砂質浅海底における砂れん型と砂移動

抄録

Sand ripples formed by waves on a sandy shallow sea bottom are classified into regular parallel ripples, nearshore irregular parallel ripples, diagonal ripples and lunate ripples. They are arranged in this order from offshore to onshore regions. The difference of sand movement mode would probably exist among these ripple types. The purpose of this study is to describe the difference of sand movement on each ripple type in connection with the ripple geometries and the characteristics of wave-induced oscillatory water flows near the sea bottom.
Field investigations were performed on a shallow offshore zone at Tatado Beach, Izu Peninsula, Japan (Fig. 1), in July and October 1979. The mode of sand movement was first observed, and then the direction of net sand drift was investigated by use of fluorescent sand composed of four ranges of grain size (i.e., -1-0φ, 0-1φ, 1-2φ, and 2-3φ) with different colors. Wave-induced oscillatory flows were measured using an ultrasonic current meter installed at a height of 36cm above the sea bottom.
Three types of sand movement were observed on a rippled bed:
Type I: sand movement in traction mode,
Type II: sand movement closely relating to vortices formed in ripple troughs, and
Type III: sand movement in the form of suspended sand cloud which entirely covers the sea bed.
Type I is suitable for coarser sand transport, while Type II sand movement is for finer sand. Type III contributes to the transport of material which can be suspended by the oscillatory water flows.
The result of measurement of oscillatory water flows showd that onshore velocity was stronger than offshore velocity on any type of ripples. In such a velocity field, Type I and III give rise to a net onshore sand drift, while Type II produces an offshore net drift due to the existence of vortices. A prevailing sand movement mode is different on ripples with different types. Consequently, the direction of net sand drift for various size of sand grains is different for each type of ripples (Table 2).