Landslides in the urban and suburban perimeter of Chefchaouen (Rif, Northern Morocco): inventory and case study

Many old and recent landslides affect the Rif region in Northern Morocco. In Chefchaouen city, this natural hazard constitutes a major obstacle for the development of the city and its suburban districts. In this study, a landslide inventory was realized to identify and characterize slope instability cases in the Chefchaouen city, their relationship to the subsoil layers and their effect on the stability of newly built districts. Special attention was given to a recent landslide in the Ain Haouzi district, reactivated in 2016 and causing damage to 35 houses. This slide was subject to further scientific investigations (borehole inclinometer and ERT) in order to better our understanding of the slopes instability in the study area. The results show that the territory of Chefchaouen city which covers an area 23.63 km2 is affected by 447 landslides most of which (300) are very old and inactive. However, many of the old landslide masses and/or scarps were partially reactivated in the last decade, which caused a lot of material damage to many constructions, estimated to be 25.74 M MAD (approximately 2.5 M$). The building damage inventory also showed how the distribution of landslides in the newly constructed suburban/semirural districts is clearly correlated with the spatial distribution of severely damaged houses. For geomorphological reasons, these housing developments prefer terrains mainly formed by weathered clay layers and scree deposits, characterized by weak geotechnical characteristics, which compromises the stability of constructions. The clandestine nature of the construction work (absence of a sewage network, random changes to the topography) increases the vulnerability of buildings to the natural slope dynamics. The layers which constitute the body of most old and recent landslides are subject to slow gravitational deformation which can be accelerated in major rain events. This was the case for the 2016 event which was reactivated by seven consecutive rainy days. The ERT profiles executed in this area coupled with borehole inclinometer results demonstrate that the plastic, weathered clay layers facilitated the slide of the scree deposits, which constitute the landslide’s body. Indeed, the development plan of the city must be adapted to the slope dynamics and the subsoil behavior, so as to avoid unnecessary material damage from taking place in the future.