I'd like to buy some land in France (Normandy!), and do some keyline design in it to benefit from the water flow optimally and regenerate a piece of land (5ha = 12 acres).
It would look a bit like that https://i.ytimg.com/vi/PJdnEtMLtHI/maxresdefault.jpg (I've been mostly inspired by Richard Perkins work in Sweden).
My terrain would have a ~10% slope downhill.
However, it seems my whole terrain is uphill from the main keypoint: the keypoint does not belong to my terrain, but is a bit downhill (you can see two views of the property in the attached file to check if I'm correct).
What would you do in this context?
Use the keypoint from the neighbouring terrain and work uphill?
The key point should be up hill, not at the bottom, what you are looking for is the point of runoff intersection, ie. where the rain runoff begins to gather, not where it ends up.
In your second photo I see a keypoint at the bottom of the photo, it looks like ^ (this) that is where you would want to start looking for the key point on your land.
Once you find that intitial point of runoff junction, you have found the place to put your initial pond and work out on each side at a 1 degree down hill pitch.
The second swale and berm would be down slope from that initial key point.
If you have other questions, I'm around so just ask away.
Most books are going to show simple systems, fairly well defined gullies make it easier to show hydrology in print.
I looked at the Google Earth Pro view of your land but their imagery makes it look rather flat land when compared to your images here, which makes it even more difficult for me to show you lines for swales.
So, what I would suggest is to go stand on the highest point, either during a rain event or just after one so you can observe how the water currently flows on the land.
By starting at the highest point, you should be able to see the areas where the water bends the grasses over and that will also show the direction of the water flow since the grass will bend over, pointing down flow.
Take some flags or stakes and mark those areas. These will be the places where convergences are occurring (key points) there will be one prime key point and it looks like you should have some secondary ones too.
With stakes placed, you will be able to visualize the contours from high points to low points, these will be the actual contours you will want to follow when constructing your swales.
I like to dig the swales and berms first then I go back and put in any ponds that are needed, that way I am making full use of the water that falls on my land and if a huge rain comes the overflow will sheet down hill to the next swale/ berm.
That last picture you put up is pretty spot on for places I would want to observe closely prior to starting placing stakes (flags).
For final lay out I use orange string to actually put down the on-contour swale/berm lines I will dig.
String allows me to really see how everything will work together when installed, I only want to dig once and the string will show if I have places that need adjustments before I do the work.
Matthieu, I hope this helps, the green/yellow line is the "Main Ridge" The green lines are the contours down the east slope, red lines are the west slope. The blue lines are where I would expect water to gather as streams or elongated ponding.
I was trying to show what I would be considering when getting ready to lay out swales and berms.