A model for wave abrasion on underwater bedrock, with an application to rapidly downwearing tephra cones adjacent to Surtsey Island in Iceland

• Published in: Earth Surface Processes and Landforms
• Main Author: Sunamura, Tsuguo
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2021
• Subjects: Earth and Planetary Sciences (miscellaneous); Earth-Surface Processes; Geography, Planning and Development; Iceland; Surtsey
• Online Access: http://dx.doi.org/10.1002/esp.5128; https://onlinelibrary.wiley.com/doi/pdf/10.1002/esp.5128; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/esp.5128

Abstract Wave abrasion is a major erosional process shaping shallow‐water coastal bedrock morphologies. In spite of various erosion studies having been conducted on underwater bedrock features, modelling research for wave‐induced abrasion is limited. Based on an existing formula for bedrock abrasion by fluid flows, this study obtains a predictive equation for the rate of wave abrasion, which is given by a function of a shear stress of wave‐induced, oscillatory bottom flow and the resistivity of bedrock. A model is derived from this equation to describe temporal change in abrasion depth of subaqueous bedrock. The model is applied to rapidly abrading summits of two submarine tephra cones, Jólnir and Syrtlingur, both residing adjacent to the Surtsey volcanic island in Iceland. Nearly horizontal platforms develop as a result of abrasion. Jólnir is a site exposed to storm‐wave attack, while Syrtlingur is a sheltered site. The modelling based on design storm waves is found to well describe temporal change in erosion depth in the middle to late stage of the platform evolution at the two sites. The result of model calculations indicates that: (1) deep‐water wave energy of waves causing abrasion at Syrtlingur is 33% that of waves responsible for abrasion at Jólnir; (2) the efficacy of abrasion occurring at Syrtlingur is about four times higher than that at Jólnir; and (3) the wave base (i.e. the maximum water depth for bedrock erosion) is 51.5 m for Jólnir and 34.4 m for Syrtlingur.