My swales (described in an earlier post http://www.permies.com/t/38956/permaculture/swale-headaches) have filled up again after a recent rain. They seem to be performing their duties fairly well - water is (more or less) evenly spread out throughout their length, and once they fill up, that water is infiltrated / percolated over a week or so (i.e., within a week there's basically no or very little water left in the swale).
(Having said that... given that the soil is clay, I wonder if percolation could slow down during periods when the soil is already saturated - and if so, I wonder what that could do to the trees growing at the base of the berm, i.e., downhill of the berm [I know I could have planted the trees on the berm to keep the roots drier, but unfortunately that is not an option, as the berm is vole condominium, and everything that I planted on the top got either gnawed or dug under & lifted by the little devils.])
Anyway, seeing all that water in the swales I recalled a suggestion that Bill Mollison makes in his Designer's Manual: "...small tanks can be sunk in swale bases for watering livestock or trees" (page 167); and "swale sections can be over-deepened, so that although the swale lip is surveyed level, its floor may rise and fall. Deepening is most effective in clay-fraction soils, and may result in shallow ponds for water-needy crop" page 168
My question is: has anyone here at Permies tried - or has anyone seen - this type of swale variations?
I'd like to try to play around with these ideas and make the most of the water that collects in the swales. At the moment, access to water is actually an issue on my plot, as we are not (yet) connected to the village mains - we may never connect, as I'm planning a system for roof water -, and the 10 metre deep well that we dug a year ago has only about 1 metre of water in it. Also, a pond larger than a few metres across doesn't seem feasible, because the plot is only 0.5 hectares (about 1.25 acres) and quite sloped. At the same time though, when it rains or during snow melt, there can sometimes be a lot of water around - the trick is separating it from the clay soil before the two form a mushy mix under your feet.
Any thoughts or suggestions will be welcome, thanks in advance !
why do swales have to be wider and shallower in sand and deeper and narrower in clay fraction soils? PDM says on p. 167 to cope with infiltration speeds but that doesn't make it clear for me.
This was my reply, and no one else seemed to add to the topic but the reply I gave got 5 supporting likes. I have not tried this at all mind you as I don't have access to land yet.
3 hours to 3 days is the suggested drainage range. So what you are seeing here is observations which lead to faster drainage.
Because clay is so dense and doesn't flow water well I would expect a deeper narrow swale would drain faster because of water pressure on the lower level of the trench. If you instead had a shallow wide swale it would sheet off water and I suspect wash out your mound.
Because sand is porous it doesn't take much for it to drain but it cannot support a deep swale so you maximize the drainage by maximizing the area the water has to penetrate, for the given mound size.
Try to imagine the mound size being fixed and then imagine what would happen if you removed the amount of 'soil' found in the mound from the trench of the swale and adjust for both narrow and wide trenches and both soil types. This might help you figure out why this is suggested, and it is how I came up with the answer
It might be worth adding using a ripper or yomans plow on clay soil in an attempt to promote better biology to break up the clays. I will check my soil food web notes when I get home as I recall there being a section on how to deal with clay and if I recall it involved ripping or injecting biology via compost or tea.
My swales were dug in clay soil and are narrow (70-80cm, so less than 3ft) but fairly deep for their width. I can afford making them narrow and fairly deep (hence with steep sides for both swale and berm) simply because clay is fairly stable and so the swale & berm keep their shape. I could NOT have afforded making my swales too wide AND have 4 of them, because my space is limited. I wanted to have 4 swales with trees along the berm, with wide enough "alleys" (interswales) in between. So the design choice was straightforward: I had to make them narrow.
Narrow and deep is what seems to be required for clay rich soils. Not sure exactly how deep though and I imagine it would vary from site to site so the 3 day water infiltration test would need to be used at different depths to see what works best.
One of the soil test you do as part of the soil food web course is an infiltration test at different depths to see just how well water can penetrate the soil.
We've gotta get close enough to that helmet to pull the choke on it's engine and flood his mind! Or, we could just read this tiny ad:
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