ABSTRACT

A statistical study of foreshore sediments from two adjacent open-ocean pocket beaches near Niteroi, state of Rio de Janeiro, Brasil, was undertaken to investigate possible sand leakage around the rocky headland separating the beaches. According to May and Tanner, the rocky headland theoretically could act as a cell boundary. A statistical analysis should reveal whether or not the rocky headland actually is a cell boundary.

Three samples collected from beach no. 1 (E. of headland) and seven samples collected from beach no. 2(W. of headland) were sieved and the four grain-size moment measures were determined for each of the samples. The moment measure analysis was marked by the following pertinent points:

1. Mean grain-size decreases in both directions away from the headland.
2. Mean grain-size for beach no. 1 (1.577 phi) is significantly different from the mean grain-size for beach no. 2 (1.297 phi).
3. Sorting for both beaches appears to improve weakly towards the headland (0.370±0.020 to 0.450 ± 0.020).
4. Skewness increases from a negative minimum (-0.150 ± 0.020) away from the headland to a positive maximum (0.100 ± 0.020) at the headland.
5. Kurtosis decreases away from the headland (1.000 ± 0.020 to 0.200 ± 0.020).
6. Two-factor regression analysis of the four moment measures (y) vs. linear distance (x) away from the headland revealed weak negative linear trends for mean grain-size and sorting, and strong negative linear trends for skewness and kurtosis.
7. Analysis of variance indicated that the variation in mean grain-size is significantly different between the two beaches.

Interpretation of the statistical analysis leads one to conclude that the rocky headland is a cell boundary (e.g. little or no leakage) separating two pocket beaches, each of which is in dynamic euqilibrium.