Role of forested land for natural flood management in the UK: A review

Abstract

Natural flood management (NFM) is the use of natural processes and environments to mitigate flood risk by reducing and delaying peak flood. This review introduces the concept and history of NFM and looks at the current state of research into the potential for using different types of woodland to fulfill the aims of NFM. Four woodland types (catchment, cross-slope, floodplain, and riparian) are discussed with reference to studies carried out, mainly in the United Kingdom, to determine the relative merits of each type and their effectiveness in mitigating flood risk. We then discuss how trees interact with the hydrological cycle, along with a discussion of modeling methods which seek to determine the amount of water intercepted by different types of forest cover. We find that while there is some evidence that carefully planned and managed woodland can mitigate flood risk, the published data for this evidence base is somewhat sparse. This may be either due to the long timescales needed for comprehensive studies or the relative infancy of the research on NFM. More research needs to be carried out in each of the four woodland types, especially in the UK, as policy makers are increasingly looking towards nature based solutions to mitigate the potential impacts of climate change. The concept of a combined canopy/hydrological model which can be scaled from stand to watershed level and incorporate different types of woodland is suggested as it would be beneficial in guiding woodland creation policy in the future, both at the local and regional scales.

Knowledge gaps in our perceptual model of Great Britain's hydrology

“…This effort is unlikely to impact GB hydrology as a whole, but it might matter locally depending on where trees are planted. It ties in with a shift in management interventions towards more natural solutions that can reduce the frequency and severity of flooding, though questions about the overall benefit and possible negative side‐effects of such measures remain (Cooper et al, 2021; Page et al, 2020). Open questions also remain regarding the efficacy of such approaches and the sensitivity of runoff extremes or low flows to land cover change for catchments across space and time scales, as well as their wider (possibly unintended) impact on ecosystem services.…”

Modelling the Hydrological Effects of Woodland Planting on Infiltration and Peak Discharge Using HEC-HMS

“…The findings of this project are significant when considering the way in which woodland areas are currently managed, regarding the growth and felling of trees and the removal of mature woodlands to make way for either newer areas of woodland or, more significantly, impermeable developments [4,15,78]. It is not uncommon for mature(ing) woodland to be removed to make way for impermeable developments, which significantly alters the local hydrology of an area, sealing-off once permeable areas and excluding them from participating in infiltration processes [4,15,79,80]. Felled trees as a result of development are commonly 'balanced out' by planting saplings in alternate locations, however newly planted saplings will not have a comparable moderating impact on flood risk compared with the felled mature trees [18,74,80,81].…”

“…It is not uncommon for mature(ing) woodland to be removed to make way for impermeable developments, which significantly alters the local hydrology of an area, sealing-off once permeable areas and excluding them from participating in infiltration processes [4,15,79,80]. Felled trees as a result of development are commonly 'balanced out' by planting saplings in alternate locations, however newly planted saplings will not have a comparable moderating impact on flood risk compared with the felled mature trees [18,74,80,81]. This project has demonstrated the above through presenting modelled results, showing that young trees, whilst they do reduce peak flows compared to impermeable land cover, have not yet developed the root systems, and influenced surrounding infiltration rates, to the extent they will with maturity.…”

Modelling the effectiveness of land‐based natural flood management in a large, permeable catchment

“…As well as in‐channel and riparian measures, such as leaky dams, offline storage areas and floodplain woodlands and hedgerows (e.g., Hankin et al, 2020; Lavers et al, 2022; Lavers & Charlesworth, 2017; Nicholson et al, 2020), NFM includes diffuse land management that reduces runoff generation by increasing surface infiltration, enhancing soil storage, increasing evapotranspiration and inhibiting lateral surface flow (Burgess‐Gamble et al, 2017; Martin et al, 2020). Diffuse land management NFM measures focus on the conversion of farming land to woodland, crop choice and rotation, and soil management (Archer et al, 2013; Cooper et al, 2021; Koschke et al, 2013).…”