Information gathered on past flood events is essential for understanding and assessing flood hazards. In this study, we present how citizen science can help to retrieve this information, particularly in areas with scarce or no authoritative measurements of past events. The case study is located in Yeumbeul North (YN), Senegal, where flood impacts represent a growing concern for the local community. This area lacks authoritative records on flood extent and water depth as well as information on the chain of causative factors. We developed a framework using two techniques to retrieve information on past flood events by involving two groups of citizens who were present during the floods. The first technique targeted the part of the citizens' memory that records information on events, recalled through narratives, whereas the second technique focused on scaling past flood event intensities using different parts of the witnesses' bodies. These techniques were used for three events that occurred in 2005, 2009 and 2012. They proved complementary by providing quantitative information on flood extents and water depths and by revealing factors that may have contributed to all three flood events.

Flood hazard assessment is at the core of flood risk management. In order to develop an efficient flood hazard assessment, it is of primary importance to have a well‐defined flood scenario encompassing all processes that could occur during an event. Understanding and assessing these processes requires meteorological, topographical and land‐use data as well as historical observations. Nowadays, flood delineation is based upon hydrological and hydraulic modelling, ground data collection, and remote sensing. Despite the advantages of these tools, they also present some specific limitations, either intrinsic to the approaches or linked with constraints of the local context. With the rapid advancement of web 2.0 technologies (e.g., Flickr and Wikimapia) and the increase of the use of participatory research, citizen science has the potential to provide valuable and complementary information at all levels of flood risk management and in particular for flood hazard assessment. After reviewing the capabilities and limitations of the current tools used in flood hazard assessment, this paper demonstrates the role that citizen science can play in providing key information on factors leading to flooding and on flood hazard parameters.

Flooding has emerged lately as a major threat for poor people in the suburban area of Dakar (Senegal), a densely populated area. In this region, flood events are mainly controlled by rainfall intensity and groundwater level fluctuations.To assess flood risk at a neighbourhood level, accurate data on flood extent, exposure and vulnerability is required. The objective of the present study is to obtain these data combining remote sensing data and local knowledge. Field work data were collected through interviews with inhabitants. About 500 respondents were surveyed with the support of a local association and very detailed mapping was carried out to clearly identify elements at risk. Information collected at the household level concerns: 1) socio-economic data, 2) information on the property, 3) flooded houses and 4) strategies of risk reduction.This research demonstrates that local knowledge is an important tool to obtain accurate data useful for understanding flood hazard and vulnerability patterns. It provides quantitative data at the household level that can be used to complement conventional GIS and remote sensing data.

Abstract. Information gathered on past flood events is essential for understanding and assessing flood hazard. In this study, we present how citizen science can help retrieving this information, in particular in areas with scarce or no instrumental measurements on past events. The case study is located in Yeumbeul North (YN), Senegal, where flood impacts represent a growing concern for the local community. This area lacks instrumental records on flood extent and water depth as well as information on the chain of causative factors. We developed a framework using two techniques to retrieve information on past flood events by involving two groups of citizens who were present during the floods. The first technique targeted the part of the citizens’ memory, which records information on events, recalled through narratives, whereas the second technique focused on scaling past flood event intensities using different parts of the witnesses’ body. These techniques were used for 3 events, which occurred in 2005, 2009 and 2012. They proved complementary by providing quantitative information on flood extents and water depths, and by revealing factors that may have contributed in aggravating floods for 3 events which occurred in 2005, 2009 and 2012.

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