Appendix B.1. Storm Servicing and Stormwater Management Memorandum

Transcription

1 Appendix B.1 Storm Servicing and Stormwater Management

2 AECOM 410 York Street, Citi Plaza tel London, ON, Canada N6A 6K fax DRAFT To Doug Law (City of London) Page 1 CC Subject Karl Grueneis (AECOM), Peter McAllister (AECOM) 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 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 City s 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. Mem SWM MEMO Final

3 Page 2 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 London s 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 rightofway 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 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, asbuilt 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 mm to mm and services Watson Street and a portion of Wellington Road from Frank Place to Grand Avenue (subcatchments A01 to A05). 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 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 Wellington Road and 645 Commissioners Road East. Mem SWM MEMO Final

4 Page 3 The Oatman Drain storm sewer decreases in size from 1350 mm to mm near the intersection of Rowntree Avenue and Wellington Road. The larger pipe size upstream of the mm pipe is the result of previous sewer replacements and road reconstruction works. The 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 mm sewer that captures drainage from Grand Avenue (subcatchment D01). According to the City of London infrastructure database, the 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 London s 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 (subcatchment P01), 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 City s 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 2year 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. Mem SWM MEMO Final

5 Page 4 Table 1. Existing Storm Sewer Capacity Storm Sewer Existing Catchment Area (ha) Existing Outlet Capacity (L/s) 2Year Design Flow (L/s) Watson Street ( mm) 5.5 1,425 (1) 620 Oatman Drain ( mm) Grand Avenue (650 mm) The current mm diameter outlet has additional capacity available; however, the 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 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 2year design flow along with an additional 1,610 m of the upstream trunk sewer also undersized. In particular, the reduction in pipe diameter at the intersection of Wellington Road and Rowntree Avenue from 1350 mm to mm significantly reduces the sewer capacity. Significant upgrades are required in this area to prevent sewer surcharging and potential flooding. Based on a review of the storm sewer network and field verification of site conditions, it is recognized that portions of the Oatman Drain outlet drainage system are performing below current design standards. This has led to historical basement flooding within the catchment area. The following conditions exist: Undersized storms sewers along the Wellington Road corridor; 2year design storm flows conveyed overland on Wellington Road and through private properties; Storm sewers with diameters decreasing in the downstream direction; Storm sewers located on private property with limited access and no easement; and Limited formal stormwater management systems for quality control currently in place along the corridor. 2.3 Existing Major System The existing major system is shown in Figure 3. Within the study area, major flow routes are along the road and follow the general path of the existing storm sewer with the exception of Oatman Drain from Moore Street to its outlet at Watson Park. Through this area the major system follows the historical alignment of Foxbar Creek and is located on private property for a large portion of the remaining length. Several sag locations are present within the Oatman Drain major system that force flow overland across private property. The sag locations are as follows: Emery Street at Wellington Road; Wellington Road at Moore Street; Wellington Road at Bond Street; Wellington Road at Weston Street; and Wellington Road at Watson Street. Mem SWM MEMO Final

6 Page 5 Removal of the sag locations or high points is required to prevent spill onto private property and potential basement flooding. Alternatively, additional capacity could be provided in the minor system storm sewers to convey the 100year event in areas of flooding concern. 3. Proposed Conditions The purpose of the EA study is to review alternatives for the upgrade of the Watson Street storm sewer outlet. These alternatives consider the following potential improvements: Abandonment of the Oatman Drain storm sewer and redirection of minor and major flows to the Watson Street outlet; Increased imperviousness in the Watson Street outlet and Oatman Drain catchment areas due to infill development/intensification, and future rapid transit planning; and Providing water quality control for existing development and future road widening. 3.1 Design Criteria Stormwater Management Relevant standards and design guidelines used in the development of the stormwater management strategy include: The Ontario Ministry of the Environment Stormwater Management Planning and Design Manual (2003); The Draft Central Thames Subwatershed Study Delcan 2014; The City of London Private Permanent Systems Policy (2010); The City of London Design Standards (2017); A comparison of future IDF curves for Southern Ontario, Coulibaly et al, February and Low Impact Development Stormwater Management Planning and Design Guide (CVC, 2010); It is noted that the Ontario Ministry of Environment and Climate Change (MOECC) have recently released an Interpretation Bulletin in February 2015 regarding the ministry s expectations on stormwater management and the use of LID practices. The MOECC recognize that conventional stormwater management practices (pipe and pond) that focus on peak flow mitigation and water quality, do not fully achieve watershed protection due to increased volume of runoff and water balance requirements. Moving forward stormwater management plans submitted for Environmental Compliance Approvals (ECAs) will consider the preservation of the natural hydrology through the use of LIDs and other source controls, where practical, feasible and appropriate. The Ministry have also clarified that LIDs are applicable and will be expected to be applied on soils with low infiltration rates (less than 15 mm/hr). The MOECC is currently preparing a LID guidance document that is expected to be release in late Ahead of this document, the CVC s guidelines for LID (2010) have been used for the evaluation of LID as a SWM alterative. Mem SWM MEMO Final

7 Page 6 The 2016 document A comparison of future IDF curves for Southern Ontario includes the following conclusion: Based on the ensemble of results developed through this study, the variability among future projections should be regarded as significant, resulting in a high level of uncertainty with respect to the future IDF curves. This high level of variability is associated with levels of uncertainty, which is greatest for shortduration and moderatetohigh return period events, for example the 15 minute to 1hour 25 to 100year storms, making it difficult to interpret a single design threshold for water management applications. Based on this study s results, it is recommended that further study is needed in the selected study areas to better understand and refine the uncertainties involved in the future IDF statistics. This appears necessary before major change in infrastructure design standards in the study areas. The City of London has updated their design IDF curves in The update includes the most recent climate data and trends and has been adopted for the purpose of developing design concepts. However, consideration of climate change resiliency has been incorporated in general terms. SWM objectives have been defined for the study area: Erosion Control: Ensure that there is no adverse impact from an increase in rates and volume of runoff; Water Quantity Control: The existing outlet discharges to the oxbow wetland at Watson Park which discharges directly to the South Branch of the Thames River. Quantity controls are not required for discharges to the South Branch of the Thames River, as the study area peak flow will occur significantly earlier than peak flows from the South Branch watershed; and Water Quality Control: Ensure that there is no adverse impact in water quality. There are currently no water quality controls provided in the Watson Street, Oatman Drain and Grand Avenue outlets drainage areas. The Central Thames Subwatershed Study (Dillon 2014) requires normal level water quality control (70 % TSS removal) for the Oatman Drain catchment. Strategic implementation of water quality control is recommended to improve existing conditions where practical. Infill and intensification within the catchment will trigger quality control measures as per the City of London Private Permanent System policy. Water Balance: Best efforts for improvements to the water balance will be made where practical, feasible and appropriate. Consideration of resiliency to climate change. Conveyance The minor system will be designed to convey the 2year storm and the major system the 100year storm safely with ponding no greater than mm depth. Localized upsizing of the storm sewers may be required to also accommodate 100year major flows where high or low points along Wellington Road and adjacent streets cannot be removed. Mem SWM MEMO Final

8 Page Opportunities and Constraints Watson Park Aquatic Habitat The Watson Street, Oatman Drain and Grand Avenue outlets provide the primary water supply for the oxbow wetlands within Watson Park. Maintaining the existing hydrologic regime is preferred to ensure the longterm sustainability of the wetland ecosystems. Relocating the Oatman Drain outlet could reduce or increase runoff volume to certain areas of the oxbow wetlands. Alternatives should consider maintaining the existing hydrologic regime at these sites. Opportunities to maintain and enhance the receiving water quality to the Watson Park oxbow wetlands are preferred for aquatic habitat enhancement and preservation. There are currently no water quality controls within the Watson Street outlet, Oatman Drain outlet and Grand Avenue outlet catchments. Improvement in water quality can be achieved through the strategic implementation of oil grit separators (OGS), best management practices (BMPs) (e.g. grassed swales, vegetated filter strips, pervious pipe systems, and infiltration basins) and/or end of pipe solutions such as a stormwater management facility (SWMF). The maximum treatment area for an OGS is typically 5 ha, therefore OGS may not be feasible for quality control in the Oatman Drain catchment because of the large number of units required. Future Development The Oatman Drain catchment area is comprised primarily of single detached residential landuse with some areas of higher density townhouses and commercial plazas. Infill and intensification of landuse will trigger the implementation of the City s private permanent systems policy (PPS) for stormwater management, providing water quality and quantity control for these areas. The City of London s rapid transit planning includes a rapid transit route along Wellington Road. The introduction of a rapid transit line will require widening of Wellington Road with a total of up to six (6) lanes. Rapid transit will also likely promote intensification along the corridor. Widening Wellington Road would result in an increase in imperviousness (< 5%) that would lead to an increase in peak flows, a decrease in water quality, and a change in water balance conditions (increased runoff volume and decreased infiltration and evaporation). Because Oatman Drain discharges directly to the Thames River, peak flow control will not be required. The decrease in water quality through increased sediment loading and spill contamination could be addressed through oil grit separators and BMP s developed during future rapid transit projects. Alternatively, this increase could be addressed through an endofpipe system in Watson Park also designed to retrofit the existing catchment and provide water quality control for future infill development. The change in water balance conditions would need to be addressed through LID strategies which are discussed further below. Wellington Road Low Impact Development (LID) Options The follow opportunities and constraints exist for the implementation of LID strategies to address water balance requirements for the future widening of Wellington Road: Mem SWM MEMO Final

9 Page 8 The proposed Wellington Road crosssection will have a narrow boulevard and will limit opportunities for inclusion of LIDs within the boulevard landscape. Wellington Road is an arterial road providing a major transportation corridor within the city. This increases the potential for spill contamination and overall suspended soils loading rates to potential LIDs. OGS units will address this concern; Soils within the study area were determined from bore hole logs obtained during the 2017 Infrastructure Renewal Program Contract 9 (Base Line Road East) and are comprised of clayey silt or sand / gravel fill over clayey silt native material. The soils are anticipated to have a moderate hydraulic conductivity promoting infiltration; The bore hole logs also identified the groundwater table to be greater than 6.0 m in the southern portion of the study area (Base Line Road East) and is anticipated to be at a significantly lower elevation than any potential LID inverts. Shallower groundwater may be present in the northern portion of the study area in the vicinity of the South Branch of the Thames River, specifically in Watson Park which is within the regional flood plain; and The Thames Sydenham & Region Drinking Water Source Protection Mapping identifies some parts of the study area are located in a Highly Vulnerable Aquifer and Significant Ground Water Recharge area, however it is not identified as being in a Well Head Protection zone. Based on these opportunities and constraints implementation of LID strategies is limited. An exfiltration LID solution could be employed to increase infiltration under proposed conditions and address the increase in runoff volume. The exfiltration system would be comprised of a perforated pipe beneath the boulevard, curb line or travel lane. The objective would not be to match a predevelopment infiltration target or minimum retention target but rather to achieve best efforts from a practical and implementable standpoint. Further detail on the exfiltration trench is provided in Section 3.3. It would be proposed to incorporate LID to address water balance and reduce the volume of runoff particularly for frequent events while maintaining the 2year design storm criteria for the local and trunk storm sewers. This would introduce redundancy into the design and increase resilience to climate change. Realigning Oatman Drain The Oatman Drain storm sewer is not fully contained within the road rightofway with approximately 1.1 km located on private property. This creates future maintenance concerns and can lead to additional basement flooding where surcharging of the minor system occurs on private property. Realigning the Oatman Drain storm sewer to Wellington Road provides the opportunity to improve capacity to meet the City of London design standards and develop a dedicated trunk storm sewer for the Wellington Road corridor. Although the current alignment is located within private property, easements are not currently in place. Realigning Oatman Drain would remove the requirement for future easements. Upper Thames Region Conservation Authority Planning Policy UTRCA regulations stipulate that SWM facilities, with the exception of outlets, should be directed to areas located outside of the defined limits of the natural hazard, which includes the regulatory flood plain, unless it can be demonstrated that there is a net public benefit in selecting the flood plain location and if all other potentially viable locations have been dismissed. In this case, the Watson Park location is the only viable area where sufficient open space is available for the location of a Mem SWM MEMO Final

10 Page 9 SWM facility as the upstream catchment is fully developed. The SWM facility would provide a net benefit to water quality for the currently untreated 72 ha of the Oatman Drain and Watson Street drainage areas. The UTRCA regulations also state that the encroachment of SWM facilities in to the flood plain must be justified with a catchment scale assessment as part of a Catchment Strategy, Area Plan, Subwatershed Plan, Master Drainage Plan or Environmental Assessment Act process. In this case, the location of the SWM facility is being assessed under the current Municipal Class EA. City of London Works Yard The City of London operates a works yard within Watson Park that includes laydown areas for construction materials and snow storage areas for winter snow removal. Any proposed works within Watson Park, including a SWMF, will reduce the available area for the works yard. Development and assessment of alternatives are to have regard for the use of this area by the City. Back to the River The Back to the River design competition includes Watson Park in future planning for the South Branch of the Thames River corridor. Any proposed works within Watson Park should consider the interaction with the future use of any park for public activity and enjoyment. Increased Storm Sewer Capacity Similar to the Oatman Drain, trunk storm sewers in immediately adjacent catchment areas have capacity issues leading to historic flooding concerns. Upsizing and realigning the Oatman Drain and the Watson Street outlet provides an opportunity to redirect minor system flows from neighbouring trunk storm sewers to the Oatman Drain catchment to alleviate capacity concerns. Sizing of the Watson Park outlet considers additional catchment areas where feasible. The following table indicates some of the key aspects and their opportunities and constraints as they pertain to storm servicing and stormwater management. Table 2. Stormwater Opportunities and Constraints Key Aspect Opportunity Constraint Future Development / Rapid Transit Retrofit quantity and quality control to catchment PPS triggers stormwater management for infill / intensification development. Upsize and relocate existing storm sewers to meet minimum standards Potential for cost sharing with the rapid transit project Opportunity to introduce a Wellington Road local and trunk storm sewer servicing arrangement Increase in storm runoff Existing infrastructure condition Identified basement flooding No water quality controls in catchment No water balance controls in catchment Mem SWM MEMO Final

11 Page 10 Key Aspect Opportunity Constraint Wellington Road Low Impact Development (LID) Options Soils have moderate to high hydraulic conductivity promoting infiltration Study are is not located within a well head protection zone Groundwater table is anticipated to be significantly lower than proposed LID inverts Meets policy directives from MOECC with respect to water balance. Introduces climate change resilience. Proposed Wellington Road crosssection constrains inclusion of LIDs within the boulevard landscape Wellington Road is an arterial road and major transportation corridor in London which increases potential spill contamination and sediment loading. Highly Vulnerable Aquifer present within the study area Realignment of Oatman Drain Develop new trunk storm sewer Remove existing storm sewers from private properties Reduce/alleviate basement flooding Reduced maintenance cost Direct flows to new outlet with scour protection Outlet Location Rationalization of the number of outlets and Retrofitting Water Quality Controls Back to the River Design Contest Watson Park Works Yard Increased Storm Sewer Capacity location of outlets for maintenance Reduced maintenance costs Improve scour protection at Oatman Drain Outlet Improve water quality of receiving waters Meet provincial targets for quality control Enhance water quality, improve downstream ecological function Provide improved aesthetics in Watson Park to meet with the design objectives of the Back to the River plan Provide water quality treatment for existing works yard and snow storage runoff Alleviate / reduce basement flooding in upstream catchment Reduce overland flow on Wellington Road and through private properties Alleviate capacity issues for neighbouring trunk storm sewers Provide capacity for future infill development and intensification Potential conflicts with existing watermains, sanitary sewers, and sanitary forcemain Capital cost Existing grading Existing condition Maintain downstream slope stability Maintain water source to oxbow wetlands Relocation of City Works yard Approvals Capital cost Encroachment of City Lands Duplication of City PPS policy to address infill and intensification. Approvals Maintenance cost Capital cost Impacts future plans for Watson Park by removing land for SWM controls Reduce available land for works yard and snow storage May require relocation of works yard Conflicts with existing infrastructure Maintenance cost Capital cost Mem SWM MEMO Final

12 Page Preliminary Storm Servicing and Stormwater Management Alternatives Preliminary storm servicing and stormwater management alternatives have been developed in consideration of social, economic, natural heritage and technical constraints. Storm servicing may include overland flow routes, storm sewers and/or SWMFs. Prescreening The implementation of multiple oil grit separator (OGS) units throughout the Oatman Drain catchment to treat the various lateral storm sewers feeding the Oatman Drain trunk sewer was initially considered. The maximum catchment area for an OGS unit is typically 5 ha. Initial evaluations identified that the lateral sewers feeding the Oatman Drain trunk storm sewer range in catchment area from 2.5 ha to 8.3 ha. Based on this area each lateral sewer would require a dedicated OGS unit resulting in approximately 14 OGS units within the Oatman Drain catchment. Due to the large number of OGS units required, retrofitting the existing Oatman Drain catchment with OGS units to provide water quality control was not considered feasible and further evaluation has been screened. The following preliminary alternatives were considered. Alternative 1: Do Nothing Replace Existing Watson Street Storm Sewer Outlet No solution would be implemented to improve stormwater servicing or water quality control for the study area and a new Watson Street outlet would be sized to convey the 2year event for the existing Watson Street outlet drainage area. Oatman Drain will continue to discharge at the existing location. The existing mm outlet would be replaced with a similar sized outlet pipe with headwall. Summary Constraints for this solution are: The Watson Street outlet would not provide capacity for future realignment and upsizing of the Oatman Drain trunk storm sewer; No provision for future widening of Wellington Road for rapid transit; No additional capacity provided for future infill development; No retrofitting of water quality controls to the study area would be provided in accordance with provincial policy; and The existing outlet provides sufficient capacity for the 2year event, however, some upsizing of the Watson Street storm sewer would be required. Mem SWM MEMO Final

13 Page 12 Alternative 2: Relocate Outlet to the Wellington Road Bridge, Realign Oatman Drain and Provide SWM Controls for Wellington Road RightofWay Only (OGS Units and LID Strategies) Storm servicing could be addressed via installation of a trunk storm sewer along Wellington Road to the Wellington Road bridge, installing a new outlet at the Wellington Road bridge discharging directly to the South Branch of the Thames River and abandoning the existing Watson Street, Oatman Drain and Grand Avenue outlets as part of the new trunk storm sewer upgrades. A separate local storm sewer would also be installed on Wellington Road to allow water quality control via OGS units for a widened Wellington Road rightofway under rapid transit planning. Water quality control would only be provided to the Wellington Road corridor. No retrofitting of the existing Oatman Drain catchment would be provided and future infill development and intensification would require private permanent system (PPS) controls. Further efforts would also be made to remove the overland flow path from private property through the removal of sag locations along Wellington Road and/or the use of oversized storm sewers to convey the 100year event in sections of the Wellington Road South trunk storm sewer. Watson Street Outlet The Watson Street storm sewer would be abandoned and replaced with the Wellington Road South trunk storm sewer. The existing outlet would also be abandoned and a new outlet constructed at the Wellington Road bridge. The new outlet could be upsized up to a 2550 mm outlet sewer, complete with a headwall subject to detailed design. The increase in flows to the new outlet location could lead to increased erosion and scour downstream of the outlet. Energy dissipation scour bowls and backwater pools for aquatic habitat enhancement would be recommended to be integrated into the new storm outlet if appropriate and feasible. Moving the outlet away from Watson Park could starve the oxbow wetlands of base flow and lead to deterioration of these natural heritage features. Base flow could be provided through secondary low flow outlet that runs from Wellington Road, along Watson Street to the existing Watson Street outlet location. A local Wellington Road storm sewer would also be installed within the Wellington Road rightofway. The local storm sewer would connect to the Wellington Road South trunk storm sewer at four locations: Intersection of Commissioners Road and Wellington Road (catchment = 3.0 ha); Intersection of Moore Street and Wellington Road (catchment = 4.1 ha); Intersection of Watson Street and Wellington Road (catchment = 2.2 ha); and Intersection of Front Street and Wellington Road (catchment < 1.5 ha). An OGS unit would be installed at each location to provide water quality control for a future widened Wellington Road rightofway. Oatman Drain Outlet The Oatman Drain from Percy Street to the outlet at Watson Park would be abandoned and replaced with the Wellington Road South trunk storm sewer. The Oatman Drain outlet would also Mem SWM MEMO Final

14 Page 13 be abandoned. The new outlet location at the Wellington Road bridge would be more readily accessible for construction and maintenance than the current Oatman Drain outlet location. Grand Avenue Outlet The Grand Avenue storm sewer outlets directly to the Watson Park at the east end of Grand Avenue. The existing outlet would be abandoned and the storm sewer would instead be directed west to Wellington Street and connected to the Wellington Road South trunk storm sewer. Existing Wellington Road Bridge Outlet An existing 1500 mm storm sewer outlet is located at the Wellington Road bridge. The storm sewer services an area of approximately 14.7 ha including Wellington Road, High Street and Grand Avenue (west of Wellington Road). The existing outlet and associated storm sewer along Wellington Road would be abandoned and the system incorporated into the new Wellington Road South trunk storm sewer. Additional Drainage Areas Five external subcatchments have been considered for addition to the Wellington Road South trunk storm sewer; P01, P02, P03, P04, P05 as shown in Figure 5. The combined catchment area is 31.8 ha. P01 is located south of Commissioners Road East and currently flows through the Victoria Hospital private storm sewer. Catchment P02 is currently undeveloped land owned by London Health Sciences and flows through the Victoria Hospital private storm sewer. Under ultimate conditions it has been assumed that catchment P02 would be fully developed as commercial or industrial landuse and a runoff coefficient of 0.9 has been applied. Catchments P 03 and P04 are located along High Street and could be redirected along Baseline Road and Thomas Janes Drive, respectively. Catchment P05 is located at Windsor Crescent. The minor system for P05 could be directed down Frank Place to the Wellington Road South trunk storm sewer. Storm Servicing Preliminary trunk storm sewer sizing was determined using the City of London storm sewer design sheet. The Wellington Road South trunk storm sewer would be located fully within the Wellington Road rightofway, and range in size from 1650 mm to 2550 mm. It is expected that the additional capacity should alleviate some basement flooding concerns where the existing storm sewer is undersized and is located on private property. Three local Wellington Road storm sewers would be installed to reduce the local sewer catchment area to a maximum of 6 ha to allow water quality treatment using OGS units. The local storm sewers would range in size from mm to 825 mm. Storm sewer sizing would account for infill development within the Watson Street and Oatman Drain catchments. Infill development was assumed to be a transition from predominantly low density residential (runoff coefficient = 0.5) to a combination of low, medium and high density residential (average runoff coefficient = 0.7). For subcatchment B01 it was assumed that the Victoria Hospital lands would be fully developed with a high imperviousness (90 %). Subcatchments where runoff coefficients were increased to account for the change in land use are shown in Table 3 and identified in Figure 5. Mem SWM MEMO Final

15 Page 14 Table 3. SubCatchment Existing and Proposed Runoff Coefficients SubCatchment Catchment Area (ha) Existing Runoff Coefficient Proposed Runoff Coefficient A A A A B B07a B07b B B B B B C D * Only catchments with a change in runoff coefficient area included. There are several sag locations along Wellington Road that create trapped low points for the overland flow path. During detailed design, further assessment should be completed to determine if the sag locations can be removed to allow overland flow to be contained within the road rightofway or if the Wellington Road South trunk storm sewer should be upsized to provide capacity for the major system (100year event) to alleviate ponding in these locations. Storm Water Management Controls Water quantity control is not required for catchments discharging directly to the South Branch of the Thames River due to the much longer time to peak for the South Branch of the Thames River at this location. MOE Level 2 protection (70% total suspended solids (TSS) removal) is required for the Oatman Drain / Fox Bar Creek catchment (Delcan, 2014). Water quality control would be provided for the Wellington Road local storm sewer only. Four (4) OGS units would be sized to provide MOE Level 2 protection. No additional retrofitting of water quality controls would be provided. The proposed location of OGS units is shown in Figure 5. Water quality control calculations accounted for an increase in the area of the Wellington Road pavement of 4.2 ha to account for up to 6 lanes of traffic under rapid transit planning. Low Impact Development (LID) Strategies The increase in runoff volume associated with the widening of Wellington Road would be addressed with a LID exfiltration trench. Initial flows would be directed to the exfiltration trenches comprising of perforated pipe wrapped in geotextile surrounded by gravel, and plugged at the downstream end. A conceptual arrangement of the exfiltration trench is shown in Figure 4. Flow from Wellington Road would be captured in catchbasins and conveyed through a local storm sewer to an OGS unit for pretreatment. The OGS unit would discharge to a manhole containing an diversion arrangement Mem SWM MEMO Final

16 Page 15 comprised of a sump, an outlet to the trench at a lower elevation, and an outlet to a lateral storm sewer at a higher elevation. Once the capacity of the trench is exceeded, flows would be directed to the Wellington Road trunk storm sewer via the lateral outlet. Figure 4. General Arrangement for the Exfiltration Trench The sizing of the exfiltration trench and perforated pipe would be determined during detailed design and would be sized to provide best efforts in mitigating impacts to the water balance. Some key considerations in the design will include: Confirmation of soil and groundwater conditions. The Wellington Road lateral and main storm sewers will be designed and sized with no consideration for flow reduction provided by the LID (assumes exfiltration system has failed and/or abandoned); OGS units will provide pretreatment to limit loading to the LID; Appropriate spacing of catchbasins to manage drainage areas directed to the exfiltration system; Orientation of perforations to limit clogging; Consideration of maintenance activities to allow flushing of the system with existing City operational capabilities; and Appropriate grade of perforated pipe to encourage sediment accumulation at the downstream end for ease of maintenance. Summary Constraints for this solution are: Existing Wellington Road profile may require steep storm sewer pipes and / or drop structures to provide sufficient cover and grade for proposed trunk storm sewer; Sag locations on Wellington Road will need to be removed to provide an appropriate overland flow path for the major system without causing flooding of adjacent properties. If sag locations cannot be removed, sections of oversized pipe will be required to convey the 100 year storm; Upsizing of existing sewers is required to provide sufficient capacity to convey the 2year storm; Potential conflicts with future rapid transit corridor; The opportunity for both local and trunk storm sewers along Wellington Road is dependent on the final rapid transit design and proposed crosssection; LID opportunities are dependent on local site conditions and will be investigated during detailed design; Potential conflicts with the watermain, sanitary sewer and utilities along Wellington Road; Retrofitting water quality controls to the Oatman Drain catchment area will not be provided and no benefit to water quality will be achieved. Future water quality control for infill development would need to be addressed through PPS; Potential for erosion and channel instability at the outlet location to Watson Park; and Mem SWM MEMO Final

17 Page 16 Potential to starve the oxbow wetlands of base flow by amalgamating the three (3) Watson Park outlets. Alternative 3: Realign Oatman Drain and Provide SWM Controls for Wellington Road RightofWay Only (OGS Units and LID Strategies) Storm servicing could be addressed via the installation of a trunk storm sewer along Wellington Road to the existing outlet at Watson Street, abandoning the existing Oatman Drain and Grand Avenue outlets and upsizing and replacing the Watson Street outlet as part of the new trunk storm sewer. A separate local storm sewer would also be installed on Wellington Road with water quality control for a future widened Wellington Road rightofway for rapid transit provided by two OGS units. Water quality control would only be provided to the Wellington Road corridor. No retrofitting of the existing Oatman Drain catchment would be provided and future infill development and intensification would require private permanent system (PPS) controls. Further efforts would also be made to remove the overland flow path from private property through the removal sag locations along Wellington Road and/or the use of oversized storm sewers to convey the 100year event in sections of the Wellington Road South trunk storm sewer. Watson Street Outlet The Watson Street storm sewer would be abandoned and replaced with the Wellington Road South trunk storm sewer. A new outlet will be constructed at the existing Watson Street outlet location. The new outlet could be upsized up to a 2550 mm diameter outlet sewer, complete with a headwall subject to detailed design. The increase in flows to the new outlet location could lead to increased erosion and scour downstream of the outlet. Energy dissipation scour bowls and backwater pools for aquatic habitat enhancement would be recommended to be integrated into the new storm outlet where appropriate and feasible. A local Wellington Road storm sewer would also be installed within the Wellington Road rightofway. The local storm sewer will connect to the Wellington Road South trunk storm sewer at two (2) locations: Intersection of Commissioners Road and Wellington Road (catchment = 3.0 ha); Intersection of Moore Street and Wellington Road (catchment area = 4.1 ha); and Intersection of Watson Street and Wellington Road (catchment area = 2.2 ha). An OGS unit would be installed at both locations to provide water quality control for a future widened Wellington Road. Oatman Drain Outlet The Oatman Drain from Percy Street to the outlet at Watson Park would be abandoned and replaced with the Wellington Road South trunk storm sewer. The Oatman Drain outlet would also be abandoned. The new outlet location at Watson Street or the Wellington Road bridge would be more readily accessible for construction and maintenance than the current Oatman Drain outlet location. Grand Avenue Outlet Mem SWM MEMO Final

18 Page 17 The Grand Avenue storm sewer outlets directly to the Watson Park at the east end of Grand Avenue. The existing outlet would be abandoned and the storm sewer would instead be directed west to Wellington Street and connected to the Wellington Road South trunk storm sewer. Additional Drainage Areas Five external subcatchments have been considered for addition to the Wellington Road South trunk storm sewer; P01, P02, P03, P04, P05 as shown in Figure 6. The combined catchment area is 31.8 ha. P01 is located south of Commissioners Road East and currently flows through the Victoria Hospital private storm sewer. Catchment P02 is currently undeveloped land owned by London Health Sciences and flows through the Victoria Hospital private storm sewer. Under ultimate conditions it has been assumed that catchment P02 would be fully developed as commercial or industrial landuse and a runoff coefficient of 0.9 has been applied. Catchments P 03 and P04 are located along High Street and could be redirected along Baseline Road and Thomas Janes Drive, respectively. Catchment P05 is located at Windsor Crescent. The minor system for P05 could be directed down Frank Place to the Wellington Road South trunk storm sewer. Storm Servicing Preliminary trunk storm sewer sizing was determined using the City of London storm sewer design sheet. The Wellington Road South trunk storm sewer would be located fully within the Wellington Road rightofway, and range in size from 1650 mm to 2550 mm. It is expected that the additional capacity should alleviate some basement flooding concerns where the existing storm sewer is undersized and is located on private property. Two local Wellington Road storm sewers would be installed to reduce the catchment area to a maximum of 6 ha to allow water quality treatment using OGS units. The local storm sewers would range in size from mm to 825 mm. Storm sewer sizing would account for infill development within the Watson Street and Oatman Drain catchments. Infill development was assumed to be a transition from predominantly low density residential (runoff coefficient = 0.5) to a combination of low, medium and high density residential (average runoff coefficient = 0.7). For subcatchment B01 it was assumed that the Victoria Hospital lands would be fully developed with a high imperviousness (90 %). Subcatchments where runoff coefficients were increased to account for the change in land use are shown in Table 3 and identified in Figure 6. There are several sag locations along Wellington Road that create trapped low points for the overland flow path. During detailed design, further assessment should be completed to determine if the sag locations can be removed to allow overland flow to be contained within the road rightofway or if the Wellington Road South trunk storm sewer should be upsized to provide capacity for the major system (100year event) to alleviate ponding in these locations. Storm Water Management Controls Water quantity control is not required for catchments discharging directly to the South Branch of the Thames River due to the much longer time to peak for the South Branch of the Thames River at this location. Mem SWM MEMO Final

19 Page 18 MOE Level 2 protection (70% total suspended solids (TSS) removal) is required for the Oatman Drain / Fox Bar Creek catchment (Delcan, 2014). Water quality control would be provided for the Wellington Road local storm sewer only. Three OGS units would be sized to provide MOE Level 2 protection. No additional retrofitting of water quality controls would be provided. The proposed location of OGS units is shown in Figure 5. Water quality control calculations accounted for an increase in the area of the Wellington Road pavement of 4.2 ha to account for up to 6 lanes of traffic under rapid transit planning. Low Impact Development (LID) Strategies The increase in runoff volume associated with the widening of Wellington Road would be addressed with a LID exfiltration trench. Initial flows would be directed to the exfiltration trenches comprised of perforated pipe wrapped in geotextile surrounded by gravel, and plugged at the downstream end. The design of this system is consistent with Alternative 2 above. Summary Constraints for this solution are: Existing Wellington Road profile may require steep storm sewer pipes and / or drop structures to provide sufficient cover and grade for proposed trunk storm sewer; Sag locations on Wellington Road will need to be removed to provide an appropriate overland flow path for the major system to prevent flooding of private property. If sag locations cannot be removed, sections of oversized pipe will be required to convey the 100year storm; Upsizing of existing sewers is required to provide sufficient capacity for the 2year storm; Potential conflicts with future rapid transit corridor; The opportunity for both local and trunk storm sewers along Wellington Road is dependent on the final rapid transit design and proposed crosssection; LID opportunities are dependent on local site conditions and will be investigated during detailed design; Potential conflicts with the watermain, sanitary sewer and utilities along Wellington Road; Retrofitting water quality controls to the Oatman Drain catchment area will not be provided and no benefit to water quality will be achieved. Future water quality control for infill development would need to be addressed through PPS; Potential for erosion and channel instability at the outlet location to Watson Park; and Potential to starve the oxbow wetlands of base flow by amalgamating the three (3) Watson Park outlets. Mem SWM MEMO Final

20 Page 19 Alternative 4: Realign Oatman Drain and Provide End of Pipe SWM Controls (SWMF Facility) Storm servicing could be addressed via the installation of a trunk storm sewer along Wellington Road, abandoning the existing Oatman Drain and Grand Avenue outlets and upsizing and replacing the Watson Street outlet. The major overland flow path will be moved from private property to Wellington Road and follow the general path of the trunk storm sewer. Retrofitting the existing catchment with water quality controls would be achieved through an endofpipe stormwater management facility (SWMF). Watson Street Outlet The Watson Street storm sewer would be abandoned and replaced with the Wellington Road South trunk storm sewer with a new outlet at the existing Watson Street outlet location. The new outlet will be upsized up to a 2550 mm outlet sewer, complete with headwall, subject to detailed design. Oatman Drain Outlet The Oatman Drain storm sewer system would be abandoned and replaced with the Wellington Road South trunk storm sewer. The Oatman Drain outlet would also be abandoned. The new outlet location at Watson Street would be more easily accessible for construction and maintenance than the current Oatman Drain outlet location. Grand Avenue Outlet The Grand Avenue storm sewer outlets directly to the Watson Park at the east end of Grand Avenue. The existing outlet would be abandoned and the storm sewer directed west to Wellington Road and connected to the Wellington Road South trunk storm sewer. Additional Drainage Areas In order to provide relief to adjacent storm sewer networks external drainage areas would be redirected to the Wellington Road South trunk storm sewer. Five external catchments have been considered for addition to the Wellington Road South trunk storm sewer; P01, P02, P03, P04, P05 as shown in Figure 7. The total catchment area is 31.8 ha. P01 is located south of Commissioners Road East and currently flows through the Victoria Hospital private storm sewer. Catchment P02 is currently undeveloped land owned by London Health Sciences and flows through the Victoria Hospital private storm sewer. Under ultimate conditions it has been assumed that catchment P02 would be fully developed as commercial or industrial landuse and a runoff coefficient of 0.9 has been applied. Catchments P03 and P04 are located along High Street and could be redirected along Baseline Road and Thomas Janes Drive, respectively. Catchment P05 is located at Windsor Crescent. The Windsor Crescent minor system could be directed down Frank Place to the Wellington Road South trunk storm sewer. Mem SWM MEMO Final

21 Page 20 Storm Servicing Preliminary trunk storm sewer sizing was determined using the City of London storm sewer design sheet as defined in the City of London Design Specifications and Design Manual (2012). The Wellington Road South trunk storm sewer would be relocated from private property to the Wellington Road rightofway. A 2550 mm outlet size would be required, subject to detailed design. It is anticipated that additional capacity in the trunk storm sewer and removing Oatman Drain from private property will alleviate some basement flooding concerns in the study area. Storm sewer sizing would account for infill development within the Watson Street and Oatman Drain catchments. Infill development was assumed to be a transition from predominantly low density residential (runoff coefficient = 0.5) to a combination of low, medium and high density residential (average runoff coefficient = 0.7). For subcatchment B01 it was assumed that the Victoria Hospital lands would be fully developed with a high imperviousness (90 %). Subcatchments where runoff coefficients were increased to account for the change in land use are shown in Table 3 and identified in Figure 7. There are several sag locations along Wellington Road that create trapped low points for the overland flow path. During detailed design, further assessment should be completed to determine if the sag locations can be removed to allow overland flow to be contained within the road rightofway or if the Wellington Road South trunk storm sewer should be upsized further to provide capacity for the major system (100year event) to alleviate ponding in these locations. Storm Water Management Controls Water quantity control is not required for catchments discharging directly to the Thames because of the much longer time to peak for the South Branch of the Thames River at this location. Water balance requirements would not be met by this strategy. The proposed widening of Wellington Road would result in increased runoff volume and decreased infiltration and evapotranspiration. Under Alternative 4, water quality controls would be provided via an endof pipe SWMF for existing development and future road widening within the catchment. The new Wellington Road South trunk storm sewer outlet would discharge into a forebay, directing flows through a permanent pool and active storage before discharging via an outlet structure to the oxbow wetlands in Watson Park. MOE Level 2 protection (70% total suspended solids (TSS) removal) is required for the Oatman Drain / Fox Bar Creek catchment (Delcan, 2014). The MOE SWM guidelines were used to determine preliminary sizing for the SWMF volume and SWM block area. The preliminary SWMF location is shown in Figure 8. To SWMF sizing calculations accounted for a widened Wellington Road (additional 4.2 ha increase in impervious surfaces) and redirection of an additional 43.8 ha of external drainage area to the Wellington Road trunk storm sewer. The pond has not been sized for future infill development or intensification which would require permanent private systems to address water quality concerns. The permanent pool and extended detention pond volumes for each scenario are provided in Table 4. Mem SWM MEMO Final

22 Page 21 Table 4. Proposed Stormwater Management Facility General Design Characteristics Characteristic Scenario Scenario Existing Conditions Proposed Conditions Imperviousness 69% 72% Service Catchment Area (ha) Level of Water Quality Protection Level 2 Level 2 Water Quality Protection Volume (m 3 /ha) Total Volume Required (m 3 ) 9,286 13,860 Required Permanent Pool and Forebay Volume (m 3 ) 6,395 9,696 Erosion Control and Baseflow Volume (m 3 /ha) Extended Detention Volume (m 3 ) 2,892 4,164 SWM Block Size (ha) Summary Constraints for this solution are: Does not address water balance requirements and will result in a change to the existing hydrologic cycle through increased runoff volume from the future widening of Wellington Road. Existing Wellington Road profile may require steep storm sewer pipes and / or drop structures to provide sufficient cover and grade for proposed trunk storm sewer; Sag locations on Wellington Road will need to be removed to provide an appropriate overland flow path for the major system without causing flooding of adjacent properties. If sag locations cannot be removed, sections of oversized pipe will be required to convey the 100 year storm in the minor system; Upsizing of existing sewers required to provide sufficient capacity; Potential conflicts with future rapid transit corridor; Potential conflicts with utilities, watermain, sanitary sewer along Wellington Road; Significant portion of the Watson Park area required for the proposed SWMF; Location of the SWMF constrained by the woodland community within Watson Park; Retrofitting water quality controls to the Oatman Drain catchment area would be provided for existing development and future road widening, however, water quality control for future infill development would still need to be addressed through PPS; Potential to starve the oxbow wetlands of base flow by amalgamating the three (3) Watson Park outlets; Interaction with the Back to the River design contest and future planning; Removal of a large portion of the current City of London works yard. Mem SWM MEMO Final

23 Page 22 References AECOM. (2017). City of London, 2017 Infrastructure Renewal Program, Contract 9, Base Line Road East, Balderstone Avenue, Percy Street and Sylvan Street. Delcan. (2014). Water Resources and Slope Stability Components for a Central Thames Subwatershed Study. City of London (2017) Design Specifications & Requirements Manual. City of London. (2010). Private Permanent Systems Policy. Coulibaly et al. (2016). A comparison of future IDF curves for Southern Ontario. Credit Valley Conservation. (2010). Low Impact Development Stormwater Management Planning and Design Guide. Ontario Ministry of the Environment and Climate Change. (2003). Stormwater Management Planning and Design Manual. Toronto: Queen's Printer for Ontario. Ontario Ministry of the Environment and Climate Change. (2015). Interpretation Bulletin: Expectations Re: Stormwater Management. Ontario Ministry of the Environment and Climate Change. (2017). Thames Sydenham & Region Drinking Water Source Protection Interactive Mapping (available at Mem SWM MEMO Final

24 Appendix A Figures

25 WESTDEL BRNE Hill Street William Street South Street Nelson Street Maitland Street Colborne Street Wellington Street Clarence Street Waterloo Street Carfrae Street Philip Street Carfrae Crescent Front Street Woodland Place Thames River (South Branch) Kennon Place Gerrard Street Grand Avenue Watson Park Oatman Drain Outlet Watson Street Outlet Grand Avenue Outlet Gerrard Street Belgrave Avenue Bellevue Avenue Ferguson Place McClary Avenue Weston Street Tecumseh Avenue East Tecumseh Avenue East McKenzie Avenue Langley Street Bond Street Fairview Avenue Trevithen Street Baker Street Raywood Avenue Foxbar Road Windsor Crescent Windsor Avenue Alexandra Street Garfield Avenue Ridout Street South Fairview Court Lincoln Place Herkimer Street Moore Street Emery Street East Lyndhurst Place Southgate Street Emery Street East Thompson Road Whetter Avenue Wellington Road Thomas Janes Drive High Street Chester Street Westminster Avenue Rosel Crescent Chester Street Rowntree Avenue Chester Street Wellington Court Marla Crescent Percy Street McKeon Avenue Connington Street Hickson Avenue St Neots Drive Balderstone Avenue Belhaven Road Sylvan Street Baseline Road East Baseline Road East Averill Crescent Averill Place Mountsfield Crescent Victoria Hospital Entry Adare Crescent Kingsford Crescent Mountsfield Drive Commissioners Road East Torrington Crescent Highland Heights Huntingdon Drive Cowan Avenue Barons Court Rockingham Court Cowan Court Western Counties Road Key Map Huntingdon Place Waterman Lane Cowan Place Chiddington Place Waterman Avenue Chiddington Avenue OXFORD ST W WONDERLAND RD N Compton Place Upper Queen Street Cheviot Road COMMISSIONERS RD Chiddington Gate Compton Crescent Compton Court Lhsc Driveway Wilkins Street OXFORD ST E DUNDAS ST HAMILTON RD SOUTHDALE RD W BRADLEY AVE Study Area HWY 401 E ADELAIDE ST N HIGHBURY AVE N Sandringham Crescent Wilkins Street DRAFT Legend UTRCA Regulatory Floodline Study Area Lot Line Watson StreetStorm Sewer Outlet Class Environmental Assessment Schedule B Drainage and Stormwater Management Project No Sheet No. Scale: Figure 1 1:7,500 Date: µ1/9/2017 Study Area