Ž . Geomorphology 29 1999 275–292 The nature of small-scale flooding, muddy floods and retention pond sedimentation in central Belgium Gert Verstraeten ),1 , Jean Poesen 1 Laboratory for Experimental Geomorphology, Katholieke UniÕersiteit LeuÕen, Redingenstraat 16, B-3000 LeuÕen, Belgium Received 16 October 1998; accepted 13 January 1999 Abstract This study investigates the spatial variation of small-scale flooding and muddy floods in rural areas in a medium sized Ž 2 . study area 5516 km 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 m 3 ha y1 year y1 for basins ranging from 25 to 5000 ha. q 1999 Elsevier Science B.V. All rights reserved. Keywords: small-scale flooding; muddy floods; retention ponds; sediment yield; central Belgium; economic costs 1. Introduction For the last few decades, off-site problems of soil erosion have arisen in many parts of north-western Ž . Europe Boardman et al., 1994 . This is certainly true for the case of central Belgium. Storm runoff is often too high compared to the capacity of drainage ditches or the sewage system. This leads to small- ) Corresponding author. Fax: q32-16-32-64-00; E-mail: gert.verstraeten@geo.kuleuven.ac.be 1 Both authors are working for the ‘Fund for Scientific Re- search, Flanders’. scale flooding of lowlands such as meadows and in Ž . housing properties of dense populated areas Fig. 1 . If there is pronounced soil erosion in the drainage basin, these local floods can take the form of ‘muddy floods’, covering the streets and even floors inside Ž . houses with a blanket of mud Fig. 2 . This not only results in financial costs to the government and private households for cleaning up, but also psycho- logical damage to those who are frequently con- fronted with it. Therefore, different government levels take con- struction control measures in order to reduce or even avoid the damage caused by these floods. The most common way to do this is by the construction of 0169-555Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-555X 99 00020-3