The nature of small-scale flooding, muddy floods and retention pond sedimentation in central Belgium

This study investigates the spatial variation of small-scale flooding and muddy floods in rural areas in a medium sized study area (5516 km2) and the linkage with controlling factors. A questionnaire set up in central Belgium indicates that 43% of the municipalities have to deal from time to time with muddy floods generated from direct runoff from arable land and 36% with flooding of permanent streams. A strong relation exists between the nature of the problem and the site in relation to topography and landuse. Areas suffering from muddy floods have significantly steeper cultivated slopes compared to areas suffering only from small-scale flooding. The high spatial and temporal frequency of small-scale flooding and muddy floods results in emotional and significant economic damage to private households. As a control measure more than 100 retention ponds have been constructed with 50 more to be built in the near future. The mean cost for the construction of a retention pond amounts to 380,000 EURO. These retention ponds store large quantities of sediment from runoff events and must thus be dredged regularly with costs of the order of 1.5 million EURO yearly. The dredged sediment volumes can be used to assess and predict sediment yield for these drainage basins; values vary between 0.19 and 6 m3 ha−1 year−1 for basins ranging from 25 to 5000 ha.