Seasonal Changes in Sediment Grain Size of New Hampshire Atlantic Beaches: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report

TitleSeasonal Changes in Sediment Grain Size of New Hampshire Atlantic Beaches: BOEM/New Hampshire Cooperative Agreement (Contract M14ACOOO10) Technical Report
Publication TypeReport
AuthorsWard, LG, Corcoran, NW, McAvoy, ZS, Morrison, RC
Number of Pages208
Organization NameDepartment of Interior, Bureau of Ocean Energy Management, Marine Minerals Division
LocationSterling, VA

The beaches along the New Hampshire Atlantic coast are essential to the local and regional economy and are one of the major attractions of the seacoast. Beyond their economic importance, the beaches also have great aesthetic and ecological value that are vital to the character and history of New Hampshire. Unfortunately, climate change and an acceleration in sea-level rise, coupled with a major reduction in sediment supply and extensive development (including engineering structures along the coast), has led to loss of elevation and narrowing of many of the beaches. The forecast is that these trends will continue and likely become worse. It is also very likely that engineering solutions will be sought to reduce the impact of sea-level rise and coastal erosion in the near future as the loss of the beaches become more critical and coastal flooding becomes a more frequent threat.

An option that will undoubtedly play an important role in efforts to mitigate the impacts of beach erosion, flooding and storm damage is beach nourishment. Essential to beach nourishment success is a thorough understanding of the natural sediments that compose the beach. This includes studying the grain size distribution under low energy conditions (typically summer) when the beaches tend to be accretional, and under higher energy conditions (typically winter and stormy periods), when the beaches erode and finer sediments are winnowed.

A preliminary inventory of the grain size of the natural sediment composing the major New Hampshire beaches was carried out by Ward et al. (2016). However, this study was conducted in summer 2015 after a prolonged period of accretional or stable conditions. In addition, samples were taken only in the upper ten centimeters of the sediment column. Here, a seasonal study (completed in 2017) of sediment grain size from seven major New Hampshire beaches is presented. A total of twenty-eight elevation profiles were measured and one hundred forty sediment samples collected at cross-shore transects in late winter – early spring following an extended period of beach erosion. In late summer twenty-two of the profiles were rerun and ninety-seven sediment samples collected following an extended period of accretion. Six stations were not rerun due to a late summer storm which eroded the beach. The samples were collected along shore-normal transects from the seawall or foredunes to the low tide swash. Large samples were typically collected (~1 kg to 24 kg) from the upper 20 to 30 cm of the sediment column.

Results of cross-shore elevation profiles at each beach verified that all locations sampled in late winter – early spring 2017 had been eroded by winter storms and often had sediment lag deposits. Conversely, all the beaches sampled following the summer accretional period had recovered and gained elevation. Along with the deposition of sediment there was a general fining of grain size, especially at bimodal beaches. This decrease in grain size by late summer was related to the deposition of fine to medium sand that migrated onshore, often in ridge and runnel systems. The bimodal beaches tended to show the largest change in grain size overall due to scattered pebbles or pebble lag deposits being buried by the sandy accretional wedge.