Maps of the flood

Modifié le 06/11/2015

Carto crue

Source: IGN - Limits of the highest waters recorded during the 1910 flood in the Paris region

Modelling of the flood phenomenon is complex and its representations are varied. This is managed by the DRIEE [Regional and Inter-departmental Department of Environment and Energy]. It includes:

  • The map of the Highest Recorded Water Levels (Plus Hautes Eaux Connues - PHEC), a historical map. This is aimed at the general public (public information), by representing the areas that have actually flooded in the past.
  • The maps of the Flood Risk Prevention Plan (Plan de Prévention du Risque Inondation), which has regulatory value. The objective is to prevent an increase in the number of people and properties exposed, and to prevent aggravation of the risk. It includes the level of water reached by a reference flood.
  • Map of the four ORSEC [Civil Security Response Organization] scenarios. Constructed on a digital simulation of water flow, they present floods in a simplified way to enable planning. They are represented on a regional level, so that operators can develop their action plans. They use coefficients ranging from 0.6 to 1.15, related to a reference flood (1910).
  • Three maps related to the transposition of the European Flood Directive, according to the extent of the risk: frequent, average or extreme probability. According to one of these hypotheses, a line of water is reconstructed on a digital model of the land, which defines the flood-risk areas and the categories of water level (less than 1m, between 1 and 2m, above 2m).
  • Mapping related to each measurement station, corresponding to the flow coefficients of the reference flood, cross-matched with a digital model of the land, which define the Potential Flooded Areas (Zones Inondées Potentielles - ZIP), showing the contour of a flood without giving indications as to the water levels.
  • A Height Categories Zone map (Zone Iso Classes Hauteurs - ZICH) showing information on the predicted height, per height category of 50cm. For the EU SEQUANA 2016 exercise, the last set of maps will be used.

Despite the precision of the mapping, caution is necessary and a margin of error must be tolerated, due to several uncertainties:

  • Topography of the natural land;
  • Possible flood via the utility networks;
  • Breach-leaks of low walls;
  • Floods due to accumulation of groundwater not taken into account in the modelling.

Furthermore, all the scenarios correspond to replicas of the 1910 flood, and are therefore all equivalent in terms of extent, for each of the tributaries. However, the extent of the phenomena varies according to sector, so that a flood corresponding to scenario R10 can be found on the Seine Moyenne and R0.6 on the Marne.

Likewise, the mapping for the flooded area always represents the maximum flood level for the entire duration of the flood, without taking into account the time factor: it does not give a representation at a given moment in time.