Leonardo da Vinci said, ?Water is the driving force in nature.?
Even in the 15th century, the artist, scientist and Renaissance man, realized the profound importance of water as an element that sustains the natural world. As a driving force in nature, water shapes the landscape and is a fundamental necessity for all forms of life. The presence or absence of water determines the viability of habitats, dictates the composition of vegetation communities and modifies microclimate. The power of water has shaped the Earth?s landscape over millennia to create wonders of the world such as Niagara Falls and the Grand Canyon. Yet for all of its influence, power and potential, water is a fragile resource. In Canada, a country surrounded on three sides by vast oceans and abundant with rivers and lakes ? including the world?s largest freshwater lakes ? clean water is largely regarded as an infinite resource. However, this perspective requires reconsideration.?
The Impacts of Urbanization
Since the Earth and its atmosphere are a closed system, the volume of water on Earth and in the atmosphere is fixed and finite. Water cannot be added or removed; it can only be converted from one form to another. Although water covers 75 percent of the Earth?s surface, only 3 percent can be used as drinking water.
Our supply of clean drinking water is limited and, as populations grow, the conversion of land from natural to rural to urban results in changes to hydrology. Since water is the universal solvent, it absorbs pollutants that are present in the air and on the ground, impairing surface water quality and groundwater reserves.
When lands are urbanized, infiltration rates can be reduced by 35 percent, and evapotranspiration decreased by 10 percent, increasing runoff rates by as much as 45 percent. In comparison to the natural condition, peak flow rates increase, magnifying the potential for flooding and erosion, impacting both water quality and the stability and sustainability of downstream watercourses.
In addition to this profound increase in runoff rates, runoff from urban areas contains a suite of contaminants including suspended solids, metals, nutrients and chemicals. Table 1 provides a list of contaminants commonly found in urban runoff juxtaposed to the water quality guidelines set out by the Province of Ontario, Canada.
In response, Low Impact Development (LID) solutions have been implemented to improve water quality and achieve runoff quantity control objectives. LID solutions include permeable pavement, bio-retention cells, infiltration galleries, rainwater harvesting and recycling systems and green roofs.
Close to the Source
Within urban areas, the landscape affords the potential to reduce the amount of runoff generated from a precipitation event. In the past, cities employed conventional storm sewer systems that conveyed runoff downstream with maximum efficiency. In contrast, effective landscape-based stormwater management systems manage stormwater as close to the source as possible, instead promoting contact with soil, filter media or plant material to encourage filtration and cleansing.
Throughout the Greater Toronto Area (GTA), a number of projects successfully employ landscape-based stormwater management solutions as the primary means of meeting water quality and quantity control objectives. These projects demonstrate the effectiveness of a wide variety of tools that address stormwater management in different ways.
Honda Canada: Unique Aesthetics
The Honda Canada Campus in Markham, Ontario, encompasses more than 45,000 sq-meters of floor area and includes parking lots that accommodate 1,100 cars in addition to vehicle storage and an extensive loading and service area. Here, in place of a conventional storm sewer and pond system, Honda deployed landscape-based solutions to manage the runoff from the entire Campus and protect the Rouge River.
The stormwater management system comprises biofilters, permeable pavement and a rainwater harvesting and recycling system. The network of biofilters replaces the conveyance function of a conventional storm sewer system, effectively creating an elongated, sub-terrain stormwater management pond. Permeable pavement installed in the main entrance plaza and parking lot areas provides for the pre-treatment of runoff, enhancing water quality. The granular sub-base below the permeable pavement installations affords additional storage volume.
Runoff from rooftop areas is directed to a rainwater harvesting tank that supplies the irrigation system for the site. The rainwater storage system consists of three tanks, 3-metres in diameter and 30 meters long, which together can store approximately 1,500 cubic metres of stormwater. When the stored water is redistributed through the irrigation system, evapotranspiration is optimized, contributing to the site-wide water balance.
At the Honda Canada Campus, the presence of water is not obvious since the attenuation and filtration of runoff takes place largely below grade. However, the stormwater management system itself is legible within the landscape, creating a unique and identifiable aesthetic signature that reflects Honda?s corporate focus on efficiency and innovation.?
York University: Imperceptible Workings
In contrast, at the York University Life Sciences Building, the stormwater management system was designed to be barely perceptible. Its objectives are similar: the system moderates runoff rates, promotes infiltration and achieves water conservation, yet the permeable pavement, subsurface infiltration gallery and rainwater harvesting system are all but invisible. The permeable pavement pre-treats runoff and the porous pipe system conveys it first to an infiltration gallery and finally to a storage tank. The system is integrated with internal building services; the infiltration media cleanses the water, rendering it suitable for non-portable re-use within the building.
Mississauga Streetscapes
In Mississauga, Ontario, Canada, two pilot projects aim at replacing conventional road drainage systems with landscape-based solutions. In the established community of Lakeview, for example, ditches lining suburban streets were replaced by bio-retention cells designed as planted rain gardens, which not only contribute to the functional effectiveness of the stormwater system as a whole, but also establish a unique streetscape for the community. On Elm Drive, the LID retrofit project uses a series of stormwater planters to treat runoff and facilitate infiltration. This design represents a hybridized urban streetscape treatment that relies on permeable pavement and porous bottom catch basins to pre-treat runoff prior to discharge into the planters.
The Learning Curve?
Because each of these projects presented specific challenges, creative collaboration amongst the various disciplines comprising the consulting team was essential. The regulatory approvals process differed from project to project and issues related to operation and maintenance needed to be addressed to ensure reliable performance throughout the lifecycle of the installation.
During the construction phase, contractor education is key since many of the system elements are new technologies. In particular, filter and bio-retention media must be correctly mixed, and infiltration media protected from contamination (In practice, partially constructed biofilters and infiltration galleries have proven to be attractive locations for the storage of building materials, equipment and other construction-related elements).
Proof Positive??
In order to gauge their effectiveness, the Toronto and Region Conservation Authority (TRCA) under the Sustainable Technologies Evaluation Program (STEP) is monitoring many examples of these systems. Ongoing monitoring has confirmed the performance of a number of technologies including permeable paving, bio-retention cells, rain gardens, infiltration trenches and vegetated swales.
The STEP program has been particularly useful in dispelling myths regarding the perceived lack of reliability of LID solutions. The STEP program provides local performance data that is painting, for the most part, a positive picture. LID systems have been proven through monitoring to work and yield excellent results, primarily for the smaller, more frequent storms which tend to be most impactful in terms of water quality and erosion parameters. Lifecycle performance is also proving to be impressive ? 20-year plans of operation with less than a 5 percent reduction in efficiency for infiltration systems and permeable pavement installations.
Just as water is the driving force in nature, through the application of a creative, integrated approach to design, the application of LID technologies within the urban landscape can be a driving force to sustain this fragile and finite resource.
Mark Schollen is a principal at Schollen & Company Inc., a landscape architectural consulting practice involved in the development of Low Impact Development strategies and sustainable stormwater management solutions.
