Hydraulic modeling using software has played a significant role in delivering optimized options for the Shanghai World Bank Loan project, the West Trunk Line renovation project
Phase 2, which is part of the World Bank-funded Shanghai Urban Environment project (with a value of $512 million).
Built in 1970s with a design capacity of 700,000 m3/day, the West Trunk Line collection system is one of the two main wastewater systems in Shanghai. Due to the city's rapid urban development, this capacity is already below the current wastewater load received from its designated drainage area, which is causing significant environmental problems for the Shu Zhou Creek, the important river that flows through the city center.
The key aim of the West Trunk Line renovation project is to look at short and long-term engineering solutions for improving collection, trunk sewers, pumping stations and wastewater treatment plants, as well as restructuring the drainage areas to operate jointly with the neighboring Combined Sewer System Phase 1, particularly during the wet seasons. As the new interceptor is inter-related to the West Trunk Line, and they affect each other, they both had to be studied as part of the project.
Sogreah Consulting Firm on behalf of Shanghai Municipal Government and their wastewater utility Shanghai Municipal Sewerage Company worked together with Shanghai Water Planning and Design Research Institute to establish hydraulic models to simulate the complicated West Trunk Line system under different projected time and design flow conditions using the InfoWorks CS software solution.
Project overview
The city of Shanghai has a total area of 6340.5km2, and a projected population of 20 million in 2020. The Shanghai wastewater system consists of three major drainage catchments: the Shi Dong catchment, the Zhu Yuan catchment and the Bai Long Gang catchment.
Shanghai is a coastal city sited on very flat terrain, mostly about 6ft (2m) to 9ft (3m) above sea level. The city's aging wastewater systems suffer additional stresses due to the area's poor ground conditions, urban construction and storm and groundwater seepage.
The West Trunk Line System was built in 1971 and was the major wastewater interceptor for the central city until the 1990s, running along the line of the Shu Zhou Creek. The egg-shaped trunk line is some 14.4 miles (23.2km) long, with pipe diameters ranging from 2.5m to 3.6m. It serves an area of 74 square miles (193km2) and a population of about 1.8 million.
The interceptor drains to the Shi Dong Kou wastewater treatment plant, which has a design capacity of 105MGD (400,000 m3/d) - less than the collection system, which has a design capacity of 185MGD (700,000 m3/d).
After construction of the adjacent Combined Sewer System Phase 1 in 1993, it was found that much of the excess flows from the new system drained to its neighbor. In addition, because of the rapid urban growth, the West Trunk Line System faced several major challenges including significant flow pattern changes, sedimentation in some zones, and problems caused by the construction of incompatible drainage infrastructure.
The Combined Sewer System Phase 1 has a design capacity of 309 MGD (1,170,000m3/d). It operates as an interceptor sewer, collecting wastewater from the existing combined sewer network (industrial wastewater is pre-treated before entering the system), with treated flows discharging to the Yangtze River. The main trunk line is 21 miles (34.4km) long, comprising 10.5km of gravity sewer of diameters ranging from 3ft (1m) and 13ft (4m), with the remainder consisting of dual rectangular pressurized pipes 11.5ft (3.5m) high and 14ft (4.25m) wide.
Many challenges
Shanghai's rapid urban development has sparked swift population growth, which has exceeded the estimates of the 2001 Wastewater Master Plan. The demands imposed on the interceptors by this growth, combined with the network restructuring and past poor maintenance, had created significant problems in terms of both operating and improving the system. Any uncertainties in the data, however small, could create significant differences in terms of the engineering solutions, both environmentally and financially.
Unfortunately, poor data quality is common in most wastewater projects in China, and engineers normally use traditional simplified methods to consider local solutions. Network modeling has not normally been considered a necessary tool.
However, learning from past experiences and technical exchanges with the Hong Kong Drainage Services Department and several European companies, the project team recognized that computer network modeling was the best way to get a complete picture of the existing systems, resolve the various conflicts between them and visualize the results of any suggested improvements.
A modeling system
The team recognized that the success of computer modeling relies on integrating all available information (or data) and the functionality of the modeling software.
The project team therefore chose the InfoWorks CS modeling solution to look at various scenarios. All relevant information, including pipe and control structure data, demographic data, field investigation data, images and operational records were easily integrated into the network model.
The modeling project simulated typical performance for both the existing and new design elements of the West Trunk Line in dry and wet weather over both the short and long term. It also checked the behavior of the Combined Sewer System Phase 1 under its design flow.
An interlinked model looked at the effect of receiving extra flows from the Shi Dong Kou sub-catchment in wet weather conditions, when it was proposed that the West Trunk System would transfer part of its flows to the Combined Phase 1 System. Because of the considerable variation in the inflows and velocities in the pipe network, ways of avoiding possible sediment build-up were also evaluated.
The results from the model have been used to examine the feasibility of the renovation scheme and refine the improvements to the West Trunk System.
By using the established InfoWorks CS models under various scenarios, several critical findings were made that helped to identify sound engineering decisions. An important element involved reviewing predicted 2020 wastewater flows in the West Trunk Line service area. The model also indicated that the dry weather flow in the West Trunk Line System was causing sedimentation problems and thus reducing its capacity. It also predicted that the system renovation would result in flow pattern changes and indicated actions that would need to be taken in connecting sub-catchments.
There were two main proposed project options: one was to construct a treatment works north of Wen Zhaobang, and the other was to build a regulating tank in the same place. The modeling results demonstrated the major differences between these two options and showed that the regulating tank scheme would suffer poor hydraulic conditions, be complicated to operate and create a high risk of environmental pollution.
Building a treatment works was shown to have many advantages - it would provide better hydraulic conditions, simpler operation, and would enable excellent utilization of the two interceptors, as well as being an extremely adaptable and flexible solution.
Conclusion
Through the research undertaken using InfoWorks CS, it is much easier to examine the performance of various elements of the system and make financially and environmentally sound engineering decisions.
