It seems the community has been crying out for a post of this nature and so here I am; gallantly riding into the maelstrom of a politically sensitive issue, on the back of my watery steed, wielding my lance of science and wearing my armour of academic integrity.
For starters, this is a complex topic. The exact ramifications of river dredging upon geomorphology will be a function of the river typology, sediment characteristics, the dredging technique employed, existing floodplain connectivity and antecedent environmental conditions – the points made within this post are theoretical and generic, just like my post on the geomorphic impact of weir removal. I’d invite you to highlight examples where what I suggest should happen, hasn’t happened.
Another point to make is that the academic literature has relatively little information on the geomorphic effects of river dredging in low-energy rivers. A few key papers have highlighted the following morphological effects of dredging in high-energy rivers:
- A reduction in bedload supply and therefore channel slope downstream of the dredging activity.
- An increase in channel slope upstream of the dredging activity, potentially creating a knickpoint
- Reduced sediment availability leading to a simplified channel planform in inherently braided systems
- Vertical incision leading to river bank instability and potential channel widenning
- Greater streampower leading to the transport of coarser sediment downstream of the dredging activity, combined with a greater fine sediment load to cause bed armouring
In the UK the principal reason for river dredging is flood defence. The notion is that by removing the amount of sediment within the channel you are able to increase its capacity. Lane et al. (2007) demonstrated in their study of an upland, coarse sediment-dominated river that the magnitude of low-frequency (1 in 0.5 and 1 in 2 year) events is increased in river reaches that experience aggradation (sediment accumulation) and therefore reduced channel capacity. So there is scientific evidence to suggest that the presence of sediment increases flood risk, but it is quite heavily caveated and I’m going to use one of my classic MS Paint diagrams to illustrate why it doesn’t exactly work the other way round – dredging doesn’t always reduce flood risk.
If we assume that a dredging activity increases channel capacity then it can also be assumed there will be a reduction in the frequency of overbank flow events as more water can be stored within the channel. However, during extreme events, perhaps upwards of the 1 in 5 year frequency, dredging is ineffective for flood defence purposes because even a river that has twice the capacity than it would naturally cannot contain a high magnitude flow; the capacity of the river floodplain is far greater than the capacity of the river channel. So dredging has a relatively minor effect on the frequency of big flood events. Dredging can also have a number of unwanted geomorphic consequences.
Dredging can have a number of undesirable geomorphic consequences, many of which are detailed above and in an excellent paper by Wishart et al. (2008). Mmom et al. (2012) have shown that vertical incision and bank destabilisation can occur following dredging activities. In the simplest terms this occurs because the energy the river would be using to transport sediment (which has been dredged out) is now acting against the river banks and bed. This can be particularly problematic where spoil material is placed either side of the main river channel, creating an embankment. Embankments can prevent overbank flow, increasing bankfull discharge, streampower and sediment transport capacity.
Restricting the river in this way can significantly increase rates of geomorphic change, with greater local erosion and sediment transport during high-flows. However it is also possible that the reach that was dredged may experience accelerated sediment deposition during low flows, particularly where the dredging activity has widened the channel. Fine sediment loads are likely to increase, as the river is unable to deposit this material on the floodplain. Few of these changes are likely to benefit the river system in the short-term and may require further management. So should we dredge or shouldn’t we?
Dredging inhibits the natural functioning of the river system and is unsustainable in the long-term. It can act against legislation such as the Water Framework Directive and is costly in a time of austerity and reduced funding for river maintenance.
The trick, as my colleague Lucy Shuker put it to me on Twitter, is to realise the value of sediment and to view it as an asset. Gravel has a role to play in providing spawning habitat for fish, aerating water with oxygen by increasing local roughness and inducing turbulent flows. Fine sediment is important for species such as lamprey, whilst the process of floodplain deposition is vital for nutrient transfer between the aquatic and terrestrial environment. As examples, the gravel supports fisheries, which pay their way through rod-licensing and the fine sediment promotes grass growth in water meadows, which provide an ecosystem service in the form of pasture for cattle grazing and improved in-stream water quality.
If we’re worried about flooding then what we need to do is stop building in the river floodplain, and not only infrastructure. There is an argument to say that in the long-term, even land that is currently used for arable agriculture will have to adopt a different land-use to accommodate a less predictable climate.
Things cannot change over night, but they must change eventually. A gradual, responsive, phased approach that recognises the need to compromise flood defence and biodiversity priorities may be our only option if we are to undo decades of river mismanagement.