I'm not sure that there would be much point even attempting to retain
flood water like that. I know Yeomans in his books recommends developing
Keyline systems to capture 100% of rainfall. But I can't see the point
of designing for a 1 in 100 year flood event when those flood events
always occur in above-average rainfall seasons.
I would be more inclined to design for say a 1 in 10 year event - say
15-20% of average annual rainfall - because these can readily occur in
the midst of a drought when water is more precious. And ensure the
design allows the 1 in 100 year events to safely flow away.
If you have never learnt which climate systems drive drought in your
area, it sounds like you don't have significant drought-related problems :)
I am still on the list, and still interested in the topic. I just have
more important things to be working on for now. I hope to have some
spare time next winter to work on projects like these.
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