The descriptive model of coastal morphodynamics for the historic, present, 

 and future conditions at St. Joseph is illustrated in Figures 35 - 37. For each 

 period, the descriptive model summarizes the findings of the lake bed surface 

 comparisons, dredging and nourishment volumes, recession rates, and repre- 

 sentative estimates of alongshore sediment transport. Any variation in the 

 transport rates between the different periods only reflects changes to the repre- 

 sentative nearshore profiles used as model input with the average annual wave 

 climate from 1956 to 1987. Qualitative information from the analysis of lake 

 levels, profile comparisons, and COSMOS 2D/3D modeling presented in Chap- 

 ter 4 is also incorporated in the descriptive model. 



Historic Conditions 



1945/6 to 1964/5 



Descriptive model results for 1945 to 1965 are summarized in Figure 35. 

 Current understanding of coastal processes for cohesive environments would 

 suggest that the shoreline of Sector A has been eroding since the glaciers 

 receded several thousand years ago. Analysis of air photo information dating 

 back to 1939 indicates that the recent long-term recession rate is about 0.8 m/ 

 year (see Section of Chapter 2 entitled "Shoreline Recession"). The average 

 lowering rate for Panel 1 was found to be 2.7 cm/year for the period from 

 1945 to 1965. This rate of erosion compares well to the situation in Sector G 

 during this period (i.e. at the south end of the project area), which featured a 

 recession rate of about 0.9 m/year and an averaged lowering rate of 2.6 cm/ 

 year. Based on this finding, and on the fact that these recession rates are 

 similar to those found in areas further to the north and south of the harbor 

 (i.e., well beyond any zone of harix)r influence), the authors suggest that these 

 two sectors are representative of the "background" erosion conditions related to 

 cohesive shore processes and are not strongly influenced by the presence of the 

 harbor jetties, at least for historic periods. This is an important finding 

 because the Section 1 1 1 program is only intended to mitigate erosion related to 

 the presence of the structure and not the background erosion. 



The fillet beaches immediately north and south of the SL Joseph River 

 mouth (Sectors B and C) have been stable or accreting at least since the con- 

 strucfion of the jetties in 1903. Numerical modeling results indicate that sig- 

 nificant quantifies of sediment may be deposited in these areas during storms. 

 The bypassing analysis showed that the combination of the long jetties and the 

 deep navigation channel acts as a total littoral barrier, trapping all sediment 

 reaching this area from either the north or the south. 



Somewhere in Sector D (i.e., the feeder beach), the shore changes from 

 sandy to cohesive as the bank of the incised river vaUey is encountered. 

 MDNR calculations of long-term recession rates indicate that the entire reach 

 of Sector D has been eroding, with' recession rates between 0.35 and 1.15 m/ 

 year over the last 50 years (with the larger rates occurring immediately north 

 of the Waterworics revetment). 



Chapter 5 Interpretation of Results - A Descriptive Model of Coastal Morphodynamics 



79 



