Quantifying the Overwash Component of Barrier Island Morphodynamics: Onslow Beach, NC

A quantification of the role that barrier island overwash plays in the evolution of Onslow Beach, a barrier island located on Marine Corps Base Camp Lejeune, North Carolina, is presented. Ground-penetrating radar (GPR) and sediment vibracores provide an estimate of the relevant-sand prism above a silty/peat contact underlying the island. The average thickness from the surface, as determined from lidar, to this geologically-defined base, is less than 1 m and equates a total volume of approximately 1.8 ± 1.1 × 106 m3 over the 4.8 km stretch of Onslow Beach from 1 km north of the New River Inlet to Riseley Pier (~ 2 km2). Approximately 39% of the relevant-sand prism (680 ± 215 x 103 m3) is contained within the area of the island currently exhibiting signs of overwash events (i.e., the active overwash complex). Based upon the average cumulative thickness of distinct washover facies within 12 sediment cores (52 cm) and the surface area of the active overwash region, it is estimated that the volume sedimentologically distinct washover deposits equals 199 ± 88 × 103 m3 (approximately 29% of the active overwash complex or 11% of the entire relevant-sand prism). A time series of aerial imagery from 1938 to 2008 details the spatial and temporal trends in migration of both the wet/dry line (a shoreline proxy) and the vegetation line (indicating the landward extent of overwash). Long-term shoreline erosion rates in excess of 3 m/yr occurred over the southern portion of Onslow Beach while the northern portion experienced up to 1.7 m/yr of accretion within the same 80-year time span. Between 1938 and 2008, the vegetation line moved an average of 85 m landward over the length of the entire island and over 450 m in overwash sites at the southern end of the island where shoreline erosion rates are highest. A comparison with long-term shoreline change rates suggests that a simple linear relationship between spatial and temporal variability in shoreline behavior and volume of the relevant-sand prism does not exist. Trends based upon the past 80 years suggest that a positive correlation exists between storm frequency and overwash extent. Furthermore, the region experiencing the highest rates of shoreline erosion and the highest occurrence of overwash does not coincide with the area regularly subject to military training activities. These data suggest that natural forcings (sea level, wind and wave energy, geology, etc.) exert first-order control on the evolution of this barrier island. The ability to quantify and evaluate the relative importance of such forces is paramount to understanding how, and over what timescales, the nearshore environment responds to changes in external forcings (e.g., sea-level rise, storms, etc.) and, in turn, is fundamental to the development of reliable forecasts of shoreline trends and storm susceptibility models