Impacts of transect location and variations in along-beach morphology on measuring volume change

Real Time KinematicGPS 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 ∼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 ∼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 ∼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 ∼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.