CAUSES OF STREAMBANK EROSION

Streambank erosion is a natural phenomenon but can become accelerated because of changes at the watershed and reach level. One should try and establish background conditions using old photographs and maps and compare rates over time, in the context of floods, and watershed and reach level changes. These are combined with recognizing reach level changes in stage of channel evolution, scour patterns, sediment deposits, side slopes, slumping, and cultural features. Changes at the watershed level might be caused by logging, mining, road building, Changes at the reach level might be modifications of the riparian area by vegetation change or loss, animal use, stream channel straightening, or channel excavation and removal of material (stream bed mining), or avulsion. For accelerated erosion try to determine whether bank instability is caused by cantilever failure, rotational failure, planer failure, preferential flow failure, high pore pressure or seepage forces causing failure or popout failure. One needs to understand if cantilever or other failure is accelerated by a stratigraphy, flow condition (peak or duration), waves, or ice condition. Focus on banks materials (composition and stratigraphy), height, overhang, root density and height relative to bankfull depth. Is there evidence of high pore pressure, seepage forces, liquefaction, or upwelling flow, that impacts soil strength associated with rotational and planer failure. Accelerated streambank erosion at the reach level is often modified by local sedimentation patterns that redirect flow, to the extent that avulsions occur where not expected. Landslides and debris flow often provide such a large quantity of coarse material to streams that the stream finds it easier to eroded the streambank, than to move the course material. Watershed level changes like logging can cause increased sedimentation downstream that completely bury the natural streambed and streambanks and create a whole new cycle of streambank erosion. After having evaluated hundreds of miles of streambank, the author has concluded that stratigraphy (especially washing fines and sand out of gravel strata) is the principal case of cantilever failure, for most gravel bed river streambanks. In addition it is apparent that streams with a Rc/Bkf of less than 2.5 are the most prone to failure along meandering streams.