5 Sediment Transport 



There are numerous sediment-transport relationships for calculating potential 

 transport in the coastal zone. Some of these relationships are theoretically derived 

 from energy that is expended by the water flow in transporting sediment. Others 

 were developed from empirical measurements from laboratory experiments. Still 

 others combine laboratory observations with theory to arrive at a transport 

 potential. 



The Port Huron study site is unique to the problem of calculating potential 

 sediment transport, because it is influenced by open-coast breaking waves and 

 strong unidirectional currents that imitate riverine transport. Therefore, three 

 approaches to calculating potential sediment transport were considered: 

 (a) steady-flow transport depending on a Shield's criterion approach for initiation 

 of motion; (b) standard USAGE breaking-wave transport potential approach as 

 recommended in the USAGE Engineer Manual (EM) 1 1 10-2-1502 Coastal 

 Littoral Transport (USAGE 1992); and (c) a composite breaking wave-longshore 

 current approach also described in USAGE (1992). 



The potential longshore sediment-transport rate depends on the quantity of 

 material available for transport. Therefore, the term "potentiaf" suggests that the 

 calculated rate is the amount being transported if an unlimited supply of sediment 

 were available. Lack of feeding sediments updrift of the area of interest and 

 sediment trapping (from groins, jetties, harbors, navigation channels, etc.) all 

 negatively impact the calculated rate. When any of these exist at a project site, the 

 best approach to identify a more accurate transport rate is to conduct long-term 

 shoreline change analysis, topographic and bathymetric surveys to examine rates 

 of sediment trapping, and dredging-volume analysis. 



Steady-Flow Transport 



Steady-flow sediment transport has been researched and studied for over 

 50 years. Most practical steady-flow transport formulas are based on experiments 

 of total transport load. Sediment transport can occur as either bed load or 

 suspended load, but differentiating between the two is not practical in an applied 

 sense. Experimental data suggest that for relatively coarse materials (0.7 mm), 

 suspended load contributes little to total transport (Nielsen 1992). At Port Huron, 

 the dominance of coarse material suggests that suspended-sediment transport is a 



22 Chapter 5 Sediment Transport 



