Natural Flood Management (NFM) Knowledge System: Part 2 - The Effect of NFM Features on the Desynchronising of Flood Peaks at a Catchment Scale

Janice Blanc, Grant Wright, Scott Arthur

Research output: Book/ReportCommissioned report


Executive SummaryBackground to researchNatural flood management (NFM) is currently being promoted as a cost-effective catchment scale approach to managing flood risk and The Flood Risk Management (Scotland) Act 2009 places an emphasis on all statutory bodies to consider the use of NFM approaches where possible. Whilst this emphasis has already led to a number of initiatives aimed at assessing and promoting the more widespread implementation of NFM techniques within Scotland,there remains significant uncertainty regarding the effectiveness of NFM measures at the catchment scale. There is therefore a clear need to improve the evidence base of NFM performance, design and implementation.Objectives of researchThis report is one of three produced for CREW to verify the current state of knowledge on NFM. It focuses on establishing the effectiveness of NFM features at a catchment scale, particularly in relation to how they may be used to desynchronise flood peaks and therefore reduce downstream flood risk.

Key findings and recommendationsThe key findings of this research are as follows:- Identifying the impact of specific NFM measures (independent of other factors) is difficult, but there is evidence to suggest that they can reduce flood risk at the local scale, e.g. Wheater et al.(2012) predict that afforestation can reduce peak flows by up to 60%, and Acreman et al.(2003) report that river restoration can reduce peak flows by up to 15%. However, the scale and direction of such impacts are necessarily site specific, e.g. Bullock and Acreman (2003) report that nearly half of wetlands studied have the potential to actually increase flood risk.- Farming practice and antecedent soil conditions can have a significant impact on the effectiveness of NFM at the local level, e.g. Jackson et al. (2008) report that strategically placed tree shelter-belts can reduce peak flows by up to 40% and Hümann et al. (2011) state that site and soil characteristics can have a greater influence on runoff generation than forest type. However, the impacts are again very site specific.- NFM measures need to be above a minimum size to achieve noticeable effects on flood risk at the local scale, e.g. Bathurst et al. (2011a) consider that forest cover must change by at least 20-30% to provoke a noticeable change in peak discharge.- There is currently no conclusive evidence that NFM features can be used to reduce flood risk at the catchment scale, and Mayor et al.(2011) note that “...extrapolation of runoffvalues between scales or between catchments of different sizes is meaningless”.- NFM measures can impact on the time to peak of local flows, but their location within a catchment and the spatio-temporal variations in rainfall and runoff have a significant impact on the ability of NFM measures to assist flood peak desynchronisation, e.g. whilst Thomas and Nisbet (2006) predict that forestation of a catchment can increase time to peak by as much as 77%, Pattison & Lane (2012) consider that location and spatio-temporal rainfall/runoff characteristics may mean that “...the pursuit of generalisations between different land-management practices and flood risk may be a meaningless and unachievable aim.
- There is currently no conclusive evidence that NFM features can be used to desynchronise flood peaks, and there is concern that simplistic application could result in unforeseen outcomes, e.g. Nisbet & Thomas (2008) caution that “...a possible downside of de-synchronisation, however, is that by extending the flood hydrograph there is a risk of consecutive flood events contributing to higher flood peaks if they coincided with the delayed recession limb of the flood hydrograph”..
- Whilst the current lack of evidence and uncertainties does not imply that NFM measures cannot make a significant contribution towards flood risk reduction, it does highlight the need for continuing research in this area. There are now a number of ongoing and planned UK catchment scale ‘demonstration projects’ that have the potential fill this current knowledge gap.
Key wordsNatural Flood Management; Local; Catchment; Peak Desynchronisation
Original languageEnglish
PublisherCREW - Centre of Expertise for Waters
Commissioning bodyCentre of Expertise for Waters
Publication statusPublished - 2012


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