A dominant feature o£ the surf zone along many of the Great Lakes 

 shelves is a bed form distinct from those mentioned, and is properly 

 referred to as a longshore bar. Longshore bars can occur singly, but 

 are often found in a parallel sequence that follows shoreline trends 

 almost continuously for many miles. The spacing between bars in a 

 parallel sequence approximates a geometric series, beginning with about 

 100 feet between inner bars and progressing up to a spacing of several 

 hundred feet between the outermost bars. Longshore bars form along tidal 

 and tideless coasts wherever there is a sufficient supply of sand- sized 

 material in the nearshore zone and a bottom slope of the proper steepness. 

 Within a range of bottom steepness from 0.002 to 0.02, the number of bars 

 that can form increases as slope decreases (Bascom, 1953; Zenkovitch, 

 1967). 



In contrast to other types of bed forms, the formation of longshore 

 bars is a direct response of the bottom to shoaling waves. Currents in 

 the littoral zone modify bar geometry but are not necessary for bar 

 formation. Local deepening of bar troughs by longshore currents (Knaps, 

 1959) and reduction of bar crest heights by rip currents (Shepard, 1950) 

 occur. Bars can build in the absence of longshore currents. This is 

 amply proven by the generation of bars in wave tanks solely under the 

 influence of wave action (Keulegan, 1948; McKee and Sterrett, 1961). In 

 the laboratory, wave -generated currents cannot develop the natural circu- 

 lation systems established in nature but these experimentally produced 

 bars have the same shape and geometry as prototype longshore bars. Hence, 

 it can be concluded that bars are a direct result of shoaling wave action. 



On some coasts, longshore bars appear only seasonally. They are 

 built up in winter by relatively steep storm waves and destroyed in the 

 summer by a net shoreward transport under swell which tends to move the 

 sand onto the upper beach. On other coasts, notably those with restricted 

 fetch (Great Lakes; Baltic, Black, and Mediterranean Seas), bars persist 

 throughout the year (Davis, 1964; Bajorunas and Duane, 1967; Berg and 

 Duane, 1968; Otto, 1911-12; and King, 1959). 



According to theory and experiments in wave tanks (Keulegan, 1948), 

 bars have a fixed and definite relationship to the Stillwater level. A 

 change in water surface elevation induces a like change in bar elevation. 

 The reaction of bars in the vicinity of east central Lake Michigan 

 (Pentwater Harbor) to rising lake levels supports this relationship — 

 bars moved inward and upward in such a manner as to preserve a fixed 

 depth beneath the slowly changing lake surface (Saylor and Hands, 1970). 



Coastal engineers are interested in longshore bars since they control 

 the position of breaking waves and remove much of the wave energy before 

 it reaches the shore (Munk, 1949). In some instances bars represent 

 temporary storage bodies for sand that are eventually returned to the 

 upper beach by natural processes (Bascom, 1960, 1964; Coastal Research 

 Group, 1969; Inman, 1953; and Shepard, 1950). Bars control the position 



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