BOSTON HARBOR SALIENTS: MORPHOLOGY, FORMATION AND EVOLUTION

Coastal salients are accretionary features that are common on the drumlin island shorelines in Boston Harbor. Although salients are found in a range of settings worldwide, there is a general lack of understanding of why and where they develop, how they migrate, and their stratigraphy and evolution. Studies of these features in Boston Harbor indicate that they form in response to a variety of processes including change in shore orientation, changes in wave approach directions or energy along the shoreline, or longshore sediment saturation. Salients in the Massachusetts Bay region form on fetch-limited shorelines where dominant waves approach the shore at a high angle. The correspondence of salients in these sheltered settings is associated with low-energy, short-period waves with steep angles to the shoreline, commonly coming from more than one direction. When there is a strongly dominant wave direction, the salients often migrate in the downdrift direction. Areas of bi-directional longshore sediment transport often occur in low-energy environments where fetches are limited. In particular, the lee side of islands is often a site for drift convergence or longshore saturation, resulting in salients that are more stable in position. Bathymetry combined with wind information, obtained from Logan Airport and the NOAA data buoy 44013, is used as input data to the SWAN (Simulating WAves Nearshore) wave model in order to correlate wave energy fluxes in the harbor with the distribution and behavior of salients. Sediment volume calculations of the salients are compared with local bluff erosion rates and composition in order to evaluate sediment transport rates as determined from historical aerial photographs and charts. These data are used to evaluate sediment transport rates as determined in the wave study. Ground-penetrating radar transects, stratigraphy determined from trenching, and sediment data allow us to construct a conceptual model of salient evolution.