Overland flow velocity and soil properties in established semi-natural woodland and wood pasture in an upland catchment

Management of upland land-use has considerable potential for mitigating flood risk by increasing topsoil storage and slowing overland flow. Recent work has highlighted the potential for vegetation to impact the velocity of saturation-excess overland flow. Woodland creation is widely proposed for Natural Flood Management (NFM), but data on saturation-excess overland flow in woodland habitats is lacking. Here we measure soil properties and overland flow velocities in established broadleaf woodland and wood pasture with an understorey dominated by either grass or bracken. We show that wood pasture dominated by bracken has overland flow velocity 12–20% lower than established broadleaf woodland and 19–27% lower than grass-dominated wood pasture. Established woodland soils exhibited eight times higher saturated hydraulic conductivity than bracken-dominated wood pasture and 80 times higher than grass-dominated wood pasture. We conclude that upland habitats can be managed to reduce flood risk, first by storing storm water in the soil and then by reducing overland flow velocity through rough surface vegetation. These factors combine to reduce floods by delaying the onset of overland flow runoff and slowing its delivery to streams. It is clear than Manning's n is far from constant in these shallow overland flows, the development of overland flow datasets is, therefore, also beneficial for improving the theory and practice of hillslope rainfall-runoff modelling.