The water quality benefits of working with natural processes

Since 2012, the Sussex Flow Initiative (SFI) project has been using Natural Flood Management (NFM) to help local communities to reduce flooding. What most people don’t realise is that our work to store and slow down flood water in the landscape also helps to improve water quality.

Did you know that one of the most widespread forms of freshwater pollution is particles of sand, silt and clay, known as ‘fine sediment’? Although it’s a natural and important component of freshwater ecosystems, human activities (e.g. agriculture, forestry, urban areas and wastewater treatment plants) often lead to elevated levels of fine sediment in surface waters, impacting the physical, chemical and biological characteristics of these ecosystems.

River with a high suspended sediment concentration compared to adjacent watercourse (photo: NASA)

Elevated fine sediment levels can have direct detrimental effects on aquatic organisms, by burying or dislodging them; by clogging gills or feeding appendages; and indirectly by introducing contaminants attached to the fine sediment particles (either adsorbed to the surface of particles or absorbed in them). It can also limit light penetration, reduce oxygen concentrations in water and substrate, and more. The type of contaminants attached to fine sediment varies depending on the activities upstream, but in catchments which are primarily agricultural, the most abundant contaminants are likely to be nutrients (i.e. nitrogen and phosphorous compounds). In water, these nutrients can lead to eutrophication and oxygen depletion, and can even be directly toxic to aquatic organisms. However, on land and in wetlands these nutrients can be locked up in soils, or more rapidly transformed and removed/utilised by plants and microbes.

Sensitive gills on a stream macroinvertebrare (mayfly; Heptageniidae) (photo: Ross Piper)

These are all good reasons for reducing soil erosion and fine sediment delivery by embracing more sustainable land management practices. Soils are some of our most precious natural resources, sometimes taking hundreds of years to form, and yet we have lost unprecedented levels of them in recent decades. In addition to tackling the sources of the problem, we also need to look at restoring the natural processes that would normally be helping to regulate and remove sediment once it’s in our watercourses.

This is where Natural Flood Management can help, and why it’s considered a more holistic approach compared to more engineered flood risk management. NFM includes a range of techniques that work with natural processes to slow and temporarily store water in the landscape. This is good news for water quality, because by slowing water velocity, the ability of sediment to remain suspended in water is reduced, leading to it becoming deposited along with any contaminants that are attached to the sediment.

For centuries human civilisations have exploited river floodplains and wetlands for agricultural production due to the naturally nutrient rich soils, which are the result of sediment and nutrient deposition from flood waters. These floodplains and wetlands are vital for healthy rivers and streams, as they can act as vast stores for large proportions of a watercourses sediment and nutrient load, as well as carbon sinks. However, many wetlands have been drained, flood embankments built to reduce localised flooding, or levees accumulated over time from dredging spoil, resulting in many rivers being cut off from their floodplains, and the movement of water through the catchment being sped up. 

​By carrying out NFM we can lower embankments and levees and partially block drainage ditches, with the aim of reconnecting rivers to their floodplains and restoring wetlands. In doing so, not only do we open up large areas for flood water storage, we allow the important water purification processes to occur.

Flood embankments on the River Ouse

Woodlands can also be important areas for water purification, but these too have often been ditched to rapidly transport water from the land, reduce the height of the water table and maximise accessibility. This combined with historic watercourse management practices which included clearing woody material from streams and ditches, reduces the ability of woodlands and its spongy, organic and microbe rich soils to store and breakdown pollutants in water. The good news is that NFM can again help to restore these processes, by using woody debris dams that slow the flow, and encourage water out of stream/ditch channels during flood events.

Many watercourses have been significantly modified (e.g. channelised) to the point where instream habitat and sediment deposition are relatively uniform. More natural streams have a sequence of erosional and depositional areas, providing greater habitat diversity. By reintroducing woody material to these streams we can encourage greater hydraulic diversity and the return of these natural processes, allowing fine sediment to be deposited and stored in some areas (rather than along the entire length), but also creating areas which are scoured of sediment and can support organisms which are less tolerant of this type of substrate.

All of this means that by using NFM to reduce flooding in towns and villages, we can also be reducing the impacts of water pollution on aquatic organisms and the effort needed to clean our drinking water (saving money and energy!). Nevertheless, despite the amazing ability of the natural world to purify water, there is an upper limit to its capacity to do so, and whilst we should be working with, and attempting to restore the natural processes that regulate water purification, this doesn’t reduce societies responsibility for minimising the inputs to the environment. NFM is just one of the useful tools that can help us begin to reduce water pollution, but to protect this important resource we need to do much more!

Area of woodland without drainage ditches and with abundant dead woody material, acting as temporary water storage during floods