annual net transport rate of 80,000 m-^/year to the south is blocked by the 

 jetties, and must therefore be eroded from the lake bed and shoreline south of 

 the jetties, this leaves an additional 178,000 m^/year of lake bed erosion that 

 must be related to offshore losses. It is likely that the majority of these losses 

 are related to offshore dispersal of silt and clay associated with the erosion of 

 the cohesive sediment. From these simplified sediment budget calculations, 

 the average annual erosion of the cohesive profile may have been as high as 

 20 m /m/year for this 9-km-long section of shoreline south of the harbor 

 (Panels 3 to 8). 



1964/65 to 1991 



The second period of lake bed evolution corresponds with the introduction 

 of beach nourishment at St. Joseph. From the lake bed surface change plot 

 given in Figure 31 and the volume estimates in Table 7, the 1964 to 1991 

 comparison clearly represents a period of reduced erosion rates combined with 

 nearshore deposition, both north and south of the harbor jetties. There are 

 several possible explanations for the nearshore deposition: (a) migration of 

 nearshore bars during the high water levels in 1991; (b) deposition associated 

 with the alongshore transport of beach nourishment; and (c) error in bathyme- 

 try or datum conversion. 



Since the 1991 bathymetry did not extend north of the jetties, the 1964/65 

 bathymetry was compared to the 1995 survey for Panels 1 and 2. Although 

 Panel 1 continued to erode, the average annual lowering was only 12 percent 

 of the 1945 to 1965 rate (see Table 7). The depositional trend also continued 

 for Panel 2; however, only at 27 percent of the 1945 to 1965 rate. The long 

 narrow depositional feature in Panel 2 (see Figure 31) is associated with a 

 change in bar location (profile change) in response to the difference between 

 the 1964/5 low water levels and the average water levels in 1995. 



At the southwestern comer of Panel 2, which corresponds with the end of 

 the north jetty, the 1964/65 to 1991 bathymetry comparison in Figure 31 

 revealed localized deposition in the range of 1 to 2 m for the 27-year period. 

 A similar trend was also evident in the 1945/6 to 1964/5 comparison, although 

 the zone of high deposition was located closer to the shore (see Figure 30). A 

 decrease in the offshore depths at the end of the jetty is the direct result of the 

 growth of the fillet beach deposit. This process may also have contributed to 

 the development of a sediment pathway for channel infilling during north- 

 westerly wave attack. 



For Panel 3 south of the jetties, the annual erosion rate averaged 0.2 cm/ 

 year. Panels 4 through 7, which experienced the most extreme erosion 

 between 1945 and 1965, continued their lowering trend, but at dramatically 

 reduced rates. The annual lowering rates for Panels 4 to 7 ranged from 0.2 cm 

 to 1 . 1 cm. The total lake bed lowering for the isolated case of the large 

 depression in the lake bed (R12 to R20) was in the range of 1 - 2 m for this 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



71 



