Evaluation of annual mean shoreline position deduced from Landsat imagery as a mid-term coastal evolution indicator

• Annual mean shorelines allow correctly quantify mid-term beach changes.
• Tens shorelines can be automatically obtained from Landsat imagery.
• 270 shorelines automatically obtained from Landsat imagery have been analysed.
• The proposed method improves both run-time cost and visual analysis.
• Landsat shorelines lay about 4 or 5 m seaward.

The shoreline is a useful indicator of mid-term coastal evolution. Every shoreline is affected by instantaneous sea-level, the length of the run-up, and beach profile changes. In this work, annual mean shorelines are evaluated in a manner that avoids these effects by averaging the instantaneous shoreline positions registered during the same year. A set of 270 shorelines obtained from Landsat imagery between 2000 and 2014, using the method described in Pardo-Pascual et al. (2012), have been used. It has been shown that the use of annual mean shorelines enables the same rate of change to be obtained as when using all the shorelines, but that the data is simpler to manage and more useful when visualising local changes. It has also been shown that annual mean shorelines largely remove the short-term variability, and are therefore useful for analysing mid-term trend quantifications. In addition, we propose a methodology for annual mean shorelines, obtained from Landsat imagery, that minimises the effects of sea-level variation on the shoreline positions. Both shorelines – instantaneous and mean annual – appear to be about 4 or 5 m seaward from those obtained using more precise sources.