Long term shore platform surface lowering rates: Revisiting Gill and Lang after 32 years

In 1978 E.D. Gill and J.G. Lang installed 53 micro-erosion meter (MEM) sites on shore platforms along the Otway coast in south eastern Australia. Surface lowering rates were originally determined from 2 yr of measurements (1979 and 1980) and showed that the mean annual rate of lowering from all sites was 0.37 mm/yr− 1. We re-measured these MEM sites 32 yr after the first set of readings were taken in February 1979. We relocated 49 of the original 53 bolt sites and were able to measure 45 of them. From the sites we measured, the mean annual rate of shore platform lowering was 0.31 mm/yr− 1. Rates of surface lowering on shore platforms are commonly reported from studies of only two or three years and only one other record exceeds 30 yr (Kaikoura Peninsula, NZ; 1.09 mm/yr− 1), in that case fewer than half of the original bolt sites were functioning because erosion had removed bolts. Along the Otway coast rates of erosion are much slower hence the greater number of still readable sites after 32 yr. Compared to other micro-erosion meter studies the rates reported here are at the lower end of the range of values from around the world but nevertheless consistent with rates from the Otway Coast and others in similar lithology. We found no statistical difference between erosion rates calculated over 2 yr (1979 to 1980) and 32 (1979 to 2011) years supporting the view that shore platform lowering rates measured over 2 yr are representative of decadal scales.