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.

世界的海岸线被多岩石的海岸所占据，多石的海岸形成了海岸线，并形成了岸台平台海滩。这些环境的固有特征使其极易受到海洋强迫变化的影响。由于海岸线指标是针对宽阔和低洼的沙滩开发的，因此评估长期海滩演变可能是一项艰巨的任务。例如，由于大多数岸台的海滩都受到悬崖或结构的限制，因此它们具有静态的陆上边界（海岸线），因此禁止使用任何海岸线指标来替代海滩数量的变化。此外，由于海岸线的位置高度依赖短期海平面的波动，因此使用海岸线指标经常会面临高度不确定性。

这项工作提出了一种新的评估指标，即提出一种新的指标-沙滩脚趾（BT）-评估海岸平台/栖息海滩的固沙变化，从而以非常规方式评估海岸平台海滩的演变。BT的位置以及轮廓模型提供了一种有效的工具，可以随着时间的推移评估沙滩的数量。

拟议的海滩指标在位于卡斯卡伊斯海岸（葡萄牙）的12个岸台海滩上进行了测试，可以认为是此类海滩的例证。

结果表明，该新方法提供了一种客观的指标，可以评估海滩的配置和体积的变化，可以使用常见的垂直图像轻松实现。沙滩趾和高水位线指标与现场数据的比较表明，BT指标具有更好的性能，支持在海岸平台海滩上使用建议的指标。所提出的分析可以轻松地复制到其他类似的岩石海岸，因此代表了研究平台海岸上的海滩时急需的进步。