STREAMBANK EROSION CAUSES, AND LARGE WOODY MATERAL (LWM) SOLUTIONS FOR STREAMBANK EROSION AND SEDIMENT REDUCTION

Watershed and reach level changes can impact streambank erosion. Most of the focus of streambank erosion at the reach level is the bank instability caused by cantilever failure, planer failure, rotational failure, or preferential flow failure. These mechanisms are modified by high pore press; liquefaction; high bank height exceeding a critical bank height; stratigraphy; Rc/Wbkf < 2.5; flow condition (flow duration, helicoidal flow, cavitation, prewetting); boundary shear stress along eroding bank; root density and depth; waves; ice condition; desiccation, and animal burrows. For the gravel bed rivers of the western states the predominant cause of bank failure is catilever failure. The cantilever is created because the fines and sands are washed out of the gravel and cobble stratigraphy to create the cantilever overhang. Local sedimentation, especially from debris flows, can cause streams to erode their streambanks, because it is easier to erode the streambanks than it is to erode the coarse material of the debris flow. Large woody debris jams in stream channels can cause stream to change course and to create stream avulsions. The avulsions cause extensive streambank erosion by downcutting and widening. Tight curve meanders with a Rc/Wbkf < 2.5 tend to fail unless they have extensive deep toe protection. For environmental reasons, bank protection in recent years has tended to use large woody materials. This could be the placement of logs, stumps, rootwads, whole trees with branches, along with spacer logs, deflectors logs, and vertical wood post for stability. The trend in recent years is to install rootwad and boulder buttressed structures, log vanes, and engineering log jams. Critical to the success of these structures is how well the toe protection works. Most large woody material structures, tend to have associated streambank Soil Bioengineering treatment with fascines, brush mattresses, and stake and pole planting. It is essential that this type of Soil Bioengineering material be applied above the bankfull stage of the river, otherwise failure is likely.