Going Beyond: West Don Lands Stormwater Treatment Facility and Storm Outfall Project

To achieve the City of Toronto’s new treat­ment objectives for suspended solids removal and disinfection, stormwater facilities are becoming more complex, costly and larger in scope.

In the case of the West Don Lands Stormwater Treatment Facility and Storm Outfall project, the treatment process has incorporated techniques and equipment that are normally associated with water and wastewater treatment. Furthermore, to reduce the system’s process equipment size and to achieve cost savings, the main shaft of the new outfall conveyance system was used as storage space for stormwater.

Waterfront Toronto’s 800-hectare revitalization

Waterfront Toronto’s Revitalization Project is the largest urban renewal project in Canada and one of the largest waterfront projects in the world. It includes 800 hectares of former industrial lands, the creation of 40,000 residential units over a period of 25 years, and $30 billion in private and public investment.

Of this land, 32 hectares will be dedicated to the West Don Lands precinct, which will be the site of the 2015 Pan Am Athletes’ Village. The precinct will include 6,000 residential units, as well as commercial and other buildings.

The stormwater quality facility for the West Don Lands precinct reflects the advanced criteria the city of Toronto incorporated in their 2006 Wet Weather Flow Management Guidelines. Waterfront Toronto is the first developer to be subject to the combination of the stringent stormwater sediment removal and disinfection provisions of the guidelines.They require 80% total suspended solids removal and disinfection to 100 E.coli per 100 mL.

Early on in the West Don Lands project, it was identified that filtration and UV disinfection would be needed to meet the requirements. But it was also recognized that these measures would be costly and difficult to achieve within the allocated space.

Flood protection required

regrading and new outfall pipe

For development of the West Don Lands to proceed the lands first had to be protected from flooding from the Don River. A 750-metre long flood protection landform was constructed parallel to the river (see site context, page 13). The landform project, undertaken by Ontario Realty Corporation, protects the West Don Lands area of 32 hectares, as well as a larger area of the city beyond.

Because the landform blocks the land drainage that previously flowed easterly towards the Don River, it was necessary to regrade the site to slope it westerly to Cherry Street. At Cherry Street there is a new outfall to Lake Ontario at the Keating Channel. The regrading of the site and new outfall and stormwater quality facility project was undertaken by Waterfront Toronto, with R.V. Anderson Associates as the infrastructure engineers.

The new 450-metre long storm outfall pipe follows the alignment of Cherry Street from north of the railway embankment to Lake Ontario (Keating Channel). The alignment required crossing the Lakeshore Rail Corridor, a major commuter line that runs into Toronto. It also crosses high voltage underground and overhead hydro lines that feed into the city, a 500-mm diameter high pressure gas main, Lake Shore Boulevard (a high-volume arterial roadway), and the piers of the elevated Gardiner Expressway — as well as the usual suite of utilities and services.

To add to the project’s complexity, the area is lakefill, which increased the possibility of encountering old wharfs, shore walls and assorted buried structures. Further, the overburden soil geotechnical and environmental conditions are poor.

For these reasons, tunnelling the outfall in the bedrock at a depth of approximately 25 metres was recommended by R.V. Anderson Associates. This 3-m diameter concrete-lined tunnel has been constructed and includes capacity for the West Don Lands and adjacent future development areas on the waterfront.

Main shaft used for TBM and stormwater storage

The main shaft of the stormwater outfall system is located immediately south of the railway embankment and east of Cherry Street (see above and cross-section p.16). This 12-m diameter shaft was used for launching the tunnel boring machine (TBM). The shaft is also lined with concrete and is to be used as a stormwater storage and pumping facility. This was a key feature of the project. It creates 3,000 cubic metres of storage to allow attenuation of the flows to the treatment system (clarifiers and UV), resulting in cost savings by enabling a reduction in the size of the treatment equipment. The storage shaft will be used every time there is any appreciable rainfall.

An overflow from the storage shaft was achieved by constructing a vertical standpipe in the centre of the storage shaft, with a funnel-like weir structure located at the top of the shaft, as shown in the cross-section (p. 16). When the storage is full, flow will overtop the weir for discharge to the outfall tunnel. Such discharges are incorporated into the design and occur after the first flush rainfall event is captured. Pumps located in the storage shaft convey the stormwater to the at-grade treatment facility. A flushing system cleans the storage facility after use.

The arrangement of the tunnels (see p. 14 and above) all three tunnel headings to be advanced from the main shaft. This was achieved by tunnelling on an upwards gradient from the main shaft to conduct all construction dewatering at the main shaft. All mining work was done from the main shaft, which resulted in considerable cost and schedule savings.

Four smaller shafts, 2.4 metres to 3 metres in diameter, are incorporated at the tunnels. Three of these shafts were made by drilling (flushing shaft, Cherry Street shaft, and Keating Channel shaft), and the fourth (West Don Lands shaft) by secant pile shoring. The tunnelling contractor was able to back the TBM out of each tunnel upon completion, achieving savings in cost and schedule.

Innovative vortex oil and grit separators

Immediately upstream of the tunnel system is a large oil and grit removal system consisting of two large vortex separators that incorporate a screen to also facilitate the removal of trash and buoyant materials associated with urban runoff. This system is the first stage of treatment and prevents damage to downstream facilities. According to the manufacturer, Echelon Environmental, the installation is the largest of its kind in Canada. The structure is 13 metres deep, excavated into bedrock. The challenging site is immediately adjacent to the railway embankment, requiring the excavation to be shored using secant piles and soil anchors. Stop logs were incorporated at the inlet for maintenance and a stop log storage area was integrated into the structure.

Despite the use of oil-grit separators, a unique flushing shaft was incorporated upstream of the tunnel that crosses under the railway. This flushing shaft fills as the main storage shaft fills. The volume of water is retained by a vacuum pump at the surface to avoid having moving parts at depth. Once the storage in the main shaft is drawn down, the vacuum is released and the volume of water in the flushing shaft is discharged, creating a wave to thoroughly flush the tunnel. This innovative system was developed in England and this is the first time it has been used in Ontario.

To help expedite the schedule, precast concrete components were incorporated for the standpipe and weir structure in the main shaft, and for the box culvert outfall to the Keating Channel.

Tunneling had advantages

Tunnelling in rock was a key element of the project because it avoided the risk of unknowns associated with working in the overburden soils and avoided soils of poor environmental quality. Compared to open cut construction, tunnelling also meant only having to dispose of overburden soils at the shaft locations. The geo-environmental report showed that it was important that groundwater not be mobilized, so all the shafts incorporate watertight shoring systems by the use o
f secant piles or steel liners. Incorporating watertight shoring for an open-cut solution would have been extremely expensive due to the length required.

The outfall discharges to the Keating Channel, a navigable waterway. Due to concerns of the Toronto Port Authority, large (3 m x 5 m) precast box culvert sections were incorporated into the outfall pipe to reduce the velocity of the discharge into the channel.

Treatment to high standards

The first step in the treatment process occurs in the previously described oil-grit separator. The next step is storage and pumping the stormwater from the main shaft to the treatment facility building, which is located beside the main shaft. Within the 300-m3 treatment building area there is fine screening and two clarifiers. The final treatment step is ultraviolet (UV) disinfection.

The treatment equipment includes two ballasted flocculation clarifiers to rapidly settle suspended solids. While this process is regularly used for water, wastewater and combined sewer overflow treatment, this is the first use of this proven technology on stormwater in Canada.

The Total Suspended Solids (TSS) loading is estimated to vary from 10 to 500 mg/L with an average concentration of 130 mg/L. The two parallel clarifiers with a combined flow of 400 L/s will discharge effluent at TSS of 20 mg/L for effective UV disinfection.

The UV disinfection system will be the first to be incorporated into a full-scale end-of-pipe treatment system implemented under Toronto’s Wet Weather Flow Management Guidelines. Since disinfection is only required during the swimming season, the low-pressure UV system will be located in an open channel outside the treatment building. After UV treatment, the flow will be directed to the outfall tunnel and into the Keating Channel.

Small footprint,

architecture and tendering

The stormwater quality facility building was designed within a small footprint using only a very small parcel of land at the main shaft site. This land would not have been suitable for development and the facility footprint is compact to avoid impacting adjacent development lands.

Waterfront Toronto incorporates architectural design excellence into their projects in an era when many municipal infrastructure buildings are designed to blend into the surroundings. R.V. Anderson Associates as prime consultant is responsible for architectural coordination and interior architecture and retained gh3 Architects to undertake the building exterior and landscape architectural design. The architectural design excellence incorporated into the project with its theme “the story of water” received a Canadian Architect Award of Excellence in 2011.

The tendering of the tunnelling component of the project was advanced beyond the project’s original aggressive schedule to ensure that the contract was “on the street” ahead of other tunnelling projects. Even with a busy tunnelling season in 2011, the bids received from the four short-listed contractors were very competitive. The tunnels, shafts and oil grit separator are completed and the treatment building will be tendered and constructed in 2012.cce

Peter Langan, P.Eng., is a principal with R.V. Anderson Associates and project manager of the West Don Lands project.

Client/Owner: Waterfront Toronto

Prime consultant: R.V. Anderson

Associates (Peter Langan, P. Eng.,

Geneviève Kenny, P. Eng., Tyler

Lahti, P. Eng., Grazyna Krezel)

Geo-environmental engineering:

Conestoga Rovers and Coffey

Exterior building architecture

and landscape: gh3

Contractor: Eastern Construction

(construction manager), C&M McNally (tunnels and shafts), Varcon

(oil-grit separator)