7. description of the preferred alternative

Toronto and Region Conservation Authority and the City of Brampton Downtown Brampton Flood Protection Environmental Assessment Environmental Study Report

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7. Description of the Preferred Alternative

7.1 Project Components The Preferred Alternative consists of the following components to improve conveyance:

Widening and deepening the By-pass channel by approximately 50 m and approximately 1.2 m, respectively

Re-aligning Ken Whillans Drive and re-grading Church Street and the valley

Bridge replacements at Church Street, Scott Street, and Queen Street

Modifications to the CN rail bridge, should TRCA and the City be responsible for implementation

Relocation or replacement of the pedestrian bridge in Centennial Park

These components are shown in Figure 7.1, on the grading plan contained in Appendix F, and described below.

7.1.1 By-Pass Channel

The design of the By-pass channel is based on the two-dimensional hydraulic modelling completed (Appendix G). The modelling used the following assumptions to meet the conveyance requirements while minimizing property impacts:

Roughness of 0.013 (Manning’s n co-efficient)

Design flow of Regulatory Flood plus 0.5 m freeboard (which is equal to a 25% contingency for climate change or future development)

Side slopes at 2:1 ratio (horizontal to vertical)

The modelling also makes assumptions regarding infrastructure intersecting or adjacent to the By-pass channel:

Sanitary trunk sewer along the right bank: Ensure that alignment maintains integrity, minimum cover, and maintenance access.

Roads: Ensure the alignment does not compromise existing roads. For example, a 2 m offset was provided from the curb line of Scott Street/Nelson Street east of the By-pass channel.

Railway limitations: Widening of the CN rail crossing must be to the east due to signaling and safety issues related to adjacent intersections.


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Figure 7.1: DBFP Project Conceptual Design


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Similar to its current condition, the widened By-pass channel will include a low flow channel sized to convey base flow and the surrounding channel that will convey storm events. The low flow channel may be designed to have a naturalized appearance (i.e., a sinuous shape with some vegetation), but will be hardened to ensure that it does not erode or move within the surrounding channel. In order to maximize flood conveyance, the rest of the By-pass channel will be constructed using a material slightly rougher than concrete (e.g. armourstone, stamped concrete, etc.), with minimal vegetation below the design flow water elevation to not impede flow. There are no restrictions on vegetation plantings above the design flow water elevation.

Preliminary cross-sections of the proposed By-pass channel, including the low flow channel, are shown through the valley system upstream of Church Street (Figure 7.2), at Church Street (Figure 7.3), Scott Street (Figure 7.4), Queen Street (Figure 7.5), and the CN rail bridge (Figure 7.6). The cross-sections maintain the existing 2:1 slope for the sides of the By-pass channel. The design of the By-pass channel may be modified provided that the overall conveyance objectives related to flow are maintained3 and the width at the top of the channel does not increase. Geotechnical investigations must be completed during Detailed Design which will inform construction methods and help determine stabilization requirements for the walls of the By-pass channel.

Upstream of the By-pass channel, the natural channel will be lowered and re-naturalized to match the grade of the By-pass channel. As shown in Figure 7.2, the existing top of bank width at the upstream end of the natural channel is not anticipated to change significantly (ranging from 28 to 35 m), while the low flow channel will be approximately 3 m to 5 m wide and 1 m in depth and lowered slightly compared to the existing natural channel. The low flow channel will gradually transition to meet the depth of the new concrete By-pass channel.

At the Church Street bridge, the top of bank of the By-pass channel will be widened by approximately 10 m, as shown in Figure 7.3.

Between Church Street and Scott Street, the top of bank of the By-pass channel will be widened to a total width of 50 m. The width of the By-pass channel and of the low flow channel remain roughly the same from Scott Street south to the CN rail corridor.

3. The design flow water elevation shown in the cross-sections accounts for a 25% increase in the Regulatory flow to

reflect impacts from climate change or future upstream development. Any change to the design must accommodate the Regulatory flow plus the worst of either a) half metre freeboard or b) a 25% volume increase.


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Figure 7.2: Valley System Cross-Section


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Figure 7.3: By-Pass Channel Cross-Section at Church Street


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Figure 7.4: By-Pass Channel Cross-Section at Scott Street


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Figure 7.5: By-Pass Channel Cross Section at Queen Street


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Figure 7.6: By-Pass Channel Cross Section at CN Rail


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7.1.1.1 Construction of the By-pass channel

During construction, the By-pass channel will be widened and deepened from within the existing channel to limit the need to obtain private property for construction purposes. Access from existing points north of Church Street and from Centennial Park could be provided to the upstream and downstream portions of the By-pass channel, respectively. Area parks are expected to serve as laydown areas for construction equipment and materials. Construction will be undertaken using typical construction equipment such as excavators and dump trucks.

During Detailed Design, a detailed construction staging plan will be developed and will include contingency plans in the event of large storm events.

A partial coffer dam could be used to isolate the construction area for temporary flow diversion. Alternatively, flow from upstream of the works area could be conveyed downstream by a flume (gravity pipe). Once the construction area is isolated, existing concrete would be removed, followed by excavation of earth to match future grades, and pouring of the new concrete form.

7.1.2 Valley System

As shown in Figure 7.1, a new valley system will be created between Church Street and the driveway to the retirement residence to provide flood conveyance capability. Figure 7.2 shows the proposed re-grading of the valley system, with a 1% slope heading east from the re-aligned Ken Whillans Drive to the By-pass channel. As described in Section 6, Ken Whillans Drive will be re-aligned to the west and raised by approximately 1 m compared to the existing ground elevation.

Ken Whillans Drive is proposed to remain as a two-lane Minor Collector Road and would occupy a 23 m right-of-way in accordance with the City’s road standards, shown in Figure 7.7. During Detailed Design the location of bike lanes and sidewalks or walking trails will be finalized based on input from the UDMP.

Cross-sections depicting the proposed works around Church Street are shown in Figure 7.3 and Figure 7.8. The valley system will be re-graded to the realigned Ken Whillans Drive to provide additional flood storage capacity. Church Street will be raised by up to approximately 1.25 m to provide flood protection.


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Figure 7.7: Ken Whillans Drive Preliminary Road Design


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Figure 7.8: Church Street Re-Grading


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7.1.2.1 Construction of the Valley System

Construction of the valley system will begin with clearing and grubbing of all vegetation, followed by excavation of soil in areas where the grade will be lowered. This soil will be relocated to fill in areas where the grade is being raised, where feasible, to minimize offsite disposal and import of clean fill. Once the future grades are established for the valley system, the new alignment of Ken Whillans Drive will be constructed. This will include the relocation of utilities within the existing alignment.

The existing alignment of Ken Whillans Drive will remain in place until the re-aligned Ken Whillans Drive is completed. Once the re-aligned Ken Whillans Drive is completed, the pavement will be removed along with any utilities that have been relocated and the area will be re-graded to match the proposed floodplain. Modifications to the grades at Church Street will take place prior to the removal of the existing alignment of Ken Whillans Drive.

Once all grading in the valley system is complete, the valley system will be seeded and planted with vegetation to replace what was removed during construction.

7.1.3 Church Street Bridge

The conceptual design for the Church Street bridge replacement structure is shown in Figure 7.9. The Church Street replacement bridge will have abutments spaced at 32.0 m. For a single span bridge of this length, a concrete topping slab supported by side-by-side concrete box girders presents the thinnest superstructure type achievable. Accordingly, using girders with a 175 mm thick concrete topping slab results in an overall superstructure thickness of approximately 1.175 m. The proposed bridge will have a minimum vertical clearance of 0.75 m above the design flow water elevation for the Regulatory Floodplain, plus 25% contingency.

Since a concrete slab on girder bridge can be constructed in stages, the Church Street bridge structure could remain open during removal/reconstruction at a reduced capacity of one traffic lane for both directions, which would in turn require a signalized crossing. Alternatively, the bridge may be closed for a single construction season and replaced all at one time. Closing the bridge will require a shorter construction period than keeping the bridge open during construction.

7.1.4 Scott Street Bridge

The conceptual design for the Scott Street bridge replacement structure is shown in Figure 7.10. The Scott Street replacement bridge will have abutments spaced at 35.0 m. For a single span bridge of this length, a concrete deck supported by prestressed


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Figure 7.9: Church Street Bridge Conceptual Design


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Figure 7.10: Scott Street Bridge Conceptual Design


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concrete girders presents the most cost-effective superstructure type. Accordingly, using girders with a 225 mm thick concrete deck results in an overall superstructure thickness of approximately 2.0 m. The proposed bridge will have a minimum vertical clearance of over 3.0 m above the design flow water elevation for the Regulatory Floodplain, plus 25% contingency.

Like the Church Street bridge, the Scott Street structure could remain open during removal/reconstruction at a reduced capacity of one traffic lane for both directions, which would in turn require a signalized crossing. Alternatively, the bridge may be closed for a single construction season and replaced all at one time.

7.1.5 Queen Street Bridge

The conceptual design for the Queen Street bridge replacement structure is shown in Figure 7.11. The Queen Street replacement bridge will have abutments spaced at 35.0 m. Similar to the design for Church Street, prestressed concrete girders presents the most cost-effective superstructure type. Accordingly, using girders with a 225 mm thick concrete deck results in an overall superstructure depth of approximately 2.0 m. Given the lowest point of the existing Queen Street profile above the flood zone, the proposed bridge will have a minimum vertical clearance of approximately 2.25 m above the design flow water elevation for the Regulatory Floodplain, plus 25% contingency.

The Queen Street structure could remain open during removal/reconstruction at a reduced capacity of one traffic lane in each direction.

7.1.6 CN Rail Bridge

As described in Chapter 6, discussions are underway between TRCA, the City, and CN as to which entity will design and construct any changes to the CN rail bridge. The design described below is presented in the event that TRCA and the City undertake design and construction should CN not move forward with the addition of the permanent third track.

To accommodate the widening of the By-pass channel at the CN rail bridge, a multi-cell box culvert or storm bypass culverts will be installed adjacent to the existing CN rail bridge (Figure 7.12 and Figure 7.13). The conceptual design includes three (3) multi-plate round pipe culverts of approximately 6 m in diameter being installed to the east of the existing bridge. These culverts will be used to convey water during larger storm events.


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Figure 7.11: Queen Street Bridge Conceptual Design


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Figure 7.12: CN Rail Corridor – Plan and Elevation


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Figure 7.13: CN Rail Corridor – Cross Section


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A wider By-pass channel will be required in the approach to the culverts as well as the tie-in to the downstream segment of Etobicoke Creek. The existing By-pass channel will be deepened through the existing bridge to match the proposed downstream invert. The existing bridge will be widened to accommodate a third track, allowing the bridge to remain operational, with reduced service, during construction of the proposed culverts.

The addition of the third track requires improvements to adjacent crossings, specifically the John Street at James Street at-grade crossing and at the Centre Street crossing (Figure 7.14).

Construction will need to be staged, as follows, to allow for rail traffic to run unencumbered:

Install additional span at Centre Street on the south side;

Widen existing Etobicoke Creek bridge with additional span on the south side;

Install the south 1/3 portion of the multi-cell bypass pipe culverts, with necessary shoring, to the east of the existing By-pass channel;

Extend existing third track from east of Centre street, including temporary crossovers, to east of Queen Street as a temporary diversion of existing south track;

Extend the pipe culverts across existing tracks by means of open cut and additional shoring, temporarily restricting all rail traffic to the new south track, including cancellation of daytime off-peak hourly GO service;

Reinstate the racks to the current configuration, with the third track embankment to remain to provide for permanent future third track; and

Excavate the By-pass channel.

During construction of the bypass pipe culverts, measures will be taken to protect the existing sanitary trunk sewer. The culverts will be designed to maintain the minimum cover (1.2 m) over the sanitary trunk sewer in consultation with Peel Region.

7.1.7 Pedestrian Bridge in Centennial Park

The existing pedestrian bridge in Centennial Park may need to be relocated or replaced to accommodate the changes at the CN rail bridge described above. The need for changes to the pedestrian bridge will be confirmed during Detailed Design.


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Figure 7.14: CN Rail Corridor – Improvements to John Street and Centre Street Crossings


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7.2 Summary of Design Parameters As mentioned previously, modifications to the design of the By-pass channel and other elements of the design may be feasible, provided they meet the parameters described in Table 7.1. Modifications to the design may arise during Detailed Design as a result of new information becoming available.

Table 7.1: Summary of Design Parameters

Project Component

Fixed Components of the Design Opportunities for Flexibility

By-pass channel

• Flow volume • Amount of cover overtop of

existing sanitary trunk sewer

• Channel alignment and width (50 m)

• Location and treatment (e.g. provision of fish habitat) of low flow channel within By-pass channel

• Steepness of By-pass channel side slopes (provided that future geotechnical investigations support increasing slope)

• Type and amount of vegetation planted within the low flow channel and above the Regulatory Floodline

• Treatment of By-pass channel slopes • Location of trails within and adjacent to By-pass

channel

Valley System

• Area, dimensions, and elevation of the valley system

• Alignment and profile of Ken Whillans Drive

• Type and amount of vegetation planted within valley system

• Type of park programming within valley system • Location of sidewalks, bike paths, and other

elements of Ken Whillans Drive cross-section

Bridges / Culverts

• Abutments must span design flow water elevation

• Vertical clearance above design flow water elevation

• Type of bridge • Design elements of bridge

* Note: Any flexibility with design must ensure conveyance, maintenance, and long term stability of channel are not compromised.

The UDMP is exploring opportunities for sustainable open space, built form, and mobility within the DBFP Project Location and elsewhere within downtown Brampton. Such opportunities could include new trail systems, parks, and green infrastructure features, among other public realm improvements. The UDMP process is building upon


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the design of the Preferred Alternative to identify and develop these urban design features.

As the UDMP will be completed after the Notice of Completion is issued and the ESR is submitted for public review, the ESR does not specify how the features developed through the UDMP will be integrated with the Project. The Preferred Alternative has been designed to allow flexibility to accommodate these features, such as trails adjacent to and within the By-pass channel, provided that the design parameters described in Section 7.2 are met.

Should any changes to fixed components of the design be proposed, an addendum to the ESR to assess the effects of such changes may be required.

7.3 Preliminary Cost Estimate Capital costs for this Project are estimated to be between approximately $85 million and $106 million. A breakdown of this estimate is included in Appendix H.

7. description of the preferred alternative

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