Effect of neap-spring hydrodynamics on salt and suspended sediment transport in multi-branched urban estuaries.

Unique neap and spring tide hydrodynamic features were studied in the estuarine areas of multi-branched urban rivers in Tokyo, Japan. Intensive measurements of salinity, turbidity, dissolved oxygen, velocity, particulate organic matter (POM), and nutrients were conducted for 13h in five stations simultaneously on July 3 and July 11, 2017. Water sampling, analysis and calculations of salt and sediment transport were performed. Results showed that the Arakawa and Sumida Rivers follow the typical natural estuary hydrodynamics while Shakujii River followed a typical artificial urban estuary. Shakujii River upstream estuary had lower flow velocity during spring tide (-0.05 to 0.04m/s) than neap tide (-0.09 to 0.16m/s) because of the channel slope that does not allow the transit of water to upstream even during high tide. Shakujii River downstream estuary had hypoxic (DO<2mg/L) bottom depths during neap and spring tide. Sumida River and Shakujii River are adjacent yet the nutrient and POM had different nature. Shakujii River is highly influenced by freshwater from combined sewer systems carrying POM, nutrients and sediments. The freshwater was 54% (14,650m3) and 100% (28,671.1m3) by volume during neap and spring tide, respectively. The POM and nutrients in Sumida River is influenced by tidal processes and tributary rivers. SS was transported from Sumida to Shakujii River during flood phase of spring tide. Freshwater from Arakawa River flows to Sumida River during ebb tide. Salt (-0.066kg/m/s) and SS (-0.16 to -2.435kg/m/s) were retained in the estuaries which may lead to river deterioration and occurrence of scum, hypoxia and odor. The results of the study, particularly on Shakujii River, are significant and could serve as basis for water quality management in similar urban estuaries.