this site is such that this is not an issue. Lake bed lowering is on the order of 

 meters, which is far greater than any possible relative datum or measurement 

 errors in the surveys. 



From the extent of the hydrographic surveys, eight panels (or sediment 

 compartments) were created north and south of the harbor jetties. Wherever 

 possible, the panel boundaries were selected to delineate changes in the lake 

 bed evolution characteristics (i.e., south fillet beach. Panel 3 and feeder beach. 

 Panel 4). Data from historic and recent bathymetries were used to create 3-D 

 surfaces representative of the lake bed conditions at the time of the surveys. 

 From the comparison of the 3-D lake bed surfaces, net volume changes were 

 calculated for each individual panel (i.e., a positive value is obtained when the 

 net change for a panel was deposition for the period of the comparison). To 

 provide a relative basis of comparison for the calculated change in panel vol- 

 umes between the individual periods (i.e., 1945/56 to 1964/65 and 1964/65 to 

 1991) and panels, the total change in panel volume is divided by the surface 

 area of the individual panel. This provides an averaged depth change for the 

 entire panel, representing an annual rate of erosion or deposition. 



1945/46 to 1964/65 



The 1945 to 1965 bathymetry comparison provides a description of the lake 

 bed evolution downdrift of the harbor jetties before the implementation of the 

 Section 111 Plan for beach nourishment at St. Joseph (Figure 30). 



North of the jetties, the average annual lake bed lowering for Panel 1 was 

 2.7 cm/year. This rate of lake bed lowering is very similar to Panel 8 (2.6 cm/ 

 year), at the southern limit of the bathymetry comparison (see Table 7). This 

 may indicate that Panels 1 and 8 were representative of the background rate of 

 erosion at St. Joseph from 1945 to 1965. Panel 2 corresponds to the northern 

 fillet deposit, adjacent to the northern jetty at the mouth of the St. Joseph 

 River. The average increase in lake bed elevation was almost 2 cm/year, with 

 1.0 - 3.0 m of deposition recorded near the northern end of the jetty over the 

 20-year period. 



The southern fillet deposit. Panel 3, experienced minor volume increases, 

 amounting to an average of 0.5 cm/year for the panel, with the majority of the 

 deposition located adjacent to the shore and southern harbor jetty (see 

 Figure 30). 



From Panels 4 to 7, severe lowering of the lake bed occurred between 1945 

 and 1965. From profiles R8 to R17, a 3-km-long depression in the lake bed 

 developed, with lowering in excess of 4 m recorded over the 20-year period 

 (see Figure 30). Average lake bed lowering for the four panels (4 to 7) ranged 

 from 5.7 - 10.0 cm/year. 



The total average annual sediment volume lost from Panels 3 to 8 was 

 258,000 m /year between 1945 and 1964/65. Assuming the long-term average 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



69 



