Spatial and temporal variation of muddy floods in central Belgium, off-site impacts and potential control measures

Numerous villages in the European loess belt are confronted with floods caused by runoff from agricultural land. Seventy-nine percent of the municipalities in central Belgium experienced at least one muddy flood during the last decade. Of these flooded municipalities, 22% have been affected more than 10 times during this period. Twenty municipalities have been selected for a detailed analysis. A database of 367 locations affected by muddy floods has been compiled, and the connectivity between cultivated areas and inhabited zones could be assessed for 100 flooded locations. Roads and drainage network facilitate runoff transfer between cultivated and inhabited areas in 64% of cases. Three types of areas producing muddy floods have been identified: hillslopes (1–30 ha) without thalweg where runoff is generally dominated by sheet flow; small catchments (10–300 ha) characterised by runoff concentration in the thalweg and medium catchments (100–300 ha) with multiple thalwegs dominated by concentrated runoff. About 90% of muddy floods are generated on hillslopes and in small catchments. A critical area–slope threshold for triggering muddy floods has been computed for hillslopes. A logistic regression shows that muddy floods are generated in small and medium catchments with 99% probability after 43 mm rainfall. Rainfall depths required to trigger muddy floods are lower in May and June (25 ±12 mm) than between July and September (46 ± 20 mm), because of different surface conditions (crusting, roughness and crop cover). Each year, muddy floods lead to a total societal cost of 16 × 10⁶–172 × 10⁶ € in central Belgium, depending on the extent and intensity of thunderstorms and monetary values damaged. Recent datasets suggest that the phenomenon is becoming more frequent in central Belgium, because of land consolidation, urban sprawl and expansion of row crops, sown in spring, at the expense of winter cereals. The huge costs induced by muddy floods justify the installation of erosion control measures. It is suggested to install a grassed buffer strip at the downslope edge of cultivated hillslopes to protect houses and roads. In small and medium catchments, it is preferred to install a grassed waterway and earthen dams in the thalweg.