RINA - Démonstrateur d’utilisation de l’intelligence artificielle pour la gestion opérationnelle des risques naturels gravitaires géologiques | FEREC 2020

5. Standardized area of a profile of the cliff every 10 m 6. Standardized Concavity Index (SCI) of a profile of the cliff every 10 m 7. Topographic Wetness Index (TWI) of the top of the cliff every 10 m 3.1. Previous works Many works have studied the temporal occurrence of rock falls using statistical methods [Damato 2015, Rosa 2012, Westoby 2020] and some of those studied specifically the occurrence of rock falls at the RN1 (Réunion Island) as it is a site with many well documented incidents. Two studies of rock falls at the RN1 are highlighted hereafter: an operational study and a statistical study. 3.1.1. Operational rules for road RN1 maintenance The most relevant studies of rock falls temporal occurrences at RN1 were made in order to create rules for closing the road in order to avoid accidents [Batista, 2016]. The rules are updated with time as better performing rules are created and as conditions change, notably the installation of protective structures that help diminish the number of blocks that hit the road. Each version of the code is composed of 2 rules. The first indicates a maximum amount of rain in the former 24 h and the second indicates the maximum amount of daily rain in the former 48/72 h. Two rules will be highlighted in this work, the R7 and the Rps4b. The R7 was developed through a study in 2003 and started being applied in 2004. It was based on data acquired between 1998 and 2002 and was developed after analysing a study made by CEREMA. The R7 dictates that the road will be closed if: ● The daily rain in the former 24h is superior to 15 mm ● The daily rain in the former 72h is superior to 30 mm This rule was able to reduce by 30% the duration when the road was closed and maintained the same intensity of 0.151 blocks falling per day on the period that the road is open leading to the road being closed during 11% of the time. In 2009 a new suggestion was presented. As the protective structures were built, and dimensioned to reduce the number of blocks that hit the road by almost 80%, less restrictive rules could be used in order to have longer periods of allowed traffic on the road whilst maintaining a similar level of risk for drivers. The Rps4b dictates the road will be closed if: ● The daily rain in the last 24h is superior to 30 mm ● The daily rain in the last 48h is superior to 50 mm This permits the road to be closed during only 5,8% of the year, about half of the time given by R7. Between 2010 and 2015, the intensity of rockfalls per day while the route was open was inferior to 0,015 being less than 10 times the historically accepted risk. This work provides a basis of what can be done on this case study with two expert rules. 13