I have 15 acres with a hollow running through it. There are about 300+ acres behind me and it all runs off through the hollow. The rain takes water, dirt and everything else out across the street and into a small creek, and the neighbors’ front yards.

I am thinking about a dam to catch the topsoil runoff. I plan to scoop out most of the topsoil that is already there. (I have need of it on other parts of the property.)
The soil dam, as I call it, will consist of a wall of old loosely stacked concrete. You can get it at most road construction sites; sometimes they will even deliver it free rather than have to pay to take it to the dump.

The side of the dam facing the oncoming water will have a layer of material like weed mat; some material that stop the soil particles and let the water flow through.

After a good rain I plan to take any compostable materials and dump them in still soggy mud and bury it. Then scoop up the mess and pile it and let it decompose.

Further down the hollow I plan to dig out and build a pond with a real dam, low over 4 ft and I will have to hire an engineer. This will help limit the flooding across the street.

Any ideas or input, it would be appreciated.

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We have a thread discussing it here.

Without knowing the volume of water involved, it is impossible to come up with advice that would fit your circumstances. If you are in an arid or semi arid region that gets rainfall in small doses, your design would be much different (and easier) than if you live in a tropical region that gets sudden, heavy downpours, where any dam would need to be designed by a pro that understands fluid dynamics, and conservation issues.

For a low flow system, one easy and inexpensive way to collect the run-off top soil is to partially fill the hollow with cut brush. This slows the flow of water so that the topsoil has a chance to let gravity sink it to the bottom rather than spewing across the road and away.

However, if you live in an area that occasionally gets 4" in a single day, you may not want to do anything that could result in disaster.

Depending on the slope you might want several swales to handle the runoff and collect silt. However, John is correct about it being difficult to make suggestions without knowing the details of the runoff. In any case I would not mix compostables into the silt/soil as you're better off handling them separately.

I did something similar to what you propose and for exactly the same reason: I was losing a lot of soil in the form of erosion and kind of decided I was sick of it. So while I did place multiple rock check dams further up the hill to prevent it from happening in the first place, I also built a dam so that IF the water and soil did make it to the lowest part of the field, I could redistribute that lost top soil. And yes I do have before and after photos!

I call the area my "Super Swale", but it comes with one caveat. In some photos you may see a rock Ford. This is because I needed road access to a small field AND because this is where two fields come together at a gate where I need to bring sheep through for rotational grazing. So if you see that, just realize I had a lot going on in this project!

The first picture shows what I started with, and what I wanted to recover: the topsoil lost that is.

The second picture shows the mid way point in my super swale which was how I reclaimed the lost topsoil and generally formed the shape of my super swale

The third photo shows the same spot after the rock check dam was installed, the Super Swale sodded in, and the Rock Ford was made. (Note the stakes are so I can see the lowish rock check damn when I am bushogging with my tractor)