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

Abstract

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.

The influence of land management and seasonal changes in surface vegetation on flood mitigation in twoUKupland catchments

“…The influence of vegetative roughness was also shown in the management scenario simulation, alongside permeability and interception. Woodland has been found to have lower surface roughness (the ground level understory of woodlands can be shaded out and is not dense) and higher permeability than grassland (Bond et al, 2020; Bond et al, 2021; Monger, Bond, et al, 2022). In Swindale, scenario 2_2 reduced overland flow and total flow peaks by up to 24.2% and 20.5% respectively, and increased subsurface peaks by up to 16.1%.…”

“…Selective grazing by animals changes the structure and volume of vegetation present, which may alter surface roughness. Surface roughness is an important modifier of overland flow with research showing that vegetation has the capacity to significantly reduce overland flow velocity with varying effectiveness depending on season and management (Bond et al, 2020; Holden et al, 2008; Monger, Bond, et al, 2022). Vegetation roughness retains water so that duration of overland flow during storm events can be longer in rougher vegetation (Bond et al, 2021), potentially delaying flood peaks.…”

A Geospatial Modelling Approach to Assess the Capability of High-Country Stations in Delivering Ecosystem Services

“…Flat areas are usually prone to flood and have a high risk of sediment delivery as they receive quick overland water and drain flow from the steep slopes, which are dominant runoff pathways in hillslopes [47]. Overland flow from steep slopes down to flat areas can be controlled by trees or shrubs in the foothill area, as the type of vegetation contributes to intercepting the overland flow and reducing the flood risk due to its density (roughness) [48,49]. For instance, woodlands have greater permeability, consequently, greater potential in flood mitigation than grazed grasslands due to improved soil porosity and structure [50] and improved soil hydraulic properties [47].…”

Rigid vegetation affects slope flow velocity

“…The increased ow velocity owing to the compression of the water cross-sectional area by vegetation at low coverage cannot replace the reduced ow velocity owing to vegetation form resistance [34]. Under these conditions, the compression of the water ow space by high coverage vegetation becomes more pronounced [52], leading to increased ow velocity.…”