Rocky coastal cliffs reinforced by vegetation roots and potential collapse risk caused by sea-level rise

Evaluation of the effect of vegetation roots on the stability of rocky coastal cliffs is necessary to understand geomorphological, environmental and ecological changes in coastlines caused by sea-level rise (SLR). To better understand these topics, coastal cliffs with and without vegetation were investigated. Study cliffs were selected on Taketomi-jima Island and Kuro-shima Island, both in Okinawa, southwestern Japan, because their environmental settings are similar except for the vegetation cover: the islands are only 10 km away from each other and comprise uplifted coral limestone, with an elevation below 5 m. Pandanus odoratissimus (PO) covers the seaward edge of the top surface of the coastal cliffs in Taketomi-jima but is absent in Kuro-shima. Field investigations were conducted on both islands to map the distribution of blocks with the dimension of one side exceeding 1 m to obtain the parameters required for stability analysis. The results indicate that the actual coastal cliff elevation is higher in Taketomi-jima than that in Kuro-shima by approximately 1 m, which results in the presence of PO on the seaward edge of the coastal cliffs in Taketomi-jima and its absence in Kuro-shima. The PO root system likely reinforces the cliffs in Taketomi-jima because the stability analysis results revealed that these cliffs are unstable in terms of their dimensions. Therefore, the possible disappearance of PO in Taketomi-jima caused by an SLR of 1 m in the following several decades or hundred years can increase the apparent frequency of cliff collapses by approximately tenfold.