Figure A- 1 I . Schematic of a straight river 

 showing zones of potential erosion. 



Larger sized granular material (such as coarse sands, gravels, cobbles) re- 

 quire higher velocities to be eroded because of the increased weight of the 

 particles. With vertical cut banks, large diameter materials often build up at 

 the base of the bank. This build up is because the finer materials holding 

 them in place are eroded away while the larger sized materials cannot be 

 transported. Material finer than fine sands (silts and clays) are often more 

 resistant because of the cohesion between particles. 



If the buffer material is uniform throughout, then historical erosion 

 rates do not need to be modified for soil composition effects. If there are 

 areas of significantly finer or coarser sized materials, the historical ero- 

 sion rate should be modified accordingly based on the discussion in the pre- 

 ceeding paragraph. 



97 



