Temporal and spatial variations of stranded debris on the sandy beaches of Sendai South Bay revealed by a monitoring method with an UAV.

Abstract

Using aerial photographs taken by a drone, i.e., unmanned aerial vehicles (UAV), we examined the seasonal and spatial variations of debris stranding on the sandy beaches of Sendai South Bay, which was damaged by the tsunamis caused by the Great East Japan Earthquake, 2011, We classified the debris into ten types, enumerated seasonally these from the shoreline to the embankment at two beach areas and described the seasonal changes in the abundance and spatial distribution patterns of the debris. Throughout the study period, the most abundant debris was driftwood, which was probably discharged from the rivers flowing into Sendai South Bay and accounted for 70％ of the total debris enumerated. Among the debris of anthropogenetic origin, i.e., marine garbage, plastic caps and bottles were the most abundant, followed by processed timbers, styrene forms and plastic bags. Most of the garbage was generally found within 30 to 60 m from the shoreline and increased in summer, while the driftwood was often found even near the embarkments and tended to accumulate on the beaches. The principal component analysis revealed that among the garbage, spatial and temporal variations of the plastic caps and bottles were somewhat related to those of the driftwood, suggesting the likelihood that the latter entraps the formers on the beaches. These results also indicate that aerial images taken by a UAV are useful for examining and quantifying the spatial and temporal variations of the marine debris drifting on beaches.