context, the term, equitihvium profile refers to a curve of fixed size and 

 shape which "adequately" represents the "average" profile shape before per- 

 turbation by a shift in water level. By assumption, shore erosion eventually 

 returns the profile to this same shape after it is displaced as a result of 

 the water level change (see Fig. 13). 



A willingness to accept equilibrium as a reasonable approximation is not 

 inconsistent with recognition of seasonal, storm, or other temporary profile 

 fluctuations. Careful judgment should be made on a case-by-case basis, if 

 field profiles claim to represent quasi-equilibrium conditions. Generally, 

 the claim will be more reasonable the longer the time frame of the study. The 

 spatial extent of the study is also important. Usually, the longer the 

 stretch of shore, the more likely that longshore variations will also average 

 out, thereby, providing an overall equilibrium. 



This discussion has shown that the Bruun concept is theoretically sound 

 but difficult to apply in the field. The next subsection examines how some of 

 the difficulties discussed above are avoided in the Lake Michigan data. 



3. Suitability of Present Data for Testing the Sediment Balance Concept. 



The ways that previously discussed difficulties (items a to e) affect Lake 

 Michigan data are outlined here, before an actual application of the data in 

 the next subsection. Difficulties (d) and (e) should not limit application of 

 the model to any data set for reasons discussed in the last subsection. 



Establishing a realistic closure depth (item c) depends on accurate 

 repetitive profiling. Profile errors increase with distance from shore. 

 Fortunately, the bottom drops off to suitable depths relatively rapidly in the 

 present study area. Furthermore, the Great Lakes are free from tidal varia- 

 tions as well as from long-period swell. The Great Lakes are notorious for 

 their large storm surges and seiches; however, based on extensive water level 

 measurements in 1969, it was concluded that these disturbances are not a 

 significant problem in the present study. By choosing the right time of year 

 and surveying only when conditions are calm, it is possible to avoid datum and 

 bottom ambiguities. Note the absence of confusing wave interference on the 

 raw fathogram in Figure 14. 



The difficulty of determining sediment losses (item b) on the Great Lakes 

 is greatly simplified by the absence of submarine canyons, hurricanes, and 

 overwash events. Fluvial sediment input is also no problem because all rivers 

 entering eastern Lake Michigan flow through deep inland sediment traps. 

 Dredging at Pentwater Channel is well documented. On the average, 60,000 

 cubic meters is removed annually, and some of this is returned to adjacent 

 beaches. Inlet losses have only a small effect on the overall sediment budget 

 for the broad study area. Thus, in the present application Q (eq. 4) will 

 have a negligible effect. 



The only process supplying new sediment to the active profile is shore 

 recession. Furthermore, shore deposits and backshore bluffs within the study 

 area contain less than 1 percent silt, making it unnecessary to correct for 

 any unstable fine fraction (i.e., Ra = 1; eq. 1). Thus, a number of site- 

 specific attributes simplify sediment balance for the study area. 



30 



