TEMPORAL AND SPATIAL VARIABILITY IN SEDIMENT TEXTURE ON NORTHERN, PARAGLACIAL BEACHES: NEW HAMPSHIRE

Typical of paraglacial environments, beaches along New Hampshire’s (NH) ocean coast are strongly influenced by bedrock geology, previous glaciations and associated deposits, and a complex sea-level history. As a result, the morphology and sedimentology of the beaches are highly variable and exhibit rapid spatial and temporal changes over the ~30 km coastline. Field observations from this and earlier research indicate sediment composing NH beaches varies spatially between two main beach morphologies. North of Great Boars Head (a large glacial drumlin), pocket beaches and welded barriers tend to be bimodal sands with larger pebble populations. South of Great Boars Head, the coast is an extended barrier spit/island system composed of unimodal medium and coarse sand. The offshore bathymetry has a major influence on the morphology and sedimentology of the northern beaches. Recently mapped bathymetric highs extending from headlands, likely interrupt longshore sediment transport, confining sediment movement to predominantly onshore-offshore. The barriers to the south (Hampton Beach and Salisbury Beach) exhibited rapid temporal change, accumulating finer-grained sediments during extended periods of accretion (typically summer) and significantly eroding during extended periods of erosion (typically winter). The coarser grained, pebble rich beaches north of Great Boars Head tend to have lower volumetric change over time. However, finer sediments tend to accumulate during extended calm conditions, burying the coarser, more gravelly sediments. To quantify changes in grain size spatially and temporally on NH beaches, extensive sediment sampling was conducted during 2017 in spring/early summer and late summer/fall with support from Bureau of Ocean and Energy Management (BOEM) and the University of New Hampshire. Twenty-four shore-perpendicular transects spaced over seven major NH beaches were sampled, resulting in ~240 large volume sediment samples. This new database when fully analyzed, combined with beach profile monitoring since 2015, will allow a better understanding and quantification of the stability and the textural characteristics of the NH coast. Ultimately, this will provide important input to beach management decisions concerning beach nourishment or other engineering modifications.