Decadal scale micro erosion rates on shore platforms

Knowledge of the long-term rates of erosion of shore platforms are needed to assist modeling platform development, understand rates and modes of development and decipher issues of inheritance from former sea level. Few studies report rates from more than 2 years of monitoring. We report decadal scale erosion rates using both a micro-erosion meter and a traversing micro-erosion meter with measurements from 30 years and 10 years, respectively. Gross mean surface lowering rates measured over 10 years with a traversing micro-erosion meter were 0.901 (S.E. = 0.116) mm/yr compared with 1.130 (S.E. = 0.217) mm/yr over two years. Over thirty years (1973-2003) 12 micro-erosion meter sites provide a mean surface lowering rate of 1.09 (S.E. = 0.126) mm/yr compared to 1.43 (S.E. 0.128) mm/yr calculated over twenty years (1973-1993) from 15 bolt sites and 1.53 (S.D. = 1.45) mm/yr from 31 bolt sites. Rates calculated over longer time periods are reduced compared to short-term rates as a result of faster eroding bolt sites being removed from the data set. Comparison of means between short and long time periods for existing bolts show no significant difference. That is, short-term measurements over two years are representative of decadal scale erosion rates. In fact decadal scale rates may under represent rates of platform development because of the loss of faster eroding bolt sites. The loss of such bolts means that obtaining long-term erosion rates is problematic. Future modeling of shore platform development can use MEM rates with more certainty, but we recommend using higher rates from shorter term studies, unless longer records are complete.