beaches retreated an average of 14 meters. 



No adequate explanation can be given at present for the other instance 

 of a net lakeward advance of the shoreline at station 17, located on the 

 tip of Little Sable Point. The shore opposite station 17 alternately 

 prograded, receded, and then prograded between the 1969, 1971, 1975, and 

 1976 surveys. On the final survey, the shoreline was 3,6 meters lakeward 

 of its initial position, but it had fluctuated through a range of 16.3 

 meters. Photos taken at the time of the 1975 and 1976 surveys at station 

 17 (Fig. 12) suggest that as a result of the longshore passage of a sand 

 wave (i.e., a lakeward protrusion of the shoreline, sometimes referred to 

 as shore rhythm, crescentric planform, beach pod, etc.), the shore can 

 alternately prograde and recede over such distances. Additional shore- 

 line protrusions occurred where bars merged, at their updrift ends, with 

 the beach face (Fig. 13). Smaller shoreline undulations marked the lo- 

 cation where the inner bar frequently forms a cellular pattern in plan 

 view. Shoreline undulations seem slightly more prominent on Little Sable 

 Point than elsewhere throughout the study area; however, this is not a 

 complete explanation for the shoreline behavior at this locality. While 

 station 17 showed net progradation, stations 14, 16, and 19 on Little 

 Sable Point were among the most rapidly retreating during the study 

 period. Shore retreat would probably have been extensive at station 18, 

 had the property owners not rebuilt and maintained an earlier (1950-51) 

 timber bulkhead to protect a cottage near the edge of the high bluff which 

 backs this site. Likewise, terrace erosion reported by Davis (1976) for 

 a point between stations 16 and 17 (D6 in Fig. 4) was among the highest 

 he determined in a 1970 to 1973 study of 17 sites spread over almost the 

 entire length of the eastern shore of Lake Michigan. Other measurements 

 in the vicinity of the point also showed rapid retreat during recent 

 years. It is not known why Little Sable Point during the last several 

 thousand years has been the site of massive sand dune accumulation (ap- 

 parently fed by a convergence of littoral transport from both the north 

 and south) should now be the site of the most rapid shore retreat. It is 

 equally difficult to explain why a shorter section of shore on Little 

 Sable Point (represented by station 17) alternately prograded and re- 

 ceded, producing only a small net change in the midst of this presently 

 rapidly receding section of shore. 



2. Temporal Variations in Average Retreat Rates . 



Engineers are sometimes criticized for placing too much reliability 

 in average retreat rates derived from a limited number of measurements 

 widely spaced along the shore. If the dynamics of beach cusps, rip cells, 

 or the possible effects of edge waves were of interest, then obviously 

 the temporal and spatial scales of these processes would have to be con- 

 sidered in planning the response measurements. More often, however, the 

 practicing engineer is interested in overall conditions affecting a large 

 section of shore, and in long-term results affecting the lifetime of a 

 project or structure (e.g., 30 years). It is worth pointing out that as 

 the temporal scale increases some of the problems that originally contam- 

 inated data tend to cancel one another rather than accumulate as the 



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