Abstract
Flash floods and muddy floods may cause severe human and material damage despite their small spatial extent and low occurrence. In late May 2008, a major event has affected the area of Liege. This paper describes the methodology used to reconstruct the hydrosedimentological parameters of the flood from the geomorphological evidences observed in the field. Bed load and suspended load transport rates estimated during this extreme event were compared to the average values observed in other Walloon rivers and more specifically in the Ardennes Massif. The spatial distribution and the temporal evolution of the flash flood and muddy flood events are then analysed across Wallonia based on several works compiling press archives since the early twentieth century. The biases associated with this type of historical sources and the consequences of flash floods and muddy floods on the vulnerability and the risk of flooding in Wallonia are finally addressed.
Highlights
In addition to the well-known location of flash flood events in the northern part of Wallonia, he has highlighted a high density of events in the slopes of the tributaries of the Sambre and Meuse rivers in the Condroz region, where the population density exceeds 100 inhabitants.km-2 (Figure 7, showing the population density by municipality in 2012 with a classification highlighting the lowdensity areas that are subjected to the database incompleteness bias)
49 The major flash flood event that occurred in Liège on May 29, 2008 was analysed from geomorphologic evidences
Flood marks, block deposits and fine sediment accumulations allowed the reconstruction of the hydrological characteristics of the flood in these ungauged watersheds
Summary
According to the flood areas mapped in 2004 on the basis of residents survey (Peeters et al, 2006), no significant overflow was identified in the urban areas of these watersheds in the last 20-25 years corresponding to the period of analysis. Such an intense flood on the riparian vegetation are important: some tree trunks were ripped off by the water flow transported by flotation and impacted other trees on the banks (Figure 2c). A fluvial fan was developed in the Ourthe river bed while a 10- to 15-cm thick sediment layer was observed in the downstream part of the Blanc Gravier Creek, due to the accumulation zone created by the flat surface of the road
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