Relationship of Grain Size to Slope in Bimodally Sourced Beaches

Abstract:

Beach slope and grain size are defining features of beach morphology and have long been an area of focus with respect to oceanographic and geologic controls. Sandy beaches have been studies in detail; however, less understanding exists for mixed sand and gravel systems (MSG). Here we present approximately 100 paired summer and winter transects of intertidal grain size and slope from 18 separate MSG beaches in southern New England. Postglacial materials provide the principal local sediment source to beaches in this region and are the predominant control on their sedimentary characteristics. Stratified drift and outwash are the primary sediment sources to sandier beaches, while till predominantly source the coarser gravel-dominated systems. When aggregated, grain size measurements exhibit a bimodal distribution of medium sand and medium gravel with an absence of grains between 1-10 mm. This bimodality is also common to and inherited from the post-glacial and fluvial deposits sourcing the beaches. Beach face slope is observed to increase with median grain size for finer sandy systems, followed by little-to-no correlation for coarser mixed sand-and-gravel beaches where bulk D₅₀ is greater than ~1 mm. This finding is consistent with previous trends observed in global beach data sets and highlights the limits of using bulk D₅₀ to describe bimodal systems. When gravel is removed from the grain size distribution and the median grain size recomputed for the remaining sand fraction, beach slope exhibits a positive correlation with median grain size, suggesting that characteristics of sands predominantly control processes governing cross-shore intertidal beach morphology for mixed sand and gravel systems. Not until a relatively high fractional content of gravel becomes available does beach face slope again begin to correspond to the bulk median grain size. We suggest that grain size distributions of sand serve as the primary governor of both beach face permeability and transport in bimodal systems, which together result in sand having the greater control on beach morphology for mixed sand and gravel systems.