I think that the intention is to dig a swale, or trench, and then fill it with wood. This is what I have done, and there is nothing to collapse. It is just a trench filled with wood and organic matter. As I understand swales, they are not necessarily meant to
hold back water, but to collect and spread it. Perhaps if one was to use wood in a swale
berm, it might collapse or shrink, but I think it would only collapse and wash away if it were on a steep slope which had masses of water rushing down it. Or if you buried wood in the wall of a dam, well, that wouldn't be wise at all.
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Burying wood in a trench or swale is basically like building a below-ground hugelkultur, and seems to be a great way to get some organic matter into otherwise sandy and humus-lacking soil - which is exactly what I have. My first 85' garden bed filled with logs and organic matter performed beautifully this past summer. Our sand tends to 'eat up' smaller organic matter that is incorporated into it, so this was the best quick way to get some soil building action happening. Also, our sand tends to 'shed' water, so it runs right down the hill and off the property. So, the buried wood beds also help to capture the run-off and get it into the ground where it belongs, similar to a swale.
Good luck with your venture!
Cheers
Tracy