest influence. The quantitative results are presumably roughly applicable to the lower part of turbidity currents. However, the exchange with higher levels is excluded in the experiments, and the paddles cause increased turbulence. Spiral flow is another drawback. For the deposition of a bed with median size of 200 microns, a velocity of 2 to 3 metres is required, for a median of 1 mm (that is to say a gravelly turbidite) something of the order of 10 m/sec, much depending on the concentration of lutum. At rippling velocity, about 50-60 cm/sec, the median of the deposit is around 80 microns. There appears to be no or only insignificant exchange between deposit and current in so far as concerns the grains that have become immobilized. No flutings or groovings develop during the process of deposition. Even while rippling is going on there is no or only slight local erosion. The climbing ripples of many turbidites with deposition on the stoss-side betoken non-erosion. No indications of the existence of a traction carpet were found or observed, a circumstance that is attributed to the absence of grains larger than those carried all the way in suspension.