Challenges and new strategies in assessing multidecadal shore platform sandy beach evolution from aerial imagery

Abstract

The world's coastlines are dominated by rocky coasts, where embayed and shore platform beaches develop. The intrinsic characteristics of these environments make them extremely vulnerable to changes in oceanographic forcing. Assessing long-term beach evolution can be a challenging task because coastline indicators were developed for wide and low-lying sandy beaches. For example, as most shore platforms beaches are limited by a cliff or structure, they have a static landward boundary (coastline), thus inhibiting the use of any coastline indicator as a proxy of beach volume changes. Also, as shoreline position is highly dependent on short-term sea-level fluctuations, the use of shoreline indicators is frequently subject to high uncertainty.This work presents an unconventional way to evaluate shore platform beach evolution by proposing a new indicator - the beach toe (BT) - to assess changes in sand retention for shore platform/perched beaches. BT position, together with a profile model, provide an effective tool to assess beach volume over time.The proposed beach indicator was tested at 12 shore platform beaches located at Cascais coast (Portugal), which can be considered illustrative of this type of beach.Results show that the new approach provides an objective indicator to assess both the beach configuration and changes in volume, being easily implemented using common vertical imagery. The comparison of Beach Toe and High-Water Line indicators with field data show a better performance of the BT indicator, supporting the use of the proposed indicator on shore platform beaches. The proposed analysis can be easily replicated to other similar rocky coasts, thus representing a much-needed advancement in the study of beaches perched on platform shores.