砂質浅海底における波による振動流と砂れん型

抄録

Various patterns of sand ripples found on sandy shallow sea bottom are classified into planar parallel, lunate, diagonal, nearshore irregular parallel and regular parallel ripples. They generally appear in this order with increasing water depth. Relations between ripple types and intensities of wave-induced oscillatory water flow have been studied by severall researchers. In these studies, however, flow intensities are computed from wave data on the basis of wave theories.
In order to consider the relation between ripple types and sand movement, more detailed characteristics of oscillatory water flow over each ripple type is necessary. Tatado Beach, Izu Peninsula, Japan (Fig. 1) was selected as a study site. The flow measurement was conducted using an ultrasonic current meter installed at heights of 26 to 80cm above the sea bottom. The characteristics of oscillatory water flow, such as the flow intensity, the directional variation and the velocity asymmetry, were examined for the flows over regular parallel, nearshore irregular parallel, diagonal and lunate ripples, and also over a flat bed.
As shown in Fig. 2, the horizontal flow velocity is lowest for the flows over regular parallel ripples and is highest for those over a flat bed. The relation between ripple types and flow intensity represented by Manohar's (1955) non-dimensional function Ψ₁' is examined (Fig. 3): Ψ₁' is calculated using actually measured velocity. Fig. 4 shows the similar relation, in which the maximum horizontal velocity, U_m, is calculated from wave data using wave theories. As compared with Fig. 4, Fig. 3 shows: a smaller variation in the value of Ψ₁' for each ripple type and gives more clear boundaries for the ripple-type demarcation. This suggests that the calculated value of U_m in the nearshore zone involves considerable error.
The directional variation of flow oscillation is examined in relation to the flows over each ripple type using a rose diagram with 16 directions (Fig. 6). This figure indicates that little difference in the directional variation exists among different ripple types. Especially, the range of directional variation in the oscillatory flows is too narrow to explain the directional angle between main and sub-crests of diagonal ripples, which intersect nearly at right angles to each other (Photo. 1). This result does not support the opinion that diagonal ripples are generated by waves advancing from two different directions. The relation between ripple types and velocity asymmetry of the oscillatory flows is shown in Fig. 7, which indicates no general tendency.