Abstract :
[en] Between 1998 and 2004, Europe suffered from more than hundred major inundations, responsible for some 700 deaths, for the moving of about half a million of people and the economic losses of at least 25 billions Euros covered by the insurance policies. Within this context, EU launched the 2007/60/CE directive. This directive aims at a better evaluation of the risks and a better coordination of prevention, protection and crisis management.
In most countries, inundation maps only include rivers’ overflowing. In Wallonia (southern part of Belgium), it was decided to include overland flows and mudflows in the flood hazard map. Indeed, the cleaning operations for a village after a storm can lead to an estimated cost of 11 000 €. Average construction cost of retention dams to control off-site damage caused by floods and muddy flows was valued at 380 000€, and yearly dredging costs associated with these retention ponds at 15 000€. A specific study in Gembloux (25 000 inhabitants) estimated the mean annual cost for the runoff damages to 20 000€. This cost only consists of the physical damages caused to the settlements and movable properties of the residents as well as the emergency operations of the firemen and the city. On top of damages to public infrastructure (clogging of trenches, silting up of retention ponds) and to private property, runoff and mud flows generate a significant loss of arable land. Yet, the soil is not an unlimited resource. Moreover, sediments’ transfer to watercourses alters their physical and chemical quality. And that is not to mention the increased psychological stress for people.
But head catchment hydrology is not well known. Mapping the overland flood and mud flow hazard over a 17000 km² region is a real challenge. This contribution will present the pragmatic methodology used in Wallonia. In accordance with the Directive, different maps are produced (25, 50 and 100 years of return period and an extreme scenario). Local characteristics are taken into account: rainfall statistics, soil data, land use and relief. They are used to assess runoff production and transfer to an outlet identified as the point where runoff enters the permanent river network. Peak discharge values are used as basis for the mapping. The maps locate the water paths using a colour chart based on the peak discharge. Summer 2011 and spring 2012 storm events as well as a survey made at the municipalities’ level allowed us to validate in some ways the maps produced.
Whereas this first approach at regional scale includes uncertainties, the aim of these maps is currently to prompt recognition of the runoff inundation hazard. It is of major importance for soil conservation and citizens’ protection. Above all, it should contribute to lower the damages by early prevention during the design of town-planning projects.
