Assessing landslide hazard in the High City of Antananarivo (Madagascar)

The High City of Antananarivo is one of the most important built cultural heritage sites of Madagascar, and therefore is part of the UNESCO Tentative List. Built on the hilltop of a granite ridge elevating above the Ikopa river valley, it’s renowned for its baroque-style palaces the Rova royal complex and gothic cathedrals dating back to the XIX century. During the winter of 2015, the twin cyclones Bansi and Chedza hit the urban area of Antananarivo, triggering floods and several shallow landslides, which caused thousands of evacuees and many casualties. Between 2018 and 2019 several rockfalls occurred from the rock cliffs of the Analamanga hills destroying housings and killing over 30 people. Both events showed that landslides can pose a high risk to the safety of the inhabitants, the infrastructures, and the cultural heritage of the High City of Antananarivo. To assess landslide hazard an integrated approach was adopted by means of the following actions: i) creation of a multitemporal slope-scale landslide inventory; ii) geotechnical characterization of the involved materials; iii) analysis of shallow landslide susceptibility; iv) runout analysis of debris flows channeling within large creek gullies; v) landslide kinematic analysis of the rockmass; vi) simulation of rockfall trajectories; vii) analysis of rainfall data. The results show that the main factors affecting landslides are slope, lithology, creek-gully erosion and anthropization, while most of the landslide events are clearly triggered by heavy rainfall events. The landslide-prone areas (including shallow landslides, rock falls and debris flows) are located primarily along the cliff bounding the western hill slope, the southeastern sector, where abandoned quarries form large slope cuts, and subordinately in the steep creek catchment just east of the Rova. The produced thematic maps represent fundamental land use management tools to be used as a first step towards a geo-hydrological risk reduction strategy by the institutions and actors involved in the High City protection and conservation. The conducted study represents an important contribution for improving the knowledge on landslides processes in an area with limited data such as Madagascar and Antananarivo in particular, and may be reproduced in cultural heritage sites characterized by similar geomorphological and urban scenarios.