Evaluation of Controlling Effect of Sand Transport by Detached Breakwaters Built on Dynamically Stable Beach

The contour-line-change model was applied for predicting the beach changes around headlands on the Shimizu coast, which were built using pairs of detached breakwaters. Before constructing the headlands, there was a dynamically stable beach, at which longshore sand transport of 1.3 x 10⁵ m³/yr flowed continuously alongshore, but it was altered to a new equilibrium state, in which longshore sand transport was reduced to 7 x 10⁴ m³/yr owing to the construction of the headlands. The effect of countermeasures against beach erosion was also predicted using this model. It was found that the extension of the present detached breakwaters by 50 m in the updrift and downdrift directions to narrow the interval between the detached breakwaters is effective for reducing the necessary amount of sand nourishment by (0.5–1.0) x 10⁴ m³/yr.