The example of the Pontbren catchment in upland Wales is a well-documented NFM project initiated by farmers planting trees in the 1990s to create shelter for their sheep (Clark, 2018). It provides empirical evidence of tree planting increasing soil infiltration(Marshall et al., 2014). Two treatments, Figure 4: A farm in the Pontbren catchment (Forest Research, no date)
removal of sheep and removal of sheep with tree planting were compared to a control using
12mx12m plots for each treatment in four different sites. It was found that runoff was reduced by 48% on the ungrazed plots and 78% on the ungrazed and tree-planted plots. The median soil infiltration rate 5 years after treatment was found to be 67 times greater in the plots planted with trees, compared to the grazed pasture. The scale of increase may seem implausible, but grazing can create a thin near impermeable layer of soil and the soil infiltration rates for the control areas were very low. The soil infiltration rates before treatment were only measured once, compared to three times post treatment, so this may add some uncertainty to the result. Hillslope studies elsewhere revealed soil infiltration rate increases due to woodlands just 5-8 times greater than grazed pasture (Ford et al., 2020). These studies were of mature woodlands compared to newly planted trees in Pontbren. Furthermore, before reaching a conclusion on the differing results. there is a wide range of variables to investigate, such as soil properties, topography and tree species composition.
Slow the Flow at Pickering is a Defra NFM pilot scheme in response to the 2008 Pitt Review. Cost-benefit analysis indicated that a traditional engineered flood management scheme would not meet the thresholds for implementation. The main elements of the Pickering project were bunds, but additionally there was planting of riparian and farm woodland and installation of over 150 large woody debris (LWD) dams. The project was unable to meet its planting goals for reasons discussed later. However, modelling showed that planting 50 ha of riparian woodland and construction of 100 LWD dams would reduce the peak discharge in a 1 in 25 year event an amount equivalent to 21% of the necessary margin to reduce the flood peak to the level at which the town of Pickering starts to flood (Nisbet et al., 2015). It is not a complete solution, but it would still be a significant contribution to flood prevention.
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