The multidecadal spatial pattern of erosion on sandstone shore platforms in south-eastern Australia

Abstract

Knowledge of the spatial pattern of erosion on shore platforms is needed to better understand their evolution. The long-term spatial and temporal variation of downwearing remains difficult to interpret from erosion data commonly collected over 1–3 years of monitoring. We report erosion data recorded with a micro-erosion meter (MEM) over 40 years (1979–2019) on sandstone shore platforms in a micro-tidal environment along the Otway coast, Victoria, Australia. This well-preserved monitoring network (43 out of 44 original MEM sites) allowed a detailed investigation over a multidecadal timescale not previously considered. Over 40 years the mean erosion rate was 0.25 mm/yr. This value was statistically similar to erosion rates calculated over 2-year (0.32 mm/yr) and 32-year (0.26 mm/yr) periods. In the vertical plane, two significant but opposite relationships between elevation and erosion rates were displayed in the central intertidal zone. There was also a peak in the erosion rate near mean higher high water level. This spatial pattern was strongly associated with wetting and drying controlled by tides and waves. At elevations lower than mean higher low water, downwearing rates were one or two orders of magnitude lower than the average rate of the entire platform. The low erosion rates may be caused by the longer wetting condition of surfaces at lower elevation. Fast erosion rates were recorded in the upper intertidal zone where the bedrock was only inundated by spring high tides or storm waves. Salt weathering may be the primary mechanism but the effect of thermal weathering cannot be neglected at higher elevation. Based on the experience from the near complete MEM network in the study area, suggestions were provided to extend the longevity of monitoring network for future long-term erosion records on shore platforms.