Waves and Current Transport 



A more likely mechanism for sediment transport at Port Huron is from the 

 combined effect of waves and currents. As noted from the current-only 

 calculations above for the representative sediments, though the currents are strong 

 at Port Huron, they do not appear to be moving sediments independently of other 

 hydrodynamic forces. Breaking waves probably destabilize sediments making 

 them available for transport as suspended or bed load by the unidirectional 

 currents that transport the sediments southward along the beach. USAGE (1992) 

 documents early work from Grant (1943) that stated nearshore transport results 

 from the combined effects of waves and currents — waves placing the sand in 

 motion and currents producing a net sand advection. Inman and Bagnold (1963) 

 developed the formulation for longshore transport on beaches resulting from the 

 combination of breaking waves and longshore currents. This relationship, which 

 was modified from Equation 6-15b in USAGE (1992) for use in this study for 

 0.78 breaking criterion, converting wave height from root-mean-square to 

 significant and for fresh water rather than salt water, is given in Equation 5. 



Q, =3.329x10' Hi' V, (5) 



where 



Qi = volumetric sediment-transport rate, mVday 

 Hbr = breaking-wave height 

 V; = longshore current 



The longshore current value used in Equation 5 was taken as the most near- 

 shore depth-averaged current measured with the ADCP from the transects north of 

 the boat basin — namely, Transects G4, G5, G6, G7, and G8. Table 4 summarizes 

 the currents and transport rates calculated by Equation 5. Wave data used in 

 Equation 5 for each case are identical to that presented in Table 4, so only 

 breaking height and period are repeated for reference in Table 5. Similar 

 uncertainty estimates are made on the calculation of Qi in Equation 5. Plus/minus 

 5 percent for wave height and ±10 percent for current velocity translates into a 

 ±20 percent uncertainty for Qi. 



These rates appear to be reasonable approximations of a likely potential 

 longshore sediment-transport rate. Considering the limited supply of sediment to 

 the north and the presence of sediment-trapping structures, an actual longshore- 

 transport rate on the order of 100,000 m^/year (±50,000 m^/year) is probably 

 appropriate. It should be noted that this estimate should be considered now 

 preliminary, as it is not supported by any field data (measured sediment transport 

 or historic bathymetric and topographic surveys for trapping rates). 



Chapter 5 Sediment Transport 27 



