Storm Water Management
Where land is undeveloped, runoff from storms can be absorbed by soil and roots and normally does not cause flooding or other problems except in extreme cases. In cities, however, the extensive construction of roads, parking lots, and buildings diminishes the area of land able to absorb runoff, and is instead diverted into a storm water system after picking up residues of oil, animal waste, fertilizers, pesticides, road salt, exhaust chemicals, and other contaminants. In many areas, especially with older sewage systems, the storm water combines with sewage in "combined sewers." Ideally, the combined sewer water is all treated before being released, but during heavy rain or melting events, the sudden spike in volume exceeds capacity of the treatment plants and the excess is released untreated, polluting the waterways where this waste is dumped. This case study documents some of the innovative approaches being undertaken to mitigate this problem in Chilliwack, BC, and Toronto, ON.
Sustainable Development Characteristics
There are a number of approaches to mitigating wastewater pollution, ranging from source to end-of-pipe, small to large-scale, low to high-tech, public education, and building large public works. In order to capture a wide range of innovative approaches to reducing the problem, this case study looks at the medium-sized community of Chilliwack, BC and the large urban area of Toronto, ON. How a community treats all of its waste, including wastewater, is an essential component of sustainable community development, and indeed adjacent communities, particularly where water systems and watersheds are interconnected .
The Policy and Design Criteria for Surface Water Management in Chilliwack1 is a policy for surface water management that aims to replicate the natural infiltration rate of the land surface to prevent peak flows in flooding in streams within the municipality. The policy is also tied into a process of stream classification based on the water course’s suitability for fish habitat, its flow regime and cross sectional characteristics. A parallel policy to the surface water management is, therefore, the derivation of suitable riparian setbacks to protect stream quality.
The policy aims to transfer the costs of drainage to the private developers of new subdivisions and property improvers and away from the city. This means that developers are responsible for designing systems that retain water within a new development area and release it in a slow and controlled fashion into the soil, and ultimately the natural drainage system. The standard for this has been set at a flow rate of less than 1 litre / sec from the outflow from a subdivision.
The Great Lakes hold one fifth of the world's fresh water, covering 23,000 km2 and draining an area of 745,900 km2, making them ecologically important on a global scale. Its shores are home to 35 million residents, almost half of whom draw their drinking water directly from the lakes (Sierra Legal Defence Fund, 2006). Although some improvements have been made in recent decades, the Great Lakes are still a disposal area for a wide range of industrial, residential, and public effluents. Since only about 1% of the Great Lakes' water is changed every year, the effects of water pollution are cumulative and long-lasting (Gorrie, 2006). Of all the pollution sources in the Great Lakes, one of the most significant is untreated sewage: the 20 cities studied in the Great Lakes Sewage Report Card (Sierra Legal Defence Fund, 2006) dump 92 billion litres of untreated sewage annually into the Great Lakes.
Toronto ranked 12th of these 20 cities for sewage treatment performance, with particularly low performance on combined sewer overflows (containing a mixture of sanitary and storm water), occuring 30-50 times per year for a total of 9 billion litres.
In order to mitigate these impacts, in 2003 Toronto embarked on a 25-year, 1-billion dollar Wet Weather Flow Management Master Plan. The basic goal of this Plan is to meet the Provincial Water Quality Objectives within the City of Toronto area surface waters. More specifically, it aims to (City of Toronto, 2006):
The Plan includes efforts at every stage of the wastewater management system: source, conveyance (moving of the waste water), and end-of-pipe facilities. This plan is part of a longer-term 100-year watershed-wide strategy to mitigate the impacts of wet weather flow. The size and population of Toronto present both a significant burden on the local watershed and a particular opportunity to develop new solutions given the significant financial and human resources available to the City.
Critical Success Factors
|Capital Cost ($)||Operating Cost ($)||Total Cost ($)|
|End of Pipe Facilities||38,075,000||9,700,000||47,775,000|
Table 1: Toronto Wet Weather Flow Management Plan costs
Chilliwack is an agricultural community of 70,000 running along the south bank of the Fraser River in BC. The community recognises three creek based watershed planning areas:
Throughout the municipality there are 600 km of streams in a land area of 260 km2. The goal of the project was to create an urban form that simulated similar run-off profiles to the natural forest eco-system – or a land surface that is only 10% impermeable. Or to put it another way a land surface that can infiltrate the first 30mm of rainfall in any precipitation event.
The Department of Fisheries and Oceans has a policy of restricting development along riparian zones to protect fish habitat, this led the City of Chilliwack to formulate classification of streams to allow for development to create appropriate riparian set-backs and also instigate a more sustainable surface water management system that improved water quality entering water courses and reduced flood events.
Chillwack was selected as a case study for the application of Stormwater Planning: A Guidebook for British Columbia2 funded jointly by the Provincial and Federal Governments and forms part of the Georgia Basin Ecosystem Initiative. Chilliwack has been concerned about planning for stormwater management since 1995, when it released guidelines stating that:
“all new development must restrict flows from the subdivision or development to pre-development volumes.”
The goal of the current policy is to:
“implement integrated stormwater management that maintains or restores the water balance and water quality characteristics of a healthy watershed, manages flooding and geotechnical risks to protect life and property, and improves fish habitat values overtime.”
This is supported by 5 management objectives.
The objectives and goal are supported by a master planning exercise providing strategic plans for surface water management at creek watershed and sub-watershed levels and a functional plan for individual creek catchment areas.
Design criteria are implemented so that at the site level rain fall is kept on site to allow it to infiltrate into the natural subsurface hydrological systems. At a neighbourhood scale runoff from a subdivision is held in order to reduce storm run-off peaks into the municipal drainage system and creek network. This has the goal of reducing flooding in the municipality. These also reflect the severity of rain events. Most rainfall events should be exclusively retained on site and infiltrated into subsurface hydrological system. Moderate events will be stored within neighbourhood in retention ponds, extreme events will be safely conveyed into and through the natural surface water system avoiding flooding in urban areas.
Not only does this reduce flood risk, but also prevents surface run off from washing pollutants into the creek system from road surfaces and residential lots. In addition holding water in the subsurface and on site allows for water recycling by households and aids the recharging of the groundwater. Although this approach, termed the 'water balance approach', has been tested in Chilliwack no private developer has used it to date, preferring to implement there own design solutions.
Design lessons from the policy formulation process included the provision of a minimum depth of spoil in subdivision landscaping. Careful design of drainage on driveways to ensure water is channelled into soak-a-ways rather than into storm drains. Pilot projects in five new sub-divisions between 2001 and 2003 were implemented to test some of the design criteria. These included:
Narrower roads also have other benefit by reducing development costs and reducing the amount of clearing of natural vegetation required. The data from the test subdivisions has confirmed that the drainage is adequate for all storm events and prevents flooding in the downtown and other sensitive areas.
Since the formulation of the design guide and policy developers have been obliged to implement water retention and infiltrations systems in all new developments. This has usually been in the form of permeable pipes and storage tanks allowing slow release of water into the ground and adjacent water courses. There are however exception to this. On the hillsides within the municipality the infiltration into the soil was not an option as slope stability concerns become an issue. Therefore impermeable concrete storage tanks have been used to hold the water which is then released in a controlled fashion in water courses.
Toronto is a large urban centre of 2.6 million people and a $7.6 billion operating budget (City of Toronto, 2006). Its municipal sewer and storm water systems are extensive: the storm water system is a 4,500km network with 2,600 outfalls (http://www.toronto.ca/water/protecting_quality/wwfmmp/index.htm), connected in an unknown number of locations to the sanitary waste system which handles 400 billion litres of sewage annually.
Its land base is almost entirely developed except for preserved areas, and residential and commercial development is in the form of re-development of already urban areas, rather than the building of new subdivisions as in Chilliwack. Due to the high degree of existing development in Toronto, and the greater budgetary resources available, the strategies available for wastewater pollution mitigation are both limited and more extensive in comparison with those available to Chilliwack.
In developing its Wet Weather Flow Management Master Plan, Toronto considered an extensive list of available strategies and technologies, under the categories (City of Toronto, 2003):
This 275-page document outlines 86 different wastewater management strategies, explains what each one is, its mechanism of effectiveness, expected benefits, related management plans, application requirements, proven effectiveness / experience elsewhere, cost considerations, and references for further information. The document also lists which of the 13 objectives of the plan each strategy addresses:
Natural areas and wildlife
It is a comprehensive and valuable resource of available wastewater management strategies. Selection and adoption of these strategies is decided by a 24-member steering committee including public members, city councillors and staff, and representatives of provincial agencies and interested NGOs (City of Toronto, 2003b). The strategies this committee has selected include both large and small scale projects, both centralized and diffuse (Toronto Water - Community Program for Stormwater Management http://www.toronto.ca/water/news/cpsm/index.htm). There are too many projects to list in this case study, but the following is a brief summary from the 2004-2005 Implementation Report.
Source Controls and Pollution Reduction
Analysis of this case study leads to the following observations: