The Overlooked Role of Alongshore Beach Non‐Uniformity in Short‐Term Shoreline Variability

Abstract

Long‐term beach monitoring and satellite‐derived shoreline products are typically validated, and shoreline models calibrated, along cross‐shore transects. Open sandy beaches, however, are rarely uniform alongshore, because crescentic megacusps migrate past fixed transects, and their effect on single‐transect records remains unquantified. We combine a 30‐year, daily‐to‐weekly single‐transect record with monthly surveys across 11 transects spanning 400 m at Hasaki Beach, Japan, and use the alongshore range of shoreline position as a proxy for megacusp magnitude. Between monthly surveys, 70%–76% of single‐transect observations lie within this range and cannot be distinguished from alongshore non‐uniformity at a single transect. The range shows no significant correlation with concurrent wave forcing, and the skill of wave‐driven equilibrium models declines as the range grows relative to the single‐transect variability. Densely spaced transects are therefore needed to isolate the forcing‐driven cross‐shore response from megacusp‐induced variability, in both in situ and satellite‐derived shoreline analyses.