A combined approach to cliff characterization: Cliff Stability index

In this work, a combined multidisciplinary method to characterize coastal cliff environments is presented. It combined two complementary approaches – engineering and geomorphological. The first one is represented by the wave power values along the cliff face. For that purpose, the deep-water wave climate is statistically characterized, and high-energy sea states are numerically propagated to the cliff with a state-of-the-art model. The variations in wave power at the cliff face are controlled by the varying cliff orientation and by the irregular morphology, which influences wave propagation through refraction and shoaling processes. Based on wave power, four engineering exposure levels, from low to extreme, are defined and mapped onto the cliff. The geomorphological approach is based on an index developed ad hoc for this work, the Cliff Stability (CS) index, which takes into account the cliff geometry, lithology, structure and degradation state, as well as the hydrological conditions. Based on the CS index, four geomorphological exposure levels are defined and mapped, from low to extreme. The combined approach is shown through the application to a study site in NW Spain. The two perspectives, engineering and geomorphological, are found to yield similar results in some sections of the study area, but not all. It may be inferred that, despite the importance of wave action in shaping the cliff, the additional elements included in the CS index also play a significant role. In practical terms, the significance of these results is that the two approaches, engineering and geomorphological, should be combined to properly characterize coastal cliffs. This combined approach represents a multidisciplinary tool to define and characterize the exposure levels and, thus, prevent damages in cliff environments across the world.