Monitoring Low Impact Development at the Elm Drive demonstration site

The Elm Drive low impact development (LID) road retrofit, necessitated by water flow and drainage issues, is located on Elm Drive West, just south of the Square One Shopping Centre in Mississauga, Ontario. This project involved a partnership between three stakeholders, including the City of Mississauga, the Peel District School Board (PDSB), and the Credit Valley Conservation (CVC). The area is experiencing the climatic impacts related to extreme precipitation. Water drainage and flow patterns are changing and increasing areas of impervious surfaces are impacting the inflow and infiltration of precipitation. This compelled the development of alternative stormwater management techniques (e.g., bioretention and permeable pavement). The Elm Drive project incorporates both permeable paver lay-bys within the road right of way (on City of Mississauga property) and bioretention planters on the adjoining property owned by the Peel District School Board (PDSB). Runoff flows from Elm Drive West onto the permeable paver lay-by and into the bioretention planters. Preliminary monitoring indicates that LID features are performing well.

Understanding and Assessing Impacts

Local climate projections were one of the drivers for CVC’s LID monitoring that led to the monitoring of Elm Drive; given the local climate projections of increasing precipitation events and the risks (to infrastructure, people and ecosystems) associated with increasing surface runoff (e.g., water quality concerns and erosion), the stormwater management retrofit was designed to capture stormwater runoff and convey it through permeable pavers and bioretention planters before discharging any remaining runoff to the existing storm sewer system.

Identifying Actions

Coordination with project partners, stakeholders and local Councillors is important with early LID adoption. Prior to and during the design process, project partners worked together to negotiate the terms of the project, including the roles and responsibilities of each party. To facilitate this process, CVC worked with the Ward Councillor and the local PDSB trustee. Support from these representatives helped ensure that the project had buy-in from both City and PDSB staff. It also led to a successful agreement granting the City access to PDSB property for stormwater management and maintenance activities.

Prior to implementing the retrofit project, Elm Drive consisted of a roadway with soft shoulders and a grass drainage ditch. The stormwater management retrofit was designed to capture stormwater runoff and convey it through permeable pavers and bioretention planters before discharging any remaining runoff to the existing storm sewer system. This was achieved by implementing a road cross-section which is sloped to one side of the road (using a “side shed” configuration) towards the permeable pavers and bioretention planters with all runoff conveyed to LID features via overland flow. Construction took place over a period of seven months.

Elm Drive is just one of the many projects implemented and monitored by CVC as part of the Infrastructure Performance and Risk Assessment (IPRA) program. IPRA is a multi-year stormwater monitoring program focused on gathering detailed information to evaluate stormwater facility performance in various land use types, climate conditions and development stages. The IPRA program also evaluates the effectiveness of stormwater facilities in flood control, erosion protection, nutrient removal, cold climate performance and the maintenance of pre-development water balance.
The monitoring program is based around a set of objectives that have been developed with an advisory committee consisting of municipalities, provincial and federal environmental agencies, academia, and engineering professionals.

Implementation

Implementing this project included broad phases of design; construction drawings; construction inspection and supervision; site layout and surveying; erosion and sediment control and protection of infiltration areas; and monitoring.

Bioretention planters and permeable pavement were installed as part of the LID project to enhance stormwater management. Bioretention planters consist of layers of varying types of aggregate. The excavated trench is lined with non woven geotextile and the first layer of aggregate is high performance bedding. The bedding is comprised of angular washed limestone free of dirt or small fines. Above the high-performance bedding are retaining walls for bioretention planters. Non woven geotextile lines the first layer of the wall. A 150 mm thick coarse concrete sand filter layer is then placed on top, followed by the 450 mm filter media mix. The permeable pavement was installed in the lay-by as well as the sidewalk, totalling an area of 530 m2. The subbase aggregate is lined in geotextile and contains a 150 mmØ subdrain, with a 400 mm layer of 50 mmØ clear stone, followed by a 250 mm layer of 19 mmØ clear stone. Runoff is filtered and dissipated as it percolates through the numerous layers below the pavers. Any runoff from higher flows is directed to bioretention planters through catch basins and is temporarily stored before discharging to Cooksville Creek.

Capital costs – consultant fees ($60,000); roadwork ($240,000); permeable pavement lay-by sidewalk ($55,000); storm sewers ($50,000); bioretention planters ($150,000); boulevard & misc. ($30,000); TOTAL ($585,000).

Completed Bioretention Planters

Completed bioretention planters, showing flow dissipater (foreground) and salt-tolerant native plants (background).

Completed bioretention planters, showing flow dissipater (foreground) and salt-tolerant native plants (background).

Outcomes and Monitoring Progress

The broad areas of successes/outcomes achieved with this project include:

  • Innovation – one of the first green street retrofits to take place in Ontario.
  • Joint partnership and collaboration – the City of Mississauga, the PDSB and CVC. These partnerships were necessary for the success of the project.
  • Demonstration showcase – The LID features at Elm Drive have been showcased through numerous presentations, events and site tours.
  • Performance – Preliminary monitoring indicates that LID features are performing well, and that for the majority of rainfall events (~90% of all events) little to no stormwater runoff leaves the site.

Barriers and issues encountered with this project include design, construction and commissioning, and operation and maintenance. Some lessons learned in overcoming some of the observed barriers include:

  • To provide additional clarity and reduce the potential for error, drawings should include a profile view of the storm services through the bioretention cells, and detailed dimensions of any non-standard items.
  • Warranty provisions need to be more specific with respect to LID features (i.e. plant watering and weeding) and need to be adhered to by all parties.
  • Aesthetics are key – original landscaping had to be supplemented with additional plantings, including trees, to improve aesthetics and add seasonal variety to cells.

Future LID projects would benefit from weekly on-site meetings to review the design and direct the contractor on any issues they have. Engineers and contractors are also encouraged to read CVC’s LID Construction Guide for further guidance with LID best practices.

Next Steps

Demonstrating that LID works in the real world and provides quantifiable stormwater management quantity and quality control benefits is critical to overcoming barriers and concerns among municipalities, regulatory agencies, developers, businesses, and other stakeholders. To help address these concerns and barriers, CVC is currently undertaking a comprehensive infrastructure assessment at Elm Drive to monitor its performance in managing stormwater runoff at the site. This infrastructure assessment is being overseen by an expert advisory committee consisting of municipalities, regional government, the Ontario Ministry of Environment, consultants, and universities. The infrastructure assessment began in 2011 shortly after construction. The assessment involves continuously monitoring precipitation, discharge from the site and water temperature. Monitoring staff maintain an on-site rain gauge, monitoring wells within the planters, and have equipped a manhole at the end of the facility with specialized equipment to measure the flow, volume and quality of stormwater leaving the site. Flow-weighted water quality samples are analyzed for total suspended solids (TSS), and a broad spectrum of nutrients and metals for all events producing discharge. Inflow is estimated based on precipitation amount and the catchment characteristics. For further information and monthly updates on the ongoing infrastructure assessment work being undertaken at Elm Drive visit cvc.ca website to access CVC’s suite of LID guidance materials.