period. The zone of the greatest lowering rates also shifted southward from 

 R9 - R17 to R12 - R20 (refer to Figures 30 and 31). 



A comparison of Figures 30 and 31 clearly shows that the highest rates of 

 deposition in the nearshore between 1965 and 1991 correspond to the locations 

 of significant lowering between 1945 and 1965. This would suggest that sand 

 moving in a cross-shore and alongshore direction is partially trapped in the 

 deeper nearshore zones, until the depressions are filled. Consequently, these 

 depressions may be sinks for sand transported from the feeder beach. 



The net volume change related to the changes in the lake bed surface 

 (within the panel boundaries) south of the harbor jetties was -25,000 m^/year 

 between 1964/65 and 1991. Disregarding the negligible sediment input from 

 shoreline recession and channel bypassing, input to the sediment budget from 

 the beach nourishment averaged 88,000 m-^/year from 1965 to 1991. Assum- 

 ing that 80,000 m /year of the nourishment sediment is transported in an 

 alongshore direction (due to the southerly directed net transport gradient), and 

 that approximately 8,000 m^/year may have been lost as annual deposition in 

 the navigation channel, there would be no net gain or loss resulting from 

 alongshore transport processes. Therefore, the annual net change in the sedi- 

 ment budget from the bathymetry comparisons of -25,000 m^/year must be 

 largely related to offshore losses. As noted above, offshore losses are probably 

 the result of the erosion of the cohesive sediment and the offshore dispersal of 

 silts and clays. Consequently, for the panels south of the harbor jetties, the 

 volume of irreversible erosion of the cohesive substrate may have been as high 

 as 2.8 m /m/year. 



1991 to 1995 



The 1991 to 1995 bathymetric comparison is limited to Panels 3 to 8 south 

 of the harbor jetties, as seen in Figure 28, due to the limited surveying done in 

 1991. During these 4 years the volume changes south of the jetties changed 

 dramatically. Volumetric losses and lake bed lowering were greater than the 

 previous peak during the initial interval (1945 to 1965) (see Table 7). 



The most dramatic erosion rates between 1991 and 1995 occurred in a 

 200-m-wide band along the shoreline, with 1-4 m of lake bed lowering. Depo- 

 sitional areas, seen in Figure 32, are further offshore and do not compensate 

 for the nearshore lowering. There are several possible explanations for the 

 reduced rate of offshore deposition: (a) sand eroded from the nearshore is 

 widely dispersed offshore; (b) sand eroded from the nearshore is transported in 

 an alongshore direction; (c) a significant percentage of the eroded nearshore 

 volumes is glacial till and provides very little sand to the local sediment 

 budget. 



The volume loss related to lake bed lowering was approximately 

 367,000 m/year for this period. Disregarding the negligible inputs f 

 shoreline recession (outside of calculated panel volumes) and harbor 



Chapter 4 Analyses of Coastal Processes and Geomorphology 



73 



