d (Y sediment Y fluid* 



-A tan <J> ( 5) 



The left-hand side of this equation is commonly referred to as the Shields 

 parameter, after A. Shields, widely acknowledged as the first to develop 

 guidance on criteria for initiation of sediment motion. Experimental studies 

 have shown that the right-hand side (specifically C 1 and C 2 ) of this equation 

 varies with the boundary or grain Reynolds number R g . The grain Reynolds 

 number relates the degree to which sediment grains project into the zone 

 immediately above the viscous sublayer of the boundary layer and is typically 

 expressed as 



u.d ... 



R a = — ^- (6) 



where 



u* = apparent or shear velocity, ft/sec 



v = kinematic viscosity, ft 2 /sec 

 Plotting experimentally obtained values of the Shields parameter versus the 

 grain Reynolds number yields the well-known Shields diagram. Although the 

 original work of Shields (1936) was done for inorganic particles of uniform 

 size in a unidirectional flow, numerous others have conducted similar research 

 to broaden the applicability of the Shields diagram, with median diameter d 

 used to define sediment size. Figure 2 shows a curve fitted to the Shields 

 data as well as data from other investigators. The curve itself represents a 

 reasonable approximation of conditions for impending sediment particle motion 

 in a unidirectional flow. Conditions that plot above the curve correspond to 

 regimes where sediment motion does occur while no motion occurs for conditions 

 which fall below the curve. 



Critical Conditions for Sediment Transport 

 Under Oscillatory Flows 



7 . When the motion of particles in unidirectional flow is compared to 

 the motion of particles subjected to oscillatory flows, obvious differences 

 are seen in both the flow of the fluid and the path of the particle. In 

 steady flows, sediment transport is related to flow characteristics and 

 sediment/fluid properties. In oscillatory flows, additional forces are 



10 



