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 m uddy floods in rural areas in a medium sized study area (5516 km(2)) and th e linkage with controlling factors. A questionnaire set up in central Belgi um 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% wit h flooding of permanent streams. A strong relation exists between the natur e of the problem and the site in relation to topography and landuse. Areas suffering from muddy floods have significantly steeper cultivated slopes co mpared 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 contr ol 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 re tention pond amounts to 380,000 EURO. These retention ponds store large qua ntities of sediment from runoff events and must thus be dredged regularly w ith costs of the order of 1.5 million EURO yearly. The dredged sediment vol umes can be used to assess and predict sediment yield for these drainage ba sins; values vary between 0.19 and 6 m(3) ha(-1) year(-1) for basins rangin g from 25 to 5000 ha. (C) 1999 Elsevier Science B.V. All rights reserved.