Canadian Consulting Engineer

Feature

Edmonton’s W12

Like many cities built during the pre-WWII era, Edmonton relies in part on combined sewers for its stormwater and sanitary drainage. Until recently, overflow from these combined sewers discharged into the North Saskatchewan River at an average...




Like many cities built during the pre-WWII era, Edmonton relies in part on combined sewers for its stormwater and sanitary drainage. Until recently, overflow from these combined sewers discharged into the North Saskatchewan River at an average rate of 2.1 billion litres a year. Not only did this situation strain the city’s invaluable river valley resource, which includes the largest stretch of urban parkland in North America, but also it threatened the quality of the water supply for Edmonton neighbourhoods and other communities downstream.

One of the five fundamental principles guiding the city of Edmonton’s Drainage Services is to “maximize environmental protection.” In addition, Alberta Environment’s policies and procedures require combined sewer overflow (CSO) control. To meet these needs, the city developed a CSO control strategy and began implementing it in 2000. The objective of this $150-million (1999 dollars) strategy is to provide a cost-effective plan that will mitigate the environmental impacts of the city’s CSOs and reduce the spillage of untreated runoff and sewage to the North Saskatchewan River.

The program is also expected to increase the average annual volume of captured and treated wet weather flows from 56% to 86% and to reduce the average annual CSO events from 89 to 46. The control strategy includes a number of components: enhanced primary treatment, real time control gates, opportunistic sewer separation, weir structure modifications, and construction of the W12 tunnel as part of the West Edmonton Sanitary Sewer (WESS) system.

Targeting overflow weirs

The WESS W12 project is part of the city’s plan to target major overflow weir structures, in this case the Rat Creek outfall, which is near downtown Edmonton. The outfall was responsible for 60-80% of the annual CSOs.

Completed last year, the W12 syphon redirects flow from a combined sewer, sending it under the North Saskatchewan River and south to an existing sewer trunk. This carries it finally to the Gold Bar Wastewater Treatment plant 4 kilometres east.

Flows into the syphon barrel from the existing combined sewer are controlled by a sophisticated real time control (RTC) structure that opens and closes the gates according to the drainage conditions and the sewer system’s state. The RTC structure consists of two control gates and a diversion gate, with shafts and an operations facility installed at an empty lot at the top of the ravine, just uphill from the Rat Creek outfall.

Using real-time measurements and operational algorithms, the RTC system dynamically manages the flow rate, improving system performance dramatically. The W12 was expected to reduce CSOs from the Rat Creek outfall by up to 80%, but so far it has exceeded that figure.

Construction below downtown Edmonton

The project presented extensive design and construction challenges. The majority of the work took place 75 metres below downtown Edmonton. In the footprint of abandoned coal mines, five shafts needed to be constructed, along with a 1.2-km, 2500-mm tunnel deep enough to pass beneath the North Saskatchewan River. Coal seams, water pockets, sub-surface voids, and methane gas under pressure all posed potential risks.

Designing and constructing the RTC structure was complex as it involved tying into a pipe whose flow could not be redirected and which was very fast even during dry weather (3.7m3/s). Finally, the central location of the tunnel’s operations facility demanded careful odour control, while the tunnel construction itself, located beneath the downtown core and river valley, required high environmental standards.

The City of Edmonton’s Drainage Services brought in Associated Engineering (AE) to design the tunnel and SMA Consulting to provide decision support and help identify the best possible construction delivery scenario. AE provided the preliminary design services for the project and for integrating the tunnel into the existing drainage network. Several innovative techniques informed their work, including advanced hydraulic and particle models, computational fluid dynamics, and air and odour modelling.

The project team developed a unique collaborative design process involving interactive workshops, risk analysis and management studies. We used 3D and 4D [3D + time] visualization, construction simulation, and constructability reviews. SMA provided this dimension of the project as well as overseeing project controls and productivity analysis.

AE and SMA identified several cost and time-saving solutions. A grit analysis by AE showed that a $7-million pump station proposed in the original design could be eliminated as a result of the scouring action of the high flow through the pipes.

Simulation modeling undertaken by SMA, including risk and cost-benefit analysis, helped to identify that the best scenario was two-way tunneling, and that the best choice for tunnel secondary liners in terms of value, schedule, maintenance, efficiency, and constructability were HOBAS pipes.

A constructability review using advanced 3D and 4D visualization narrowed down the options for the RTC structure’s configuration from five to one, accounting for value, ease of construction, and risk.

Another unique cost-saving measure involved purchasing a lot near the construction site. Building the shafts on this lot helped to ease the flow of pedestrians and traffic around the site and made it possible to avoid utilities. The lot also provides a site for operational facilities and above ground housing for the odour and gate control.

Care was taken to avoid disturbing the fragile river valley environment, including by barging equipment across the river and limiting construction traffic to existing access roads and trails. The lay down site itself was situated in a pre-disturbed area; once the work was completed, the site was extensively re-landscaped into a park.

The project’s success has exceeded preliminary targets by a wide margin. A reduction in Rat Creek CSO events from 49 per year to fewer than 10 would be considered a great achievement. Yet, in the first year alone, the project reduced CSOs from the Rat Creek outfall to one event — a 98% reduction. Not only has this improved water quality, but also it has resulted in improved aquatic habitats and far less odour and corrosion at Rat Creek.

Besides addressing and improving environmental and socio-economic conditions, the project provides newer neighbourhoods in the northwest with sanitary and stormwater services, supporting Edmonton’s skyrocketing growth and economic progress. All-in-all, this collaboration between the city of Edmonton, SMA and AE has resulted in more robust and reliable infrastructure for Edmonton’s citizens.cce

Jeff King, M.A., is a facilitator and analyst at SMA Consulting in Edmonton.

Client-owner: City of Edmonton

Prime consultants: Associated

Engineering and SMA Consulting

Tunnel design: Associated Engineering (Herb Kuehne, P.Eng.)

Decision support: SMA Consulting (Hussien Al-Battaineh, P.Eng.,

Mohammed Al-Bataineh, P.Eng.)