Georadar Analysis For Geomorphic Evidence Of Prehistoric Breaches At Nahant Tombolo, Massachusetts, USA

Abstract

Paraglacial coasts utilize their barrier systems as a defense mechanism for the mainland from the open ocean. Large storms, such as nor’easters and hurricanes, occasionally breach barriers, altering back barrier environments. Therefore, understanding both the nature and periodicity of such events is pivotal in maintaining coastline sustainability. In this study we investigated the Nahant, MA tombolo which serves as a barrier seaward of the Saugus River mouth. Nahant tombolo is a sand dominated barrier, spanning 2km long and 150m wide with a tidal range of 2.7m. Our goal was to observe the geophysical stratigraphy in order to identify potential storm driven breach channels, and to interpret them in the terms of the evolution of the tombolo. A GSSI Ground Penetrating RADAR (GPR) with a 350 MHz hyperstacking antenna was used to investigate the subsurface, and Radan was used for data analysis and processing. Subsurface profiles were generated along 14 continuous beach segments located on a paved path seaward of the done crest. Reflectors were compared to previous studies of subsurface RADAR facies. GPR records logged only three inlet features which were concentrated in a 300 m sector of the 2km tombolo. Inlet features include a basin fill, and two channel fill patterns including complex and accretionary. Prehistoric channels range from 3 to 6m in width and 5 to 6m in depth. Our data indicates that the Nahant tombolo has experienced at least three breaching events since its formation, possibly 3 to 5 thousand years ago. Such events are constrained to a specific stretch of the tombolo, and are infrequent in the geologic record.