LONGSHORE BAR GENERATION MECHANISMS

Robert G. Dean, Rajesh Srinivas, Trimbak M. Parchure

Abstract


A laboratory study was conducted to investigate mechanisms of bar formation with particular focus on infragravity wave and break point mechanisms. With one exception, all tests commenced with a planar beach composed of fine sand. Infragravity (IG) waves, the result of a biharmonic primary wave spectrum, were generated and documented. For various frequency differences of the components of the primary spectrum and thus the frequencies of the IG wave, the changes in nodal/antinodal positions were compared with the changes in positions of the bar formed during the experiment. It was found that the bar position experienced relatively small movements whereas the nodal/antinodal positions changed substantially as expected with the difference frequency of the primary waves. The approximately constant bar position was interpreted as due to the relatively constant primary wave height characteristics. The effects of the wave spectrum and wave height distribution on bar formation were investigated. The shape of the wave height probability distribution function was found to exert a noticeable control on the degree of bar relief, with those distributions characterized by a substantial concentration near the maximum wave height causing the more prominent bars. This is interpreted in terms of a transport influence function which, if held stationary, would result in a narrow prominent bar. The distributions with a small concentration near the maximum wave height resulted in a less prominent bar. A conceptual model is proposed which incorporates the transport distribution function and which results in bar characteristics which depend on the initial conditions.

Keywords


longshore bar; bar generation

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