Temporal and spatial relationship between sediment grain size and beach profile

Abstract

Sediment samples and beach profile evolution data collected along one profile line at “El Puntal” Spit, Santander, Spain, are used to analyze the spatial and temporal structure of the grain size distribution variability and its relationship with the beach profile changes. Standard principal component analysis (PCA) and three-way PCA is applied to determine the temporal and spatial scales of variability of the data. Results indicate that the sediment grain size distribution varies markedly along the beach profile both spatially and temporally. These variations are shown to be strongly related to morphological changes in the beach profile. The spatial eigenvectors determined from the profile data and those from the sediment data exhibit similar patterns with their maxima and minima located at the same position. Since eigenvectors may be regarded as representative of uncorrelated modes of variability it is concluded that the spatial variability of both sediment and profile data are strongly related. In particular, it is shown that the location of the highest variability of grain size corresponds to that of the beach profile. Also, different grain sizes are shown to exhibit a distinct degree of variability which leads to the conclusion that each sediment size responds to the same hydrodynamics differently. The temporal eigenvectors determined from the profile and the sediment data shown a seasonal dependency. However, their maxima and minima are not located at the same position. It is shown that this temporal shift is due to the different response of each sediment size to the hydrodynamics, and in particular, that the recovery of the profile starts with fine material from the bar. It is inferred that models for beach profile evolution which do not take into account the sorting processes involved in the sediment transport cannot be fully succesful. A “master” grain size sample, constructed by adding all the grain samples taken over the profile, is used to further examine the cross-shore redistribution of the sediment. The following working hypothesis is suggested: “For a beach profile within a physiographic unit the master grain size does not depend on time”.