Hourly to daily-scale microtopographic fluctuations of supratidal sandstone

Abstract

A traversing micro-erosion meter (TMEM) was used to measure short-term microtopographic changes on a supratidal rock surface at Marengo, Australia. In order to describe the characteristics of rock surface behaviour at different temporal scales, the TMEM site was monitored at 2-hourly, daily and multiday (3.5 days) periods. The rock surface was highly dynamic at 2-hourly scale, repeatedly falling and rising by up to 0.644 mm. Two-hourly surface change was also characterised by spatial heterogeneity, with contraction and expansion occurring concurrently at centimetre scale across the rock surface. Two-hourly microtopographic change was linked to the microclimate with significant relationships between surface movement and relative humidity (R2 = 0.27) and air temperature (R2 = 0.24). Expansion was observed across the rock surface when there was a fluctuation of 18% in relative humidity, and when cloudy the rock surface remained active during the day. Temporal variability in surface movement was also observed over a 24 h period with three distinct periods observed: falling (06:00–12:00), rising (12:00–20:00) and stable (20:00–06:00). The daily surface change underwent no net microtopographic change (no loss of material). In contrast, an overall contraction was observed at multiday scale. Rock surface behaviour in coastal environments is therefore complex with many cycles of expansion and contraction acting concurrently albeit at different temporal scales from hours to multiple days. © 2018 John Wiley & Sons, Ltd.