RAPID FORMATION of SAND BODY at SHOREWARD EDGE of OFFSHORE DREDGING HOLE and RESULTING LARGE CHANGE in SAND SPIT

The influences of offshore sand mining on the surrounding beach topography were investigated at the Sumiyoshi-hama sand spit extending into Moriye Bay in Oita Prefecture, Japan. It was found that sand bars could develop above the MSL on the steep shoreward slope of the dredging hole in the tide and wave height ranges obtained from the predicted probability of wave occurrence at the Sumiyoshi-hama sand spit. As a result of the numerical simulation of wave height and nearshore currents given a predominant wave direction, wave height in the nearshore zone was found to be enhanced by the formation of the offshore dredging hole, and longshore currents toward the tip of the sand spit and counterclockwise currents on the lee side of the spit were accelerated by this wave intrusion. It was concluded that sand thrown up from the steep slope of the dredging hole by wave action was transported to the tip of the sand spit, causing large changes of the spit.