Shoreline Dynamics Along a Developed River Mouth Barrier Island: Multi-Decadal Cycles of Erosion and Event-Driven Mitigation

摘要: Human modifications in response to erosion have altered the natural transport of sediment and across coastal zone, thereby potentially exacerbating impacts future erosive events. Using a combination historical shoreline-change mapping, sampling, three-dimensional beach surveys, hydrodynamic modeling nearshore inlet processes, this study explored feedbacks between periodic patterns associated mitigation responses, focusing on open-ocean inner-inlet beaches Plum Island Merrimack River Inlet, Massachusetts, USA. Installation river-mouth jetties early 20th century stabilized inlet, allowing residential development northern Island, but triggering successive, multi-decadal cycles alternating accretion along oceanfront beaches. At finer spatial scale, formation southerly migration an “hotspot” (a setback high-water line by ~100 m) occurs regularly (every 25–40 yr) refraction northeast storm waves around ebb-tidal delta. Growth delta progressively shifts focus wave energy further down-shore, replenishing updrift segments with sand through detachment, landward migration, shoreline-welding swash bars. Monitoring recent hotspot (2008–2014) demonstrates (>30,000 m3 sand) 350-m section six months, followed recovery, as migrated south. In these cycles, local residents governmental agencies attempted protect shorefront properties variety soft hard structures. The latter provided protection some homes, enhanced elsewhere. Although community is broad agreement about need plan for long-term changes sea-level rise increased storminess, real-time responses involved reactions mainly short-term (<5 threats. A collective consensus sustainable management area lacking longer-term adaptive perspective needed proper planning has been elusive. With deepening understanding dynamics, including characterization relative contributions both nature humans, we can be more optimistic that adaptations beyond mere shoreline change are achievable.