Evaluation of Retrofitted Low Impact Development Practices for Stormwater Management at an Industrial Site

In 2014, Calstone Inc. launched a low-impact development (LID) project to renovate an underutilized landscaped area to reduce the volume of runoff being sent from their property to storm sewers and Highland Creek.

Large-scale infrastructure projects to retrofit water and wastewater systems have traditionally been implemented by municipalities to address water quality and quantity issues and reduce the impacts of extreme precipitation events. However, these projects have long lead times and are often exceedingly costly for already cash-strapped municipalities. Calstone Inc.’s LID project provides a solution at the facility level in the private sector that alleviates the burden on municipal stormwater management and water treatment infrastructure. LID builds climate resilience by supplementing large retention ponds with a distributed network of smaller-scale stormwater management practices upstream and throughout the catchment to retain and treat runoff as close to its source as possible. Retrofitting stormwater source control practices that reduce the volume of runoff from a lot helps to alleviate stresses on urban watercourses and municipal storm sewer systems, replenishes groundwater resources, maintains stream baseflow during dry weather and restores a more natural water cycle.

The project involved several partnering service vendors and technical guidance from the Partners in Project Green, a working group of the Toronto and Region Conservation Authority (TRCA). Monitoring and performance evaluation of the project was conducted by the Sustainable Technologies Evaluation Program (STEP).

Renovation plans were shaped by on-site investigations and a collaborative design process with partners and vendors. Before construction commenced, geotechnical reports were consulted to estimate the water table depth below the property. Tests were performed to determine the asphalt pavement thickness and permeability of the underlying subsoil. Calstone Inc.’s expansion, improvement and maintenance plans were developed and reviewed by Grounds Covered, XCG Consultants Ltd. and STEP.

Between 2014 and 2015, the installation of the LID stormwater system was completed on Calstone Inc.’s property. With supportive grants from Earth Day Canada and the Ontario Ministry of the Environment and Climate Change, source controls were retrofitted on-site including a 9,300-litre rainwater cistern and irrigation system to provide a source of water for landscaping, a rear lot infiltration trench, three soakaway ponds and a lined decorative pond and fountain that enhance green spaces for employees and visitors. Combined, these features disconnect 4 of 6 roof drains (2,600 m² roof area) from municipal storm sewers. The targets set for the project are to capture 100% of the roof runoff and divert approximately 1.8 million litres of rainwater annually, increase the aesthetic appeal of their green spaces and help restore a more natural water cycle to Highland Creek.

Stormwater practices must be compact and adaptable on densely developed lots like Calstone’s. The selection of the LID practices implemented was based on the relatively small pervious area available, favouring approaches with small surface footprints that maximized stormwater management benefits while enhancing green spaces. Since the stormwater infrastructure servicing Calstone Inc.’s area is aging and does not meet current standards that factor in climate change considerations, this lot-level stormwater system demonstrates a practice that would help municipalities avoid the higher adaptation costs of retrofitting municipal infrastructure.

Monitoring and performance evaluation of the project was completed by STEP. Performance of the stormwater source controls was evaluated through the collection of climate and geophysical data and continuous field monitoring of rainfall depth and practice water levels over five months from July to November 2015. A tipping bucket rain gauge was installed on the roof, and pressure transducer water level loggers were installed in the cistern and three soakaway ponds, and all were set to record readings at 5-minute intervals. Monitoring data was used to assess runoff volume reduced, rainwater used/municipal water conserved, soakaway pond drainage rates and frequency and causes of overflows.

Key results of the STEP evaluation included:

• The rainwater cistern and irrigation system reduced runoff volume from the roof drainage area by approximately 64% over the 2015 growing season.
• Despite deviations from precipitation normals, monthly quantities of rainwater used for landscape irrigation remained relatively consistent, indicating that the irrigation system was being routinely operated.
• The cistern overflowed nine times when roof runoff exceeded available storage capacity. This suggests the need for enhanced storage capacity and/or the installation of an overflow pipe.
• The infiltration trench and three soakaway ponds reduced runoff volume from their combined drainage areas by approximately 89% over the July to November evaluation period.

The STEP evaluation demonstrated the viability of LID approaches to stormwater source control within the climate context of the Greater Toronto Area.

The STEP evaluation provided several research and design recommendations:

• To provide year-round runoff reduction in cold climates like Ontario, stormwater infiltration practices should be installed underground below the frost line.
• Where opportunities for year-round rainwater use exist, cisterns should be installed underground or as part of building-integrated rainwater harvesting systems.
• To improve drainage performance, stormwater infiltration practices should be designed to be continuous or connected.
• An additional overflow outlet should be added to the cistern to prevent overflowing into neighbouring lots.
• The infiltration trench should have included standpipes and monitoring well on the perforated pipe to track water level and drainage performance over time.
• To disconnect the remaining two roof drains from direct connection to the municipal storm sewer, it is recommended that they be directed to an underground infiltration trench.
• Research is needed to better understand this approach’s year-round benefits and seasonal limitations.
• Further research on the long-term performance of soakaway ponds is needed to provide better data on performance over time, maintenance needs and length of service life.
• Further research is needed to identify the types of soil, cover, maintenance practices and vegetation best suited to meet runoff control functions, and to determine how different options influence long-term maintenance.
• More research is needed to understand the influence of soil and ground cover, vegetation types and associated microbial processes in maintaining infiltration in stormwater infiltration practices.

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