- 26th October 2017
- Posted by: Joanne Woodhouse
- Category: Blog
In August, we completed a piece of work for Forestry Research which looked at ways to use woodland as a Natural Flood Management (NFM) measure. The project focused specifically on the town of Southwell in Nottinghamshire that has a history of flooding from fluvial, pluvial and surface water sources. Modelling of mature woodland plantings (150ha or 310ha), in the catchments upstream of Southwell, indicated that the number of properties flooded was reduced for the medium and larger flood events (25 to 75-year return period) when compared to the baseline (current) condition.
The economic viability of using woodland creation to help reduce flood risk was assessed by undertaking a detailed benefit-cost analysis which compared per hectare costs of woodland planting versus the flood, environmental and total benefits.
The analysis suggested that for all scales of planting and different cost scenarios, the flood benefits outweighed the planting costs with a benefit-costs range between 1 and 8. This is the same for the environmental benefits (excluding flooding) with a benefit-cost range between 5 and 40. While uncertainties exist in terms of grant availability, land opportunity costs, landowner willingness and the inclusion of environmental benefits for the Southwell case study, the analysis suggested that woodland creation could provide small but cost-effective reductions in damages from flooding.
Woodland helps flood peaks
The Centre for Ecology and Hydrology also released a report this August on the same topic. It asked, “Do trees in UK-relevant river catchments influence fluvial flood peaks?” and sought to systematically review qualitative findings from peer-reviewed studies.
The review found broad support for the conclusion that trees influence flood peaks. An increase in the amount of woodland cover leads to a decrease in flood peaks, whilst a decrease in cover results in increased flood peaks. However, the report highlights, the difference between the findings derived from model outputs and those from observations.
The majority of statements supporting both of the above conclusions are based on model-outputs, whilst observation-based statements are much more mixed, resulting in no overall significant difference. Further research is needed to assess the effects of tree cover on large flood peaks and in large catchments. These two areas currently lack observed data to test and validate models.
Working with Natural Processes Evidence Base
At the end of October 2017, the outcomes of an Environment Agency funded research project will be published. The project aimed to develop a comprehensive evidence base across a wide range of Working with Natural Processes (WWNP)/NFM measures, including woodland. The new evidence directory, underpinned by a comprehensive literature review, will describe what is known and not known about the effectiveness of woodland and other WWNP measures to reduce flood risk. It also delivers a range of wider benefits to the environment and society.
The new publication has been designed to provide the wider WWNP community with a practical and accessible information resource. A library of case studies from across the UK is included in this and can be drawn upon when planning, exploring, designing and implementing new WWNP initiatives.
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