PATTERNS OF EROSION, DEPOSITION AND SEDIMENT TRANSPORT IN ESSEX BAY, MA

During the past 25 years the southern end of Castle Neck Island, Massachusetts has retreated notably. The consequent evolution of inlet geometry has resulted in a dramatic shift in hydrodynamics within the Essex Estuary system, impacting morphology within the bay. Much of the sand eroded from the barrier is thought to have moved into Essex Bay, while large areas have been exposed to increased wave energy, affecting valuable shellfish beds and wetlands. The objective of this study is to identify recent patterns of net sand movement, in order to map and predict channel shoaling, enlargement or destruction of shellfisheries, and marsh resiliency.

A series of historical aerial imagery over the past 80 years were used to assess the evolution of the barrier and bay. The island has retreated 600 m in the last 25 years which has resulted in 1.3 x 10⁶ m³ of sand transported into Essex Bay. Concomitantly, this opening of the inlet has led to marsh erosion rates of over 1 m per year at some sites. Over 450 sand samples were gathered by hand, from along Crane Beach and in the nearshore within the bay, and using a grab sampler, from within the inlet and bay, including the main ebb channel, and across the flood tidal delta. Grain sizes analyses show that the sediment consist of medium to fine grain sands and suggest transport to the north and west within the bay. Hydrodynamic measurements of tidal currents and water depths collected within the Bay, and numerical model results, show strong ebb-dominant flows in deeper channels. However, sediment transport patterns suggest deposition in shallow and flood dominated areas. Growth of a flood delta formation beyond a bedrock constriction separating the outer and inner bay, suggests infilling both at the mouth and deeper into the Estuary.