The indispensable environmental functions of tidal flats have led to constructing artificial tidal flats as part of environmental mitigation projects in Japan. Most tidal flat studies, however, have focused on biological, ecosystem, and water quality issues, with only limited research being directed at establishing design technology ensuring the protection of artificial tidal flats against wave actions, which are known to produce significant settlement damage.
　A series of field surveys investigating the stability of natural/artificial tidal flats were conducted from August 2000 to April 2001, i.e., the profile of the seabed and the stiffness and grain-size distribution of sediment were measured. Five natural and seven artificial tidal flats were investigated including one natural and three artificial sand beaches. Major conclusions are as follows.
1) The terrace of the foreshore region in most natural tidal flats consists of a main flat part and a multiple bar-trough part (multi-bar). The former serves as an important habitat, while the latter is needed to ensure stability of the main flat part against wave actions which occupy the area from the ebb tide shoreline to 200-350 m shoreside. It is considered that a multiple bar-trough part of more than 200 m is needed to ensure a stable artificial tidal flat.
2) When designing an artificial tidal flat, the underground water level of the main flat part at ebb tide should be kept sufficiently high to maintain good habitat conditions there. Accordingly, we investigated the relationship between the width and elevation of the terrace and grain size necessary for supporting the habitat on the main flat part.
3) For the case of an artificial tidal flat with sand cover layer, we found that detailed investigations on the movement of the sand layer and the strength of the lower layer are indispensable.