50 



conditions were performed with a grain size of 2 mm. The coarse grain size 

 was selected because the 0.5-m marker depth represents conditions where most 

 of the sediment is moving relatively close to shore. 



Review of the modeling results (summarized in Table 9) shows that slight 

 increases in alongshore sediment transport are predicted in the beachfill area 

 for the post-fill Run L (Lines R9a to Rll) compared to the pre-fill Run K. 

 However, most of the profile change in the beachfill area for Run L is related 

 to profile adjustment. The gradients in potential alongshore transport are very 

 low and do not result in rapid redistribution of the feeder beach sediment to 

 the south. 



Results of Runs H and K, which differ only in the assigned marker depth 

 (2.5 m versus 0.5 m, respectively) are very similar, with only minor deviations 

 in the beachfill area (see Table 9). 



Assessment of predicted glacial till downcutting 



For each of the 3-D model runs, downcutting of the exposed areas of 

 glacial till was determined based on the magnitude of shear stress and the rate 

 of wave energy dissipation at the location of the exposures, over the duration 

 that the till was exposed. The approach used to calculate downcutting is 

 described in Parson, Morang, and Nairn (1996). 



Downcutting results for the 2 November 1991 (SW) storm are compared in 

 Figures 21a to 21k, showing the difference in predicted downcutting between 

 the 0.2-mm and 2-mm results (i.e.. Run A versus G). In general, the predicted 

 downcutting (or vertical erosion) of the exposed till areas is predicted to be in 

 the range of to 0.15 m. The occurrences and magnitude of downcutting are 

 greater for the 0.2-mm sediment for the northern section of the shoreline (i.e., 

 where a beach exists with till underneath). For the southern profiles offshore 

 of the revetment, the magnitude and occurrence of downcutting are similar. 

 This finding relates to the fact that changes to the sand cover were much more 

 limited for the lake bed offshore of the revetment for both fine- and coarse- 

 grain sediment. Modeling was also performed for the 14 January 1992 storm, 

 and the results were similar to the 2 November 1991 results. 



The downcutting predicted for these storm events corresponds to storm 

 conditions that might be expected once every 1 to 5 years. The bathymetry 

 comparisons have identified annual lake bed lowering in the range of 0.06 to 

 0.10 m/year for the revetment shoreline between 1945/6 and 1964/5. Although 

 lowering rates decreased between 1964/5 and 1991, lowering in the range of 

 0.09 to 0.13 m/year occurred between 1991 and 1995. The numerical model 

 results of this investigation imply that significant downcutting is still ongoing, 

 not only in the vicinity of Profiles R9 to R12, but also for the deeper profiles 

 offshore of the revetment between Lines R12 and R23. 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



