Polymodal Composition of Beach Sands from the East Coast of India

Abstract

ABSTRACT The size frequency distribution of subaerial sands of an actively eroding beach situated in the east coast of India is polymodal in nature. The degree of polymodality varies over different parts of the foreshore. The graphic mean size of sediments lies within 1.0 and 4.0 phi values. Analysis shows that the regressions of inclusive graphic standard deviation and inclusive graphic skewness on graphic mean size, determined separately for the different size groups, viz medium sand (1.0 phi to 2.0 phi), fine sand (2.0 phi to 3.0 phi) and very fine sand (3.0 phi to 4.0 phi), are different. Medium-grained sands are positively skewed; sorting improves with increasing size. By contrast, very fine sands are mostly negatively skewed; sorting improves with decreasing size. For the fine sand group the mean size and sorting is moderately correlated; however, the attitude of the regression lines in this and in the very fine sand group is somewhat similar. Sands of all the size groups when considered together exhibit an improvement in sorting and a change of skewness from positive to negative values with decreasing size. The distribution represents mixing of population and the polymodality depends on it. The ultimate nature of these mixed sediments is related to: 1) the permeability of the beach surface, 2) the availability of a wide range of sizes of sand particles, and 3) the original character of incoming sediments. Supply of a wide range of particle sizes in the study area has mostly resulted from extensive erosion of coastal dunes and marine terraces bordering the coastline. However, on an aggrading coast one may expect different results.