Bar morphology of dissipative beaches: An empirical model

Abstract

This study of 150 examples of nearshore bar morphology along the highly dissipative beaches ( ϵ > 33) of the southeast Mediterranean shoreline employs 32 years of aerial photography and wave data, aided by detailed short-term field studies. Three major bar types were delineated: non-rhythmic parallel/meandering bars, inner single-crescentic bars and double-crescentic bars. Each of these bar families includes variations. The bars were related to daily ship and shore-wave data, including wave spectra, from the day of observation back to 60 days prior to the observation. The increase in total bar occurrence during summer is related to crossing of a major wave-energy threshold in the spring, when significant wave heights ⩽1 m sharply increase to 70–85% in April–May. The bar morphology/wave comparisons further indicate that as the significant wave heights decrease and remain below 1 m, non-rhythmic bars form within 7–10 days, single-crescentic bars require 15 days, and double-crescentic bars require 20–30 days. This adjustment period of the bars to wave power causes a delayed response which accounts for lack of coincidence between wave energy and bar occurrence. The formation of the initial double-crescentic bar, and its transformation to the mature double-crescentic type, requires a short pulse of wave energy ( 0.5 ⩽ H 1 3 ⩽ 1.5 m ). Some bar families occur throughout the year. The aseasonal occurrence is best shown by the mature double-crescentic type, which apparently is the final stage in the crescenticbar development sequence. However, other bar families show a tendency for a seasonal distribution which reflects their sensitivity to wave energy. Inner single-crescentic and initial double-crescentic bars are largely restricted to the calmest wave months of May/April to October/November. There is an antiphase relationship between the frequencies of non-rhythmic and crescentic bars.