We are faced with water quality problems at Lake Shinji. Therefore, in order to solve these problems, we are required to understand the mechanism controlling the environment in this lake. River Ohashi connects Lake Shinji with Lake Nakaumi, so that the water exchange between these two brackish lakes, makes the problem even more complex internally. In this study, we analyzed a one-year data set to understand the dependence of water quality, specifically the variation of salinity, in function of external forces like weather and atmosphere pressure. Also, from a detailed observation at Lake Shinji, Lake Nakaumi and Ohashi River from September 26th to October 25th, we concretise important water exchange attributes between Ohashi River and Lake Shinji.
The sewage coverage rate in Tokyo Ward has reached 100 %. However, 80 % of this coverage are combined sewer systems built before 1980. Consequently, large quantities of organic suspended solids are discharged from rainwater outlets into rivers during heavy rainfalls. Suspended solid (SS) pollutants accumulate in downstream brackish-water regions, giving rise to water quality problems such as floating scum, foul odors, and cloudy water. In this study, SS runoff characteristics were investigated in the Nomi River drainage basin with a combined sewer system using MIKE URBAN software to calculates sewer runoff. First, the overflow water depth and SS were measured at two sections located in the sewer system during two storm events. The parameters for MIKE URBAN were adjusted so that the observation results were reproduced well. Next, the model was applied to all outlets in the entire catchment of the Nomi River. The estimated discharge was input into a one-dimensional unsteady river flow model. Calculations satisfactorily reproduced observational results of the water level and turbidity measurements at the upstream end of brackish-water region not only for the two storm events but also for three other storm events which occurred after the model parameter calibration was completed.
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