There is increasing interest in the use of Natural Flood Management (NFM) to reduce flood risk for vulnerable communities. NFM seeks to reduce flood risk by restoring or enhancing landscape processes and natural hydrological functions that have been damaged by human activities. Peatlands cover nearly 10% of the UK’s land cover but few of our peatlands are in a near-natural state. Most have been damaged by drainage, air pollution, fire, erosion and other land-use pressures, and the last decade has seen a dramatic increase in the number of projects aiming to restore peatland landscapes. Many communities at risk of flooding have large areas of peatland in their upstream catchments, and it is increasingly common to see claims that peatland restoration can reduce flood risk. This report therefore reviews the evidence that restoration of peatlands can reduce the peak flows of rivers and so contribute meaningfully to NFM.
We have good understanding of how storm runoff is generated from peatlands and the changes associated with peat restoration that could contribute to NFM. Restoration may provide increased storage of flood waters in peatland. This is more likely to be associated with restoring surface storage in pools, hollows and depressions than (as is often assumed) through water storage in the peat itself. Any benefits of increased storage would be limited in large rainfall events. Restoration measures can also contribute to NFM by reducing how quickly stormwater moves into river channels (‘slowing the flow’), so delaying and reducing flood peaks. The nature of the peat surface and the ‘roughness’ to stormflow presented by the peatland vegetation cover and vegetation type are key controls on this process.
Recent studies have measured or modelled the impacts of different types of peatland restoration on peaks flows in rivers and streams. Some of these have used field monitoring to directly measure change in flood peaks after restoration, but such monitoring is only appropriate in small catchments due to the need to eliminate other (non-restoration) factors. Assessments at larger catchment scales therefore use computer models to upscale the data from field measurements. There is increasing evidence from these various studies that peat restoration alters catchment runoff and can reduce peak flows, and therefore contribute to NFM, in small (<20 km2) catchments. There is some evidence from modelling that the effects could extend into larger catchments. However, the evidence is not consistent across all types of restoration. We have high confidence that the re-vegetation of bare peat and the re-introduction of Sphagnum moss to degraded peatlands can reduce peak flows by slowing storm runoff on hillslopes. We also have field data showing that drain blocking can reduce peak flows through increased storage or by slowing flows, but modelling studies predict mixed results, including potential increases in flood peaks for particular orientations and characteristics of drainage. We currently have limited field data on the impacts on peak flows of gully blocking, peat restoration by forest removal, and severe wildfire plus subsequent recovery from fire, although our current understanding of the relevant hydrological processes predicts that gully blocking will reduce peak flows, while forest removal and severe fire will have the opposite effect, particularly immediately after disruption and if no efforts are made to restore deforested or burnt peatlands.
We therefore need more field studies of the impacts of several types of peatland restoration to improve our predictions of change in peak flow and to quantify potential NFM effects, particularly over timescales that are longer than five years. There is also a need for further refinement and testing of our models and the development of modelling capacity to provide more comprehensive, catchment specific assessment of the potential NFM benefits (or adverse effects where relevant) of peatland restoration interventions. These assessments need to be extended to catchments of different sizes and geographies, including more routinely for peatland catchments containing communities at risk of flooding and for rainfall/flood events of different sizes (return periods).