Which earthworks are appropriate for our property ?

Hi everyone,
we have a 5800m2 property in the south west of France with a slope ranging from 25 to 30% in the field where we would like to establish an orchard or some sort of food forest along the countour lines. We thought swales would be good but I have recently read that they are not appropriate when slope is greater than 15%.
The property is situated in Bardos, 64520 France. Rainfall can be dramatic : https://www.infoclimat.fr/climatologie/normales-records/1981-2010/labastide-clair/valeurs/64289001.html
Average annual rainfall = 1400mm
Max rainfall in one day 100mm
A the moment it is a pasture with 3 small sheep grazing. There is a small woodland on the west side where the main winds come from.
The soil is heavy clay, very sticky when wet, so hard when dry.

UPDATE : I have read that Bill Mollison calls a 50% slope what engineers would call a 100% slope (source : ). My understanding is that our 25% slope represents a fall of 25m vertically for 100m horizontally so the ingeneer version.


compared to

Any thoughts / advice for us to grow fruits there regarding earthworks ? Thank you for your time, very much appreciated !
Aurélie

I would not use any kind of earthwork on such a steep slope.  To slow runoff I would, if material is available, build low brush windrows (brush berms) on countour, anchoring them with stakes driven into the ground.  

I would look at Sepp Holzer's earthworks which are done on slopes similar or greater than yours.  Possibly consider using terracing systems with deep rooted trees that will prevent saturated soils from slipping downhill. Provided is a thread detailing his terraces.

https://permies.com/t/55668/Comprehensive-Sepp-style-terracing

Terraces without retaining walls are suitable at slopes of 3:1, 18.4 degrees, 33% or less.   Terraces with retaining walls can go up to 2:1, 26.6 degrees, 50%.

I've been somewhat obsessed with terraces of different types and have posted about them here (building them with rock retaining walls), here (might be especially helpful because it's another discussion of what earthworks might be most appropriate for steeper hills), and here (starting them by planting trees and then weaving brush amongst them -- and letting sediment accumulate against them with floods -- so that your stakes are live rather than dead, but otherwise similar to Tyler's suggestion), if you're curious. Also, check out some of this discussion of techniques like checkdams for slopes.

We have similar slopes as you have, also heavy clay, very sticky when wet and very hard when dry. We have noticed that rainwater barely infiltrates in our soils if we do not break up the dense layers in the topsoil. Since we have to do that anyway in the areas we plant in, and we do not want to loose any rainwater, we terrace everything.

We're making small terraces, so that if maybe we'll have an accidental washout the damage is limited. All terraces we make have a slight angle, where the downslope side is higher than the upslope side. In heavy rains that means excess water won't flow over the edge, but it will flow to the backside of the terraces. Like this you can easily prevent erosion. We built stairways of rocks, which are also the spillways for excess water. When you try to retain rainwater it is very important to always have a way for excess water to escape.

The picture below shows how it looks after it's done. It works great, we also regularly have 100mm per 24 hrs rain (actually it falls in just a few hours) with one big event where we had 90mm in just 90 minutes, and we had NO runoff from the terraces. It all went into the soil. Very happy with that.