Gert Verstraeten

Long-term (105 years) variability in rain erosivity as derived from 10-min rainfall depth data for Ukkel (Brussels, Belgium): Implications for assessing soil erosion rates

intensity–kinetic energy equation (I-KE) for central Belgium. This R value is 26% larger compared to the R factor based on the I-KE equation recommended in the RUSLE. No significant monotonic trend in annual R factor could be observed over the entire period, yet a standard normal homogeneity test showed a significantly higher R factor (+31%) for the period 1991–2002 compared to the period 1898–1990. Annual variability in R factor is very high, with a coefficient of variance of 31%. For central Belgium, rain erosivity is highest in the period May–September, which corresponds well with observed soil loss rates and the occurrence of muddy floods. Especially the period May–June is critical with respect to soil erosion. The year-to-year variability in rain erosivity for May–June shows a different temporal pattern than the annual erosivity. No statistically significant increase in rain erosivity for May–June was found, and during the last decade of the twentieth century these values are lower than average. Despite the lack of a significant trend in annual rain erosivity, average 10-year erosion rates calculated with the RUSLE have increased by 24–34% from 1903–1912 to 1993–2002 for major crops grown in central Belgium, solely as a consequence of changing rain erosivity through time.

Solving the off-site impacts of soil erosion by an integrated environmental watershed management?

In central Belgium, soil erosion by water has many detrimental off-site impacts including muddy floods,silting of riverbeds and sediment deposition within small flood retention ponds. These problems cause highfinancial costs, which are increasing very fast. A controlling strategy is highly needed and given thegeographical nature of soil erosion by water and its impacts, this should be spatially oriented. Integration ofseveral incentives in an environmental management can reduce the off-site impacts of soil erosion to aminimum. If 10% of the most erodible parcels are put under fallow, the erosion risk in central Belgium could bereduced by 20-25%. By constructing small ponds in agricultural areas, much of the eroded and non-pollutedsediment can be trapped, hence diminishing sediment delivery to rivers up to 50% or more. At the same time,muddy floods in downstream villages can be reduced by diminishing the presence of sediments in runoff.

Small-scale flooding and muddy floods as a geomorphologic hazard in central Belgium: some financial consequences

Over the last decades off-site problems of soil erosion have increased in many parts of north-western Europe (Boardman et al. 1994). In central Belgium these problems are a widespread phenomenon as small-scale flooding events, often with a muddy character, are frequently occurring after moderate to intense rainfall events. It was recorded that no less than 43% of the municipalities (n = 123) in central Belgium suffer from muddy floods generated by direct runoff from arable land and 36% with flooding of permanent streams (Verstraeten and Poesen 1998). Because these hazards are not only causing financial costs but also much emotional damage to private households, different government levels take control measures, like retention ponds, to reduce or even avoid these problems for the future. During the last two decades around 100 retention ponds have been constructed with 50 to be built in the near future (Verstraeten and Poesen 1998). Frequent dredging of these ponds due to severe sedimentation in these ponds is an unexpected source of financial costs. In this paper a short overview will be given of both the nature and the financial consequences of these problems. More detailed information is presented by Verstraeten and Poesen (1999).