Hydrodynamic Implications of Beach, Beach Ridge and Dune Grain Size Studies

Abstract

ABSTRACT Stepwise linear discriminant and canonical analyses of beach, beach ridge and coastal dune (slip-face) sands from localities around the Gulf of Mexico show that beach ridges have textural affinities with both beaches and coastal dunes. However, the internal structure shows that they were deposited largely by swash action. Washover and slip-yface bedding are, in general, not present in beach ridges. Skewness is the most important moment measure entering the discriminant functions and the cannonical plots; standard deviation is the next most important. These two measures for beach ridge sands show both swash-zone and dune affinities. In these samples kurtosis reflects shell content. Mean grain size, an indicator of local availability, is of minimum importance for determining hydrodynamic conditions of transport and deposition in such a regional study. A progression of moment measures was determined for the surf zone, the upper wash zone, and the coastal dune. Changes from zone to zone are largely confined to the coarse tail; however, individual sample grain size distributions (plotted on probability paper) have important but subtle characteristics for each zone. Differences in skewness values bear this out. Our beach ridge data generally fall between the upper swash zone and the coastal dune (in most instances closer to the upper swash zone). The statistical process, from surf to beach ridge to coastal dune, is one of filtering (but not mixing). Each filter is truncated. The total result is to improve the internal sorting from zone to zone, but the terminal distribution is nevertheless still not simple. The lack of simplicity aids in making an hydrodynamic interpretation.