Why dredge rivers?

Before the evolution of the highways network, our waterways were used extensively for travel and trade via boat – requiring deep and navigable rivers that were maintained through regular dredging activities. In the extreme, this looks like entirely straightened and canalised sections of river. More minor dredging tends to involve removing excessive sediment build up. There are situations in which minor dredging may be necessary and even beneficial for localised flooding, however, as our understanding of river catchments expands it is increasingly found that dredging is becoming a less and less appropriate form of management for much of our river catchments. Dredging may also be needed in areas in which alterations have been made to the river in the past, reducing its ability to function naturally; the aim in these situations is to get the river to a point in which it can self-maintain, which may not always be possible in highly altered sections of rivers such as those passing through our towns and cities.

What is the problem with dredging?

As mentioned previously, as our understanding of river catchments as one unified system has evolved, we have come to understand the damage related to dredging activities, which are briefly summarised below:

• Dredging dislodges and destroys fine gravel and sediment beds needed for fish spawning – in rivers like the Lugg and Wye SSSI, this impacts notable species such as Salmo salar (Atlantic salmon).
• Riverbanks become unstable and dredging often causes increased erosion – leading to further sedimentation and the destruction of delicate habitat features and woody debris.
• The removal of material creates a vacuum effect, where in which the river will pull sediments from upstream to ‘fill’ the newly created void – this causes increased erosion to riverbanks upstream – a model showing the action can be viewed here: https://youtu.be/OAZ_BuyM41s
• The action of increasing the velocity of water in one area can often cause increased flooding and erosion issues downstream, as water now travels downstream faster and in greater volume than previously.
• Where large amounts of sediment are entering a watercourse, dredging seeks to deal with the consequence but not the cause – point sources of excessive sediment must be addressed at source, such as through differing land management, sediments traps and appropriate riparian buffers.
• Dredging activities stir up sediments settled on the riverbed that in many of our catchments contain high levels of phosphates as well as historic chemicals – these are then mobilised and negatively impact water quality. The removed sediment is often classified as contaminated waste and as such, is difficult and expensive to dispose of.
• Dredging is expensive! Especially when compared with Nature Based Solutions (NBS). And as it does not solve anything, it will only become more and more costly to manage rivers in this way.
• Finally, flooding is a natural part of a river’s functioning, and we must enable space for rivers to flood. By inhibiting this process, we only increase the chances of larger and more damaging floods elsewhere in the catchment.

What can we do instead?

Each year the taxpayer forks out a massive 2.2 billion for costs related to flooding throughout the country – not only are our efforts not working, but they are also costing a fortune. So, what do we do instead? There is no silver bullet when it comes to flood related issues, instead, we must understand rivers at catchment scale, as dynamic, self-maintaining systems, that we must work with rather than against.

Natural Flood Management (NFM) looks to work towards this new understanding of river management, working with a range of interventions and land management practices to influence rivers in a way in which we can find balance. Below are a list of interventions and land management techniques that NFM may employ to reduce flood risk, while promoting an ecologically healthy river system:

• Leaky woody debris or leaky dams – ‘slowing the flow’ of water and trapping sediments in the upper reaches.
• Sediment traps / pools – allowing sediments to settle and drop out of waters.
• Wetlands and riparian ponds – storing large volumes of water that can be slowly released into the catchment.
• Vegetated riparian zones – Trees, shrubs or simply longer vegetation in the riparian zone slows the flow of water and sediments off the land.
• Rethinking ditches and drainage – historically much of the land was drained for agriculture, but in some places this may no longer be necessary.
• Tree and hedge planting – particularly in the upper reaches and preferably planted cross contour, these can greatly aid the infiltration of water into soils and reduce the mobilisation of sediment.
• Soil health – by using regenerative methods to ensure soil health is thriving, our soils can hold huge stores of water.
• Reconnecting historic floodplains – by reconnecting previously disconnected floodplains we can store huge amount of water on the land, reducing flood risk downstream.
• Re-meandering and naturalising heavily altered sections of rivers – allowing natural processes to take place once again. 

We are seeing the return of many of our rivers returning to a ‘pre-dredged’ state and it is crucial we balance the needs of river systems with our need for productive farmland.

To summarise – the scale of the change needed is not possible solely with interventions, it requires a different understanding of how rivers function and the benefits they bring us. In a country as wet as Wales, it can be easy to forget just how important water is; but rivers and streams are the veins of our land, connecting and providing life everywhere they touch.

To keep up to date with the Wilder Lugg Project – get in touch with Dan at: dan@rwtwales.org

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