Bank Erosion, Bank Stabilization

[troutbert]

Here's a good post from another forum, on a topic I haven't seen discussed on here.

"Repair of stream bank erosion

At the risk of being branded a heretic, I must ask if the bank should be stabilized? Is this a natural flow or has something bad occurred upstream to cause this problem? If there have been no large upstream changes to cause this erosion, then we need to remember that streams are very dynamic systems that move around over time. Banks naturally erode and build up and this is exactly what they're supposed to do. We can cause problems when we try to stabilize and control what is, by its very nature, a dynamic and changing system. So stabilizing "your" bank could well cause problems elsewhere when that flow energy now is forced to dissipate in a different place. So I guess we'd need to know a little bit more about the system and the issue before forging ahead to fix something that may simply be a natural event in a living stream that needs to happen. In other words, proceed with much caution and do not try to control something that may actually need to occur."

 

[lycoflyfisher]

There is no simple answer to this debate. I will add that the worst thing that can be constructed in the name of bank stabilization is armoring both banks with a hard substrate with method of dissipating flows. Examples are extensive use of riprap, concrete, large cut stone blocks etc. These devices only increase the velocity of a stream and erosion issues are going to moved downstream. When deflectors, log vanes, rootwads, mud sills etc are incorporated into project designs there is typically less downstream impacts. Projects that come to mind specifically are bridge replacement projects. Typically there are significant upstream and downstream cases of bank erosion associated with bridges that maybe are undersized, poorly aligned etc. Many of those bridges when replaced in the north central region instead of just using rip rap to armor the wing walls and the upstream banks, log vanes and rock deflectors are being utilized to help shift the flow regime towards the center of the channel. This accomplishes several things:
1. Decreases bank erosion
2. Decreases scouring of bridge abutments and wing walls which decreases the frequency of maintenance
3 Typically aquatic organism passage is improved as well by forming a deeper channel vs a wide shallow riffle the width of the structure.