Please cite this article as: Lagadec, L-R., Patrice, P., Braud, I., Chazelle, B., Moulin, L., Dehotin, J., Hauchard, E., Breil, P., Description and evaluation of a surface runoff susceptibility mapping method, Journal of Hydrology (2016), doi: http://dx. AbstractSurface runoff is the hydrological process at the origin of phenomena such as soil erosion, floods out of rivers, mudflows, debris flows and can generate major damage. This paper presents a method to create maps of surface runoff susceptibility. The method, called IRIP (Indicator of Intense Pluvial Runoff, French acronym), uses a combination of landscape factors to create three maps representing the susceptibility (1) to generate, (2) to transfer, and (3) to accumulate surface runoff. The method input data are the topography, the land use and the soil type. The method aims to be simple to implement and robust for any type of study area, with no requirement for calibration or specific input format. In a second part, the paper focuses on the evaluation of the surface runoff susceptibility maps. The method is applied in the Lézarde catchment (210 km², northern France) and the susceptibility maps are evaluated by comparison with two risk regulatory zonings of surface runoff and soil erosion, and two databases of surface runoff impacts on roads and railways.Comparison tests are performed using a standard verification method for dichotomous forecasting along with five verification indicators: accuracy, bias, success ratio, probability of detection, and false alarm ratio. The evaluation shows that the susceptibility map of surface runoff accumulation is able to identify the concentrated surface runoff flows and that the susceptibility map of transfer is able to identify areas that are susceptible to soil erosion. Concerning the ability of the IRIP method to detect sections of the transportation network susceptible to be impacted by surface runoff, the evaluation tests show promising probabilities of detection (73 to 90%) but also high false alarm ratios (77 to 92%). However, a qualitative analysis of the local configuration of the infrastructure shows that taking into account the transportation network vulnerability can explain numerous false alarms. This paper shows that the IRIP method can be a valuable tool to facilitate field analysis and perform surface runoff zonings and opens interesting prospects for the use of the IRIP method in a context of risk management.3

Railways are critical infrastructures for the transportation of people and goods and network failures must be controlled in order to maintain safety and to limit economic losses. The railway network is exposed to natural hazards and particularly to intense pluvial runoff. Due to the complexity of the phenomenon, management of risks induced by pluvial runoff raises technical and scientific issues. An innovative method for runoff susceptibility mapping, called IRIP for "Indicator of Intense Pluvial Runoff", has been created and adapted to the railway context. The objective of this paper is to evaluate the relevance of the mapping method and to provide application advice. The mapping method is evaluated by comparison with the results of a hydraulic diagnosis, on a 20 km railway line, using quantitative and qualitative comparisons. On the basis of contingency tables, probabilities of detection (POD, railway sections exposed and detected by IRIP) and false alarm ratios (FAR, railway sections detected by IRIP whereas they are not exposed) are computed. POD range from 94 to 100% and FAR range from 20 to 26%. Then spatial information provided by the maps is compared with field observations and recommendations. It is shown that the mapping method can bring substantial contribution to risk identification and that the IRIP method can allow pushing forward the current risk reduction methods. Thus, the surface runoff maps open up new opportunities to manage surface runoff, such as targeting mitigation actions at the origin of the hazard in partnership with the other territory stakeholders.

Abstract. The IRIP 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.