SPATIAL-DISTRIBUTION OF COARSE WOODY DEBRIS DAMS IN THE LYMINGTON BASIN, HAMPSHIRE, UK

Debris dams of coarse woody debris have a significant influence on channel processes in forested areas but few detailed studies have been made of variations within a single basin. Results from previous research are standardised and show that average variations throughout basins include densities of debris dams up to 40 per 100 metres of channel and can involve a loading value of up to 225 kg per m2 of channel. Variations have been ascribed to distance downstream, to channel width, to land-use effects, to felling, and to the management of coarse woody debris in streams. This study of the Lymington Basin, 110.4 km2 in drainage area, shows that the input of storm debris resulting from blowdown accounts for 45% of the gross load. The remaining 55% net load varies according to distance downstream, to land use with the greatest loads in deciduous woodland areas, and according to management removal of debris from streams and multiple regression equations are provided. It is deduced that as a consequence of long-term management the present channel debris may be as little as 7% of the total net load that could have been present if no management had occurred.