Cartographical approaches to visualize successions of major historical floods on territories
ISBN 978-85-88783-11-9
Authors
1Saint-marc, C.; 2Davoine, P.; 4Lang, M.; 5Villanova-oliver, M.; 5Coeur, D.
1SNCF / LIG Email: cecile.saint-marc@imag.fr
2LIG / INPG Email: paule-annick.davoine@imag.fr
4IRSTEA Email: michel.lang@irstea.fr
5LIG / UPMF Email: marlene.villanova-oliver@imag.fr
5ACTHYS-DIFFUSION Email: denis.coeur@imag.fr
Abstract
In the field of natural risks, the anticipation and the prevention of future events involve studying in details the historical succession of past events. A map is an interesting tool to work over past natural risk events in their spatial context. Phenomenological maps enable people to better understand past events via the visualization of their locations, their features, their spatial context and some potential related phenomena. This contributes to identify threatened areas. Floods are cyclic hydrological events, each characterized by a return period (e.g. 1 in 100 year flood) that indicates the probability of occurrence of a similar event. Decision makers are particularly interested in the study of major historical floods, which are relatively rare on a human-life scale (1 in 50 year flood and more) but which have big impacts, often on large territories. The succession of numerous floods or flood’s phenomena on the same territory leads to three issues concerning the design and the legibility of a phenomenological map. First, the number of events that should figure on the map may be high, leading to dense information. Second, it is essential for the analysis with the map to show several features of the events (e.g. dating, type of flood, climatic causes). Finally, several phenomena may occur at the same place through time and may be spatially overlaid. Dynamic maps (interactive and animated displays) have developed over the last decades and have contributed to overcome the limitations of classical maps. Nevertheless, static maps remain mandatory in specific cases, such as the publication on a paper medium or the comparison of several locations of events at different dates. Thus, for illustration purpose of (Lang et al. in press), we had to map major flood events in France. Our challenge was to enable the visualization of 5 dimensions on the map, whereas the classical semiological variables (Bertin, 1967) only address 3 or sometimes 4 (e.g. 2D for the space, color and grain of the surface). In our proposals, the impacted areas (2D) are circled with different hues, which represent the dating of events (1D). Labels are used to specify the dating values and the climatic causes (1D), while the types of floods (1D) are labeled by pictograms to limit the use of sometimes-long text since a lot of labels are already on the map (names of watercourses and cities). The rules about the placement of labels on maps (Imhof 1975) are not followed, but labels are lined up on the edges of the map, in order to balance the page layout. The article will first describe the issues encountered to make maps of successions of events. It will present the related works about temporal maps and their limits when studying the return of periodic events. Then, it will focus on proposals for making a single cartographical view (or static map) of chronologies of floods that occurred on a given territory at a large time-scale. The proposals of maps design are based on the use and combination of semiological variables, such as color to represent the dating of events and of pictograms and textual labels to express the other dimensions of the information. Finally, the proposals will be discussed and outlooks will be presented.