Preprints
https://doi.org/10.5194/esurf-2022-28
https://doi.org/10.5194/esurf-2022-28
 
07 Jun 2022
07 Jun 2022
Status: this preprint is currently under review for the journal ESurf.

Constraints on long-term cliff retreat and intertidal weathering at weak rock coasts using cosmogenic 10Be, nearshore topography and numerical modelling

Jennifer R. Shadrick1, Dylan H. Rood1, Martin D. Hurst2, Matthew D. Piggott1, Klaus M. Wilcken3, and Alexander J. Seal1 Jennifer R. Shadrick et al.
  • 1Earth Science and Engineering, Imperial College London, London, SW7 2AZ, United Kingdom
  • 2School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
  • 3Institute for Environmental Research (IER), Australian Nuclear Science and Technology Organization (ANSTO), Lucas Heights, NSW 2234, Australia

Abstract. The white chalk cliffs on the south coast of England are one of the most iconic coastlines in the world. Rock coasts located in a weak lithology, such as chalk, are likely to be most vulnerable to climate change-triggered accelerations in cliff retreat rates. In order to make future forecasts of cliff retreat rates as a response to climate change, we need to look beyond individual erosion events to quantify the long-term trends in cliff retreat rates. Exposure dating of shore platforms using cosmogenic radionuclide analysis and numerical modelling allows us to study past cliff retreat rate across the late-Holocene for these chalk coastlines. Here, we conduct a multi-objective optimisation of a coastal evolution model to both high-precision topographic data and 10Be concentrations at four chalk rock coast sites to reveal a link between cliff retreat rates and the rate of sea level rise. Furthermore, our results strengthen evidence for a recent acceleration in cliff retreat rates at the chalk cliffs on the south coast of England. Our optimised model results suggest that the relatively rapid historical cliff retreat rates observed at these sites spanning the last 150 years last occurred between 5300 and 6800 years ago when the rate of relative sea level rise was a factor of 5–9 times more rapid than during the recent observable record. However, results for these chalk sites also indicate that current process-based models of rock coast development are overlooking key processes that were not previously identified at sandstone rock coast sites. Interpretation of results suggest that beaches and heterogenous lithology play an important but poorly understood role in the long-term evolution of these chalk rock coast sites. Despite these limitations, our results reveal significant differences in intertidal weathering rates between sandstone and chalk rock coast sites, which helps to inform the long-standing debate of ‘wave versus weathering’ as the primary control on shore platform development. At the sandstone sites, subaerial weathering has been negligible during the Holocene. In contrast, for the chalk sites, intertidal weathering plays an active role in the long-term development of the shore platform and cliff system. Overall, our results demonstrate how an abstract, process-based model, when optimised with a rigorous optimisation routine, can not only capture long-term trends in transient cliff retreat rates but also distinguish key erosion processes active in millennial-scale rock coast evolution at real-world sites with contrasting rock types.

Jennifer R. Shadrick et al.

Status: open (until 07 Aug 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esurf-2022-28', Anonymous Referee #1, 30 Jun 2022 reply

Jennifer R. Shadrick et al.

Model code and software

Rocky-Profile-Model: RPM-CRN with Dakota Implementation v1.0 Martin Hurst, Hironori Matsumoto, Jennifer R. Shadrick, Dylan H. Rood, Mark E. Dickson https://zenodo.org/record/5645478#.Yo3cmBNBzPg

Jennifer R. Shadrick et al.

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Short summary
Here we use two different datasets to inform a process-based model to study coastal cliff retreat rates across the past 7000 years at four different chalk coast sites across the south coast of England. Our results demonstrate how a simplified model can not only capture long-term trends in cliff retreat rates, which are linked to the rate of sea level rise, but also identify key erosion processes at real-world sites with contrasting rock types.