Grain size and beach face slope on paraglacial beaches of New England, USA

Abstract

New England lies at the boundary between the Mid-Atlantic coastal plain and paraglacial lowlands. Beaches in this region are commonly composed of a mixture of sand and gravel, but how grain size distributions relate to beach face morphology remains unclear. To fill this important knowledge gap approximately 100 paired summer and winter transects of beach face slope and intertidal grain size are examined from 18 separate beaches in southern New England from meso- and micro- tidal regimes. Paraglacial materials provide the principal local sediment source to a majority of 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 sources 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 and 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. However, where gravel mixes with the sand and bulk D50 increases beyond ~1 mm there is little relation between slope and median grain size. This finding is consistent with previous trends observed in global beach data sets and highlights predictable limits of using bulk D50 to describe bimodal systems. Upon ignoring the gravel component from the grain size distribution and recomputing median grain size for the remaining sand fraction, the familiar positive relationship between grain size and slope reemerges. Results extend globally to the large subset of beaches composed of a mixture of sand and gravel and support sand characteristics as the predominant control on slope for these mixed systems.