of retreat are attributed to the fact that profile retreat was actually lag- 

 ging behind the lake level rise. As hypothesized earlier (Hands, 1976a), 

 rising water levels establish a potential for erosion and realization of that 

 potential requires sediment redistribution, i.e., work which depends on the 

 energy being available. The convergence of measured and predicted retreat in 

 both regions, 3 years after annual lake levels had stabilized, suggests that 

 several storm seasons may be required to readjust the profile to changes in 

 mean water level of several tenths of a meter. 



According to the model, which works well here, the problem of predicting 

 the effect of lake level changes is equivalent to the problem of identifying 

 the pinch-out depth. The remarkable confirmation of theory and data in the 

 present case highlights the need to generalize a method applicable to similar 

 regional, long-term settings but where wave energies and therefore pinch-out 

 depths might be significantly different. 



5. Using Wave Climate to Estimate the Pinch-Out Depth . 



In the model, the closure depth is the point below which the bottom does 

 not adjust to changes in water surface elevation. In the field, this point 

 was approximated by averaging the upper bounds of the region of negligible 

 profile change in repetitive surveys (pinch-out depths). The closure depth, 

 thus established, is not necessarily appropriate for other areas of the Great 

 Lakes. The depth of profile closure should vary regionally with the wave 

 climate. Unfortunately, the repetitive profile record is usually not suffi- 

 cient to establish this parameter. 



In these cases, knowledge of the wave climate is useful. Wave gage data 

 obtained during profile survey periods are too short to be indicative of the 

 important long-term conditions in the study area; however, wave climate data 

 are available from other sources including hindcast studies (Saville, 1953; 

 Resio and Vincent, 1976a, 1976b, 1976c, 1977, 1978), shipboard observations 

 (Pore, et al., 1971; National Oceanic and Atmospheric Administration, 1975), 

 U.S. Coast Guard reports (Liu and Housley, 1969), and the LEO program (Weggel, 

 1979). Considering site specificity, long-term coverage, and the availability 

 of comparable data for the entire U.S. shoreline of the Great Lakes, Resio and 

 Vincent's reports were chosen as the basic reference for extrapolating profile 

 response from the present study area to those with significantly different 

 wave environments. Their wave climate parameters were generated by a 

 numerical hindcast model using wind data from the extreme storms recorded over 

 a 30-year period. The parameters thus describe only the deepwater storm 

 conditions. Because the maximum depth of profile response depends on the 

 higher waves and because only a consistent, relative measure of spatial wave 

 variability is needed, the milder waves though important in profile 

 development need not be considered here. It is reasonable to assume that the 

 maximum depth of intense bottom agitation depends on at least the wave period 

 and the shoaled and refracted wave height, but Hallermeier (1977) found that 

 the maximum depth in a number of actual design wave conditions was essentially 

 proportional to deepwater wave height alone. 



The wave height data necessary to estimate the pinch-out depth for any 

 Great Lakes site are given in Appendix C. The average pinch-out depth 

 established within the present study area is 2.1 times the average 5-year 



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