i'm designing water retention earth works in an erea with very little soil- a hill with 40 cm soil before you get to the base rock.
i have very little budget as well.
the best thing to do is to make terraces to build the soil, but it's expensive and takes a lot of effort.
i thoght of making swales using piles of mulch (that we have planty) instead of dugged soil.
the slop ratio is 1:20- 1:30
does anyone have experience with that?
Hello Saar, welcome to permies, I have no experience with that but if you can keep it from washing away when the water collects behind the swale I think it would be great. Give it a try and let us know if it works. How much rain do you get and how often?
From a newbie's perspective (if I'm wrong, I invite correction from more experienced heads.)
40 cm is enough to get a good, functioning swale. Dig swales wide but not deep maybe 20-25 cm) and use the dirt from the swale to build a berm on the downhill side. Mulch the berm heavily. Put branches over the swale, then cover with leaves (or straw) as an evaporation barrier.
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with 40 cm of soil you should be able to get a long, wide swale 8 meters long with the deepest part just hitting rock. Throw the dirt you dig out over the downhill side to create your soil mound and plant that with nitrogen fixing trees and groundcover (you should not dig the swale unless you are prepared to plant immediately. having a wide, shallow swale will minimize erosion--the angle on the up hill side shouldn't be more than the natural angle of repose and can be significantly less. The downhill side needs to be wide enough to handle the force of the water from your catchment area running into it and filling it up.
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