breakwaters which consist of rouble-mounds and caissons. The sea water exchanges in the artificial lagoon through the rouble-mound breakwater under the caissons.
The measured results shown in Fig. 6 demonstrate that the DO value significantly decreases in the bottom layer in summer, because anoxic water created by activities of organisms stays in the bottom layer owing to the presence of a strong discontinuity layer. While in winter,the DO in the bottom layer is almost the same value as that in the surface layer because the discontinuity layer disappears. In this case, the transparency in the inside of the artificial lagoon drasti- callv increases.
The results of numerical simulations are shown in Fig. 7. The DO value at the bottom layer (box 1) shows the similar tendency to the measured result. However, the simulated values are not in quantitatively agreement with the measured values. This may be caused by the inaccuracy of the parameters for benthos and seaweed. The COD value in the inside of the rock filter (box2) drastically decreases in winter. This tendency shows negative correlation to the measured transparency, because transparency may increases with decreasing organic carbone concentration.
Fig. 6 Time histories of measured DO and transparency
4. CONCLUSION
In this paper, a mathmatical model of the material cycle in the artificial lagoon and a numerical simulation method to estimate the water purification process have been proposed The feature of this method is that the ecosystem model includes compartments of benthos and seaweed, which are important factors to remove COD and nutrients. The simu- lated results showed that the artifitial lagoon have some water purification effects, such as removal of COD and nutrients, wave breaking reacration. However, the simulated values are not in quantitatively agreement with the values Which were measured m a real artieficial lagoon. This may be mainly caused by the inaccuracy of the parameters for benthos and seaweed In order to improve this method, we need to investigate the unknown rate parameters in the mathematical model in more detail.
The authors are greatly indebted to Profs. R. Hosoda and Y. Ikeda of Osaka Prefecture University and Prof. S. Ebise of Setsunan Umversity for helpful discussions. They also wish to thank Mr. K. Akai for cooperation in field measurement.
