Lithological effects on rocky coastline stability

Rock coasts are perceived to be stable, however, recent occurrence of stacks of rocks and subsequent loss of some rocky coasts poses a challenge for research. This study sought to assess the impact of waves on the compressive/tensile strength of the rocks and further investigated the lithological properties of coastal material that influence shoreline change along the heterogeneous rock coast of the western region of Ghana. The study determined how the petrology and mineralogy of the various rocks types influence the stability of rocky shoreline. Data used included available historic topographic maps and images, Geological map, directional wave data, field measurements of rock hardness and rock samples collected for laboratory investigations. Schmidt's hammer was used to measure in-situ rock hardness. Shoreline features for the study period (1974–2005) were extracted from multi-temporal dataset into a geodatabase, and change statistics computed by end point rate method using DSAS, an extension of Arc GIS software. Thin sections were produced from rock samples collected from the field, and petrographic and microscopic analyses were carried out on them. It was found that wave impact was minimal compared with the tensile strength of the rocks in the study area; thus wave is not the key geomorphic agent in the study area. The results showed shoreline accretion at few sites, whereas other parts of the rocky shoreline are eroding at varying degrees. It was observed that the site lithology of the rock coast as well as the quartz feldspar ratio content of the rocks influence the shoreline change rates, as quartz bearing rocks are often more resistant to weathering. It was also noted that the strength of the intact rock had moderate correlation with the shoreline change rates; instead the mineralogy, state of weathering and textural properties of the rocks explains the shoreline change rates along the coast.