er and poor biota. Therefore, it was necessary to use sandy material transported from outside this district.
A small experiment was conducted on the life and growth of short-necked clams. The desirable type of sand, the length of emergent dry time, and other factors was examined. It was decided to use pit sand (central 50% diameter. d=0.25 mm), taken from a nearby mountain and temporarily placed in the sea for about four years. Studies were then conducted on the stability of the local topography against the estimated maximum wave, as well as on the appropriate ground height. The basic gradient was set at 1/100. No submerged dykes were constructed along the alignment. The construction of the sandy beach began in 1979, and was completed in the same year.
In the autumn and winter of 1979, sand drifted onto the high tide zone, causing an increase in the bottom gradient. This sand movement promoted the formation of tide pools on the surface. Sand did not washed out/drift significantly toward the offshore. These phenomena indicate that a stable topography was being formed under the influence of natural waves. The beach was opened to the public in 1980. Trough shells, Mactra veneriform is and other shellfish were found after the completion of the construction until the winter of 1979. These kinds of shellfish were gradually superseded by short-necked clams, which, with the standing biomass of 800 to 1000 g/m2, have predominated since 1980. Each year, the number of clams decreases in spring, the season of shellfish gathering, and increases to the original level after following autumn. Since fry, are not bred artificially, shellfish proliferate naturally. On the other hand, due to the progress of eutrophication in the surrounding area, drifted sea lettuce has accumulated on the beach and degraded the living conditions of short-necked clams.
3 Results of Case Studies
Each of these tidal fiats is located in an area that was originally once a natural tidal flat or of similar topography. Therefore, topography with the soil and external forces (eg. wave/current forces) are considered favorably balanced for tidal flats. The kinds of benthic animals living in tidal flats vary according to soil as well as water exchange and other conditions. Waves and other external forces determine bottom particle size, and hence the species of benthic animals present. In a tidal flat of fine mud, clam worms and other polichaeta predominate. On the other hand, short-necked clams and other bivalves tend to predominate in a sandy flat.
In order to ensure the subsistence of birds, it is necessary to ensure the subsistence of benthic animals as their pray. The dietary habit of the protected birds may influence the choice between polichaeta worm-type" and "bivalve-type" tidal flats.
Tidal flats can also be used for decomposing organic matter. Particle organic matter is ingested and assimilated by bivalves and polichaeta worms. Based on the population density of several main species of macro-benthos, assimilation flux of suspended organic particles into benthos bodies can be estimated by supposing production I biomass ratio (P/B ratio) for each species.
Some examples are summarized in Table-2 after Imamura (1993), Kimura (1994) and Sasaki (1989). Estimation shows that bivalves play a very important role for assimilation capacity of tidal flats. Soluble or settled organic matter are decomposed by bacteria on the bottom sediment. Bacteria population density increases for the smaller particle size. Accordingly, bacterial activity, is brisk in a fine mud flat with high organic content. In a flat of fine-particle mud, however, the volume of seawater permeating into mud and subjected to decomposition is small. Hydraulic characteristics influence large to the decomposition flux.
Comprehensive approach is required for the benthic ecological system. Sustainabilitv, self-recovery and independency will be the key concepts for the system evaluation. Sound nutrient circulation is essential for the system stability. We should also consider the natural fluctuation of the environmental condition as well as the tidal topography. Adsorption and desorption of nutrients sediment clay may act as buffer/negative-feed-back for shock loads to a tidal flat. Environmental diversity and environmental gradient in a tidal flat may support the self-recovery, of the ecosystem through biodiversity. Our 'Design' of constructed flats will contain some kind of aerial margin or spatial capacity which assure the nature to create and alter the detailed morphology or micro-zonation of the benthic ecosystem. These processes can be summarized as Table-3.
4. CONCLUSIONS
In this paper, three cases of tidal flat construction were compared, to outline several points to be noted for construction, and the present status of the technology.
