Grain Size and Beach Face Slope on Paraglacial Beaches of New England, USA

Approximately 100 paired summer and winter transects of beach face slope and intertidal grain size were examined from 18 separate beaches in southern New England that span meso- and micro- tidal regimes. Paraglacial materials provide the principal local sediment source to beaches in this region and grain-size distribution of beaches corresponds to adjacent surficial geology. Stratified glacial fluvial deposits are the primary sediment source to sandier beaches, while till predominantly source the coarser gravel-dominated systems. When aggregated, grain size measurements exhibit a bimodal distribution of medium-to-very-coarse sand (0.25-to-1 m...  more

Approximately 100 paired summer and winter transects of beach face slope and intertidal grain size were examined from 18 separate beaches in southern New England that span meso- and micro- tidal regimes. Paraglacial materials provide the principal local sediment source to beaches in this region and grain-size distribution of beaches corresponds to adjacent surficial geology. Stratified glacial fluvial deposits are the primary sediment source to sandier beaches, while till predominantly source the coarser gravel-dominated systems. When aggregated, grain size measurements exhibit a bimodal distribution of medium-to-very-coarse sand (0.25-to-1 mm) and medium-to-very-coarse gravel (10-to-64 mm), with a paucity of grains between 1-10 mm. This bimodality is also common to and likely inherited from the glacial fluvial deposits sourcing the beaches. Beach face slope is observed to increase with median grain size (D50) 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 the familiar positive relationship between grain size and slope reemerges. Results support the growing appreciation for sand characteristics as the primary governor of intertidal slope for mixed sand and gravel systems due to its predominant control on beach face permeability and resulting transport processes.