Constructed Flat at Hiroshima Port and c) Kanazawa Beach at Yokohama Port, summarized in Table-I.
Table-I Summary of case study sites
2-2 Gamoh Tidal Flat in Sendai Bay
The Gamoh Tidal Flat is connected with the mouth of Nanakita River which is flowing into Sendai Bav. This flat was originally a part of the river channel. The river was running parallel to the coast line behind a 100 to 300 m-wide sand dune. In the course of construction of the New Sendai Port to the north of the rivermouth, steps were taken in 1967 to dig a shortcut path of the river to the ocean and fix the river mouth at its present location by a jetty. A lagoon was left. It was separated from the river by a small dyke. Then, connecting pipes were placed under through the dyke to ensure the linkage between the lagoon and the river mouth. As a result, the lagoon started feeling the tidal fluctuation and was developed into a tidal fiat repeatedly emerging out of, and submerging under, seawater. As shown in Fig.-I, this new Flat consists of sandy dune, a lagoon, and the tidal fiat proper. The lagoon,
extending from the pipes, has a length of 850 m, a width of 200 m, and an area of approximately 15 ha. Water depth increases with the distance from the pipes.
At the deepest portion of the lagoon, the bottom is approx. 70 cm lower than the mean water level of the sea. Sediment diameter is ranging from 0.3 to 0.005 mm, and decreases with distance from the entrance pipes. Fresh water and nutrients are brought into the lagoon by the river and fishpond effluent. Salinity and oxygen concentration also become lower toward the inner end of the lagoon. The surrounding area of 48 ha, including the lagoon water surface, has been designated as the Gamoh National Sanctuary. In the Gamoh Sanctuary, Japanese black pines are found on the beach, and reeds grow along the lagoon. The lagoon is also famous as a transit point for snipe, plover and other migratory birds, and as a habitat of small tern. The entrance area up to 400 m from the dyke with pipes is densely populated by polichaeta worms, on which snipes and plovers feed; that is, the habitat of worms is the same as the area on which these birds stay.
As stated above, the construction of the pipes connecting to the river mouth caused the formation of the tidal fiat. This fact suggests that tidal flat conditions can be controlled by adjusting water conveyance. Based on the results of this step and related data, Kurihara (1992) presented a lagoon ecosystem as a cybernetic model. For the ecosystem, each environmental requirement has its optimum point with an appropriate range. If an environmental condition fluctuates and shifts from its optimum, then negative feedback systems make the condition return to the optimum stage. The stability of the ecosystem needs this kind of feedback support. Furthermore, he pointed out the following requirements for maintaining sound ecological activities:
?Topography in which the bottom emerges from, or submerges beneath, the lagoon water due to tidal fluctuations,
?Salinity suitable for benthic animals, attained through the mixing of seawater and fresh water,
?Supply and accumulation of nutrients and organic particles,
?Supply of sufficient oxygen, ensuring the subsistence of fauna in the lagoon,
?Clay minerals absorbing and retaining nutrients and organics, which are supplied to biota at slow pace.
Fig.-1 Plain view of present lagoon and tidal flat at Gamoh, Sendai
