Modelling the processes of cliff-top erosion and deposition under extreme storm waves

At exposed sites on the deep water coasts of the British Isles, cliff-top storm deposits (CTSDs) occur on cliffs at a variety of elevations above sea level and may occasionally reach up to 50 m above sea level. Time-series field mapping of CTSDs has demonstrated their formation over the historical period and their continued modification during major past storms. This paper seeks to clarify the morphogenetic context of CTSDs, model the wave conditions and forces encountered at the cliff-face and cliff-top platform and propose mechanisms to link wave processes to cliff-face quarrying and landwards cliff-top transport of quarried blocks to deposition zones at the rear of the cliff-top platform. We report on wave-tank experiments using scaled cliff and wave conditions from a cliff in the Shetland Islands to focus on three situations: incident waves lower than the cliff edge height; at the same height as the cliff edge height; and higher than the cliff edge height. The modelling suggests that steep waves of 10 m and above impacting on a 15 m high cliff will result in impact pressures sufficient to promote crack propagation, block detachment and lifting of large blocks. Large, but not necessarily steep, waves of the same height as the cliff edge produce sufficient impact pressures and water flow over the cliff edge and platform to entrain blocks, transport and deposit them on the cliff-top. Where cliff-top height is below wave crest elevation green water bore flow occurs, sufficient to force rotation or lifting of blocks out of cliff-top and cliff-top platform 'sockets'. High flow velocities rapidly accelerate and transport blocks inland until the flow attenuation results in deposition of blocks at the limit of run-up. The model results provide a good explanatory framework to account for the quarrying of the upper part of the cliff-face and cliff-top platform under storm wave conditions and provide an insight into the exceptional velocities experienced over the cliff-top platform under bore flow conditions. The modelling results show that extreme storm waves are capable of quarrying, transporting and depositing large blocks at altitude and significant distances inland and so present serious questions about the use of such deposits as diagnostic of palaeo-tsunami. (c) 2008 Elsevier B.V. All rights reserved.