Field Tracer Observation of Fragmented Sediment Retention in a Dynamically Unstable Coastal Environment under Anthropogenic Disturbance

Coastal sediment retention mechanisms are increasingly relevant under rising storm intensity and sea-level variability in extreme weather events, such as typhoons, tornados, tsunamis, earthquakes, landslides. Nearshore sandy beaches are often treated as depositional environments, but can also retain and repeatedly re-circulate material at very small spatial scales. Increasing anthropogenic disturbance has altered sediment supply, seabed structure, and nearshore circulation, potentially compromising this function. This study reports a field-scale tracer observation derived from the accidental loss and recovery of a dense consumer object (≈240 g) smartphone; sand-density when waterlogged) in a shallow nearshore environment in Okinawa, Japan. Despite exposure to strong wave forcing associated with distant typhoon swell, the object re-emerged approximately 14 days later within ~1 m of its initial loss point. Environmental data, physical damage patterns, and site geometry indicate that retention and return occurred only under conditions provided by a rock-bounded nearshore enclosure (~34 × 6 m), formed by large coastal rock masses (~3 × 6 m). Using field observations, density-matched dummy tests, metal detector operations, sediment stratification assessment, and reanalysis wind-wave-current data, the event is interpreted as evidence of a nearshore sediment retention cell governed primarily by oscillatory wave transport rather than net advection. This retention operated only within a locally semi-closed area that preserved natural wave-sediment interaction. Outside this enclosure, comparable objects were permanently lost to the open ocean and did not return. This observation suggests that subsurface sediment structure and bottom boundary layer processes played a decisive role in whether the object returned or was permanently lost. The findings reveal beach-scale retention has largely collapsed in the study area and now survives only as fragmented, conditional pockets. Loss of return mechanisms implies that debris and pollutants no longer reappear onshore for removal, instead dispersing irreversibly into the ocean and increasing long-term environmental risk. The study further discusses how anthropogenic disturbances-including dredging, seabed excavation, sediment extraction from both ocean and mountains, and coastal construction (~300-1000 m offshore)-may disrupt retention functions of nearby human, marine, natural environment, leading to irreversible sediment loss, increased erosion risk, and ecological degradation. Many coastal assessments rely primarily on remote sensing and plan-view data, which underrepresent subsurface sediment structure and benthic function. As a result, seabed areas that appear visually simple or biologically sparse may be dismissed despite playing critical roles in sediment retention, wave-energy dissipation, and coastal stability. This study highlights how such observational blind spots contribute to irreversible degradation before functional loss becomes detectable. Finally, a monitoring framework is proposed to address gaps in high-frequency, site-specific coastal data.