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  • Appendix B.1 Storm Servicing and Stormwater Management Memorandum

  • AECOM 410 250 York Street, Citi Plaza 519 673 0510 tel London, ON, Canada N6A 6K2 519 673 5975 fax www.aecom.com

    Memorandum - DRAFT

    Mem-2017-10-03-SWM MEMO-60507735-Final

    To Doug Law (City of London) Page 1

    CC Karl Grueneis (AECOM), Peter McAllister (AECOM)

    Subject

    Watson Park Outlet Municipal Class Environmental Assessment Storm Servicing and Stormwater Management

    From Adam Spargo. B.Sc., Chris Moon P.Eng.

    Date

    3 October, 2017 Project Number 60507735

    1. Introduction The City of London has engaged AECOM Canada Ltd. (AECOM) to complete a Municipal Class Environmental Assessment (EA) study for the replacement of the Watson Street storm sewer outlet that currently discharges into Watson Park. The EA study will consider the entire Watson Park drainage area and review existing storm sewer capacity, future intensification and infill development, areas of poor drainage and known flooding concerns. Various alternative strategies and solutions will be assessed that consider environmental and site specific constraints and public and agency input. The study will also focus on water quality and erosion control and consider the Citys ongoing rapid transit planning along the Wellington Road corridor. This memorandum provides a review of the existing drainage conditions and preliminary storm servicing alternatives for the evaluation of alternatives in the Municipal Class EA.

    1.1 Study Area

    The study area for the drainage and stormwater management investigations includes the entire Watson Park drainage area and some adjacent external catchments (refer to Figure 1). Three (3) storm sewer outlets currently discharge into Watson Park; the Watson Street outlet, Grand Avenue outlet, and Oatman Drain outlet (also referred to as Foxbar Creek). The EA will consider future redirection of external minor system catchments that could be directed to the Watson Park system to relieve capacity in adjacent storm sewers. The study area generally encompasses the area from Watson Park and Grand Avenue in the north, the CN rail line from Watson Park to Commissioners Road in the east, Commissioners Road from the CN rail line to High Street in the south, and High Street from Commissioners Road to Wellington Road in the west. The study area is located within the Upper Thames Valley Conservation Authority (UTRCA), and falls within the Central Thames Subwatershed. Watson Park is fully within the UTRCA regulated flood limit. The Watson Street outlet, along with the Oatman Drain and the Grand Avenue outlets, discharge into the Watson Park oxbow wetlands before ultimately flowing into the South Branch of the Thames River.

  • Page 2

    Memorandum October 3, 2017

    Mem-2017-10-03-SWM MEMO-60507735-Final

    Land use within the study area is predominantly low density single detached residential, with some areas of commercial development, pockets of natural heritage features, and the Victoria Hospital and surrounding lands. While the majority of the study area is developed there is potential for future redevelopment and infill development including intensification of residential areas and expansion of the Wellington Road corridor under the City of Londons rapid transit initiatives.

    2. Existing Conditions 2.1 Existing Minor System

    The local minor system drainage is conveyed via storm sewers through the study area corridor. Portions of the Oatman Drain storm sewer network follow the original Foxbar Creek alignment and are located outside of the road right-of-way on private property. The study area is flat and back lot drainage is common within existing residential developments. Under existing conditions, there are three (3) minor system outlets into Watson Park. The EA study will consider the amalgamation of these outlets, among other alternatives. The drain ranges in size from 300 mm to 1350 mm diameter, however, in some locations the storm sewer diameters decrease in the downstream direction, which is not a recommended practice. Existing catchment areas have been determined based on the City of London digital elevation model data, 1 m contours, as-built drawings, site servicing plans, and field verification. Refer to Figure 2 for catchment details. General minor system conveyance for existing conditions is as follows: Watson Street

    The Watson Street storm sewer ranges in size from 300 mm to 900 mm and services Watson Street and a portion of Wellington Road from Frank Place to Grand Avenue (sub-catchments A-01 to A-05). The catchment area is approximately 5.5 ha and consists of primarily low density residential and commercial land use. The Watson Street outlet is a 900 mm diameter concrete pipe that discharges into the historical oxbow at the western side of Watson Park. Some incidental water quality enhancement may occur in the oxbow wetland before entering the South Branch of the Thames River.

    Oatman Drain

    The Oatman Drain storm sewer follows the historic alignment of Foxbar Creek and drains an area of approximately 66 ha. The storm sewer runs along Wellington Road from Commissioners Road East to Frank Place. The system captures minor system flows from Weston Street, Bond Street, Raywood Avenue, Foxbar Road, Moore Street, Emery Street, Whetter Avenue, Thomas Janes Drive, Chester Street, Rowntree Avenue, Percy Street, Baseline Road East and Commissioners Road East. The catchment is dominated by low density residential and large commercial properties with some higher density residential areas and small pockets of undeveloped land. The catchment generally has no water quantity and quality controls with the exception of private permanent systems (PPS) providing quantity control at commercial properties 387-401 Wellington Road and 645 Commissioners Road East.

  • Page 3

    Memorandum October 3, 2017

    Mem-2017-10-03-SWM MEMO-60507735-Final

    The Oatman Drain storm sewer decreases in size from 1350 mm to 450 mm near the intersection of Rowntree Avenue and Wellington Road. The larger pipe size upstream of the 450 mm pipe is the result of previous sewer replacements and road reconstruction works. The 450 mm pipe also marks the location where the Oatman Drain storm sewer runs beneath private property. The reduction in size creates a significant decrease in the system capacity. The Oatman Drain outlet discharges into the south west corner of Watson Park providing the majority of runoff to the oxbow wetlands. Grand Avenue

    The Grand Avenue storm sewer is a 600 mm sewer that captures drainage from Grand Avenue (sub-catchment D-01). According to the City of London infrastructure database, the 600 mm sewer crosses the Watson Park oxbow twice before discharging directly to the Thames River. This unusual alignment occurs because the sewer was originally a combined sewer that discharged to the former Watson Park pollution control plant (PCP). The PCP has since been removed and the sanitary component discontinued. During field inspections, AECOM staff were not able to confirm if the full length of the Grand Avenue storm sewer is in operation. It is possible that the downstream portion of the sewer has been abandoned and the outlet discharges directly to the oxbow wetland at the northern boundary of Watson Park.

    Victoria Hospital

    A review of the City of Londons storm sewer database and the Victoria Hospital site servicing plan identified that the storm sewer running along Wellington Road, from Wilkins Street to Commissioners Road East (sub-catchment P-01), is connected to the Victoria Hospital private storm sewer. This sewer crosses the hospital grounds from south west to north east and connects to a 1650 mm storm sewer located in the parking lot east of Baseline Road. The 1650 mm pipe conveys the minor system east to a storm water management facility that ultimately discharges to Dayus Creek. While the Wellington Road storm sewer from Wilkins Street to Commissioners Road East is not currently connected to the Oatman Drain, it is the Citys intention that this catchment will be reconnected to the future Wellington Road South trunk storm sewer.

    2.2 Existing Minor System Capacity

    The purpose of this study is to define the required sizing of the Watson Street outlet to accommodate future expansion of the Watson Street and Wellington Road storm sewer system. This could include replacement of the Oatman Drain and Watson Street systems with a trunk storm sewer, increased development in the catchment, and redirecting external catchments into the system. The existing outlet sewer capacity and 2-year design flow was reviewed for all outlets, and is summarized in Table 1 below. Refer to Appendix B for detailed calculations. These capacities have been determined using the City of London storm sewer design sheet.

  • Page 4

    Memorandum October 3, 2017

    Mem-2017-10-03-SWM MEMO-60507735-Final

    Table 1. Existing Storm Sewer Capacity

    Storm Sewer Existing Catchment Area

    (ha) Existing Outlet Capacity

    (L/s) 2-Year Design Flow

    (L/s)

    Watson Street (900 mm) 5.5 1,425(1) 620

    Oatman Drain (1200 mm) 65.9 5152 7887

    Grand Avenue (650 mm) 1.0 405 110 1. The current 900 mm diameter outlet has additional capacity available; however, the 600 mm diameter sewer immediately upstream of the outlet is undersized. Based on the outlet capacity analysis, residual capacity is available in the Grand Avenue and Watson Street outlets, however, in the Watson Street sewer the 600 mm diameter sewer immediately upstream of the outlet is currently undersized and could not receive additional flows. The Oatman Drain outlet is currently undersized for the 2-year design flow along with an additional 1,610 m of t