A method to use proxy data of runoff-related impacts for the evaluation of a model mapping intense storm runoff hazard: application to the railway context

The TRIP method, or "indicator of intense pluvial runoff" in English, is a geomatics method that allows mapping the susceptibility of a territory to surface runoff and that provides three maps of susceptibility to the generation, transfer and accumulation of runoff. It is based on the combination of binary maps that represent the impact of a given factor (favourable or not favourable) on runoff. These factors are summed up to provide susceptibility maps for runoff with levels ranging from 0 to 5. To be used for risk prevention, the quality and limitations of the produced maps must be assessed. However, direct runoff data are very scarce and not available everywhere in a territory. Proxy data of impacts related to runoff can provide information useful for the evaluation of the IRIP maps. However, both pieces of information cannot be compared directly, and a specific methodology to compare susceptibility maps and proxy data must be proposed. This paper presents such a method, which accounts for the hazard level, the vulnerability of the study area and possible mitigation actions taken to reduce the risk. The evaluation method is assessed using a comprehensive database of runoff-related impacts collected on an 80 km railway line in Normandy (north of France) and covering the whole 20th century. The results show that the evaluation method is robust, relevant and generic enough for evaluating a non-quantitative method of runoff hazard mapping using localized runoff-related proxy data. In addition, the good performance of the IRIP model in the case study confirms that the susceptibility maps produced by the IRIP model provide relevant information related to runoff and that they can be used to design risk management strategies, as illustrated in the railway context.