One thing to be very aware of is to respect steep slopes when building swales. In general, don’t build large earthworks on steep slopes unless you have a water-savvy engineer to help you. Remember, you are controlling literally tons (that is literally, tons) of water in a swale like this, and sending it somewhere other than where it would normally go. For context, a five gallon bucket of water weighs about 42 pounds. How many five gallon buckets would fit in this swale?
This swale could blow out in a large storm event. Water is very heavy and is persistent in trying to seek the lowest level possible. And like anything else, it will gain speed coming down a steeper incline.
Think - waterfall vs a slow moving river. And if there is water sheeting down the upper hillside, the weight and speed of that is added to the combined weight of whatever is in the swale. If there is any weak point in that berm, it is liable to blow out. Whatever is below that point will get a stream of all the water from the swale and hillside aimed at it until or unless another point blows out.
This means that moving water hits with its full weight as if it was metal at impact.
I don’t have the details of this particular project - they may have engineered it. But I’m aware of a number of projects similar to this where the berm failed at one point and it caused damage.
Here’s a graphic example of a washout (thank you, permaculturists for videoing your mistakes as well as your triumphs!): https://www.youtube.com/watch?v=IrACuvWcx8g.
You always want a spillway - you want a design that directs the water where it would leave the swale, if it overflows. But note that spillways must be done right. It wasn’t the size of his spillway (in the above video) that was the biggest problem. He needed to secure it, with rock or concrete block or something other than soil that could be eroded. It’s water’s job to erode soil at the weak point or lowest point.
You can see the force of the water going over that spillway. Don’t create a spillway with native soil. It’s got to be stable, and erosion proof.
There are formulas for how wide to make the swale and spillway, etc., but remember, a major rain event like a tropical storm can overwhelm just about any small scale water catchment that you might create, so be sure not to locate your spillway so that it leads into your front door! 🙂
Carefully consider any water catchment above dwellings. We decided not to install a dam in a really perfect landform at Pine Ridge Reservation because if it ever blew out, it would dump directly on the owner’s home and his major work area. It was just too great a risk. There were other places on his land that he could catch water without that level of risk.
Any earthwork, no matter how small or subtle, has the potential to cause unexpected flooding if enough rain falls, or to blow out and cause a rush of water somewhere. When you move earth, you are engineering something, no matter how small.
Small earthworks hold the least risk for harmful flooding or blow outs. The larger the earthwork, the more important engineering becomes. This is where “small and slow” is good to apply.
It’s good to observe some videos of how moving water in a flood can literally move cars with only 18” of water, or entire buildings from their foundations if there is enough mass in the water (like in the tsunami that hit Japan, and with storm surges in hurricanes).
Remember, what the water actually weighs is not the weight of whatever is touching the building or object but the weight of the water behind it as well. It is all one mass. https://www.cnet.com/roadshow/news/australia-flood-sends-cars-down-river-video/