A new method to study microtopographical changes in the intertidal zone: one year of TMEM measuremets on a limestone removable rock slab (RRS).

A new approach to the problem of measuring microtopographical changes on vertical limestones in the intertidal zone, simple and cost-effective, is discussed. It is based on the positioning of a rock removable slab (RRS) in the intertidal zone. The RRS method allows to use the traversing micro erosion meter (TMEM) on vertical rock surfaces, temporarily removing the slab from the operating position, so that measurements can be collected horizontally. Sources of errors and preliminary results, besides the limits and applicability of the method, are discussed. We have tested the RRS method in the Gulf of Trieste, on a micro-tidal rock coast, collecting data from 7 stations between -0.75 m and +0.75 m m.s.l. The rock surface of the slab was surveyed between November 2007 and November 2008. During this period, the surface varied between -0.127 mm/yr (rising) and 0.034 mm/yr (lowering), according to the elevation of the stations. In the same period, standard deviation increased downward and varied between 0.003 and 0.045. Beside the TMEM surveying, we have acquired near-monthly photographic images, in order to observe the biological colonisation of the slab. Monthly photographic comparison of the vertical slab indicates that biological colonisation affected mainly the mid-lower portion of the slab, sometimes preventing the collection of measurements. Preliminary data suggests that the limestone surface lowered mostly in the upper part, while the lower part is strongly affected by algal growth. Consequently, data indicating surface rising is statistically less significant than data indicating surface lowering. Even the RRS used in the intertidal zone provides new interesting data to study weathering and erosion processes on limestone coasts, the first year of surveying suggests that this technique requires longer periods to address the TMEM data to explain geomorphological questions.