Environmentally sensitive plot-scale timber harvesting: impacts on suspended sediment, bedload and bank erosion dynamics

The impact on sediment pollution of environmentally sensitive harvesting of a 15ha plot (20% of the catchment area) of mature coniferous plantation fo rest in the 0.89 km(2) Afon Tanllwyth catchment, Plynlimon, was investigate d for 12 months before harvesting began and a further 18 months after. The results revealed., (a) a steepening of the suspended sediment concentration vs. discharge rating curve resulting in a 39% increase in suspended sedime nt yield (as compared to the adjacent forested Hafren catchment) during the year in which the harvesting operations took place; (b) a statistically si gnificant increase in main channel bank erosion rates, as compared with the nearby Afon Cyff, main channel banks are estimated to have contributed aro und 80% of the total catchment suspended sediment yield during the two year pen. od (1995-1996), and (c) no significant change in bedload yields over the duration of the study, though a longer post-harvesting time series of b edload data will be required to properly assess the impact of the harvestin g operation upon bedload yields. Forest management implications of these fi ndings are discussed in detail with respect to the existing Forest and Wate r guidelines. (C) 2001 Academic Press.