STORM-DRIVEN SHORELINE DYNAMICS ALONG A DEVELOPED RIVER MOUTH BARRIER ISLAND: MULTI-DECADAL CYCLES OF EROSION AND MITIGATION

Human modifications in response to storm erosion have altered the natural transport of sediment to and across the coastal zone, thereby potentially exacerbating the impacts of future erosive events. Using a combination of historical shoreline-change mapping, sediment sampling, monthly three-dimensional beach surveys, and hydrodynamic modeling of nearshore and inlet processes, this study explores the feedbacks between periodic coastal erosion patterns and associated mitigation responses, focusing on the open-ocean and inner-inlet beaches of Plum Island and the Merrimack River Inlet, Massachusetts.

Installation of river-mouth jetties in the early 20^th century served to stabilize the inlet, allowing for the residential development of northern Plum Island, but triggering >100 yr of successive, multi-decadal (25–40 yr) cycles of beach erosion and accretion. Specifically, an erosion “hotspot” (a setback of the high-water line by ~100m) forms proximal to the inlet and migrates south along the beach over a period of 5–10 yr in response to the refraction of northeast storm waves around the ebb-tidal delta which is positioned more seaward than expected due the presence of constricting rock jetties. Growth of the delta progressively shifts the focus of storm wave energy further down-shore, replenishing updrift segments with sand through the detachment, landward migration, and shoreline-welding of ebb delta-associated swash bars. Monitoring of the most recent period of hotspot migration (2008–2014) demonstrated erosion of >30,000 m³ of sand along 350 m of beach in six months, followed by recovery, as the hotspot migrated further south.

Since the 1950s, in response to these erosion cycles, local residents and managers have attempted to protect the residential area with a variety of hard structures, which have provided protection to homes, but further enhance erosion elsewhere. Although there are general perceptions held by the local community about the need to plan for long-term coastal changes associated with sea-level rise and increased storminess, real-time mitigation tends to be reactive, responding to short-term (<5 yr) erosion threats. A collective consensus for best managing this area is lacking, and the development of a longer-term perspective needed for proper planning and adaptation has been elusive.