Soft rock cliff retreat under changing climate drivers

Abstract

<p>Shoreline retreat can happen rapidly in cliffs composed of loosely consolidated glacial and pre-glacial sediments. Typical centennial-scale average retreat rates for some cliffed coastlines of East Anglia, UK are 2 - 5 m a<sup>-1</sup> where cliffs have no protection from storm energetics. Recent research using pre- and post-storm clifftop (geomorphological) surveys, as well as aerial photographs, has shown that in single events retreat can be 3 – 4 times the long-term average, with up to 15 m of retreat in a single event. Periods of clifftop stasis are thus interspersed with short term shocks, when meteorological conditions generate energetic drivers of change (elevated still water levels, high onshore waves, high rainfall inputs). Furthermore, short term shocks deliver enhanced sediment supply to the nearshore region, which is an important factor to take into account within future management planning strategies.  </p><p>This paper uses the latest Earth Observation data to quantify and evaluate rates of soft rock cliff retreat, thereby identifying periods when short term shocks have been delivered to the cliffs. It then explores the climate drivers of these shocks and assesses the associated synoptic meteorological scenarios. Finally it considers the implications for quantities of sediment released, in terms of both overall magnitude and alongshore variability.</p><p>Results suggest that three recent events stand out as having a significant impact on rates of cliff retreat and associated sediment release on the Suffolk coast, southern North Sea. The “Big Freeze” of the UK winter of 2010-11 involved a protracted period of easterly air flow from mid-November and into December, 2010. The process drivers were high magnitude onshore winds, generating nearshore waves of around 4 m. The 5 December 2013 North Sea surge similarly resulted in rapid cliff retreat and sediment release. In this event winds were alongshore so the wave impacts were lower, but the elevated water levels generated by the surge meant that wave action could be directed onto the cliffs. By far the biggest recent event in terms of storm forcing energetics was the February – March 2018 “Beast from the East” and “Mini Beast”, where persistent onshore winds generated waves of almost 4.5 m at Southwold Approaches (highest on record) that coincided with two phases of high spring tides (no surge). When regional-scale Sudden Stratospheric Warming (SSW) generates strong and persistent easterly winds there are widespread potentially irreversible consequences for cliff and beach sediments around the western North Sea coastline.</p>