We live on a slope in a pretty dry area where it barely gets any rain. I like the swales idea, but I'm worried about landslide danger. Our house sits on top of a slope, and any part of that slope coming down might damage or destroy the house (not to mention us with it). We do have some solid rock in some areas but I'm not sure where. Is there any engineering advice as to how to do them without the danger of landslides?
Hard to say for sure. Somethings that will affect the slide chances are how well drained your soil is, the amount of trees and other vegetation to help hold the soil together, and if the soil is sitting on a layer of impermeable material such as a slab of rock.
If the soil is well drained then the swale can fill up during the heavy rains but quickly drain and the soil never has to hold the extra weight for long. But if it is all clay then the water will just sit there.
My place has all clay soils and the soil already stays saturated from around Dec through March or April. Due to this I decided not to use swales on the slopes. They would not really hold any more water and I was worried about causing a landslide.
Swales are often referred to as tree growing features. If you plant trees on the downhill side (berm side) your trees will benefit from the increased water and help hold the slope together. So if you do put in swales I would recommend planting some trees.
My place has very few trees which is another reason why I'm not putting in swales on the slopes. I'm not ready to plant the trees in the field at this time.
Smaller plants with tap roots and ones with big expansive root systems can also help hold the slope together. Lupins are a good example of a small tap root plant that also fixes nitrogen which can improve your soil.
If your soil is not very thick and resting on a layer of impermeable material then the water can lubricate the boundary between these two layers and make a slide more likely. I would not recommend a swale in this situation.
If you decide that swales are not the best option you can do other things to hold more water with less risk. At my place I'm going to create a series of ponds at the base of the slopes. I'm also going to plant my trees and shrubs on contour and place rows of slash on the top of the ground also on contour. The trees will likely be planted behind these. I will cover the slash with lots of leaves and though I say slash I will stick with medium to large size logs.
These will mimic swales without holding so much water as to cause slide issues. I will place them more often than if I used swales.
Hope that helps you to think through your decisions. You might also want to check out these two books on Water Harvesting for Drylands and Beyond:
What is the % of your slope? This makes a big difference. A slope that is too steep is better not to have full cross slope swales unless you have a lot of deep rooted trees already. Daron brought up a lot of valuable points. Another thing that can be done, instead of swales is smaller fish-scale type swales for individual plants or small guilds, or pitting and planting in and around the pit with drainage lines connecting the pits on a diamond/pattern which is called net and pan. These all infiltrate water without saturating the entire slope in a solid band (which would potentially trigger a slide). Any slope over 20 % is recommended to be forested regardless. If you are working the soil and planting annuals, stick to less than 20%, and less than 15% is super safe. Deep rooted plants/trees are your friends. :)
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I would look into underpinning your structure whatever you do if you think there's a significant chance of the slope collapsing.
Also, I would like to second the idea about planting on-contour. Anything you plant that puts its roots down into the soil will both hold the soil together and increase water infiltration.
Laying debris on-contour will create sediment traps, collecting organic matter and particulates that will become soil, all without disturbing the soil. Doing it this way also lets you play with placement ideas before you disturb the slope. If you notice evidence of erosion or movement in the debris atop the slope, it could point to problems you can address before they happen.
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Thank you all so much for your advice! I was actually thinking about doing small pits in a diamond pattern and fill them up with mulch, but wasn't sure. Another idea was to dig large 3'x3' pits, put good dirt in, and plant fruit trees in them, in a diamond pattern about 6 ft apart.
Now, here are some pictures of the slope that we have. It's 4 acres, and all of that goes down the eastern slope. All of it is terraced. This means, there are large 20-ft wide steps that then drop off to the level below. The hill is pretty steep at around 45 degrees in some places, then levels off, then goes up again. On the lower part of the property, the slope goes less steep, I'm not sure how steep, but there's an acre or so of usable lot that slopes slightly. On that more level spot we wanna put paddocks for chickens with paddock shift design, pecan trees, and just as many fruit trees as we can possibly fit. Given that about 50% dies of everything I plant, we need a bunch. On the steeper slope up the hill we also need to plant fruit trees, and possibly make it so we don't have to water them. Fruit trees that aren't watered give fruit that's healthier anyway.
So this is the steeper terraced area:
This is looking up the hill and the terrace is rimmed with rocks where it levels off.
This is the area at the bottom of the hill:
It still has terraces that are about a foot tall.
In this area, all of the soil is suffering from overgrazing that happened over hundreds of years. There's barely any grass, the soil is rocky, the topsoil is 6" tops, and most of the organic matter washes down into rivers and lakes. I will probably need to bring some wood chips to cover most of the land. We are about 40 miles from town, so I don't know if any arborists would be willing to drop their woodchips on our lot.
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