Assessing the stormwater impact to an urban river ecosystem using estuarine water quality simulation model

The reduced richness of fish species in the Love River has long been associated with the impact of storm flooding. During the 3-year time period of 1993-1995, it has been observed that the devastating conditions of low DO levels in the Love River frequently appear at a certain time after the emergence of peak rainfall. While the official investigation with regard to estuarine and river ecology indicates that more than 85 fish species could be found in the neighbouring river systems in South Taiwan, only 17 fish species appeared in the estuarine system of the Love River. To integrate the knowledge of environmental and ecological impacts in these extreme storm events, a two-dimensional numerical model - Love River Hydrodynamic and Water Quality model (LRHWQ) - was developed to simulate the fate and transport of contaminants in the Love River estuarine system. It eventually produces the responsive information of water quality and the dissolved oxygen deficit with respect to the combined sewer overflow (CSO) impacts along the river corridor. To justify the impact of DO level on the fish community, a laboratory analysis was conducted with respect to the living threshold of young Liza macrolepis and Chanos chanos that are two representative fish species in the group of abundant species in the Love River. The LC 50 found in this study lies in between 0.8-1.1 mg/L; yet the saturated oxygen concentration is normally near 9.0 mg/L in the study area. The fish community is sensitive to the dissolved oxygen deficit and simulation studies confirm that simply building a sewage interceptor system to handle routine wastewater effluents without regard to the CSO issues in the storm events cannot fulfil the ultimate goal of environmental restoration in this urban river system.