Monitoring sediment transport and grain size dynamics along the Israeli continental shelf with multibeam bathymetry and backscatter data

In 2017, the Israel Oceanographic and Limnological Research (IOLR) started an annual seafloor monitoring program. The aim of the program is to evaluate the rate of erosion/deposition and the influence of man-made infrastructures on the seabed along the Israeli continental shelf south of Akko. The survey program onboard R/V Bat-Galim includes a multibeam (Kongsberg EM2040), sub-bottom (Knudsen 3260 Chirp) mapping and box-core sediment sampling along 13 transects across the shelf, from WD 10-100 m. The multibeam was operated at 400-kHz yielding a horizontal resolution of 0.25-1.0 m (depending on water depth), and vertical uncertainty of several centimeters. Using the QPS FMGT software, both angular response curves (ARA) and 0.5 m horizontal resolution of Backscatter data (BS) were derived. The multibeam acoustic return intensities (BS) were locally calibrated at selected reference areas using in-situ sediment sampling. 
The main source of sediments along the Israeli continental shelf is the Nile Delta which undergoes erosion since 1960 when the Aswan dam was constructed. Along the Israeli inner-shelf, these sediments are transported northward and westward by wind-derived currents and storms. The analysis of the bathymetric surfaces from the consecutive years 2017-2020 shows that the shelf is stable in terms of sediment processes except along the marine infrastructures and natural seafloor features (e.g. rocky bottom outcrops) where patterns of sediment accumulation and erosion are observed. The variability along the marine infrastructures is mostly seen in the shallow water (less than 30 m) where yearly changes of up to +/-0.4 m of sediment accumulation/erosion in the vertical axis were measured.
The locally calibrated multibeam BS enabled grain size mode evaluation ranging from very fine gravel (-1 phi) to clay (9 phi). Additional in-situ sampling validated the reliability of the grain size classification method for the Israeli, continental shelf. Accordingly, we show that the Israeli continental shelf south of Haifa Bay is characterized by a sandy seafloor strip at WD 0-35 m and a muddy strip that extends west up to WD 100 m (in agreement with previous studies). Gravelly areas are identified at the coast-parallel Kurkar outcrops (Calcareous sandstone rocky ridges or rock patches) in water depths of 10-15m and 35-40m and in some places even at WD of 90 m. This demonstrates that grain size classification by locally calibrated multibeam BS is likely to be a very useful and fast method for monitoring changes in seafloor characteristics over large areas over time.