More Skewed against than Skewing
- S.-D. Nio,
- R. T. E. Shüttenhelm and
- Tj. C. E. Van Weering
Published Online: 29 JUN 2009
Copyright © 1981 The International Association of Sedimentologists
Holocene Marine Sedimentation in the North Sea Basin
How to Cite
Clark, M. W. (1981) More Skewed against than Skewing, in Holocene Marine Sedimentation in the North Sea Basin (eds S.-D. Nio, R. T. E. Shüttenhelm and Tj. C. E. Van Weering), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303759.ch8
- Published Online: 29 JUN 2009
- Published Print: 23 DEC 1981
Print ISBN: 9780632008582
Online ISBN: 9781444303759
- modification of size-frequency distribution - truncation, filtering and mixing;
- truncation- an extreme form of filtering;
- increased viscosity due to wave breaking;
- hindered settling-settling velocities of particles in suspension;
- settling convection-groups of particles - fall at a relatively high rate
Many beach sands are found to have negative skewness when analysed in terms of the weight frequency of their phi values. Tanner suggested a number of mechanisms which could lead to the modification of a size-frequency distribution: truncation, filtering and mixing. Truncation can be regarded as an extreme form of filtering. The concept of a filter may be broadened to identify two types, which differ in the range of application of the filter.
The ‘mixing’ hypothesis has received some popular support, notably in the form proposed by Visher, where a number of individually truncated distributions may be mixed. However, it is possible to derive similar results from filtering mechanisms; it is also possible to support these mechanisms by physical processes related to deposition in the nearshore. Much of the material in the nearshore passes through a suspension phase, where the key filters which would modify the distributions are (1) concentration, (2) turbulence and (3) shape. The most effective filter is probably associated with the increased viscosity due to wave breaking.
These several filters may act in concert, but the measurement base used in the analysis also contributes to the asymmetry. If the size-frequency distribution is converted to a settling velocity distribution, its shape becomes more symmetrical, although some skewness may still be present. While there is an implicit assumption behind most of these transformations that the ‘fundamental’ or ‘intrinsic’ distribution of sand sizes is lognormal, or at least symmetrical in its logarithms, this need not be the case. Analyses show that negative skewness could be derived from positively skewed intrinsic distributions.
Filtering can provide a more parsimonious and physically justifiable model for the modification of grain size curves than mixing.