Geologic control in the nearshore: shore-oblique sandbars and shoreline erosional hotspots, Mid-Atlantic Bight, USA

Abstract

Beach and nearshore morphology, defined primarily by slope and sandbar development, is very dynamic and is largely controlled by waves, currents and regional sediment characteristics. Results presented here challenge this long-established concept and suggest that underlying, framework geology may also exert a first-order control on nearshore morphology by influencing the stability and/or persistent re-establishment of large-scale sandbar morphology and position as well as surface sediment characteristics. Repeated sub-bottom chirp and swath bathymetry surveys of the nearshore (2–10 m depths) covering over 56 km of the North Carolina Outer Banks and Southeastern Virginia indicate the following: (1) development of shore-oblique sandbars adjacent to large gravel outcrops that are surface exposures of the underlying geologic strata, (2) identical re-development or sustained maintenance of large-scale sandbar morphology and position before and after very energetic conditions, (3) vertical and horizontal heterogeneity of lithology and grain-size and a minimum volume of sand, ranging from 0 to 1.5 m thick, and (4) close spatial alignment between the location of outcrops/shore-oblique bars and shoreline erosional hotspots. A hypothesis is proposed from these findings that links framework geology to bar morphodynamics and sorted bedforms and, ultimately, erosional hotspots. Sediment transport and shoreline evolution models based solely on waves and currents and an assumption of unlimited and uniform sediment may be inadequate in similar heterogeneous, sand-limited regions.