Journal of Water and Environment Technology, Vol.13, No.3, 2015 Modeling of Water Quality in Tidal River Network in Osaka, Japan Masayasu IRIE, Tomo YAMAGUCHI, Shuzo NISHIDA, Yusuke NAKATANI Department of Civil Engineering, Division of Global Architecture, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ABSTRACT An urban tidal river network is one of the most difficult targets to simulate the flow and water quality in various kinds of water areas because the model must simulate both the vertical feature of the intrusion of sea water and the horizontal feature of water quality that is affected by human activities. In this paper, we determine the applicability of a three-dimensional flow and water quality model when reproducing the process of water with high concentrations of nitrogen, phosphorus and other constituents flowing through an urban tidal river network. We applied a three-dimensional model to Neya River, its tributaries and its downstream rivers in Osaka. The model can simulate well the vertical stratification of temperature and salinity near three river mouths, the mixing process of two different sources of water originating from Yodo River, and Neya River and its tributaries, the longitudinal profiles of chemical oxygen demand (COD), dissolved oxygen (DO), total nitrogen and phosphorus. Calculated water levels in the upstream area of Neya River system, however, are not coincident with the observations. A 3D water quality model, which is a potent tool in ocean and lakes, proved to be a versatile enough tool to simulate complicated and tidal urban river networks. Keywords: Neya River, three-dimensional water quality modeling, tidal river, urban river network, water quality INTRODUCTION Most rivers flowing through large cities have been influenced by drainage water and loadings after human water usage. When there are sewage facilities and influence of tides, the temporal and spatial change and the characteristics of water quality in the river water flowing down are complicated. The water quality is poor in some rivers that are polluted and affected by human activities. Once the river water is highly contaminated by organic or toxic substances, removal of those substances is difficult if the characteristics and makeup of the water are not clarified. Tidal rivers with vertical stratification of salinity are one of the most difficult target areas to simulate with hydraulic models. The difficulties are caused by a large variety of hydrodynamic characteristics. Tidal rivers are located close to coastlines, mostly consist of more than one water channel and often have numerous branches that require many open boundary conditions in simulations. As open boundaries at the end of a stream are much wider than the river widths, the assessment and simulation of tidal force need an utmost care in the settings for the model. The ratio of the change of surface water level to depth is larger than that in lakes and oceans. If the upstream water discharge is small and the tidal river is long, the friction on the river bottom becomes a dominant parameter, especially when the model represents the perturbation of surface height in the upstream. While the geometry of river networks requires a two-dimensional (2D), vertically integrated, hydrodynamic model at the very least, the stratification does a two-dimensional, horizontally integrated model. Therefore, the tidal river network Address correspondence to Masayasu Irie, Department of Civil Engineering, Graduate School of Engineering, Osaka University, Email: [email protected] Received January 7, 2014, Accepted November 17, 2014. - 231 - Journal of Water and Environment Technology, Vol.13, No.3, 2015 requires combinations of unsteady, longitudinal one-dimensional (1D) and vertical 2D models, shallow water 2D and vertical 2D models, or a three-dimensional (3D) model. There are many open-source 3D hydraulic model codes with a sub-model of water quality, which can be applied to such dynamically complicated areas. Successful applications are mostly for large estuaries such as those done by Lin and Kuo (2003) and Shen and Haas (2004) for the York River system. There are, however, only rare cases in which the reproducibility of temporal and spatial distributions of substances in flow is modeled in small tidal rivers. Huang and Spaulding (1995) simulated pollutant transport induced by combined sewer overflows (CSO) in Mt. Hope Bay, Liu et al. (2008) simulated hydrodynamics in Danshuei River of Taiwan, and Huang et al. (2011) performed 3D numerical experiments in a shallow tidal river located in Tampa Bay. Since the river networks of large cities in Japan have complicated geometry and narrow widths because they were originally used and modified as canals for waterway traffic, it is extremely difficult to simulate the flow and water quality characteristics in the entire area at one time. In this study, current, salinity, temperature and some kinds of nitrogen and phosphorus content are simulated using 3D hydrodynamic models combined with a biochemical reaction compartment downstream of Neya River and Yodo River, which flow through Osaka City, the third largest city in Japan. The annual average discharge of Neya River and its tributaries is about 2 m3/s compared to 18 m3/s of treated sewage water. The objective of the present study is to develop a model for water quality in urbanized, tidal and polluted rivers in order to clarify the characteristics of organic matter, DO, phosphorus and nitrogen. OUTLINE OF TARGET AREA AND METHODS Target area: tidal rivers of Neya River and its tributaries and downstream Target rivers in this study are shown in Fig. 1. Neya River and its tributaries have a low-lying basin with an area of 267.6 km2 surrounded by Ikoma Mountains on the east, Yodo River on the north, Yamato River on the south and Uemachidaichi Hill on the west. Neya River has many tributaries such as Furu River, Diani-Neya River, Onchi River and Hirano River (hereafter referred to Neya River and its tributaries as Neya River system which does not contain other water system except surface runoff). There is only one downstream end, Kyobashiguchi, which is located at the north end of Uemachidaichi Hill and the water flows into Okawa River which is one of the former main streams of Yodo River. Yodo River was cut and the present main stream was constructed as a wide floodway connected with Osaka Bay directly after two massive floods in 1885 and 1891. Thus, Okawa River is controlled by a weir at the upstream end where it connects to the main stream of Yodo River. At the downstream end of Okawa River the flow divides into Higashi-yokohori canal and two rivers (Dojima River and Tosabori River) which meet and divide again into three river mouths. The river mouths are located in Port of Osaka. The upstream and the downstream side of Kyobashiguchi are hereafter referred to as Neya River system and Osaka City Rivers (OCR), respectively. Calculations were done between the port and the upstream end of Okawa River and the upstream weirs and dams of Neya River system which is still influenced by tides. - 232 - Journal of Water and Environment Technology, Vol.13, No.3, 2015 Fig. 1 - Neya River system and its downstream. Legally-imposed field surveys in this area have been carried out by local governments every season at 12 stations in OCR among public water areas defined by the Water Pollution Prevention Act. Living Environment Items such as BOD (biochemical oxygen demand), CODMn (chemical oxygen demand), SS (suspended solids) and DO (dissolved oxygen), and Health Items such as cadmium and total mercury have been measured in these public surveys (Osaka Prefectural Government, 2013). In both rivers BOD shows a gradual decrease after 1990s, but remains stubbornly high and is sometimes higher than the environmental standard. A field survey was conducted to clarify the distribution of nitrogen and phosphorus in OCR at the four tidal hours on April 25, 2006. Some of the results were reported by Irie et al. (2008). In this survey, water at the surface and bottom at 25 points was sampled and inorganic nitrogen, total nitrogen (TN), phosphate, and total phosphorus (TP) were measured. Water temperature, salinity, turbidity, and DO were also measured in situ by a CTD (conductivity, temperature and depth) sensor and DO meter. According to this survey, the water flowing from Neya River system flows down and back again, have to take longer time than semidiurnal tidal cycle to go down through the port. Consumption of DO occurs in the bottom water during the flow into OCR. Saline water comes into the streams up to 8 – 10 km away from the river mouths. A halocline is formed throughout this distance. Flow and biochemical model: EFDC The river network in OCR consists of narrow canals that were previously used for transportation by barge and they spread widely with many confluences and branches. A horizontal 2D model is necessary to model the process of the flow. As there are clear vertical differences of salinity, temperature and water quality in the downstream due to the halocline, a vertical 2D model is needed to represent such vertical characteristics. Thus, a 3D flow model was applied directly in this study. - 233 - Journal of Water and Environment Technology, Vol.13, No.3, 2015 The EFDC (Environmental Fluid Dynamics Code) was originally developed by Hamrick (1992), and has been supported and developed by United States Environmental Protection Agency (U.S.EPA) (Tetra Tec Inc., 2002) and provides flow, sediment transport, toxic substances and water quality information for models. The EFDC has wide applicability in the simulation of a variety of 1D – 3D water dynamics. In 3D dynamic solutions, sigma terrain-following vertical coordinate, hydrostatic, free-surface, turbulence-averaged equations are solved with the Boussinesq approximation like other 3D physical models based on Blumberg and Mellor (1987).