I have an area of land that was recently renovated and we had the surface replaced with filter sand/dirt (sorry, I don't know exactly what it consists of). We have an upper area (with the dirt filler) and a lower garden area with a ramp between. With recent heavy rain, this lighter filler dirt/sand is being washed away, leaving giant crevices and waterways, washing everything into our lower garden area. What can I do to prevent this deterioration / erosion? (Please see images for details)

I'm an amateur diy enthusiast and have some skills, and would prefer to not rely on paying someone to do it for me if possible.

Thank you.

@RpR

You have to give the water somewhere to go that doesn't wash away the fill. Some kind of drain that routes the water somewhere safe. Or redirecting the water from its source.

You should also have something that holds the fill. A terrace or wall or something similar. It looks like you have a partial wall, but you'll need a full wall or it will keep washing away.

And what were you trying to do with that fill? A patio or something similar?

Where are you at?
What type of soil is the norm in your area?
You are on a hill, you NEVER use sand as a surface soil.
If you have some type of clay, or heavy soil available, fill crevices with heavy soil and use a compacter to hammer it in place.
Post a picture show wide area so one can see the layout.

@Henderman @RpR

Thank you for your replies.

The fill was provided by the construction company (at no additional charge) that originally removed the contaminated soil. We bought land that was previously used by a construction company that rather than properly get rid of their metal trash, decided to bury it underground like a landfill. We were quite upset about it after the fact, and sadly cannot sue the original owners due to an "as is" clause in the damned contract. Needless to say the land was unsafe for our small children to play on.

The lower area we plan to use as a large garden, the upper area will be a playground area (we hope to put grass turf there at some point).

This is a screencap of Google Maps (before we had the upper and lower lands resurfaced)

**Side note: I live in Japan**

Henderman said: You have to give the water somewhere to go that doesn't wash away the fill. Some kind of drain that routes the water somewhere safe. Or redirecting the water from its source.

You should also have something that holds the fill. A terrace or wall or something similar. It looks like you have a partial wall, but you'll need a full wall or it will keep washing away.

And what were you trying to do with that fill? A patio or something similar?

I own a landscape installation company in colorado. The landscapes in Colorado are different from where you're at but this is a common problem that has a singular solution where I am. We put down a 20 year fabric weed barrier then we install 1&1/2" rock over it. The rock slows down the water and the flow of water is now happening above the ground on the weed barrier. I've attached a picture of a completed landscape that we did for your reference.

Looks like they used loess as fill.
As good as that would be to garden on... as fill on a slope... what you are experiencing is the rule rather than the exception.

Really depends on your end goal as to what you should do.
As mentioned, fist-sized rock... would fill in the trenches without washing...

Or... a nice drainage box/catch basin at the top (where the water leaves the impermeable surface) hooked to that drain line...

Or... research swales and rain gardens.

There are plenty of ways to make lemonade from lemons here... I only wish I had all that water...