Although the beaches of New England are well known to the millions of Americans who flock to them each summer, these sedimentary systems have been, until very recently, shrouded in a geological mystery. Prior to the recent publication of a Marine Geology article by NE CASC Principal Investigator Jon Woodruff and his collaborators, the factors governing the extent of beach slope--or which beaches descend gradually to the sea and which ones end abruptly in a steep drop-of--were largely unknown. The findings from Woodruff's study, which decisively unravel this mystery, are critically important for understanding how New England’s beaches will respond to the impacts of climate change, particularly rising sea levels and increased storm activity.
Many of New England’s beaches are composed of a mixture of sand and small stones. More precisely, the grain sizes on these beaches are “bi-modal” – composed of very large pieces of gravel, from 10 to 64 millimeters, and medium-to-coarse sand, from .25 to 1 millimeter, but with very little in between. As researchers have long known, grain size is one of the crucial determinants of a beach’s slope. As grain size becomes smaller, its pitch becomes more gradual--up to a point.
“The relationship between grain size and slope falls apart for coarser-grained beaches,” says Woodruff. Though many New England beaches are typically made up of coarse-grained particles, they still slope gradually to the water’s edge. Until now, no one knew why. “Past researchers have always focused on either the mean or median grain size,” says Woodruff. It’s a method that works well for finer-grained beaches. But in a bi-modal, New England beach, the median grain size falls right in that gap between 1 and 10 millimeters.
To unravel the mystery surrounding the New England beach slope, Woodruff and his team took over 1,000 samples from 18 beaches in Massachusetts from which they assembled the largest, publicly available dataset on these areas. Relying on this comprehensive data, Woodruff’s team discovered that in bi-modal beaches, it’s only the finer-grained sand that determines a beach’s slope. “That smaller handful of sand grains,” says Woodruff, “is why beachgoers have a place to sunbathe in New England.”
This new research, which was conducted in partnership with the Massachusetts Office of Coastal Zone Management and supported by the federal Bureau of Ocean Energy Management in addition to NE CASC, has far-reaching implications. “Understanding how beach sand grain size influences the makeup of our beaches is critical for making projections as to how beaches will respond to storms and sea-level rise,” says Woodruff. “Especially given the attempts to preserve beaches from erosion, which cost many millions of dollars every year, we need to know what determines the shape and defining grain size characteristics of these beaches.”
This article was adapted from the UMass Amherst website.