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Unintended de-stabilising consequences of dredging...

The WTT borrowed the fabulous Emriver kit from Severn Rivers Trust during the 2013 CLA gamefair. It allowed us to set up a whole range of common river-channel scenarios in an accurate scaled-down simulation. Dredging was one of the things we modelled:

Dredging river-bed material is something that is of particular relevance to the urban/heavily-modified channel environment. As is so often the case with rivers, what seems like the obvious and correct thing to do can actually blow up in your face (or someone else's face several miles up or downstream!).

Just as intriguing are the potential knock-on impacts of ad-hoc dredging in rural environments in an effort to increase the capacity to drain land...

Nature abhors a vacuum and removing accumulated material from a river channel can have far-reaching and unintended consequences. The first principle effect is to increase the demand for eroded river-bed and river-bank material from upstream. This can dramatically increase the rate of erosion in upstream reaches.

It also rapidly leads to the re-filling of the dug-out channel... (back to square one, so you dredge again....)

The other major/worrying effect is the interruption of the transport of sediment downstream of the dredged reach. Whilst the bed material is being re-accumulated in the dredged area, there is far less material being supplied downstream. That "cutting off" of the sediment supply causes a net increase in the erosion downstream of the dredged reach as well. This is due to stopping the supply of material that would otherwise "patch up" and fill in eroded areas to produce a more stable dynamic equilibrium state. How many people who undertake dredging understand these processes? Is the move to "fast
-track" UK-landowners' ability to dredge their own streams with a much lower requirement for external assessment likely to create more problem than it solves?

In addition to all that, the video below might give an interesting perspective on dredging works for the purpose of flood-water conveyance in areas downstream of bridges and weirs in modified waterbodies...


Comments

Tim Holt-Wilson said…
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Tim Holt-Wilson said…
Great use of a sand box as a communication tool.

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