Tuesday, 14 October 2008

Strines LWD - Status report 14-Oct-2008

Tim Jacklin, WTT, reports:

I walked the Strines section to check on how the introduced LWD had weathered the recent high water.  It was still there (phew!) despite the flows shown in the attached pic taken by Paul Gaskell a couple of weeks ago.  It was interesting to see the effects of the LWD even after a relatively short space of time; the structures have pulled the anchor cable taut and swung into positions alongside the bank, and there has been an accumulation of finer sediment between the LWD and the bank.  On the willow on the riffle there is some bed scour taking place (see other pic) and areas of clean gravel below it, which is great.  On the structures in the ‘glide’ sections there doesn’t appear to have been as much bed scour as I had hoped (e.g. the most downstream structure).  I think there is still some settling in to be done on some structures as some side branches shear off and the main trunk can reach the river bed.

In terms of having the desired effect on river morphology, I am thinking more and more that the structures need to be BIG.  This presents practical problems in felling trees of that size, and it would be much easier to retain LWD of that calibre as it falls in.  This will require DNMAC to  look out for this, and for EA Fisheries/Biodiversity to influence Ops Delivery to retain rather than remove.

Felling larger trees isn’t impossible, but it would need a tree surgeon to be able to climb and reduce the tree to a height that could then be dropped into position without causing wholesale damage to other trees, opposite banks, etc. 

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