EROSION OR ERROR: ARE BEACH PROFILES AND THE MEAN HIGH WATER SHORELINE APPROPRIATE PROXIES FOR MEASURING SUBAERIAL VOLUME CHANGE?

Proxies, such as changes in beach profiles and shoreline positions, are commonly used for measuring changes in subaerial beach volume; however, the accuracy of these proxies at representing the true volume change is unclear. Volume change associated with along-beach variations in morphology may not be captured well by changes in beach profiles, while volume change associated with across-beach variations in morphology may not be captured well by displacement of the shoreline. This study assesses the impacts of variations in morphology associated with beach cusps and nourishment material on volume change measurements from profiles and shoreline change at varying time periods (0.5-3.5 years). Results indicate that it is unlikely cross-beach transects spaced ≥150 m apart will accurately measure volume change over both short- and longer-time frames at beaches with cusps, on the border of a nourishment area, and where high temporal variability in shoreline position exists. Changes in beach profiles accurately measure volume change over longer (~3.5 year) time scales at beaches with consistent magnitudes and directions of volume change. The shoreline change proxy works best at beaches with low temporal variability in shoreline position, at beaches where there are no significant morphologic changes to the backshore (e.g. beach cusp formation/destruction), and at beaches with ramp-like morphologies, which often exists after nourishment material is graded. Topographic data collection using high-resolution methods (e.g. LIDAR) is essential at beaches where volume change oscillates between erosion and accretion on both short and long time scales because the magnitude of small-scale changes in beach volume from cusps, berms, and runnels will always be similar to the long-term net volume change.