Abstract
Woodland planting is gaining momentum as a potential method of natural flood management (NFM), due to its ability to break up soil and increase infiltration and water storage. In this study, a 2.2 km2 area in Warwickshire, England, planted with woodland every year from 2006 to 2012, was sampled using a Mini Disk infiltrometer (MDI). Infiltration measurements were taken from 10 and 200 cm away from the trees, from November 2019 to August 2021. Two individual hydrological models were built using the US Hydraulic Engineering Center Hydrological Modelling System (HEC-HMS), to model the effects of infiltration change on peak flows from the site throughout the summer and winter. The models were calibrated and validated using empirical data; the Nash and Sutcliffe Efficiency (NSE) was used as an indicator of accuracy. Results from this study show that woodland planting reduced peak flow intensity compared to impermeable land cover by an average of 6%, 2%, and 1% for 6-h, 24-h, and 96-h winter storms, respectively, and 48%, 18%, and 3% for 6-h, 24-h, and 96-h summer storms, respectively. However, grassland simulations show the greatest reduction in peak flows, being 32%, 21%, and 10%, lower than woodland for 6-, 24-, and 96-h winter storms, respectively, and 6%, 3%, and 0.5% lower than woodland for 6-, 24-, and 96-h summer storms, respectively.
Highlights
Urbanisation and the replacement of permeable and vegetated surfaces to impermeable surfaces, such as asphalt and concrete, reduces lag times and increases peak flows in receiving watercourses, influencing the likelihood and severity of high-flow or flooding events across the UK [1,2]
The winter of 2020 was the fifth wettest on record (329.4 mm/143% higher than the 1981–2010 baseline), and February of 2020 was the wettest ever recorded, with 155 mm of precipitation (258% higher than the 1981–2010 baseline) [88,89]. These dates are within the time periods that winter infiltration data were collected, and the excess rainfall received would have contributed to the study site saturation, influencing infiltration data collection
Area calculations regarding the coverage of superficial alluvium, clay, peat, and fluvial deposits throughout the UK show that 15% (39,269.24 km2 ) of UK superficial geology is similar in infiltration to the geology of the HofE site [92]. This demonstrates that the results of this study are significant, and are representative of 15% of UK geology, demonstrating that the results found throughout this study with regard to woodland planting and their runoff reduction capabilities can be extrapolated, furthering the usage of natural flood management (NFM) across the UK
Summary
Urbanisation and the replacement of permeable and vegetated surfaces to impermeable surfaces, such as asphalt and concrete, reduces lag times and increases peak flows in receiving watercourses, influencing the likelihood and severity of high-flow or flooding events across the UK [1,2]. Authorities responsible for managing flood risk in the UK have increased investment in alternative, more sustainable methods of mitigating flooding, such as natural flood management (NFM) techniques [2,5,6]. Common NFM methods can be categorised into those that (a) reduce hydrological or hydraulic connectivity; (b) create storage; or (c) increase infiltration [8]. Examples of these methods include vegetation planting to increase infiltration and interception, changing animal grazing and farming routines to reduce compaction and increase lag time, and reconnecting or introducing offline marshlands and mudflat areas, to slow the flow of flooding water during a storm event [7,9,10,11]
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