Behavior Analysis of Scum in Urban Tidal Rivers

Scum generation is a serious problem in urban rivers. It results from the presence of suspended organic substances, mainly discharged as combined sewer overflow after rain. Since scum affects the aesthetics of rivers and causes bad smell, we require new methods to remove the scum. However, the behavior of scum has not been sufficiently elucidated, which makes the prediction of where the scum surfaces and flows very difficult. Two effective ways to understand the behavior of scum are to observe the surface of river by using fixed point cameras and to perform numerical simulations.

In this study, field observations of the behavior of scum and water quality were conducted in the Neya riverine system in Osaka, Japan. Several cameras were installed at multiple locations in the tidal river network, and the behavior of scum floating on the surface of the river was continuously observed. The processes of scum occurrence, transport, and disappearance were clarified by analyzing the relationship between the behavior of scum and the weather, tide, and water quality.

Moreover, the behavior of scum was simulated using a three-dimensional hydrodynamic model called finite volume community ocean model (FVCOM). To reproduce the behavior of scum, a new feature was included in the model. The model could reproduce the flow and density structure in rivers relatively well in comparison with observations. A particle-tracking analysis demonstrated that tide and backwater affected the behavior of floating scum in rivers.