Figure 6. Longshore continuity of bars in the Great Lakes. The inner two to 

 three bars are usually visible when viewed from high bluffs and 

 dunes along the shore. In the above photo (taken about 900 meters 

 above lake level) the inner three bars can be seen following the 

 curve of the shoreline from station 10 toward Pentwater Harbor in 

 the upper right corner (a distance of about 6 kilometers). 



The continuity of the longshore bars is interrupted in the northern part 

 of the study area by the Pentwater Harbor jetties. Each year the outer bars 

 extend into the entrance channel beyond the end of the jetties. Typically, 

 40,000 to 60,000 cubic meters of sand is dredged annually from this entrance 

 bay and from the inland channel where windblown sand makes an important con- 

 tribution (Seelig and Sorensen, 1976). In recent years an increasing amount 

 of this sand has been used to nourish adjacent beaches; however, most of it is 

 taken about 1.5 kilometers offshore and dumped into 12- to 15-meter depths. 



In general, jetties which penetrate the surf zone interrupt the normal 

 longshore transport of littoral drift. There is frequent concern that long 

 jetties divert some of the drift offshore where it accumulates in water so 

 deep that the sand is essentially lost from the littoral system. The broad 

 mound of sediment opposite the Pentwater jetties lies at a depth of about 13 

 meters and may have originated as a result of such a diversion of longshore 

 currents. However, the broadly symmetrical appearance and position of the 

 mound (Fig. 7) suggest it is more likely an expression of the open water 

 disposal of the material dredged from the channel. 



An interruption of longshore bar continuity also occurs opposite Little 

 Sable Point. The bars at this location are not only discontinuous, as ob- 

 served in aerial photography, but are also much less regular and less smooth 

 in cross section. Bathymetrlcally , Little Sable Point is a transition zone 

 dividing the study area into two nearly equal stretches with distinctly 

 different bar geometry. A sequence of four well-formed longshore bars marks 

 both areas, but in the north these bars are shallower and closer to shore (all 

 within the first 400 meters). South of the point the barred zone is about 600 



15 



