developed in tideless seas such as Lake Michigan (Fig. 4-27). (Saylor
and Hands, 1970.) Keulegan (1948) found that the ratio of depth of longshore trough to depth of bar was approximately 1.69 in laboratory experiments, but most field measurements showed less depth différence, averaging about 1.23 (based on MLLW) for 2 76 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 eliminate 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). Because bars are formed by high waves, they may persist through long interval s of low waves. Once formed, bars may trigger the breaking of higher
waves, dissipating the wave energy and thus reducing beach érosion. (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 éliminâtes 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 migrâtes doser to the steeper foreshore, the breaker
begins to plunge, and a longshore bar forms.
b. Steepness Effect. The distinction between profiles with pronounced 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
dépends 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 H^/L^ exceeded 0.025
and accreted if Ho/L^
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 significantly 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
and Hands, 1970.) Keulegan (1948) found that the ratio of depth of longshore trough to depth of bar was approximately 1.69 in laboratory experiments, but most field measurements showed less depth différence, averaging about 1.23 (based on MLLW) for 2 76 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 eliminate 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). Because bars are formed by high waves, they may persist through long interval s of low waves. Once formed, bars may trigger the breaking of higher
waves, dissipating the wave energy and thus reducing beach érosion. (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 éliminâtes 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 migrâtes doser to the steeper foreshore, the breaker
begins to plunge, and a longshore bar forms.
b. Steepness Effect. The distinction between profiles with pronounced 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
dépends 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 H^/L^ exceeded 0.025
and accreted if Ho/L^
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 significantly 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