On Friday (12/18/09), Drew Goetting, Principal designer at Restoration Design Group, came down from Berkeley to give us all a lesson in bioengineering. First, dig a 3-4" deep trench at the toe of the bank slope. Toss the dirt from the trench upslope to make it easier to fill the wattle.
Next, lay out the coir fabric so the edge extends a few inches upslope past the lip of the trench. Secure the fabric to the soil with biodegradable staples (in this case, Bio-Stakes made from corn).
Shovel some of the soil from the trench over the coir fabric, then place willow pieces (branches, twigs, and leaves) over the soil. Add more willows and more soil, making sure that the willows are generally oriented along the streambank and no branches are sticking out perpendicular to the direction of the water flow. More willows and more soil, until the whole thing is the right height (see below for what "right" means).
Next, imagine that you are making a giant California roll, using the coir fabric as your sushi mat and the willows and dirt as your rice, crab (or krab), and avocado. Kick and push the mixture into place--it is pretty rugged and will withstand some rough treatment. The goal is an evenly compacted roll whose top is fairly level. The finished top of the willow wattle should be above the water surface when the stream is flowing.
Pull the coir fabric fairly tightly over the soil and willows. Secure the edge of the coir fabric with more biodegradable staples.
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The result is a willow wattle. The fabric will hold the soil and protect the bank on a temporary basis. Meanwhile, some of the willows inside the wattle and some of the willow stakes will sprout and their roots will grow to hold the streambank. After a while, the coir fabric and the staples will biodegrade and all that will be left will be willows, creating a sustainable streambank and a vegetative screen of riparian habitat.
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