We love the ‘All About’ series, published on the Severn Rivers Trust website. These blogs have been written by Unlocking the Severn volunteers and apprentices, so we figured we’d share them here too! See the original blog on the Severn Rivers website HERE.
Written by James Skitt, a volunteer for Unlocking the Severn and Severn Rivers Trust
The Benefits and Complexities of Natural Flood Management
Natural Flood Management (NFM) has a host of co-benefits outside of immediate flooding mitigation. This blog will focus on the habitats in which riparian (river) vegetation can develop and enhance, reducing erosion and promoting stable riverbanks, providing natural alternatives to more destructive hard engineering and providing additional recreational value. There are also complexities that accompany NFM techniques, which will be looked at in this blog. These include vegetation development issues, riparian zone maintenance, flood reduction reliability and the time vegetation can take to develop and establish.
Benefits of Natural Flood Management
Creating new habitats
NFM can improve aquatic and terrestrial species abundance and biodiversity through enhanced habitat generation. Bats benefit from this as they use trees as roosts, while otters use roots of riparian vegetation for protection and rest. NFM also provides corridors of habitat which can link areas of landscape together that may have previously been inaccessible. As a result, habitat fragmentation reduces because organisms can travel . Organic matter falling into the river channel from riparian zones is a substantial basis for aquatic food chains. It provides food for invertebrates, while insects on leaves are a food source for fish.
NFM techniques such as riparian vegetation improve water quality. Vegetation intercepts nutrient rich runoff from diffuse pollution sources including agricultural fields, helping reduce excessive nutrient inputs (Nitrogen and Phosphorus) to water systems and minimising eutrophication likelihood [2, 3]. Re-meandering of previously straightened river channels and using in-stream flow deflectors (woody debris) increases habitat diversity by establishing riffle and pool sequences. This leads to an increase in-stream flow heterogeneity and increases potential species abundance.
Reducing soil erosion and increasing bank stabilisation
NFM techniques including riparian vegetation, cross slope tree planting, hedgerow planting, perennial vegetation and winter cover crops reduce soil erosion, excessive surface runoff leading to the leaching of nutrients into water systems. Root systems from vegetation bind soils together, minimising excessive erosion, especially during precipitation events. Materials that erode into the river channel may introduce excessive nutrients and reduce species abundance through eutrophication. Also, siltation may occur filling pore space in gravel substrate, inhibiting and reducing fish spawning site availability .
A hard engineering alternative
NFM provides alternative techniques and approaches to flood mitigation, whilst providing a host of co-benefits that hard engineering cannot. It is less expensive than hard engineering techniques, easier to install and establish, make riverbanks appear more natural increasing the areas aesthetic value. Also, NFM techniques are less damaging to land surface and ecosystem functions compared to hard engineering alternatives .
NFM techniques provide a range of social benefits. Once developed, vegetation can increase the areas aesthetic value, provide green space for activities, provide environmental and ecological education opportunities . It is essential to include local stakeholder groups in NFM development, as increasing local knowledge and interest in their local river reach and community will inevitably improve the chances of NFM techniques being successful.
Natural Flood Management Complexities
NFM techniques that involve the use of vegetation can encounter establishment and development issues. Some vegetation when planted will die before developing into the riparian or other NFM technique that is aimed for. Precipitation or flood events can wash out a proportion of saplings, preventing establishment. Also, excessive erosion where the vegetation has been planted, for example, if the bank is not protected sufficiently, will lead to reduction in growth.
To maximise NFM technique productivity, maintenance is required, including, watering of vegetation (especially trees that require lots of water during growth) and replacement of vegetation that fails to develop. Also, leaky dams and woody debris will need to be checked and maintained as logs can rot overtime and become ineffective. Maintenance increases costs and can be time consuming, but it is essential to successful NFM projects .
Flood reduction reliability
NFM techniques have a limited flood reducing capability . For example, riparian vegetation and other vegetation techniques will only store and attenuate a certain volume of water, depending on the vegetation type and size of buffer strip. Once this threshold is reached, any further water input cannot be attenuated and will reach the river channel, potentially contributing to flooding . However, for smaller and more frequent events, multiple NFM techniques are extremely effective and provide a host of co-benefits.
Time to establish
There can be a lag time between planting and vegetation establishment, delivering services including flooding mitigation and habitat generation. Dixon (2018) states that the lag time between woodland planting and full benefit realisation can be between 40-50 years. During this time, restoration projects may have to use differing methods whilst vegetation develops .
Clearly, there are a multitude of benefits and complexities when it comes to NFM techniques. It is essential when planning and establishing NFM to consider the methodology in a catchment or watershed context, to ensure that development is successful. It is Important to consider the local climatic conditions, local geology and soil type, local land use types and composition and river flow dynamics. Despite the drawbacks that NFM techniques can present, overall due to the sustainable and environmentally beneficial aspects of NFM techniques, they are a positive flood management methodology.
 SEPA (2008). “Bank Protection: Rivers and Lochs”. Engineering in the Water Environment Good Practice Guide WAT-SG-23. Available from: www.sepa.org.uk/water/water_regulation/guidance/engineering.aspx
 Dadson, S.J. et al. (2017) A restatement of the natural science evidence concerning catchment-based ‘natural’ flood management in the UK.Proc. R. Soc. A 473: 20160706.
 Palmer, M.A. et al. (2005) “Standards for ecologically successful river restoration” Journal of Applied Ecology 2005 42, 208–217.
 Wilkinson, M., Addy, S., Quinn, P. & Stutter, M., (2019). “Natural Flood Management: small-scale progress and larger-scale challenges”. Scottish Geographical Journal, 135(1), pp. 23-32.
 Lane, S., (2017). “Natural Flood Management”. Primer, Volume 4, pp. 1-14.
 Dixon, S., Sear, D. and Nislow, K., (2018). “A conceptual model of riparian forest restoration for natural flood management”. Water and Environment Journal, 33(3), pp.329-341.