The dependence of the model on equilibrium assumptions (item a) makes it 

 difficult to get convincing field aonfivmation because waves, currents, and 

 conditions of sediment supply never remain constant for long. On the other 

 hand, because it is an equilibrium model the potential for application is 

 broadened. The model can even apply to situations where storms or man's 

 influence upset equilibrium. The predicted retreat in such cases would 

 indicate the adjustment required by just the change in water level; effects 

 due to other changes would have to be superimposed if significant. Thus, once 

 the concept is confirmed, equilibrium tends to make application easier. 



The model itself provides no indication of the time period required for 

 the beach to return to equilibrium. Errors in misjudging equilibrium, and 

 failing to account for the lag between cause and effect, are all too easy to 

 make if the data cover only a small reach of shore or a short period of time. 

 The length of time and the number of profiles studied here are thought to be 

 sufficient to avoid this problem. 



4. Application of the Sediment Balance Approach . 



a. Longshore Contributions . Wave data suitable for prediction of long- 

 shore transport rates are not available in the study area. Various indica- 

 tions of the direction of transport are compiled in Figure 15. Evidence from 

 coastal geomorphology (Hands, 1970), from longshore changes in grain size 

 (Saylor and Hands, 1970), from the pattern of channel shoaling (Hands, 1976a), 

 and from data hindcast for extreme storms (Resio and Vincent, 1976c) suggests 

 that the direction of longshore transport in the vicinity of Pentwater Harbor 

 is predominantly southward, but subject to frequent reversals; extrapolation 

 from Saville's (1953) hindcast data suggests a northward transport. Littoral 

 Environment Observation (LEO) data from Mears State Park were inconclusive — 

 too short a record and subject to the effects of a large eddy and reflected 

 waves from the Pentwater jetties. Near profile station 17, the extreme storm 

 data and the usual deflection of Silver Lake Creek crossing the beach suggest 

 that the direction of transport changes to northward on the south side of 

 Little Sable Point. Beyond the southern limit of the study area, storm data 

 from White Lake and Muskegon suggest a close balance between northward and 

 southward flows in that region. South of Grand Haven the geomorphology and 

 storm data indicate net southward transport for the remainder of the eastern 

 shore. Therefore, there is a consistent pattern of drift moving toward Little 

 Sable Point from the north (Summit Park) and from the south (White Lake) (Fig. 

 15). 



Long-term convergence of drift toward the Silver Lake dunes would be 

 consistent with the evolution of Little Sable Point from a shallow embayment 

 several thousand years ago when water levels were 7 meters above modern levels 

 (Hough, 1958) to the dune-covered coastal promontory of today. 



The areas from Ludington to Summit Park and from White Lake to Muskegon 

 appear to be natural boundary zones of longshore divergence (Fig. 15). In 

 addition to these natural boundaries, the jetties at Ludington and at the 

 pumped storage facility 4 kilometers farther south (Fig. 16) are also obsta- 

 cles to sediment input from the north. The jetties and entrance channel at 

 White Lake likewise reinforce the natural southern boundary. Present-day 

 processes, storm patterns, and engineering projects thus limit the possible 

 sources of drift converging toward the Silver Lake dunes to those beaches and 

 bluffs primarily within the present study area (see Fig. 4). 



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