Flood risk alleviation

There is strong evidence that floodplain woodlands could play a more important role in ameliorating downstream flooding. Although most of Britain's original floodplain woodland has been lost due to past river engineering and land reclamation works, organisations such as the Forestry Commission are working towards promoting its restoration. The use of floodplain woodland as a soft-engineered aid to flood control is now a key area of study by Forest Research. In addition to the potential advantages of flood control and storage, it offers a wide range of other benefits, including improvements to water quality, nature conservation, fisheries, recreation and landscape.

Background

Flood risk is a major issue in most EU countries. Extreme floods are expected to become more frequent due to increases in winter rainfall and irregular and extreme events caused by climate change. Many flood-prone areas, especially in urban locations, are protected by ‘hard-engineered’ flood defences such as flood walls and embankments. These will continue to play a key role in the future – but existing defences may no longer be able to provide the intended level of protection to towns and cities.

Hard-engineered defences are expensive and are not regarded as environmentally friendly, as they rarely improve the ecological or aesthetic value of the river corridor. Flood defence engineers are now shifting their attention to explore the potential of using ‘soft-engineering’ techniques for more sustainable development. These techniques are based on the principle of reducing run-off from the land following a rainfall event by providing areas of semi-permanent or permanent wetland in which to store floodwater, delaying the passage of the flood peak.

Forests are known to help reduce flooding. They use more water than other vegetation types, mainly through interception. Forests can also affect floods through their soils holding back and delaying the passage of rainwater to streams and rivers. They are usually drier during the summer due to their higher water use, enhancing the soil’s ability to store rainwater – the sponge effect.

While individual forestry practices and phases of forest growth can affect peak flows within small headwater catchments, the effects appear to even out at the larger catchment scale. Upland forestry alone is unlikely significantly to affect downstream flood risk.

Opportunities

The government’s Flood and Coastal Erosion Risk Management Strategy (2010), Making Space for Water (2005) and Defra's policy statement published in 2009, promote a whole-catchment approach to flood alleviation. The strategy identifies that sustainable development should be firmly rooted in all flood and coastal erosion risk management decisions and operations. Woodland provides a number of options, primarily the ability of floodplain woodland to slow down flood flows and enhance flood storage. Research suggests that the greater hydraulic roughness (the resistance of the bed of a channel to the flow of water in it) provided by floodplain woodland could make a significant contribution to flood alleviation (Thomas and Nisbet, 2007).

Using floodplain woodland as a sustainable method for flood alleviation will help to tackle the increasing threat of flooding faced by many local communities due to climate change, especially where it is not cost-effective to construct engineered defences. The ability of floodplain woodland to benefit water quality and freshwater habitats offers the potential to develop win–win solutions, such as contributing to meeting ecological and chemical quality targets under the EU Water Framework Directive. Planting floodplain woodland would help to meet the UK Biodiversity Action Plan target to create 7650 ha of wet woodland in England by 2015.

Some flood defence engineers argue that floodplain woodland would have only a small effect on flood flows, while others have expressed concern that any backing-up of floodwaters could adversely affect local properties, especially in areas where there are already such issues caused by bridges and culverts and other hydraulic restrictions. The high degree of uncertainty associated with these and other potential impacts has precluded any significant floodplain woodland planting to date, but research is ongoing to investigate and address these issues.

Practical considerations

Before undertaking a floodplain woodland creation or restoration project, work should be carried out to assess the likely impact of such a scheme on the local area in terms of flooding. This work will help determine whether floodplain woodland would benefit the area, or whether it would pose a threat by increasing the risk of flooding.

A number of actions can be taken before, during and after a floodplain woodland restoration/creation scheme.

Evaluating the impact on flood flows and flood risk in the urban area

Hydraulic models can be applied to the watercourse, and available stream-flow and high-resolution topographic data can be used to assess the effect on flood flows of planting floodplain woodland at potential sites. The results can be assessed in terms of flood risk at the urban area, and sites can be ranked according to their effectiveness at alleviating flooding.

Investigating the influence of woodland design and management factors on flood flows

The same models can be used to evaluate the effect of varying the design and management of the woodland on flood velocity, depth, storage volume and peak travel time. Factors to consider include the shape, area, pattern and spacing of tree planting, species choice, woodland structure and establishment methods. The findings can be used to guide the design and management of the planned woodland at the preferred sites.

Assessing the impact of planting floodplain woodland

Additional instrumentation can be installed at the selected sites to measure the initial effects on flood flows of site preparation and planting of floodplain woodland. Long-term monitoring enables evaluation of the longer-term effects on floodplain roughness and flood flows as the woodland establishes and matures. The results can be used to validate and improve existing models, enabling them to be applied with confidence at other UK sites.

Demonstrating and communicating the benefits for flood alleviation

Publications, group visits to demonstration woodland, and outreach seminars can help communicate and explain the benefits of floodplain woodland for flood retention to practitioners, planners and policy informers.

Case studies

Restoring floodplain woodland for flood alleviation

The Laver and Skell catchments, west of Ripon, North Yorkshire, were selected for a national, multi-objective pilot project. Together these make up a catchment area approximately 140 km2, with mainly arable agriculture in the lower reaches, pasture in the middle, and heather moor in the headwaters.

The aim was to investigate the potential for delivering flood-risk management through land-use and land-management changes at a catchment scale, while also pursuing resource protection, biodiversity and access opportunities.

The multi-objective, Defra/Environment Agency/English Nature/Forestry Commission pilot project provided an ideal opportunity for Forest Research to establish a demonstration floodplain woodland within the River Laver catchment to investigate the impact of woodland on flood flows in the river. A positive result would strengthen support for using floodplain woodland as a sustainable method for downstream flood alleviation.

Key results

  • Mean flood depth increased by as much as 0.6 m within the woodland area
  • A backwater effect of up to 330 m upstream of woodland.
  • A significant increase in flood extent
  • A considerable decrease in average flow velocities, by as much as 50%
  • A significant delay in peak flow timing: between 15–20 minutes for each woodland site, and up to a 55-minute delay for all four sites combined
  • Potential desynchronisation of flood peaks of the rivers Laver and Skell at Ripon, reducing total flow through Ripon by as much as 2%.

Modelling showed that the time lag tended to increase linearly with the width and length of floodplain woodland, suggesting that a number of smaller woodland blocks would be just as effective as an equivalent larger block in delaying flood flows.

Drawbacks

Despite the positive findings, landowners proved unwilling to submit applications for planting floodplain woodland at any of the sites identified. The main reasons were:

  • Reduction in capital value of the land and permanent nature of the land-use change
  • Loss of agricultural income
  • Risk of flooding causing damage to newly planted trees and landowners having to fund replanting at their own expense
  • Change in landscape and perceived loss of vista
  • Restricted access to neighbouring fields and loss of direct access to drinking water for livestock
  • Increased capture of rubbish and other debris by woodland, requiring landowners to arrange removal at their own cost
  • Increased cost of replacing fencing damaged by larger quantities of woody debris trapped during flood events
  • Scepticism that planting floodplain woodland would be effective at reducing downstream flood risk and could be relied on to provide the necessary flood protection
  • Concern that woodland planting would be used as an excuse for cancelling the proposed flood defence scheme at Ripon, and let local authorities ‘off the hook’.

Conclusions

The project provides further support for the potential of floodplain woodland to alleviate downstream flooding. Model results show that planting of four sites in the River Laver catchment, totalling an area of 40 ha, could delay the progression of a 1% a.p.e. flood by almost 1 hour. Although this was predicted to have a negligible impact on flood peak height in the River Laver, the time lag had the potential to desynchronise the flood flows from the tributary River Skell catchment, and so reduce the downstream flood peak as the main river flows through Ripon. It was estimated that desynchronisation could reduce the flood peak height by 1–2%, with the possibility of a much greater reduction if the woodland area was expanded. Modelling showed that the time lag tended to increase linearly with the width and length of floodplain woodland. This suggests that a number of smaller woodland blocks may be just as effective in delaying flood flows as an equivalent larger block.

Investigating the use of large woody debris dams for flood alleviation

Large woody debris is a term applied to pieces of dead wood larger than 0.1 m in diameter and 1.0 m long. It includes entire trees, logs and branches that can accumulate to form ‘leaky’ dams within small rivers. These dams may play an important role in holding back floodwater and reducing the risk of downstream flooding.

Hydraulic impact of large woody debris

  • Formation of major roughness elements within channel and on floodplain
  • Reduction in water velocity along reach
  • Increased water depth upstream of dams
  • Raised water levels, promoting overbank flows
  • Increased water storage, delaying and reducing flood generation
  • Increased variability of flow conditions, promoting habitat creation.

This study used a hydraulic model to simulate the effect of placing a series of large woody debris dams along a 1-km-long experimental reach at Great Triley Wood near Abergavenny in South Wales. Following the modelling, a series of 12 large woody debris dams was constructed within the reach to assess dam stability and effectiveness in controlling flows.

Key results

  • A significant effect on local water levels upstream of the debris dam
  • A water level increase of up to 1.46 m immediately upstream of the debris dams
  • A localised backwater effect extending approximately 165 m upstream of the dam (the effect is dependent on local conditions such as bed slope, and will vary with location)
  • Reduction in flow velocity by as much as 2.1 ms–1 at peak flow immediately upstream of the dam
  • Average channel velocities during ‘normal’ flows were reduced by 54% after the dams were placed into the channel at key monitoring points
  • Flood peak travel time increased by 15 minutes over a 0.6-km reach.

Conclusions

Large woody debris dams are an effective method of sustainable flood alleviation. The improved management of existing riparian and floodplain woodlands and the planting of new woodlands will have a beneficial effect on flood alleviation.

This improved management of existing woodlands and new planting will also have other beneficial effects, including:

  • Increased biodiversity and the restoration of native ecosystems
  • Sediment storage and erosion control
  • Cost-effective method of flood control for small, rural communities where large, hard-engineered projects are too expensive
  • Improved water quality
  • Better landscape
  • Increased economic outputs from local employment in production of timber and other woodland products.

Services

A strong science base is critical to address the issues surrounding the potential impacts of floodplain woodland planting. The Forest Hydrology Programme at Forest Research involves a number of collaborative studies on the effects of forestry on flood flows, both on the floodplain and in the wider catchment. These include continuing long-term studies of the hydrological effects of upland conifer forests in research catchments, and monitoring the effects of established, newly planted and potential floodplain woodland sites on flood flows.     

Further information

Forestry Commission (2011). Forests and Water Guidelines. Forestry Commission, Scotland.

Environment Agency (2010). Flood and Coastal Erosion Risk Management appraisal guidance. Environment Agency, UK.

Defra (2005). Making Space for Water: Innovation Fund (PDF-452K). Department for Environment, Food and Rural Affairs, London.

Defra (2007). Innovation Project SLD2316: Restoring Floodplain Woodland for Flood Alleviation. Department for Environment, Food and Rural Affairs, London.

Defra (2009). Policy Statement: Appraisal of Flood and Coastal Erosion Risk Management. Department for Environment, Food and Rural Affairs, London.

Defra (2010). Flood and Water Management Act 2010. Department for Environment, Food and Rural Affairs, London.

 

Nisbet, T. R. and Thomas, H. (2006). The role of woodland in flood control – a landscape perspective. In ‘Water and the landscape: the landscape ecology of freshwater ecosystems’. Proceedings of the 14th Annual IALE(UK) Conference, Eds B. Davies & S. Thompson, p118-125. IALE(UK), Oxford.

Somerset County Council website: The Parrett Catchment Project.

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.