results in erosion of the obstruction or bed scour adjacent to the obstruc- 

 tion (Figure 32). Complete channel relocation is also a potential response 

 to flow obstructions blocking a high percentage of the channel's cross- 

 sectional area. 



Close Flow Line 

 Spacing Indicates 

 High Velocities 



Probable Scour 



Figure 32. Schematic diagram illustrating the effects of a flow obstruction on 

 the local hydrau I ics. 



Changes Due to Gravel Removal 



Substantial changes in hydraulic geometry, slope, and flow obstructions 

 resulted from gravel removal operations at roughly 60 percent of the sites. 

 Typical hydraulic geometry changes in the mined area included increased 

 channel top width, reduced hydraulic depth, reduced mean velocity, and 

 increased conveyance. Changes in slope due to gravel removal operations took 



107 



