Some people believe dredging rivers is a straightforward solution for flooding. Unfortunately, it isn’t so simple. You can find Slow The Flow’s position statement on dredging at https://slowtheflow.net/dredging-position-statement/.
Mike Potter, the driving force behind the Slowing the Flow Pickering project, member of West Wolds Slow The Flow, and long term mentor and friend of Slow The Flow, discusses:
I have seen an excellent quote about dredging rivers: ‘This is one of those complex problems for which there is an answer that is clear, simple and wrong’. Advocates of dredging say that it’s for rivers to hold more water, but unfortunately, they don’t hold water, they move it. Much better for aquifers, floodplains, upper catchments, attenuation ponds and wetlands to hold water.
I find that people have very fixed views and choose not to believe my explanations, so now I often just ask questions. Most have simple yes/no answers, but if you answer them honestly and sequentially, it will hopefully make you think, analyse and understand for yourself. You could even research a reliable source for each answer:
- Do you consider dredging to include general channel maintenance i.e. weed and bankside vegetation control?
- Do you advocate dredging every river from source to sea? Do you agree that doing so will speed the flow of water?
- Do you agree that most cities, towns and villages are built around rivers and contain channel constrictions, i.e. bridges, culverts, narrowed sections?
- Would it be beneficial to dredge/keep rivers clear specifically in urban areas at risk of flooding when necessary?
- Do you know what a hydrograph is?
I’ll answer that one – it’s simply a graph of the water at a specific point on a watercourse (flood peaks gradually move downstream over a period of time).
The x axis on the hydrograph is time and the y axis the amount of water (stage height or flow in m3/second); if you speed the flow i.e. shorten the x axis time frame, what will happen to the flood peak on the graph? If the flood peak was made higher, would that increase flood risk downstream, particularly at any constriction of the channel? Conversely, what happens to that flood peak and consequent flood risk if you Slow the Flow (streeeeetch out the timeframe)?
- Where does the stuff come from that some advocate dredging out, why did it get there, and will it just return again?
- Have you heard dredging advocates mention sediment supply?
- What’s the cost of widespread dredging, what happens to the stuff dredged out (often contaminated, but that’s another issue) and who pays?
- How easy are all riverbanks to access?
- Is it better to apply the right measures in the right places?
More specifically to points often raised in press letters or on social media, with regard to dredging and widening of rivers:
- Is Yorkshire Water (or any other private water company) responsible for river maintenance?
- What effect would widening a river have on riverbank stability and hence sediment supply? What would happen to the many riverbank trees and why are they there?
- Who would pay to buy the extra property for widening (and loss of much agricultural land). What would happen to the many bridges and riverside buildings?
- What effect would deepening a riverbed have on adjacent bridge parapets or property foundations?
- How do you deepen a river that already goes down to bedrock?
- Does disconnecting the river from its floodplain through building floodbanks and floodwalls reduce the capacity of the river, given that the floodplain is part of the river?
Just to prove I’m not wholly opposed to dredging (or de-silting as the Environment Agency would prefer to call it)…. In built-up urban areas, invariably with multiple bridges and constrictions of the watercourse, it’s important not to lose capacity and conveyance in the channel, resulting in water levels rising from a higher base and therefore flooding more quickly. Two specific examples spring to mind:
- In early 2016, I watched from Hebden Bridge town hall as several diggers in the river channel removed a huge quantity of stone and gravel that had washed downstream in yet another devastating flood in the Calder Valley, leaving the capacity of the channel significantly reduced.
- Ropery Bridge in the centre of Pickering was rebuilt circa 1970’s. The former railway bridge became a road, with the same very low deck, but solid side walls rather than the former lattice. I’m sure it wouldn’t be allowed now, as it creates a clear flood risk, as the bridge soon surcharges as flows increase. To make matters worse, natural morphology and sediment deposition of the beck flow created a quite solid bank, reducing the cross-sectional area by circa 20%. Along with other sediment, sandbanks and resultant luxuriant vegetation in a few hundred metres of town centre Pickering Beck, a truly eye-watering quote for removal by a private contractor was given to the EA, who wisely declined. However, the Vale of Pickering IDB did the job effectively under a Public Service Cooperation Agreement (PSCA) at a fraction of the cost. However, a few high flow events have replaced much of that sediment in less than a year, while also moving other sediment downstream, in a natural cycle. Such sediment transfer therefore becomes a constant and costly exercise to maintain, particularly in highly modified or managed systems which tend to defy nature’s wishes. Limited public finances will always make this a controversial issue, as anyone familiar with the largely unnatural Somerset Levels drainage system will appreciate, and even more so in urban areas prone to flooding.
In my early, less educated days of interest in flooding issues, I recall pointing out to Prof Stuart Lane (a leading hydrologist and flood expert) that if you don’t clear your gutters, they overflow instead of moving rainwater away. I received a knowing smile, a raised eyebrow, and a comment about it being rather more complex than that. Indeed – clean gutters can quickly and effectively move water (Speeding the Flow), but what subsequently happens to that water after it goes down the drainpipe is just as important – often in combined sewers. And gutters still overflow if the rain is torrential enough.
No amount of flood defences or river maintenance can resist the power of nature if storms are intense enough, so the pinnacle of the pyramid that is Flood Risk Management (FRM) must be climate change mitigation!
I’ll happily answer any of your questions that may arise, but only after you’ve answered my questions, as I’d love more people to understand the basics. I only get mildly irritated (and sometimes in extremes, sarcastic) when people get offensive. I’d much rather that they realise this is a far more complex subject than first meets the eye.
I also know that watercourse management and maintenance become even more complex when ecosystem and biodiversity issues are included, but that’s a whole other can of worms.
By Mike Potter, Slowing the Flow Pickering and West Wolds Slow The Flow