For the protection against the eutrophication in polluted inner bays, seawater and sediment improvement effects are investigated as to the capping with clean materials, or the dredging of sediments, the construction of artificial beach or lagoon, the functional designing of port-and-harbour or coastal structures or their
laying out, the discharging of fresh water, the applying of aeration, the supporting of biological filtration activities, etc.
　This paper treats the examples to predict and estimate the effect of such improvement works, with the aid of numerical models.
　A series of numerical simulations prove that such improvement works as effluent-cut-off, release-cut-off, and fresh water discharge are very effective to polluted seawater and sediments. However, the optimum area and volume of the works should be decided with the most careful consideration, because of a huge amount of construction and maintenance costs. For the reason that the biological filteration by sea animals, and aeration, mixing and advection by coastal structures are also
effective to the polluted seawater, the above additional effects should be taken into account on designing coastal structures.