Impacts of Transect Location and Variations in Along-Beach Morphology on Measuring Volume Change

THEUERKAUF, E.J. and RODRIGUEZ, A.B., 2012. Impacts of transect location and variations in along-beach morphology on measuring volume change. Journal of Coastal Research, 28(3), 707-718. West Palm Beach (Florida), ISSN 0749-0208. Real Time Kinematic-GPS profile surveys are currently the most common method used by engineers and researchers for monitoring beach erosion. This study assesses the accuracy of volumetric-change measurements based on profile surveys at various beach morphologies along Onslow Beach, North Carolina. High-resolution digital elevation models (DEMs) were created from topographic data collected using a three-dimensional terrestrial laser scanner at six similar to 150-m-long focus sites at annual, seasonal, and storm time intervals. Profiles were extracted from the DEMs every 0.5 m along the beach, a distance equal to the grid size, and each profile was independently used to measure volumetric change at each site. Along-beach variability in the measurements of volumetric change was analyzed to test the assumption that one transect can be used to determine volumetric change for a similar to 150-m stretch of beach. Results show that the accuracy of profile-based volumetric change decreases as along-beach morphologic variability increases. At sites with beach cusps, beach nourishment, and pockets of anomalous erosion and/or accretion, less than 5% of the similar to 300 transects accurately measure volumetric change to within +/-10% of the true volumetric change. At the site with the lowest along-beach morphologic variability, that number only increased to 35% of the similar to 300 transects. Three-dimensional surveys or closely spaced beach profiles should be employed at morphologically variable, and/or recently nourished or engineered, beaches to accurately quantify erosion and accretion over short timescales.