The most southerly sector, Sector G, and the unprotected shoreUne further 

 to the south received about half of the historic net alongshore sediment supply 

 rate of coarse sediment. This is because the deep water offshore of Sector D 

 acts as a sink for about 50 percent of the coarse beachfill sediment. Therefore, 

 it is likely that the shoreline of Sector G, and particularly the shore to the 

 south of this sector, are suffering due to a depletion of the historic sand cover 

 with the associated increased exposure of the underlying till and increased 

 downcutting and shoreline recession rates. The loss of the coarse fraction 

 results in greater erosion close to shore (i.e., where slopes are steeper and only 

 the coarse-grain-size fractions remain relatively stable under most conditions). 

 The most recent lake bed comparison (1991 to 1995) revealed that the lower- 

 ing had in fact increased dramatically in Sector G compared to earlier periods. 



Comments on the Effectiveness of the Beach 

 Nourishment Program 



The fillet beach of Sector C would probably remain stable without beach 

 nourishment from the Section 111 program. At present, perhaps as much as 

 50 percent of the sand placed in the feeder beach area (particularly for the 

 dredged finer sediment) ends up back in the navigation channel from where it 

 was originally removed (and will be removed again). 



There must be a more cost-effective approach to maintaining the position of 

 the shoreline in Sector D than beach renourishment. An alternative approach 

 may also be more environmentally acceptable and less disruptive to the local 

 community (i.e., not requiring the annual trucking operation for the placement 

 of coarse sand and gravel). 



The primary local beneficiary of the ongoing nourishment is the transitional 

 part of Sector E. Here, too, there may be more cost-effective means of pro- 

 tecting this section of shoreline. The coarse sediment is much more effective 

 than the fine in protecting the till underneath the beach in this sector. The 

 coarse sediment fulfills a role (which would have been present historically) in 

 protecting the underlying till from downcutting that the fine sediment cannot 

 (i.e., over the steeper nearshore slopes). 



Sector E has been a sink, possibly for up to 50 percent of the coarse sedi- 

 ment placed in the feeder beach area. However, the effectiveness of this 

 sediment (whether the coarse grain or fine grain type) in counteracting the 

 ongoing downcutting (either presently or in the future) is questionable. There 

 may be more cost-effective means of protecting the toe of the existing struc- 

 tures. It is unlikely that the placement of the 8 million m^ of beach nourish- 

 ment required to completely fill the depression that has developed over time is 

 justifiable. 



Chapter 5 Interpretation of Results - A Descriptive Model of Coastal Morphodynamics 



87 



