Fascine, Revetments, and Wattle Hurdle
This year I have been in Haiti after a downgraded hurricane, and then in New Jersey a week after Sandy. While in New Jersey two tornadoes passed by my old house. What do they have in common?
In each case water was being limited in its flow by developement or the removal of natural structures that diffuse its energy. While working in Haiti and trying to build large enough swales to catch water, it was instantly apparent after the first five-inch rain that what we needed to do was slow it down and catch the sediment.
In many cases there is so much water so quickly, and so much erosion, that we need to find a way to spread out the energy and give the silt a chance to settle. These are like a windbreak with water. The increased deposition diffuses the focused energy that tears the soil, and reduces the velocity of the water.
The wattle hurdle, used for fencing and building, is what we used in Haiti to slow down the water. Like snow fences used in cold climates on the windward side of a freeway, it catches the drift of sediment and reduces the deposition in the swales below. Also, since they are on contour with the swales, the resulting micro-terrace will be even and prevent gullying.
On the top of this page is a picture of a fascine. A fascine is a long bundle of sticks or brush used to reduce erosion and build sediment, along a contour line in our case. We are using this in a ravine below a culvert to slow flood waters and spring run-off. Over time sedimentation will settle around the branches, plants will take root, and a natural berm will develop, slowing water in most cases.
Another term I learned while working in New Jersey is Revetments. In civil engineering these are large steps or concrete inclines along the banks of rivers and streams. Gabions are used for this, although the ones I saw along the river in Port-au-Prince were already being undercut by floodwaters. Most engineering revetments are smooth and slick only serving to move the water along while protecting the shoreline. Commonly concrete, But in general terms they can be used along any shoreline to decrease the erosion and increase sediment. Like the first scene, the revetment is a simple T-post along the waterline that is then woven with brush — honeysuckle in many cases.
Each time the water rises the revetment gathers more organic matter and small branches, building layers of intertwined sticks which then fill in with silt.
These are fairly simple and slow solutions that will continuously benefit the areas in which they are used, without disturbing the soil. As our weather gets more erratic and extreme, it will be important to do whatever we can to defuse the energy and increase the infiltration of the water while we can. Catch it on the ground, store it in the ground.