Mixed Semidiurnal Multi-branched Urban Estuaries: Neap-Spring Tidal Cycle Interaction and Its Effect to Odor and Scum Formation

The Sumida River and Shakujii River collect discharges from the most inhabited and highly industrialized metropolitan area in Japan. The rivers conform to water quality standards, yet problems on odor and scum occur. Numerous complaints on odor and scum take place during summer. To understand the hydrodynamics and water quality variations, two separate 13-hour neap and spring tide field observations on July 2017 were conducted. Velocity, salinity, suspended sediment concentration (SSC) and dissolved oxygen (DO) were measured every 30 minutes. The rivers were stratified during neap tide and well mixed during spring tide. The low flow velocity and retention of hypoxic saltwater in Shakujii River during neap tide may cause scum generation. The probable sources of scum during spring tide are the low flow velocity and low DO in Shakujii River, and transport of suspended sediments from Sumida River to Shakujii River. The study also analyzed the relationship of scum coverage to water quality and rainfall. The data were sorted according to first and second high tides and low tides; ebb and flood tide; and neap and spring tide. The data from April to October 2018 were statistically analyzed. The scum coverage during the first and second high and low tide, and during the neap and spring tide were not statistically significantly different. Only the scum coverage during ebb tide was statistically significantly different from flood tide. Scum coverage was significantly correlated only with DO. These results provided significant information on the occurrence of scum and odor in urban estuaries.