Koji Otsuka, Naoki Nakatani and Taketoshi Okuno Department of Marine System Engineering, Osaka Prefecture University
Keywords: Artificial lagoon, Water purification, Ecosystem model, Numerical simulation
ABSTRACT
The water quality in an inner part of a gulf encircled big cities is chronically bad, because the pollution load is very high. It is well known that the so-called UTSURO, an artificial lagoon using rubble-mound breakwater, has water purification functions for such an enclosed sea area. In this research, the mathematical model is constructed using ecosystem models. The result of numerical simulation shows good agreement with measured values, and also shows the system is effective for purification of water quality.
1. INTRODUCTION
The quality of seawater significantly depends on quantita-tive relations between pollution load and natural purification. The water quality in an enclosed sea encircled industrial cities, such as the northeast part of Osaka bay, is chronically bad.The pollution load in this area must be higher than the capacity of self purification of the sea In order to purify the water, several ideas have been proposed. The so-called UTSURO, an artificial lagoon surrounded by rubble-mound breakwater1), has been known as an effective water purifica- tion system, in which a tidal energy is used.
It has been pointed out by several researchers that the artificial lagoon may have some water purification functions, such as contact oxidation, wave breaking aeration, sedimenta- tion of suspended solid, etc.. The effect of the contact oxida-tion, which may be one of the most important factors for water purification, has been experimentally and theoretically investigated by Oda et al.2),3). They proposed some empirical coefficients related to the contact oxidation effects, and carried out numerical simulations of water purification in an artificial lagoon. Hone et at4) researched on the reaeration effect of the breakwater, and showed that the reaeration function increases with decreasing the reflection function which depends on the surface roughness and slope angle of the breakwater. How- ever, the interaction between these purification functions in the real artificial lagoon have not been discussed in these research works.
Recently, some field works have been carried out. Organ- isms living in the rouble-mound breakwater were observed by Ueda et al.5)in the artificial lagoon located at Tarui in south- east of Osaka Bay. Akai et al.6)measured environmental conditions in the artificial lagoon located at Nishikinohama in Osaka Bay, in order to clarify the effects of brealcwater on damping of wave loads on a floating structure which was proposed as a floating energy plant system by themselves7). Tsuji et al.8)constructed a model facility of an artificial lagoon in Mikawa Bay, and reported some results of the on-site experiments. However, they have not obtained enough data to clarify the long-term water purification functions of the artificial lagoon.
In this study, a mathematical model is constructed using ecosystem in order to investigate the water purification mechanism of the artificial lagoon. The simulated results, the water quality and the environmental conditions, are compared with the measurements9). The results show the detail of the water purification process and the effectiveness of the system.
2. MATHEMATICAL MODEL
A numerical model in this study is based on box model concept10), which assumes that all state quantities (physical quantities, chemical quantities, biomass, etc.) in each box are uniform. The calculation region of this model is divided into six boxes (see Fig. 1), surface and bottom layers of the inside and outside of the lagoon, and surface and bottom layers of the rock filter. Each. box has each ecosystem model with seven ompartments (see Fig. 2), such. as phytoplankton, zooplank- ton, detritus, nutrients, benthos, seaweeds, and dissolved oxygen (DO).
