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Can forestry reduce flooding?
 Forests the world over, have long been associated with helping to reduce flooding. Most flooding disasters have been partly blamed on the effects of deforestation. However, the emerging view amongst forest hydrologists is that forestry in general probably has a relatively limited role to play in reducing major floods.
The case for forestry reducing flooding Woodland soils typically have a relatively open, organic rich upper layer, which facilitates the rapid entry and storage of rain water Forests are known to use more water than shorter types of vegetation. This is mainly due to the interception of rainwater by their aerodynamically rougher canopies. Interception can reduce the amount of rainfall reaching the ground by as much as 45% or more for some types of forests. A reduction of even half of this amount could therefore make a major contribution to flood control. Another way that forests could affect floods is by their soils holding back and delaying the passage of rain water to streams and rivers. Forest soils tend to have a more open structure resulting from greater amounts of organic matter and the action of tree roots and soil fauna. They are also usually drier during summer periods due to the higher water use by forests. These conditions enhance the ability of the soil to receive and store rain water and is commonly referred to as a ‘sponge effect’. The case against forestry reducing floodingThe interception loss varies between forest types. While losses of 25-45% are typical of conifer forests, those by broadleaves are lower and tend to range between 10-25%. Losses from broadleaves are further reduced to 5-10% during the leafless period, when the risk of flooding is often greatest. Another important factor is that interception declines with the size and intensity of a given rainstorm, reaching a maximum of 5-7 mm in a day. Thus losses for major rain storm events may be <10% even for a conifer forest. The forest sponge effect also has its limitations. Most major floods occur following periods of exceptionally heavy and/or prolonged rainfall, often during the wet season. Forest soils are generally fully rewetted by such times with a reduced capacity to receive and store storm water. The role of forest management and scale Ploughing can provide an intensive network of smooth channels for the rapid removal of rain water to streams Another major factor is the role of forest management. Both the amount of water used by a forest and the timing of water flows are greatly influenced by the nature of silvicultural practices. For example, water use varies with species choice and forest age, reaching a maximum for mid-aged conifer stands. More important, however, is the propensity of forest practices such as site cultivation and drainage, road construction and timber harvesting to increase the speed of water runoff, opposing the effects of forest water use and soil storage. Forest clearfelling in particular can result in rapid surface runoff due to the removal of the vegetation cover and harvesting machinery having the potential to cause soil compaction and rutting. Forest scale has a direct bearing on the impact of forestry on flood flows. Most serious flooding problems arise well down the catchment of major rivers. At this scale, forest cover often forms a relatively small proportion of the total land area. Thus any ability to reduce flood flows will be similarly limited. Forest design also has a role to play in terms of the contrasting effects of the mix of forest species, tree ages and amount of open space, as well as the different forestry practices. Research resultsThe Forest Hydrology Programme involves a number of collaborative studies of the effects of forestry on flood flows. These include continuing long-term studies of the hydrological effects of upland conifer forests in research catchments at Coalburn in north England and Balquhidder in mid Scotland. Staff were also involved in a major EU funded, Europe-wide study of the impact of forests and silvicultural practices on the extreme flows of rivers (FOREX) between 1996 to 2000. The results of these studies show that 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. This is in line with research by CEH at Plynlimon in Wales, which found that upland floods in excess of the mean annual flood were scarcely affected by land use. It is also supported by the Natural Environmental Research Council Flood Studies Report of 1975, which found little evidence from regional flood studies in Britain that the area of forest was a significant factor in statistical relationships used for flood prediction. This evidence leads to the conclusion that upland forestry is unlikely to affect downstream flood risk. The impacts of rural land use and management on flood generation were recently reviewed by a research consortium for Defra. This has led to a re-examination of long-term flow data from a number of key catchments. A world-wide review of the subject of forests and floods is available from the Food and Agricultural Organisation of the United Nations. Can native woodlands help?The less intensive management of native woodlands reduces the effects of forestry practices such as drainage that act to increase peak flows. Thus it is argued that native woodlands are better placed to reduce downstream flooding. However, the lower water use of broadleaves, especially during winter, limits the scope for reducing peak flows. Nevertheless, there is evidence from research work by CEH in Wales that the sponge effect alone could be responsible for a significant reduction in rapid runoff, compared to that from heavily grazed sheep pasture. Shallow compaction caused by sheep trampling can greatly reduce the ability of the soil to receive rainwater, potentially increasing the risk of flash floods. Research is underway by the Flood Research Consortium to evaluate and model this effect, both at the hill slope and catchment scale. If soil compaction from grazing is found to contribute to increased flood flows, the planting of native woodlands could help to counter this, as well as provide a range of other benefits such as improved water quality. Will a shift to continuous cover forestry (CCF) help? A site undergoing transformation to continuous cover forestry The small-scale nature of silvicultural practices under continuous cover forestry will favour those factors that help to reduce flood flows. In particular, the maintenance of a permanent forest canopy with greater edge will act to maximise forest water use. This, together with the expected reduction in soil disturbance from cultivation, drainage and harvesting operations, could act to ameliorate downstream flooding. However, much will depend on the scale and location of forest cover within a given catchment. A recent report 'Environmental best practice for continuous cover forestry' commissioned by the Environment Agency and Forestry Commission concluded that none of the existing forest hydrology experimental catchments are suitable for conversion to CCF. Thus there may be a need for a new study to assess the impact of CCF on flood flows. The role of floodplain woodlands Standing trees, fallen logs, branches and leaf litter form a very dynamic and complex network of multiple channels and dams, which help to slow down flood flows Although most of Britain’s original floodplain woodland has been lost due to past river engineering and land reclamation works, there is good evidence to suggest that it could have an important role to play in ameliorating downstream flooding. This is based on woodland’s greater hydraulic roughness compared to other vegetation types, which acts to slow down and thus potentially reduce flood peaks. Unfortunately, floodplain woodlands also pose a number of risks for flood defence. These include enhanced upstream flooding due to the backing-up of floodwaters, restricted access to river banks for flood defence works, loss of engineered flood control, and increased downstream flooding caused by large woody debris blocking bridges and other structures. A major element of this forest hydrology programme is to evaluate the effects of floodplain and riparian woodlands on flood flows. Studies include: Restoring floodplain woodland for flood alleviationThe final report on this Defra funded project was published in July 2008. The work provides further support for the potential of floodplain woodland to alleviate downstream flooding.  Restoring floodplain woodland for flood alleviation (PDF-1974K)
Unfortunately, it proved not possible to plant floodplain woodland at the selected sites and the project had to be curtailed. Lessons learnt and recommendations for future work are set out, including the need for one or more replacement demonstration sites to be established to communicate and explain the benefits of floodplain woodland for flood alleviation.
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