These requirements must be fulfilled to ensure the self-sufficiency, stability and circularity of the ecosystem.
2-3 Constructed Tidal Flat in the Itsukaichi Section, the Port of Hiroshima
The Port of Hiroshima, facing the Seto Inland Sea, is calm due to natural topographic protection against high waves. Several tidal flats have developed near the mouth of the Yahata River, and served as a habitat for wild geese, ducks and other water birds. Land reclamation was planned for an area off the river mouth, to secure land for urban development. Since the tidal flat at the river mouth would disappear due to the reclamation, a new tidal flat with an area (24 ha) roughly equal to that of the original flat was created outside the revetment (Fig.-2). The length and width are 1km x 250m.
Fig.-2 Location of constructed tidal flat at Itsukaichi in Hiroshima Port
The reclamation site was located on the passage of flush water from the river mouth to the offshore. The particle diameter of the bottom sediment had to be determined, so as to counteract against scouring by river flushes. On the other hand, the tidal fiat had to be firm enough to allow water birds to walk on it, yet soft enough to enable the subsistence of polichaeta worms on which they feed. Locally available cohesive silt was placed inside. Then, it was covered with sand with a central particle diameter of 0.4 mm. Final surface gradient was made as 1.6/100 for the intertidal zone. The construction work began in 1987. Cohesive soil, with a clay content of 50 to 70%, was taken from the surrounding bottom in the course of reclamation. Since the dumping method for such shallow areas was not fully known, experimental work was conducted on a small scale. To ensure stability of the cohesive soil ground, it was dumped in several rounds, heaping to a height of approx. 50 cm at a time. After that, sand was gently covered in three rounds. The tidal flat was completed in December 1990. After completion of the tidal flat, the flat topography gradually changed to an equivalent stable one by the external force, mainly waves.
Living things were not artificially transplanted into the constructed tidal flat. Immediately after the completion, a large number of polichaeta worms, on which snipes feed, were found in the sediment lower than the mean L.W.L. Also, birds that stayed in the flat were almost equal in number to those that had stayed in the original fiat, though the former did not comprise as many species as the latter. In the winter of 1991, ducks were found in the flat, indicating that birds in the flat were almost equal in both number and species to those in the original one. Supposedly birds, which had evacuated to nearby fiats or woods during construction, have returned to an large area of the constructed flat, first inhabiting the fringe portion linked with the adjoining tidal flat, and then invading to the central part.
Stabilization of the tidal flat's topography has also resulted in an increase in short-necked clams and other benthic animals. A green park will be constructed in the reclaimed land adjacent to the constructed flat. This park, along with the flat, is expected to contribute to developing a wider habitat for water birds.
24 Artificial Beach at Seaside Park in the Kanazawa Section, the Port of Yokohama
Land reclamation in the Kanazawa Section of southern Yokohama Port was planned to secure land for factory relocation, housing construction and other purposes. In the past, beaches and tidal flats existed near the area to be reclaimed. This area was once famous for its beautiful variegated landscape.
The construction of a seaside park was planned as a part of the above-mentioned reclamation project. It was decided to construct a tidal flat for shellfish gathering, bathing and other recreational activities. The flat was planned as a I km-long, 500 m-wide area (with a 190 m-wide seaside and intertidal area) suitable for the subsistence of short-necked clams. The original bottom sediment was silty, with a developed anaerobic lav-
