Overestimation of short-term coastal cliff retreat rates in the eastern Mediterranean resolved with a sediment budget approach

Our understanding of sea-cliff erosion processes and their response to recent and/or projected environmental changes such as sea-level rise, climate change and anthropogenic development hinges on our ability to quantify sea-cliff retreat rates and their variability through time. Here, we focus on Israel's Mediterranean ‘Sharon’ sea-cliff as a case study for examining the significance of recent short-term (i.e. annual to decadal) cliff-top retreat rates that appear to exceed longer-term rates of ‘background’ (i.e. centennial to millennial) retreat by 1–2 orders of magnitude. We demonstrate that an inherent sampling bias in rate estimates inferred from observation intervals shorter than process episodicity can also explain such a pattern. This potential ambiguity leads to a striking paradox where despite highly accurate and robust documentation of recent cliff-top retreat, such as that obtained from aerial photographs and/or instrumental surveys, the short-term retreat rates of episodically retreating sea cliffs remain poorly constrained. To address this key data gap along the Sharon sea cliff we employed a sediment budget approach that focuses on quantifying the continuous wave scouring of cliff-collapsed material from the shore platform as a rate-limiting process for episodic retreat of the cliff above. We used four high-resolution (0.5 m/pixel) airborne LiDAR data sets acquired between 2006 and 2015 to determine short-term maximum retreat rates of up to ~0.08 m/yr during this nine-year period. These modern retreat rates compare to the cliff's background retreat rate of 0.03 to 0.09 m/yr since the mid-Holocene, as determined herein from multiple geologic and archeological observations. Our results demonstrate that previously reported twentieth century cliff-top retreat rates for this sea cliff, which range up to values of several meters per year, are biased and that sea-cliff erosion rates have not yet been significantly impacted by recent environmental changes in the eastern Mediterranean basin, such as the restriction of sediment supply following emplacement of the Nile's Aswan dam system.