Annex 3 - Key concepts of genesis of the flood hazard
o Runoff by saturation of the soil. This type of runoff occurs especially for low intensity rainfall falling on soaked soil, particularly because of the presence of a water table near the surface or after long-term rain. (Mounirou, 2012) o Runoff by exfiltration.
Figure 18: Types of runoff (adapted from Musy 2005) The concentration of flows within a catchment area (transfer function) In a given catchment area, flows occur along the catchment area's own drainage system and move towards the concentration of water at the outlet, which is the lowest point in the downstream catchment area (Figure 18). (Musy, 2005)
Figure 19: Illustration of a catchment area and its outlet (MEDDE France, Les inondations - Dossier d'information, 2004) The flood hydrograph summarizes the hydrologic response of a given catchment area for a given rainfall event. This response depends in particular on the topography of the catchment area (the more significant the slopes are, the faster the hydrological response will be - bell-shaped hydrograph) and the land use (the more urbanized the basin is and the faster the flows will be, conversely, vegetation tends to slow down the flow and favour the infiltration process).
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