AN INLET-CLOSURE MODEL ALONG A WAVE-DOMINATED BARRIER ISLAND: HISTORICAL GREEN RUN INLET, ASSATEAGUE ISLAND, MARYLAND

Upon tidal-inlet closure, many former channel zones become sites of back-barrier sedimentation, aeolian reworking, and vegetation overgrowth, which tend to mask the final stages of inlet evolution. This is true not only for relict, but also for many historical systems, such as Green Run Inlet along Assateague Island, ~3 km north of the MD-VA border. The inlet was well documented as open on NOS T-Sheets and nautical charts from 1852 to 1880. This former inlet has one of the best preserved flood-tidal deltas along the barrier. A series of supratidal ridges within the former channel zone, which are separated by swales and cat-eye ponds, extend southward from the northern extent of the former inlet and were previously identified as inlet-closure ridges. The present study addresses the origin of these unique geomorphic features through both surficial and subsurface approaches. A topographic analysis of geomorphic features through satellite imagery and LIDAR surveys, as well as high-resolution GPR and sediment cores, formed the basis for an inlet-closure model and aided in refining the genesis model of the intra-channel ridge complex. Stage 1 of inlet closure is delineated by the formation of three drift-aligned, concave-landward ridges (recurved spits), with the curvature radius of each ridge increasing seaward. Stage 2 is represented by an intermediate ridge, which transitions from a drift- to swash-aligned, concave-seaward ridge. During this stage of inlet evolution, the channel rotated 30° counterclockwise causing the associated flood ramp to extend southward. In response, the inlet channel lengthened and narrowed, and thus experienced increasing hydraulic inefficiency that caused the inlet to wane and eventually close. With a series of sigmoidal to tangential-oblique reflections (~5-m-deep conformable fill pattern), GPR records reveal a southerly channel migration of 700 m in the direction of net longshore sediment transport. Stage 3 of inlet closure is marked by four swash-aligned, concave-seaward ridges (prograding beach ridges) with an increasing radius of curvature seaward that extend across the final channel position. Stage 4 marks the subsequent progradation and eventually transitioning back to a drift-aligned orientation through beachface accretion and foredune formation along most of the island.