Effect of Gravel Nourishment and Maintenance of the Beach on Akiya Coast in Yokosuka City, Japan

Beach nourishment using 8 × 104 m3 of gravel with a median diameter of 15 mm was carried out on the Akiya coast in Kanagawa Prefecture between 2007 and 2014 to recover a sandy beach, and beach changes after the nourishment were monitored. It was confirmed that a wide beach was recovered by the nourishment. The beach changes measured between 2005 and 2019 were numerically reproduced using the BG model (a model for predicting 3D beach changes based on Bagnold’s concept). Since the grain size of nourishment gravel was as large as 15 mm, a steep slope of 1/4–1/5 and a high berm of 3 m above the mean sea level were formed. Because the relative height between the crown of the seawall and the gravel beach was reduced to 0.8 m, wave overtopping in the backyard of the seawall occurred. A portion of the gravel was also transported into a sandy beach across an L-shaped groin placed at the southeast boundary, damaging the sandy beach used for bathing. For the maintenance of the beach, raising the crown height of the L-shaped groin up to the berm height of 3 m and pushing gravel deposited over 3 m toward the shoreline are required.