Reconciling short- and long-term measurements of coastal cliff erosion rates
Year: 2024
Type: article
Abstract: Oblique terrestrial laser scanning (TLS) enables topographic change detection at scales (10−1 – 100 cm) that are appropriate for coastal cliff erosion monitoring. Despite this, published applications of TLS on cliff are limited to a small number of sites around the world. Here we report new TLS point cloud datasets from 9 years of monitoring (2014-2023) at Rothesay Bay within the Hauraki Gulf, New Zealand, which has relatively low wave energy and a meso-tidal range. The 120 m length scan area includes cliffs of 10-30 m height, formed of horizontally bedded soft sedimentary flysch rock. The cliffs are fronted by a 140 m wide near-horizontal shore platform that terminates in an abrupt seaward edge. Previous research at this site has estimated long-term cliff erosion rates within a range of 1.4±0.1 to 14.3±0.1 mm/year on the basis of measured shore platform width: the approach assumes that the shore platform has widened over time due to constant cliff retreat during over 6,000 years of stable Holocene sea level, and that the seaward edge of the shore platform has not retreated. Volumetric cliff-face erosion rates detected through 15 scans over a 9-year window (2014-2023), including intensive monthly TLS scanning between July 2021 and July 2022. Results show that the average cliff recession rate over the past decade has been 41±2 mm/year, and monthly scans show a range in erosion rates of 30 to 288 mm/year during dry and wet scan periods respectively. The cliff recession rate detected with TLS is 3 to 30 times higher than erosion rates derived based on the shore platform width. If erosion had been constant at this rate over approximately 6,000 years, a total cliff retreat of >245 m would be expected, whereas the contemporary shore platform is 140m. We discuss two possible hypotheses for the confounding width of the modern shore platform: 1) that modern cliff retreat rates are faster than past erosion rates; 2) that the seaward edge of the shore platform does not reliably demarcate Holocene cliff recession. We present new bathymetric survey mapping seaward of the shore platform edge that reveals multiple rocky features that are distinguished by steep slope breaks and planar surfaces. Further work is required to investigate the origin of these features and any possible connection to the modern intertidal shore platform.
Source: Engineering Geology
Language: English
Topic: Coastal and Marine Dynamics
Subfield: Earth-Surface Processes
Field: Earth and Planetary Sciences
Domain: Physical Sciences
SDG: Climate action
Open Access status: hybrid
Estimated APC paid: $3,900
Funder: Marsden Fund
OpenAlex ID: W4402208194