PART II: GENERAL DISCUSSION OF SEDIMENT TRANSPORT 

 Sediment Transport Modes 



5. In general, researchers agree that in order to accurately describe 

 sediment transport, it is necessary to consider the forces that initiate 

 sediment motion, subsequent transport, and the path back to the bottom. 

 Typically, sediment moves along the bed in a tumbling fashion as bedload, by 

 being lifted higher up into the water column and being moved by the water 

 particles as suspended load, or in some combination of the two. The 

 proportion of each mode of transport relative to the total amount of sediment 

 transport depends largely on the density and size of the sediment and the 

 hydraulic domain that acts on the bed. In typical coastal scenarios, where 

 the bed is predominantly non-cohesive sands, suspended transport is prevalent 

 in highly energetic hydraulic regimes, such as in the surf zone. 



Critical Conditions for Sediment Transport 

 Under Unidirectional Uniform Flows 



6. In most cases involving sediment transport, it is useful to discuss 

 the concept of critical values associated with the moment at which sediment 

 grain motion is incipient. Most commonly, near-bottom fluid velocities and 

 shear stresses between the fluid and sediment are used to describe what is 

 called the critical point, or the moment just before a sediment particle 

 begins to move. This is examined by an analysis of the forces that act on a 

 particle initially at rest in a unidirectional flow field. Among the 

 significant forces acting on a single particle in a flow situation are the 

 particle's weight and the forces attributed to fluid and particle interaction 

 (drag and lift) . Figure 1 schematically depicts these forces as they would 

 occur for a particle positioned on top of other particles. When these forces 

 are in balance or the restraining forces are greater than the net of the 

 forces trying to move the particle, no motion can occur. Analysis of forces 

 acting on a particle in unidirectional flows is fairly straightforward and has 

 been presented adequately in numerous other research efforts (Shields 1936, 

 Silvester 1974, Middleton and Southard 1978, Clark 1979, and Hales 1980). 

 This analysis involves taking moments about a "pivot point" shown as P in the 

 figure and results in the following equation: 



D (cos*) — = (W - L) sin(<t>)-| (1) 



where 



D = drag force , lb f 



d = representative grain diameter, ft 



