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Long-term drainage ditch hydrogeomorphological response to plot-scale timber harvesting in the Nant Tanllwyth Catchment on Plynlimon, Mid-Wales | Oliver Clegg

Key-words: timber harvesting, erosion, Plynlimon, aggradation, drainage ditch

Abstract: Accelerated rates of river channel and drainage ditch erosion in response to plot-scale timber harvesting in the Tanllwyth catchment, mid-Wales, have been widely reported. Reduced thermal insulation immediately following canopy removal has been shown to increase bank susceptibility to frost heave and soil desiccation, leading to temporarily heightened vulnerability to erosion and concomitant rises in suspended sediment and bedload yields until vegetation recolonisation. However, longer-term, non-erosional responses – including after riparian buffer strip introduction – are less well known. This Study shows that natural recovery in rates of erosion after plot-scale timber harvesting is followed by differential rates of aggradation within drainage ditches. An apparent binary system of ‘open’ (free-flowing with no superficial vegetational layers) and ‘closed’ (colonised by surficial Sphagnum mosses) drainage ditch types has established itself in the Tanllwyth catchment, and sediment distributions from each suggest subtle differences in depositional circumstances likely influenced by forestry-related activities. Furthermore, Tanllwyth drainage ditches have also been shown to be important sediment stores and may represent >16% of total sediment flux with implications for catchment hydrogeomorphological development dominated by low sediment entrainment competence within ditches. The findings in this Study demonstrate how increased drainage ditch erosion and sediment yields are not the only response stimulated by plot-scale timber harvesting, and that a longer-term perspective is warranted. Further research focused on more detailed investigations between how ‘open’ and ‘closed’ ditches differ may be prompted by this Study, with larger sediment sample sizes and direct monitoring of ditch water and sediment discharges a prerequisite for detailed comparisons.