Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain

Abstract

Rising sea levels and increased storminess are expected to accelerate the erosion of soft-cliff coastlines, threatening coastal infrastructure and livelihoods. To develop predictive models of future coastal change we need fundamentally to know how rapidly coasts have been eroding in the past, and to understand the driving mechanisms of coastal change. Direct observations of cliff retreat rarely extend beyond 150 y, during which humans have significantly modified the coastal system. Cliff retreat rates are unknown in prior centuries and millennia. In this study, we derived retreat rates of chalk cliffs on the south coast of Great Britain over millennial time scales by coupling high-precision cosmogenic radionuclide geochronology and rigorous numerical modeling. Measured 10 Be concentrations on rocky coastal platforms were compared with simulations of coastal evolution using a Monte Carlo approach to determine the most likely history of cliff retreat. The 10 Be concentrations are consistent with retreat rates of chalk cliffs that were relatively slow (2-6 cm·y −1 ) until a few hundred years ago. Historical observations reveal that retreat rates have subsequently accelerated by an order of magnitude (22-32 cm·y −1 ). We suggest that acceleration is the result of thinning of cliff-front beaches, exacerbated by regional storminess and anthropogenic modification of the coast.geomorphology | coasts | cosmogenic radionuclides | erosion | cliffs R ocky coasts are "erosional environments formed as a result of the landward retreat of bedrock at the shoreline" (1). They leave scant evidence of any previous state, making it difficult to interpret their history. Cliff retreat is driven by a combination of wave-driven cliff-base erosion, subaerial weathering, and mass wasting processes, whose efficiencies are dependent on lithology and climate. Sediment generated by mass wasting processes such as abrasion, plucking, landslides, and rockfalls tends to be reworked and transported away by waves and currents, particularly for softer rock types.Cliff erosion due to mass wasting threatens livelihoods and both public and private clifftop infrastructure and development; quantitative estimates of the rate of cliff retreat are necessary to assess the associated risk. Rising sea levels and increased storminess may lead to accelerated coastal erosion rates in the future, potentially increasing hazard exposure (2-5). To accurately assess coastal hazards in the face of future climate and land-use changes, it is necessary to understand the dynamics of cliff erosion over length and time scales relevant to the processes that drive change. To establish the context for modern change, we must quantify the natural variability and the long-term behavior of cliff retreat. Historical records are too short to allow us to do this: they typically span no longer than ∼150 y (6, 7), which can be less than the characteristic return period of significant coastal failures (8), and they coincide with the period over which humans have significantly modified th...

Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

“…Nevertheless, these results demonstrate that there is great potential for 10 Be measurements to provide first-order estimates of long-term rates of sea cliff retreat (Regard et al, 2012;Rogers et al, 2012;Choi et al, 2012;Hurst et al, 2016). Quantifying these factors, and how they may have changed over the millennial timescales required to accumulate significant 10 Be concentrations, may not always be possible.…”

“…Yet, by their very nature, eroding coastlines leave scant evidence of any former state, and their form reflects little about their longterm erosional trajectories (Matsumoto et al, 2016). Cosmogenic isotopes have the potential to reveal the long-term history of coastal change and to quantify process rates along rocky coastlines (Recorbet et al, 2010;Choi et al, 2012;Regard et al, 2012;Rogers et al, 2012;Hurst et al, 2016).…”

“…They minimized residuals between model results and a transect of eight 10 Be concentration measurements from across the shore platform in order to estimate long-term average sea cliff retreat rates that were similar to estimates from historical observations; however, the uncertainties on the analyses were large and limited the resolution and confidence of the comparison. Building on the approach of Regard et al (2012), Hurst et al (2016) quantified long-term rates of cliff retreat from 10 Be concentrations measured in two sample transects on the flint-bearing chalk shore platforms in East Sussex, UK. Rates during much of the Holocene were significantly slower than changes observed in the last 150 years, leading Hurst et al (2016) to suggest that cliff retreat rates accelerated some time between 600 and 150 years ago.…”

“…Building on the approach of Regard et al (2012), Hurst et al (2016) quantified long-term rates of cliff retreat from 10 Be concentrations measured in two sample transects on the flint-bearing chalk shore platforms in East Sussex, UK. Rates during much of the Holocene were significantly slower than changes observed in the last 150 years, leading Hurst et al (2016) to suggest that cliff retreat rates accelerated some time between 600 and 150 years ago.…”

Modelling the relative dominance of wave erosion and weathering processes in shore platform development in micro‐ to mega‐tidal settings

“…Edwards, 1941Edwards, , 1951Bird and Dent, 1966;Kirk, 1977). Swantesson et al, 2006;Hurst et al, 2016;Matsumoto et al, 2017) have provided a number of quantitative evidences useful to help settle the debate, but also revealed a wider range of potentially important processes and factors, including a suite of weathering (e.g. Recent studies utilizing advanced observation technology (e.g.…”