retreat averaged from all stations was 17.9 meters, of which only 3.4 

 meters was due to encroachment). Furthermore, the amount of encroachment 

 at a given station, while predictable, would have given no clue to the 

 final amount of shore recession (Fig. 11). 



The amount of recession depends on the exposure and resistance of the 

 beach to erosive forces. Within the range of conditions observed on the 

 lake, the flatter foreshores showed no tendency to recede more or less 

 than steeper foreshores. Moreover, shore recession continued in some 

 cases even after the water levels began to decline. Hence encroachment, 

 depending only on steepness of the foreshore and the change of water 

 levels, is a poor measure of total shore retreat. 



V. DATA INTERPRETATION 



1. Spatial Variation in Retreat Rates . 



The average rate of shore retreat for the whole study area was 2.9 

 meters per year (1969 to 1975), but there were wide variations (see Fig. 

 8). The maximum rate of retreat (4.6 meters per year) was observed at 

 station 16; progradation caused the shoreline to advance at three stations 

 (maximum of 6 meters at station 5) . Two of the stations where the shore- 

 line advanced lakeward are in Hears State Park, just north of the 

 Pentwater jetties. The park personnel employ a number of shore protec- 

 tion measures at this locality. Each fall a series of snow fences is 

 installed in multiple rows along the shore to catch and hold windblown 

 sand during the winter. Each spring the fences are removed, the beaches 

 are graded, and the sand that had blown inland and accumulated in the 

 camping area and parking lots is scrapped up and added to the beach op- 

 posite the swimming area. Since 1973 park personnel have also been 

 nourishing the beach with a small part of the 50 to 70X10^ cubic yards 

 which is dredged annually by the U.S. Army Corps of Engineers from the 

 Pentwater Channel. In each of the years 1973, 1974, and 1975, about 

 5,000 cubic yards of the sand removed from the channel by bucket dredge 

 was dropped across the north jetty onto park property. The park staff 

 widened the beach in the bathing area using the dredged sand together 

 with about an equal amount of sand removed from inland dunes, (G. Zeine, 

 Mears State Park Supervisor, personal communication, 1977). In 1976 the 

 channel was deepened with a hydraulic dredge, and about 7,000 cubic yards 

 was pumped onto the beach between the north jetty and station 4.5. In 

 addition to these steps, three rockfilled gabion groins were installed 

 near station 4.5 in 1973 as part of the Michigan Demonstration Erosion 

 Control Program (Brater, et al., 1977). Concern for sv;immers ' safety led 

 to replacement of the outer ends of the wire gabions with sandbags the 

 next spring. 



The effects of these various shore protection efforts at Mears State 

 Park, together with the protection the jetties afford by blocking some 

 of the beach from southern exposure and acting as a terminal groin for 

 the fill, are judged responsible for causing the shore to prograde lake- 

 ward at stations 4.5 and 5 while for the same 7-year period the adjacent 



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