Self-Forming Channels

Self-Forming Channels      

The self-forming approach is an ecological engineering design that establishes initial conditions for successional channel system development, having both a desired target stable end state and beneficial ecosystem services as successional processes evolve. In essence, the do nothing approach is the stable end state arrived at unintentionally (and most likely with minimal width). The self-forming approach typically involves excavating the bed of a channel to an over-wide width and allowing successional processes to develop bars, benches and an inset channel that is stable and sustainable by natural depositional processes (Figure 7). The self-forming approach is, in effect, creating a valley without a floodplain, which results in the spreading of channel flows at low flow rates. Herbaceous vegetation quickly establishes in the bed of the channel, which promotes sediment deposition. One of the main benefits of this approach is that it allows space for the system to self-organize to a form that optimizes existing watershed and valley conditions. Depending on these conditions, the approach may result in a well-defined channel or may represent more of a wetland-like system. Another benefit is that the benches form from sediment and associated pollutants that would otherwise be transported downstream acting as both a sediment and pollution sink. This sink occurs at an accelerated rate in the early stages of succession and diminishes to natural rates as the benches become higher. Additionally, because the benches are self-formed they have better soil structure and much higher organic matter content than might be found in constructed benches in two-stage approach.

Self-forming channel after construction.
A self-forming channel just after construction.

Self-forming channel 1 year after construction.
The same channel after the second growing season.

 

Locations where the self-forming approach is not suitable include steep channels prone to incision; channels transporting very high quantities of coarse bed material, where vegetation will be limited or sparse; where in-stream habitat and biota might be achieving their attainment status or are considered valuable enough to not substantially disturbed; or where early successional habitat is discouraged. In many cases the cost of this approach will be similar to that of a two-stage ditch.