developed in tideless seas such as Lake Michigan (Fig. 4-27). (Saylor 

 and Hands, 1970.) Keulegan (1948) found that the ratio of depth of long- 

 shore trough to depth of bar was approximately 1.69 in laboratory experi- 

 ments, but most field measurements showed less depth difference, averag- 

 ing about 1.23 (based on MLLW) for 276 measurements from the Scripps pier. 

 (Shepard, 1950.) According to Shepard, bars are not significant on slopes 

 steeper than 4° (1 on 14). 



There is evidence that longshore bars, as described, are formed by a 

 transient condition when waves of a given height and period plunge on a 

 relatively plane sand slope. Shepard found that steep storm waves elimi- 

 nate the bars rather than build them, and that bars form after the largest 

 storm wave subsided. Such a relation is consistent with plunging breaker 

 conditions predicted by the breaker type parameter (Galvin, 1972). Be- 

 cause bars are formed by high waves, they may persist through long inter- 

 vals of low waves. Once formed, bars may trigger the breaking of higher 

 waves, dissipating the wave energy and thus reducing beach erosion. (Davis 

 and Fox, 1972; and Zwambom, Fromme, and Fitzpatrick, 1970.) 



Laboratory observations show that longshore bars form when waves 

 plunge, and that these bars are absent when waves spill. With constant 

 wave conditions, a wave may plunge initially on a steep sand slope and 

 form a bar. The beach then erodes forming a flatter slope, which changes 

 the breaker type to spilling, which eliminates the pronounced longshore 

 bar. For other constant wave conditions, a wave may spill initially on 

 a gentle sand slope, and no pronounced bar forms. Later in the test, the 

 breaker position migrates closer to the steeper foreshore, the breaker 

 begins to plunge, and a longshore bar forms. 



b. Steepness Effect . The distinction between profiles with pro- 

 nounced berms (usually without bars) and profiles with eroded foreshores 

 (often with longshore bars) is well known. (Nayak, 1970, and Johnson, 

 1956.) Early laboratory results suggested that the shape of the profile 

 depends on deepwater steepness, H^/Lo, of the waves reaching the beach. 

 Between 1936 and 1956, laboratory experiments were made which led to the 

 assumption that beach profiles generally eroded if Hq/^o exceeded 0.025 

 and accreted if ho/ho was less than about 0.02. 



However, neither field data nor prototype-size laboratory experiments 

 support this widely used criterion. (Saville, 1957.) Field and prototype- 

 size laboratory data of Saville showed that beaches eroded at signifi- 

 cantly lower deepwater steepness than the value of 0.025 derived from 

 model laboratory experiments. Saville (1957) concluded that the absolute 

 size of wave height was probably as important as steepness in determining 

 the profile. 



4-80 



