Identification of hazardous zones combining cliff retreat rates with landslide susceptibility assessment

Epifânio, B., Zêzere, J.L., Neves, M., 2013. Identification of hazardous zones combining cliff retreat with landslide susceptibility assessment. Landslides are one of the main natural hazards which affect the coastal cliffs of Lourinhã council, located in Central Portugal. In this work, a methodology for hazard evaluation on coastal cliffs is presented, which combines the calculation of cliff retreat rates with landslide susceptibility assessment essential because landslide usually do not affect the cliff top. A landslide inventory was produced by aerial photo interpretation and a systematic field survey. The entire coastal stretch was split into 50m-long areas, resulting in 261 terrain mapping units. For each unit, 9 landslide predisposing factors were derived and classified. A predictive susceptibility model was developed using the Information Value method and the respective degree of fit was assessed using standard Receiver Operator Curves (ROC). Cliff retreat rates were calculated for a 60-year period using the ESRI′s Digital Shoreline Analysis System ArcGIS extension on a set of aerial photographs (which were manually geo-referenced and ortho-rectified) and a digital orthophotomap. Subsequently, predicted susceptibility data were integrated with the computed cliff retreat rates in a hazard matrix in order to create a map of the coastal cliff hazard. Cliff retreat rates ranged from 0.013 to 0.130m/yr and landslide susceptibility ranged from very high (4.2% of terrain unit) to low (0.8 % of terrain units). Approximately half of the study rocky coast is identified as being in a high hazard class. Thus, it is essential that hazard zones are considered for land use planning and management in order to reduce the probability of future incidents related to slope instability.