Several significant differences exist between spilling walls and the techniques which have already been discussed. The most important of these is the type of site for which they are appropriate. The techniques that have been discussed so far are most useful on slopes. If selected and spaced properly these can serve to stabilize slopes from 5 to 20 meters long, depending on degree of steepness. Spilling walls on the other hand are most useful on comparatively flat sites, where cutbank erosion produces short but nearly vertical drops from a fairly level valley bottom soil surface into a stream channel, as shown in the photograph below.
View along Decker Creek showing the flat even fields it flows through. The area the spilling wall is being built at is an outside curve. This is where the stream channel turns and rapidly moving water on the outside of the bend cuts into the field. A gravel bar can be seen developing opposite this point, where slow moving water deposits debris.
Spilling walls are better classified as a Biotechnical stabilization technique than soil bioengineering technique like those discussed so far. This is because of the use of nonliving materials such as steel rebar and burlap or jute geotextile (see construction photos at right) as important components of the wall. While a spilling wall could be built without these items, they offer significant improvements in strength and soil retention.
Another important difference between a spilling wall and the other techniques examined so far is the size of it. While a full fledged pole drain system would be a major undertaking, most of the other techniques, including a simple live pole drain are quite modular. These small distinct units can be mixed, matched, and assembled into a large overall plan, but on an individual basis are not too demanding. Spilling walls on the other hand , which line an outside curve, involve a large area, and require a significant amount of effort and material construct.
The final major difference is that spilling walls are built essentially in the stream channel itself, rather than up on the slope above it. If constructed improperly, a spilling wall could make conditions in a stream worse, rather than better. This could result directly from disturbance during the construction, making it too small, or as a result of effects the structure has on stream flow that cause deflection rather than absorption of the erosive energy in stormflow, and simply shift rather than solve the problem.
The series of photos at right illustrate the process of building a spilling wall. Because construction takes place essentially in the stream itself, it is a good idea to first put down a layer of straw in order to filter sediment churned up during the process. A trench is dug to provide a firm even base upon which to build. Rebar posts are then driven in, and willow whips are woven between them to raise the wall. Next a layer of burlap is put in behind the willow wall. This helps hold soil in place as the area behind the wall is back filled. Finally the terrace behind the wall provides a further opportunity for planting with live stakes (as in the photo below).
Planting live stakes on the terrace behind the completed spilling wall
Both the live stakes and the live cuttings forming the wall root into the terrace of soil behind it, binding the soil in place. Meanwhile as the willow whips grow, the presence of their branches in the channel further protects the bank by creating drag that absorbs energy from storm flow, thereby slowing it down and reducing erosion.
Continued: Soft gabions