90 



result of and would continue to be the result of an unmodified nourishment 

 program. 



The greatest flaw in the current nourishment program is that the area where 

 a supply of sediment is most urgently required is only receiving 50 percent or 

 less of the historic supply rate of coarse sediment This seems to have 

 accelerated recession rates for the shoreline south of the study area (i.e., 

 Sectors G and southward 1991 - 1995). These erosion pressures result in 

 construction of more shoreline protection by property owners. In the long 

 term, these actions only further aggravate the problem by further reducing the 

 supply rate (by eliminating the input of sediment from shoreline erosion and 

 by impeding alongshore transport as deep water develops offshore of the struc- 

 tures). 



The authors recommend that beach nourishment be placed downdrift of 

 Line R22 so that 100 percent of the fill reaches the area where it is required 

 (i.e., versus the current situation where perhaps 50 percent or less of the coarse 

 beach nourishment is deposited in Sector E without any apparent benefits). 

 The nourishment should consist of both fine (dredged) and coarse grain 

 components. By moving the feeder beach to the south, the sedimentation rate 

 experienced in the navigation channel should be significantly reduced. As a 

 result, maintenance dredging costs may be reduced if less ft-equent channel 

 dredging is needed. 



General Recommendation for Beach Nourishment 

 on Cohesive Shores Downdrift of Harbor 

 Structures 



It must be recognized that cohesive shores have very different erosion 

 characteristics from sandy shores and this has a significant impact on the 

 downdrift nourishment requirements. In addition, there are varying degrees of 

 cohesive shores (related to the extent and role of the overiying sand cover), 

 which also have an important influence on the nourishment requirements. 



Furthermore, effective downdrift nourishment requirements must be 

 determined in light of changes to the lake bed that may have occurred as a 

 result of the presence of the harbor structures prior to the initiation of a 

 nourishment program. This is not necessarily the case for sandy shores down- 

 drift of harbor structiu"es. 



Beach nourishment guidelines for the two extremes of cohesive shore 

 conditions (with respect to extent of historic, predevelopment sand cover) are 

 discussed here. A final special condition is also considered. 



In some cases, sections of cohesive shore on the Great Lakes (and else- 

 where) will feature only a "limited" sand cover. As a possible defining 

 variable, the sand cover between the 4-m depth contour and the bluff would 



Chapter 6 Beach Nourishment Design Guidelines 



