護岸建設による干潟上の流速減少に関する一考察

摘要

In Ariake Bay, a vanishment of tidal flat due to reclamations is pointed out as one of the reason for deterioration of aquatic environment, because a tidal flat has not only biological and chemical purifying effect but also strong vertical mixing by tidal current and wave. In this study, the influence of the seawall construction in tidal flat on tidal current and wave is estimated by a simple theoretical analysis and field survey. As a result, it is clarified that the construction of seawalls bring about considerable influence on the tidal current, and the influence depends on a positional relation between the mean tide level and ground heights at the seawall. In the case where the seawall is constructed farther offshore in tidal flat, tidal current decreases more drastically. In addition, the impact of seawalls on tidal current reaches farther area under the condition that a slope of tidal flat is gentler. The seawalls are located at from 0 m to 3 m high above mean water level in the northern part of Ariake Bay. It implies that the decrease of tidal current and vertical mixing due to constructed seawalls occurs mainly at around high tide in the northern part of Ariake Bay.