These log dams will work until the drainage holes block with water born debris and the silt builds up behind. The flood water will then run over the top or the dam will burst and this debris will wash down and block the next bridge.
Note the scouring of the water channel - this is caused by the peat moors above having been drained (gripped ) over the years, on government grants, causing the flood waters to hit the valleys within hours instead of days. This is a big problem in Nidderdale where I live.

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