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

Abstract

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.