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Rôle des effondrements karstiques sur les désordres survenus sur les digues de Loire dans le Val d’Orléans (France)

Abstract : About 4 million people live on the alluvial plain of the Loire, the main river located in the centre of France. Their protection against flooding is mainly based on the construction of 650 km of old levees. During the last 100 year flood of the nineteenth century, these levees were repeatedly broken. This led to catastrophic flooding, especially in the Val d'Orléans. No new 100 year flood occurred during the twentieth and twentyfirst centuries and the likely response of these old levees to future major floods is poorly understood. Located midway along the river, the study area is 35 km long and 1–7 km wide. It is protected by 42 km of embankments that have already suffered 35 breaches. In the Val d'Orléans, 6-10 m of sandy alluvium cover a karstified limestone bedrock. Nearly 600 collapses were reported whose diameter varies from 0.50 to 18 m. 62 % occurred at <2 km from the river, that is to say, close to the levees. Statistically, 35 collapses should have occurred on the levees but only ten of them have been observed. No major consequences resulted because these collapses did not occur during a flood event. The question that arises here is to estimate the potential role of karst in levees breaches occurring during flooding. However, most of these collapses appeared in the nineteenth and early twentieth centuries and the role of karst was not looked at. The risk scenario is as follows: the Loire flooding causes karst flooding (because kasrtic channels are connected to the river), which induces or reactivates collapses. For the ‘karst collapse’ hazard, some predisposing factors can be listed: density of ancient collapses, thickness and nature of the alluvium, hydroclimatic conditions (frost, snow, heavy rain, floods, low water period etc.). This paper examines this issue and seeks to determine the potential role of karst in the levee breaches. The intensity and probability of this hazard is then characterised. Several mechanisms of levee instability, potentially due to karst, are reviewed. If a causal link between flood, karst and collapse is established, it will enable us to monitor the risk of breaches during the next major flood.
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Submitted on : Saturday, February 20, 2016 - 5:43:51 PM
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Philippe Gombert, Julie Orsat, David Mathon, Rafid Alboresha, Marwan Al Heib, et al.. Rôle des effondrements karstiques sur les désordres survenus sur les digues de Loire dans le Val d’Orléans (France). Bulletin of Engineering Geology and the Environment, Springer Verlag, 2015, 74 (1), pp.125-140. ⟨10.1007/s10064-014-0594-8⟩. ⟨hal-01276847⟩



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