Sedimentology and morphodynamics of a barrier island shoreface related to engineering concerns, Outer Banks, NC, USA

Abstract Forty-nine vibracores were collected from a barrier island shoreface following 12.4 years of biweekly profile surveying. The sedimentologic architecture of the shoreface was linked to time-series elevation change and profile shape to determine relationships between morphodynamics, facies development, erosional processes, profile closeout, and cross-shore transport. The modern shoreface mass, which erosionally overlies a tidal inlet-associated complex, attains a maximum thickness of 3–4 m below the beach to middle profile before pinching out seaward between 9 and 12 m depth. Concave erosional surfaces overlain by cross-stratified fine to medium sand and gravel make up most of the lower half of the shoreface prism below the beach through middle profile reflecting longshore trough incision and subsequent current-dominated aggradation. At the landward margin, gravel-rich laminae record episodic seaward progradation of beach surfaces over coarse inner-trough settings. Seaward, a parallel-laminated fine-sand facies dominates the upper part of the prism recording intermittent shoal zone buildup, including trough filling, under high-velocity plane-bed conditions. Similarly, a bioturbated parallel-laminated, fine- to very fine-sand facies makes up the entire prism below the outermost lower ramp sector, again indicating buildup under high-velocity conditions. However, accretion of the lower ramp results from major storms that cause trough scour along landward locations and simultaneous displacement of fine sand onto the lower ramp. Conversely, lower ramp erosion typically occurs during less energetic conditions as sediment is slowly returned shoreward causing inner-shoreface buildup. Close spacing of major storms during some years led to net progradation of the shoreface. The upper and lower limits of surveyed elevation change (ULe and LLe) repeatedly develop similar limit-profile shapes over shoreface accretion–erosion cycles. The ULe reflects accretion maxima resulting from beach, bar, and lower ramp buildup. The LLe and lower sedimentologic limit (LLs) along the inner 250 to 300 m of the active shoreface are a product of storm-trough scour down to a maximum depth of ∼5.5 m. Below the lower ramp facies, the LLe and LLs are primarily products of less energetic wave erosion down to ∼5 m (shoreward) and 9 m (offshore) depths. The LLe closely matches the LLs documenting that >90% of the shoreface prism was reworked during the 12.4-year period whereas actual ages for erosional events indicate a potential of 2 to 4 years for complete reworking of the shoreface mass. Textural distribution indicates net long-term transport direction and loci of deposition for different sized material. The coarsest material is concentrated at landwardmost locations and well-sorted fine to very fine sand at seaward locations. Medium sand to gravel tends to remain within the trough zone, even during extreme storm events. The ULe and LLe also represent the upper and lower limits for profile closure events. A location of about 4.5 m depth at 300 to 350 m from shoreline marks the boundary between inner profile- and lower ramp-associated closure events, the boundary between trough-associated and lower ramp facies, and the lower ramp morphologic break, all of which correspond to the juncture between longshore-current- and shoaling wave-dominated zones. The lower-ramp zone of closure is a zone of seaward decreasing storm transport in which fine to very fine sand is the typical bedload material.