Abstract

High floods in watercourses are of great interest, because of the potential hazards they pose to people and low-lying land near rivers. In many parts of the world, inundations are common and in northwest Europe there are high discharges almost every year, occasionally with serious consequences. In Sweden, such a major flood occurred in 1985, causing a damburst. Land use in the area was mainly forestry, and fairly extensive clear-felling and drainage were believed to have amplified high discharges following heavy rain. This led to investigation of the hydrological consequences of these forestry activities. Drainage was carried out both as new drainage of virgin peatlands, and as drainage of moist and wet mineral soils, i.e. remedial drainage of clear-felled areas. The investigations were made on small catchments and utilised the calibration period and control basin technique, using linear regression relations. Peatland drainage resulted in reduced high discharges, while drainage of clear-felled areas resulted in increased high discharges, as compared to forested conditions. However, drainage of clear-felled areas partly led to reduced peak flows. A high groundwater level was an important prerequisite for increased discharges. When results from small catchments were applied to larger rivers, the effects of forest drainage were small, i.e. increases were less than 10%.