Role of Floods on Sinkhole Occurrence in Covered Karst Terrains: Case Study of the Orléans Area (France) During the 2016 Meteorological Event and Perspectives for other Karst Environments

The Loire River basin is regularly impacted by sinkholes because of its specific geological context, karstic limestone overlain by soft cover deposits. The intense rainfall event and the associated floods that occurred in this area in May and June 2016 triggered tens of sinkholes in a few square kilometers. At least 20 houses, one flood protection dyke of the Loire River and two highways were damaged by collapses. These events highlight not only the vulnerability of the area, especially in the case of a disastrous flood of the Loire River, but also the unexpected kinetics of the collapse process. Two different types of sinkholes occurred in the impacted areas: north of Orléans, collapses of the filling of vertical caves are suspected; in the Loire valley, the flood accelerated the internal erosion of alluvium in the karstic active network, triggering cover collapses (spatial and temporal sinkhole frequency increased by an estimated factor of 16,000 to 24,000). In parallel, an innovative internal erosion numerical modeling approach, based on Discrete Element - DEM and Lattice Boltzmann methods - LBM, has been developed through a partnership between the French Geological Survey (BRGM) and the Environment and Agriculture National Research Institute (IRSTEA). The upward propagation of cavities within the cover were successfully simulated. The role of different parameters (soft cover cohesion, hydraulic head, system geometry, etc.) in the sinkhole occurrence were tested by a parametric analysis.