86 



The transition area of Sector E from the Waterworks revetment to south of 

 Line R12 is distinguished by an ephemeral beach feature (i.e., a beach subject 

 to significant fluctuation in size). Table 10 indicates that in two of the three 

 years of the monitoring program, this sector experienced deposition towards 

 early fall (as a result of the beachfiU moving south) and erosion in late fall as 

 the deposit was eroded by subsequent storms. Results of the model tests 

 indicate that this sector is subject to the highest alongshore transport rates for 

 the study area shoreline. Numerical model results also indicate that this area is 

 subject to ongoing downcutting, particularly offshore of the beach deposit. As 

 noted in the section entitled "1945/6 to 1964/5," the Waterworks revetment 

 probably helps to impound sediment and maintain the beach immediately south 

 of the revetment. Numerical model tests also revealed that the coarse-grained 

 beachfill derived from upland sources is much more effective at protecting the 

 glacial till under the beach in this sector. 



Although downcutting of the nearshore profile in Sectors E and F may 

 eventually diminish owing to the deeper water that has developed offshore of 

 the shore protection, the numerical model results suggest it is still ongoing, as 

 did the 1991 to 1995 lake bed comparison (see Figure 32). Model results also 

 indicate that there may be ongoing deposition of sand in this sector, since only 

 about 50 percent of the coarse sediment transported into this sector from the 

 north is predicted to be transported southwards beyond Line R23. It was seen 

 that during the 1965 to 1991 period with higher annual beach nourishment 

 volumes, the rates of nearshore profile lowering in this sector were signifi- 

 cantly reduced (see Table 7). Table 10 shows that this sector typically 

 receives sediment sometime in mid to late fall. Based on the predicted reduc- 

 tion in potential transport rates of coarse sediment between Line R12 and Line 

 R23, we estimate that about half of the 600,000 m^ of coarse fill that has been 

 placed since the beginning of the Section 1 1 1 nourishment program has been 

 deposited in this sector. With this assumption, and assuming the deposition 

 occurs over a 500-m-wide band of the shore extending out to the 6-m contour, 

 the average gain in thickness of sand cover would be 0.067 m since 1976. 

 Based on the findings of lake bed surface comparisons and the results of the 

 numerical model tests, this annual deposition rate of 0.0035 m/year derived 

 from the beach nourishment is at least balanced, and probably outpaced by the 

 ongoing downcutting of the underlying glacial till. This was certainly the case 

 during the 1991 to 1995 period, with lower annual beach nourishment 

 volumes. 



In order to raise the profiles to the historic lake bed levels (i.e., to allow 

 unimpeded sediment transport to the south), and assuming about half of the 

 traditional coarse beach nourishment volume (i.e., about 20,000 m^/year since 

 1986) is deposited in this sector and that downcutting can be arrested in the 

 near future, almost 8 million m^ of sediment would be required over the next 

 400 years at the current rate of nourishment. The numerical model tests indi- 

 cated that the 2-mm grain size sediment was no more effective than the 

 0.2-mm sediment in protecting the underlying till from exposure and down- 

 cutting in this sector. 



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



