Desert Restoration and Gabions
As more homes are built in areas that were previously natural desert, there is more erosion. Particularly, the effects of building near washes can be a problem. Rain in the desert, while infrequent, sometimes comes in a violent deluge. Large roof surfaces repel water, while desert soil absorbs water quickly because it is so porous.
Normally, desert washes follow the natural contour of the land. Water rushes downhill in established patterns when it rains too quickly and heavily for the soil to absorb all of the moisture. Plant life grows to the edges of the washes. However, when water flows off the surface of a roof a substantial amount of water falls in a concentrated area at high velocity. This can expand the width and depth of a wash as concentrated volumes of water flow into the wash in a new, unnatural pattern less related to the natural contours of the landscape. This erodes the soil at the edges of the wash and causes exposure of the roots of the plants growing aside the wash. Then these plants get damaged or lost because they don't have enough of a viable root system left to sustain their former growth. So what was previously a safe and natural place for a plant to grow suddenly becomes a death trap, even for mature trees that have been thriving for years.
One remedy for this problem is the installation of "gabions". These are manmade permeable dams made of rocks and enclosed with chicken wire. They slow the flow of a wash and preserve the soil level to their height. Even where there is no erosion, gabions can be used to hold more water on your land so that your plants benefit from a raised water table. Below are before and after photos of a tree that suffered extensive root exposure from erosion, then recovered rapidly after a gabion reestablished soil around it's root zone.
How to Make a Gabion
Constructing gabions involves collecting or having delivered a fairly large quantity of rocks. It is easier with larger rocks but smaller rocks will also work as long as they are larger than the spaces in chicken wire so that they don't fly out. A gabion will hold up to the force of a wash no matter what size the rocks are as long as the overall weight is heavy enough.
A length of concrete reinforcement wire, lined with chicken wire, and long enough to extend up the banks of the wash by more than 1 foot on each side, needs to be laid out before the rocks are placed on top of it. Don't skimp on the length of the wire because it is too much work to get it wrong and to have it get blown out by the force of the water (as I have experienced) which can cause a further erosion of the banks as the water travels wildly down the path of least resistance. The gabion must extend up the edges of the bank or the water will travel around it, carry it downstream, and, ultimately, achieve nothing. The wire should also be wide enough to encircle the row of rocks so that you can close it at the top (photo follows). You can use heavy wire lengths (like coat hanger size) to close it. Remember, you are trying to create a heavy yet permeable enclosed structure. If the wire enclosure flies apart then the rocks will just travel downstream. If you are working with a wash that is just a foot or two deep and wide then using only chicken wire for the rock enclosure should work fine.
The more height to the bundle the better but the soil will collect to whatever height of gabion that you have constructed and it should stay in place as long as you extend it up the sides of the banks of the wash. A low gabion is fine because the water will flow freely over it but soil will still be collected to the height of the gabion. Later on you can always place another gabion on top of the first (connect the two with wire) to conserve even more soil but never go higher than the soil level surrounding the wash. These instructions should be fine if you are dealing with a wash that is about 5' wide or less and no more than about 2' deep. For a larger wash hire a professional. Check your gabion after it rains to make sure that it's staying together. Using several gabions downstream from one another also works well. Use these instructions at your own risk since I am only sharing what has worked for my situation. It is hard work but when you see how happily the plants respond it's worth it. Here is a photo of another successful gabion.
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