It should be noted that the tenfold reduction in lake bed lowering rates may 

 not be sufficient with respect to mitigation of the harbor influence on erosion 

 further downdrift. If the feeder beach sand simply ends up slowly filling the 

 large lake bed depression that has developed in Sectors D to F, the shore fur- 

 ther downdrift wiU continue to be denied the historic levels of sediment 

 supply. 



Existing and Future Conditions 



Existing conditions (1991 to 1995) 



Comparison of the lake bed surfaces from the 1991 and 1995 hydrographic 

 surveys reveals a rapid acceleration in lake bed lowering. In Sectors E to F, 

 the lake bed lowering rates are 30 to 50 percent higher than the 1945/6 to 

 1964/5 comparison period and an order of magnitude greater than the 1965 to 

 1991 period (see Table 7). Of greatest concern is the observation that the rate 

 of lake bed lowering in Sector G is of a similar magnitude to that of Sectors E 

 to F (see Figure 37). In other words, it would appear that Sector G is now 

 being influenced by the harbor structure and may no longer be regarded as 

 representative of background erosion. A review of the contour plots of lake 

 bed change (see Figure 32) also indicates that the focus of lake bed lowering 

 (i.e., that led to the development of the depression offshore of Sectors E to F) 

 has shifted to the south. 



One significant difference between this most recent period and the previous 

 comparison period was the annual average volume of beach nourishment. 

 Annual placement volumes have been reduced by approximately 50 percent to 

 40,000 m^ over the last 5 years (see Figure 2). The reduced level of beach 

 feeding may at least partly explain the accelerated erosion rates. 



Projections of future conditions 



The fillet beach south of the harbor is currently stable or slightly accreting. 

 During southwest storms, this sector receives sediment from erosion in the 

 feeder beach area. It would appear that the fillet has reached its maximum 

 extent and that any additional sand transported northwards eventually makes its 

 way into the navigation channel where it is deposited, and later dredged. 



The feeder beach shoreline is maintained at a stable average position with 

 the annual beach nourishment Without the nourishment, the numerical model 

 investigations have shown that shoreline recession would recommence, with 

 the transport of sand to the south and the uncovering and dovmcutting of 

 imderlying glacial till where it exists. Comparison of the 1991 to 1995 lake 

 bed surfaces in Figure 32 revealed -that this sector experienced erosion under 

 the recently reduced nourishment levels. A summary of the changes to the 



84 



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



