runoff diversion in animal paddock

At an animal sanctuary, nonprofit in Portugal, the livestock stables get flooded with standing water when it rains. The wet season has started so they are looking for emergency solutions. This week they are going to dig swales with a machine and fill it with compost and then put soil on top with a finished depth of 1.5 feet. They are adding compost because they want to grow trees there one day. Composted manure would also add porosity to the otherwise clay soil. And the swales (yellow lines) would direct towards two retention basins. Also, the organization doesn’t have much money to buy materials.
Because the area is bare soil and establishing vegetation, geotextiles, high terraces is not possible with six goats, 2 donkeys, and 4 pigs what are options to stabilize the swales, besides rocks on the swale edges?
I'm thinking that making homemade diversion structures with wood stakes to hold low walls of debris like logs, sticks, biomass could also be worth trying.

I'm open to any thoughts.

I wonder if it would be better to run the swales on or near to contour and try and get moisture into the soil, instead of taking it to the
low point and a retention basin.
The swales could be deeper and wider.
Are there too many animals on that plot?

Hi Sophia,

Welcome to Permies.

Welcome to Permies!

This looks like a multi-faceted issue, but the pressing one seems to be the water. I'm thinking the root cause is overgrazing of the area.

Adding organic material to clay definitely will help with water infiltration. Building swales to help the water further infiltrate will be beneficial downhill for growing roots but it is the fact that nothing can establish itself is perpetuating your runoff issue.

I don't want to eyeball it, but how big is this area of land? I believe rule of thumb is for one donkey you should have at least a half acre of pasture. Are these animals permanent residents or just awaiting homes?

Yes, there are too many animals. They are permanent residents; the organizations intention is to provide happy, natural endings for the animals. They hope to improve the existing conditions but need resources and knowledge to do so.
It is roughly .5 acres and mostly unvegetated from over grazing. About a third of it is 20-30% slope, and as far as I know steep slopes are not ideal for donkeys.

thanks for the recommendations and welcomes!