The geomorphic effects of river dredging

The River Management Blog

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.

Dredging reduces morphological diversity, with less variability in channel width, channel depth, substrate and flow velocities

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…

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3 Responses to The geomorphic effects of river dredging

  1. Barry Jones says:

    Our rivers & streams are pipelines to the sea. If you get a blockage in any designed pipeline system the only way to solve any problems is to clear the blockage.
    Surely this also applies to our waterways.
    With modern Combine Harvesters we collect the corn far quicker than when we used to cut it with a scythe.
    Modern man can surely design DREDGING equipment which could efficiently keep our natural drainage system clear of blockages such that unwanted flooding could be consigned to history as the scythe has been.

    • gwilwren says:

      Unfortunately as the Levels showed 2 years ago the water going downstream will not quickly drain out to sea because of tides and flattened bed gradients. So speeding its downward flow is only going to cause problems for people lower down the river. In the example of the Levels water that 30 years ago was taking 48-72 hours to run off the hills is now taking 24, but the rate of discharge to the sea has not increased. Unfortunately there is not a lot that can be done to speed up discharge other than open up new channels and put in massive pumps.
      The effect is that large volumes of water appear in the floodplain quickly and have to be stored until they naturally drain. In Somerset they are widening an alternative channel to bypass the pinchpoint at Bridgwater but are also preparing to sacrifice land between the town and the sea to store water until the tides allow it out.

  2. AB says:

    I have no geomorphic training, but I think if the river mouths were dredged, more water would get to the sea and less would be available to flood over river banks. I strongly agree that more porous flood palins need to be left, and more trees and plantings up river would reduce the flow, but the country has already allowed excess building on flood plains and surely opening up the river mouths again would be a very good short to medium term solution.

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