Overland flow is an important component of peatland hydrology.
Hydrological models of peatlands are being developed that require
estimates of flow velocity and its controls. However, surprisingly
little is known about overland flow velocities in peatlands. Some
peatlands have also been drained using open ditches, and these need to
be incorporated into flow models. This paper presents field data on the
velocity of overland flow and drain flow in upland peatlands. The
relationships between flow velocity, vegetation cover, slope, and water
depth are explored. Sphagnum provided a significantly greater effective
hydraulic roughness to overland flow than peatland grasses. In all
cases, a significant break in process occurred for flows with water
depths of around 1 cm so that there were two components of the
roughness curve. This is consistent with partial submergence theory for
very shallow flows where resistance increases with depth as the soil
surface first becomes fully submerged. While each surface cover type
should be considered separately, the results also suggest that a
first-order estimate of Darcy-Weisbach roughness and mean velocity can
be based on a single parameter for each surface cover. This paper
presents an empirical overland flow velocity forecasting model that can
be applied to peatlands. The model combines the partially submerged
component for flows with water depths below 1 cm with the fully
submerged component for flows with depths up to 5 cm, which are
representative of the depths of flows that occur across peatlands.