THEORY AND FIELD TEST OF BEACH NOURISHMENT USING COARSE SAND AND GRAVEL

On the Akiya coast a breakwater of Kuruwa fishing port was extended, inducing longshore sand transport from outside to inside the wave-shelter zone. Sand accumulated inside the wave-shelter zone, whereas beach erosion occurred on the nearby coast. A tracer test using sand of d₅₀=0.5, 1.5 and 15 mm showed that coarse material was deposited near the shoreline and slowly moved alongshore, whereas fine material was carried offshore. The contour-line-change model considering changes in grain size was applied to the coast to predict the effect of beach nourishment using material with different grain sizes. Gravel nourishment was confirmed to be effective for recovering a stable beach without damage to the offshore fishing ground. Beach nourishment using 7,600 m³ of gravel with d₅₀=15 mm was carried out in 2007 on the Akiya coast. The nourishment gravel was deposited near the shoreline forming a steep slope of 1/5, indicating that the nourished beach is very stable.