COMPARING TRADITIONAL AND DIGITAL GRAIN SIZE ANALYSIS OF SURFACE AND SUB-SURFACE SEDIMENT FROM WALLOPS ISLAND, VIRGINIA

Digital grain size analysis (DGSA) is an emerging technique for determining sediment grain size distribution. DGSA uses autocorrelation between pixels in digital photographs of sediment to estimate the parameters of its grain size distribution. Major advantages of DGSA include: 1) substantially shorter processing time than traditional sieving techniques, 2) small sample size, 3) finer grain size bins, 4) continuous analysis of grain size change in sediment cores. In this study we apply DGSA to a time series of sand samples collected along the Wallops Island (WI) coastline. WI is a chronically eroding barrier island located on Virginia's Eastern Shore. The erosion, combined with sea level rise is causing WI's shoreline to retreat within meters of NASA's Wallops Flight Facility.

We collected surface samples from 31 beach face and dune sites between Sept. 2011 and Aug. 2014. This period includes a beach replenishment project and two significant storm events (Irene and Sandy). With this dataset we have the opportunity to explore DGSA as a more efficient means of gathering grain size data. Here we compare the results from traditional sieving (7 bin auto-sieve, covering 0.0625-2 mm) and DGSA techniques. High-resolution digital photographs were taken of sand samples prior to sieving and of each sieved fraction for calibration and verification with DGSA. Preliminary results show mean grain sizes determined using DGSA are comparable to sieving for sizes between 2 and 0.25 mm, without calibration. Uncalibrated DGSA mean sizes are consistently higher by 0.19 to 0.21 mm between 0.125 and 0.0625 mm size fractions. Further work will include calculating autocorrelation profiles for known size fractions in the WI region for calibration. We will explore the feasibility of using DGSA to determine grain size from photographs taken of the undisturbed beach surface. We will also use DGSA to analyze sediment cores, allowing for a detailed stratigraphic record of grain size change down core. Rapid and reliable determination of grain size distributions with DGSA could significantly reduce the cost and time required for large comprehensive analyses of sediment distribution and transport in the coastal zone.