海浜流, とくに離岸流の発生理論

Abstract

In this paper, the rip current field equations are formulated using theexpressions of wave induced nearshore driving forces, which are derived employingthe so-called mild slope equation (MSE). The MSE is capable of expressing bothwave refraction and amplitude decay. By introducing the wave-current interactioninto the MSE, and employing a perturbation method with a scale parameter of thebeach slope, a set of rip current field equations was mathematically derived for boththe shoaling and surf zones. The eigenvalues for the rip current spacings weredetermined by the matching condition at the breaking point. Furthermore, theintegration constant for the solution was solved using the wave energy fluxconservation between prebreaking and postbreaking states.The rip current spacings were then expressed in terms of Dalrymple-Lozano'sparameter and by the so-called surf similarity parameter. The theoretical ripcurrent spacings agreed well with field data in the region where the surf similaritypaprameter is greater than 0.2. Compared with laboratory data, however, thetheoretical spacings were about half. This discrepancy was believed to exist theinfluence of the littoral boundaries in the laboratory basins. A thoretical solution ofthe rip currents, which include the littoral boundary effect, was then developed.Consequently, the theoretical curve of rip spacings agreed well with theexperimental values.Numerical calculations of the theoretical rip current solutions were performed inorder to desplay velocity profiles along the center line of the rip current and the ripcurrent circulation patterns at the scales of laboratory and field. The theoreticalvelocities of rip current were found to reasonably compare with laboratory and fieldmeasurements.