The fourth and final meeting of the Wessex Silvicultural Group in 2015 took place at Bower Hinton Farm, near Martock, Somerset. Here the focus of the day was how tree planting can alleviate flooding, under our study topic for 2015 ‘The Silviculture of Resilience’. Part of the River Parrett Catchment Project, tree planting at Bower Hinton was supported by grant aid from the Forestry Commission, under a scheme to alleviate flooding lower down the River Parrett. The visit benefited from the expert input of Ben Thorne, who was the Farming and Wildlife Advisory Group (FWAG) advisor at the time of planting, Dick Gossling the Forestry Commission Woodland Officer for the area and Huw Thomas, a hydrologist with Forest Research who has been monitoring the site.
Following flooding along the River Parrett in the summer of 1997 and winter of 1999, a number of agencies came together to encourage changes to land management which would improve the drainage characteristics and reduce runoff volumes of land surrounding the river. A twelve-point action plan was formulated, which included plans to construct temporary floodwater storage areas on farmland, create new wetland habitats, to raise river banks and to develop woodland along the course of the river.
Following an initial introduction to the Bower Hinton project, the group visited one of the farm’s attenuation ponds, created to slow the passage of water from surface runoff into the river. Thousands of these pond are needed throughout the river catchment to have a significant effect, highlighting the need for a joined-up, landscape approach to flood alleviation.
The soil at Bower Hinton is a Yeovil sand - a light sandy silt loam, which has a propensity to cap (compact at the surface) when cultivated. This sealing at the soil surface (which can be up to 0.5 inches thick) can result in severe runoff volumes. Owner Patrick Palmer discussed some of the agricultural practices he has adopted to reduce surface runoff on the sloping land around the river. This includes maintaining crop cover into the winter months (to improve infiltration of precipitation), use of a depressional roller after ploughing (which creates depressions at the soil surface, increasing the time taken for a cap to form) and use of a tied ridger, which creates mini-dams using small spades.
Bower Hinton Farm is situated in the upper reaches of the River Parrett catchment, and the site has been the subject of long-term monitoring by Forest Research since 2007. Huw Thomas, a hydrologist with FR has been involved with the monitoring programme, which aims to investigate the impact of planting floodplain woodland on flood flows. In 2005 an area of 4.9ha was planted on the floodplain at Bower Hinton, on the eastern side of the River Parrett. The new floodplain woodland consists of native species including ash, field maple and hazel (alder was not available at the time of planting due to concerns over Phytophthora alni) along with woody shrubs such as hawthorn, and 15% open space. Since 2005, monitoring has looked at the water level upstream and downstream of the woodland, flow velocity, floodplain water depths and the roughness of vegetation at the site.
Data collected at the site was been used to set the parameters of a mathematical model, which can be used to predict the impact of establishing the woodland on flood flows. One of the most important reference co-efficients within this model relates to land-use and vegetation roughness - the rougher the vegetation surrounding the river, the greater the effect on reducing the risk of flooding. Surface roughness changed as the site developed from grassland (the smoothest vegetation type) to woodland. Roughness also appears to vary with tree species – when broadleaved species lose their leaves in the winter, roughness is also lost, while there is no seasonal effect with most coniferous species. Further studies are required before firm recommendations can be made on species choice.
The hydraulic model predicted that the planted woodland at Bower Hinton significantly reduces river flow velocity locally, and increases flood levels within and upstream of the woodland, but the area of woodland on its own is too small to affect downstream flood risk.
Ideally, to maximise the effect of woodland creation on flood risk reduction, both sides of the river bank should be planted, something which was not possible at Bower Hinton. Also, larger/longer areas of woodland are likely to have a greater impact – at Bower Hinton, the new woodland site is relatively small (4.9ha). Strips of woodland planted across the floodplain (parallel to the direction of flow) and hedgerows near the river bank also appear to be effective in reducing flood risk. In terms of woodland composition, multi-stemmed trees planted in species mixtures (which have a higher degree of roughness) are more effective in reducing flood risk than even-aged monocultures and high forest. In this respect, there is an economic impact of planting woodland for flood alleviation, which is not necessarily compensated by income from timber harvested or by grant schemes for private land owners. However, as Patrick Palmer pointed out, at Bower Hinton the area planted was a low-lying field which was regularly flooded, and had little agricultural potential.
Looking to the future, there is a clear need to address the loss in productivity of land when planting new woodland for flood alleviation. Compensation should be made available for changes in land use, and a multifunctional approach to land management adopted, as it has been in other EU countries. New initiatives such as Somerset’s Hills to Levels project aim to work towards this goal, and to stimulate a collective ownership of water management.
Pre-meeting notes: Location Map
European Environment Agency (2015) Water-retention potential of Europe’s forests. EEA Technical Report No.13
Forestry Commission (2011). Forests and Water Guidelines. Forestry Commission, Scotland.
Thomas, H. and Nisbet, T.R. (2007). An assessment of the impact of floodplain woodland on flood flows. Water and Environment Journal. 21, 114–126