Do short-term cliff-erosion measurements reliably represent long-term erosion rates?

Abstract

<p>Oblique terrestrial laser scanning (TLS) enables topographic change detection at scales (10<sup>−1</sup> –10<sup>0</sup> cm) that are appropriate for coastal cliff erosion monitoring. Despite this, its application has been limited to a select few regions around the world. Here we report new TLS point cloud datasets from 13 years of monitoring (2009-2022) at Rothesay Bay, Hauraki Gulf, New Zealand, which has a relatively low wave energy setting with a meso tidal range. The 120 m-long scan area includes cliffs of 10-30 m in height, formed of horizontally bedded soft sedimentary flysch rock. The cliffs are fronted by an ~140m wide near-horizontal shore platform that terminates in an abrupt seaward edge. Formation of such platform morphology is based on the assumption that: a) the platform has developed during several millennia of stable Holocene Sea level (around 6,000 years); b) the seaward edge of the shore platform does not erode; c) and the rate of cliff retreat reduces over time due to increased dissipation of wave energy across a widening shore platform. On this basis it has been previously assumed that the long-term cliff retreat rate at this field site is between 1.4±0.1 to 14.3±0.1 mm/year. In contrast, volumetric cliff-face erosion detected with intensive monthly TLS scanning (July 2021- April 2022) at this site indicates a cliff recession rate of 140-660 mm/year (during dry and wet seasons), and three scans over a 12-year window (2009, 2014, 2021) indicate an average erosion rate of 47 mm/year. If erosion had been constant at this rate over approximately 6,000 years, a total cliff retreat of 282 m would be expected, which presents a striking contrast to the width of the contemporary shore platform. The anomaly is even more pronounced given: a) the widely held assumption that cliff erosion rates should decline as platform width increases or that the long-term cliff retreat rates are constant, an assumption made previously for this site; and b) that Holocene Sea level appears to have been higher than present during the past 6,000 years. This presentation evaluates two possible hypotheses for the confounding width of the modern shore platform in comparison to current cliff erosion rates: 1) that current rates of cliff retreat are faster than past erosion rates (perhaps the TLS captured an unusually fast period of erosion, or perhaps there has been some modern acceleration in cliff erosion rates); 2) that the seaward edge has not remained in its original position or does not reliably indicate the original location of the cliff. These possibilities will be assessed through offshore bathymetric mapping and numerical model experiments.  </p>