This blog has been written by James Skitt, a volunteer for Severn Rivers Trust.

What is flooding and Natural Flood Management?

Flooding is a natural process, but one of the most damaging and prominent natural hazards globally, and especially within the UK. Flooding inundation can have terrible consequences that include the loss of lives, damaging and destruction of property and infrastructural damage. Flooding can be river (fluvial), groundwater related and coastal in nature, or even a combination of the listed processes [1]. For the purpose of this blog, the focus will be on fluvial flooding. Fluvial flooding is primarily caused due to the river's channel capacity being exceeded. This is caused by the downstream flow that precipitates onto a saturated ground surface and is not able to infiltrate or be absorbed. Water enters the river channel as runoff or as precipitation directly entering the river channel, which increases the volume of water that can then exceed the channel's capacity and lead to flooding [2].

Natural flood management (NFM) has been promoted as a more sustainable alternative to traditional flood management techniques, that interrupt natural river processes. NFM is developed based on the notion that it is feasible to manipulate river flow at the river on a catchment scale (catchment-based flood management, CBFM), instead of defending floodplains at a specific point, to reduce flooding downstream [2]. This is achieved by using techniques that work with the rivers' natural morphological, hydrological processes and catchment characteristics, to modify the pathways and sources that runoff takes to enter a river channel that has experienced human modification [3]. NFM has the potential and scope to reduce the associated long-term costs in relation to river system infrastructure and the adaptive nature of NFM techniques can help negate the impacts of climate change [4].

NFM principles and techniques

The attenuation of flow and runoff is key to the success of NFM. According to Lane (2017), NFM has three main options to reduce the speed to which runoff enters the river channel.  

1. The first is the storage of water during high river flow events within the catchment, therefore reducing volume of water available to enter the river channel and cause flooding. This can be achieved by creating and maintaining runoff capacity in ponds, swales, floodplains and ditches. This enables them to fill during precipitation events and reduce the amount of runoff initially entering the channel. The stores can then empty slowly over the following 24 hours or so [5].

2. Second is the attenuation of water flow by limiting and reducing the ease of connection between precipitation runoff sources and areas that are prone to inundation, by increasing resistance to flow. There are various methods to achieve this is. For example, planting trees and hedgerows (riparian buffer zones) installing woody debris dams (leaky dams), blocking grips on moorland and on a larger scale wetland features (amongst others) [5].

3. Third is reducing how quickly run-off is generated on hillslopes that feed into the main river channel. This can be achieved by increasing the infiltration capabilities of catchment soils and therefore reducing the amount of runoff and increasing the attenuation of runoff within the catchment that may contribute to flooding [2,4]. Improving soil moisture quality, soil health and soil structure can increase the depth at which water can be stored and therefore increase the volume of water stored in the soil. This is crucial to increase runoff attenuation, as it will take increasing amount precipitation input to reach saturation, therefore surface runoff can be reduced [5].  

Land cover and land-use-type play a major role in flooding and the attenuation of runoff. A more urbanised catchment will have increased levels of impermeable surfaces (such as roads and concrete surfaces) that do not allow infiltration and water storage. A significant problem in urban streams is an increase in peak flows, due to runoff from impervious surfaces in the watershed [6]. A feature of urbanised catchments are storm drains which reduce the time it takes for surface runoff to enter a river channel and therefore increases the potential for flooding under heavy precipitation events. Increasing the amount of vegetation and decreasing the level of impermeable surfaces within a catchment is clearly an effective and useful way to reduce the potential for flooding under heavy precipitation events.  

The River Teme project

The Severn Rivers Trust have combined with the Environment Agency and landowners across the river Teme catchment, working collaboratively to deliver NFM techniques. The aim is to help mitigate potential flood risk from the Teme.  The Severn Rivers Trust are working with individual landowners to determine what variety natural flood management techniques meet the needs of the landowner and reduce flood risk, as well as providing a host of other benefits.  

The project has used a variety of NFM techniques, mainly centred around vegetation. The introduction of new riparian vegetation or the continued development of existing riparian vegetation has been a prominent technique involved, especially at agricultural sites. As well as helping mitigate flood risk, riparian vegetation provides numerous co-benefits including intercepting precipitation, providing habitat for aquatic organisms and intercepting agricultural runoff. To help prevent excessive bankside erosion, the riparian vegetation is aided with revetments, which allow the vegetation to become established, stablising the bank, preventing erosion and helping minimise excessive sediment input to the river channel. Cross-slope tree planting has been incorporated as well as further woodland planting and hedgerow creation. In the upstream areas of the Teme catchment, the Severn Rivers Trust are implementing 'leaky dams' to help store and extend the time it takes the runoff to reach the river channel.

To date, the Severn Rivers Trust project on the River Teme has planted over 5,000 trees this season within the catchment. They have also installed infrastructure, such as fencing, to help vegetation develop. Combined, all of these efforts and small interventions will ultimately help to prevent more flooding in the future.

Thank you to Matt Marston, Senior Farm Advisor at Severn Rivers Trust for his help and expertise in providing information for this article.

References

1. 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.

2. Lane, S.N. (2017). Natural flood management, Wiley interdisciplinary reviews. Water, vol. 4, no. 3, pp.  

3. SAIFF (2011). What is meant by restoration, enhancement, and alteration under the Flood Risk Management (Scotland) Act 2009. Edinburgh: Scottish Advisory and Implementation Forum for Flooding.

4. Janes, V.J. et al. (2017). The impacts of natural flood management approaches on in-channel sediment quality. River Res. Applic. 33: 89–101.

5. Calder, I.R., Reid, I., Nisbet, T.R. and Green, J.C. (2003) Impact of lowland forests in England on water resources: Application of the Hydrological Land Use Change (HYLUC) model. Water Resources Research 39, 1319

6. Palmer, M.A. et al. (2005) Standards for ecologically successful river restoration. Journal of Applied Ecology 2005 42, 208–217.