and man-made control structures are additional factors that influence the 

 exceedingly complex cycles of water level changes in the Great Lakes. As a 

 result, the concept of mean water level is not applicable to these inland Great 

 Lakes, and attempts to predict lake levels have not been entirely successful 

 (Walton 1990). 



Historic water levels have been used by Hands (1979, 1980) to examine 

 the changes in rates of shore retreat in Lake Michigan and to predict 

 beach/nearshore profile adjustments to rising water levels. Additional 

 research is being sponsored by the International Joint Commission to model 

 how changing water levels affect erosion of various bluff stratigraphies and 

 the nearshore profile. 



Current Records 



Current data are often critical for evaluating longshore and cross-shore 

 sediment transport and for evaluating hydraulic processes in inlets and other 

 restricted waterways. Currents, which are generated by a variety of 

 mechanisms, vary greatly spatially and temporally in both magnitude and 

 direction. Four general classes of unidirectional flow affect coastal 

 environments and produce geologic changes. These include: 



• Nearshore wave-induced currents, including longshore and rip currents. 



• Flow in tidal channels and inlets, which typically changes direction 

 diurnally or semidiurnally, depending on the type of tide along the 

 adjacent coast. 



• River discharge. 



• Oceanic currents, which flow along continental land masses. 



This section will briefly discuss the first two of these topics and present 

 data examples. The third and fourth are beyond the scope of this report, and 

 the reader is referred to outside references for additional information. 



Nearshore wave-induced currents 



In theory, one of the main purposes for measuring nearshore, wave- 

 induced currents is to estimate longshore transport of sediments. At the 

 present level of technology and mathematical knowledge of the physics of 

 sediment transport, the direct long-term measurement of longshore currents by 

 gages is impractical. Two main reasons account for this situation. First, as 

 discussed earlier in this report, deployment, use, and maintenance of instru- 

 ments in the nearshore and the surf zone are difficult and costly. Second, the 

 mechanics of sediment transport are still little understood, and no one mathe- 

 matical procedure is yet accepted as the definitive method to calculate sedi- 

 ment transport, even when currents, grain size, topography, and other 



Chapter 5 Analysis and Interpretation of Coastal Data 



95 



