58 



was completed by CERC for the shore south of the harbor jetties to assess 

 short-term trends in the changing volume of sand and gravel above the 

 cohesive profile. 



Long-term profile change 



Four locations were selected for the long-term comparison of beach profiles 

 (see Figure 10). 



At almost 3 km north of the St. Joseph harbor jetties. Line 1 is located 

 outside the zone of influence of the harbor structures. From 1945 to 1965, 

 there was severe lowering of the lake bed (see Figure 22). This lowering was 

 probably the result of erosion of the underlying till. In contrast, the 1965 to 

 1995 comparison showed little or no lowering but does feature bar migration 

 related to water level fluctuations and wave action. 



Line 2 is located 1 ,350 m north of the harbor jetties, in a transition zone 

 between shoreline influenced by the fillet beach and unaffected shoreline. Due 

 to the low density of soundings in the 1945/6 survey, the bar features cannot 

 be discerned. However, a depositional trend is evident from 1945 to 1995, 

 especially in the nearshore zone (see Figure 11). The water level at the time 

 of the 1995 survey was approximately 1 m above the historic lows recorded in 

 1964/65. The 1995 nearshore sandbar is located approximately 100 m inshore 

 of the 1964/5 nearshore bar (see Figure 11). 



Line 3 is located in the middle of the large offshore depression in the lake 

 bed that developed between 1945 and 1995. Severe erosion (vertical displace- 

 ment) of the sand and cohesive substrate occurred between the 10-m depth 

 contour and the shoreline (see Figure 12). The 5-m depth contour moved 

 inshore by 450 m in 50 years, for an average annual contour recession rate of 

 9 m/year. With the construction of the revetment several decades ago, the 

 shoreline position has been fixed at Line 3; however, the nearshore profile has 

 continued to erode and the nearshore slopes have become progressively steeper 

 from 1945 to 1995. 



Line 4 is located 8.2 km south of the jetties in a zone which, up until 

 recently, may not have been significantly influenced by the harbor jetties. 

 From 1945 to 1965, profile lowering occurred from the shoreline out to the 

 -7-m depth contour (see Figure 13). There appears to be some recovery of the 

 nearshore profile from 1965 to 1991, which corresponds to the period when 

 the Section 1 1 1 beach nourishment program was introduced updrift of Line 4. 

 A second possible explanation for the apparent gain in nearshore sand levels is 

 the onshore migration of bar features due to water level rise between 1 964/65 

 and 1991 (refer to Parson et al. (1996)). From 1991 to 1995, the profile com- 

 parison revealed significant lowering of the nearshore profile over a 

 200-m-wide zone. 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



