The abundance of the freshwater mussel Inversiunio jokohamensis was investigated in a reservoir where management has been abandoned. The investigation was carried out on September 12, 2009, and composition of microalgae investigated on November 8, 2009. The results obtained were summarized as follows: 1) individuals in shore side habitat were more abundant than in offshore side; 2) there was a noticeable absence of individuals with < 50 mm body length; 3) the ratio of old individuals was high while diversity of generation was minimal; 4) no fish were caught using a trap method in the reservoir. This research indicated that recruitment had not occurred for several years.
When improvement works for overage irrigation tanks are performed, existing embankment is first excavated, then a new outlet conduit is established and is buried with backfill material. In this case, the stress generated in the backfill between outlet conduit covered with reinforced concrete and the existing embankment may decrease. This phenomenon is called arching action and causes a large reduction of stress. Consequently, the backfill under the arch becomes a weak point in the structure and poses a risk of hydraulic fracturing. In this study, the arching action is evaluated quantitatively. Parametric studies on inclination angle of excavation, excavation width, stiffness of foundation and existing embankment and compaction stress are performed to reveal the influences on the stress condition. Moreover, the effect on the arching action of the stiffness of flexible conduit, which has recently become popularly used in the improvement of irrigation tank, are discussed.