engleski

Comparing grain-size distribution from digital image analysis on natural and artificial coarse-clastic beaches

This study presents new data on grain size distribution and dynamics from automatic digital image analysis of pebble beaches. Since coastal managers responding to strong erosion effects with replenishment strategies must be chiefly concerned with sediment qualities rather than aesthetic concerns to avoid repeated erosion events, it is essential that the research community provide these stakeholders with high quality data on the behavior of beach-fill sediments in different environments and weather conditions. To achieve this goal, we define the grain-size distribution of two coarse-clastic beaches, one artificial (Marina di Pisa, Italy) and one natural (Srebrna, Croatia), in order to compare the evolution of sedimentological parameters in the short and long-term. These beaches are similar in configuration, being confined on both sides, respectively, by artificial groynes and natural cliffs ; no offshore structures are present at either site. Both beaches are constituted by pebbles 40 to 90 mm in diameter. The major difference between the two sites is length: 180 m at Marina di Pisa and about 50 m at Srebrna. As traditional grain-size analysis techniques for coarse sediments are complex and extractive (sieving) or extremely time-consuming (caliper measurements), we use digital image analysis to quantify grain-size along three transects at each site spanning the entire backshore. An iron square for size calibration was placed at 45 cm intervals along the transects and photographed. The three-dimensional positions of each transect were also recorded on a DPGS. The images were then analyzed with pyDGS, an open source python framework for digital grain size estimates, which scans lines of pixels looking for monochromatic changes in intensity and then uses wavelet analysis to provide mean grain size, standard deviation, and a number of other statistics for each image. We use the R statistical computing environment to combine the granulometry data with the transect height profile obtained from the DGPS, then perform quantitative and qualitative analysis in terms of coastal sediment dynamics. In Pisa, this method has also been extended to the sub-aerial environment by a diver using a tape measure and depth gauge. Early results indicate that grain-size tends to decrease after medium-energy events at both beaches, inferring a seaward shifting of coarser particles. Even though longshore transport is not negligible, no major differences in grain-size are reported between the transects in consecutive surveys, which suggests that longshore movement mainly involves particles of similar size. This tendency has already been reported at several mixed sand and gravel beaches, where gravel moves down the beachface during storms and back up during storm decay. Since the steepness of the investigated beaches is significant, such behavior cannot be observed here ; coarser pebbles that can be found at the higher level of the backshore are likely transported there during highest energy events. This study improves our understanding of grain-size distribution and evolution on coarse-clastic beaches in the short and long-term, which can inform the selection of material for artificial beach replenishment to achieve better outcomes, minimize future erosion issues, and generally increase benefit to cost ratios.

artificial, beach, grain size, natural

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