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Stormwater Management

Stormwater refers to rainwater and melted snow that flows over roads, parking lots, lawn and other sites.

Under natural conditions, stormwater is intercepted by vegetation and then absorbed into the ground and filtered and eventually replenishes aquifers or flows into streams and rivers. Later, part of it is returned to the atmosphere in the form of evapotranspiration. In urbanized areas, however, impervious surfaces such as roods and roofs prevent precipitation from naturally soaking into the ground. Instead, the water runs rapidly into storm drains, municipal sewers and drainage ditches into streams, rivers and lakes and on its way it picks up pesticides, road salts, heavy metals, oils, bacteria, and other harmful pollutants and transports them through municipal sewers into streams, rivers and lakes.  The sheer force and volume of polluted runoff causes:

Impacts of Urbanization on Hydrologic Cycle

  • Increased downstream flooding risks
  • River bank and bed erosion
  • Increased turbidity
  • Aquatic Habitat destruction
  • Changes in the stream flow regime
  • Combined sewer overflows
  • Infrastructure damage
  • Contaminated streams, rivers,
    and coastal water


In order to mitigate the undesirable impacts of urbanization on watercourses and associated infrastructure stormwater management practices need to be implemented:

  • To preserve the natural hydrologic balance in newly developing areas and re-establish it, wherever possible, in already developed areas;
  • To protect and enhance quality of stormwater discharged to lakes and streams; and
  • To reduce the volume and frequency of combined sewer overflows in older urban areas.


Stormwater Management Criteria

The Stormwater Management Criteria provides guidance in the planning and design of stormwater management infrastructure for developers, consultants, municipalities, and landowners, and outlines the processes and infrastructure needed to address flooding, water quality, erosion, water balance, and natural heritage.  It was developed based on the principle that the establishment of appropriate, effective, and sustainable SWM practices requires a solid understanding of the form, function, and interrelation of the water resources and natural heritage systems.

SWM Practices & Low Impact Development

There are numerous practices that can be incorporated into a proposed development. SWM practices that take an integrative treatment train approach are essential ingredients for implementation of successful stormwater management strategies. Accepted practices are described extensively within the Stormwater Management Planning and Design Manual and the Low Impact Development Stromwater Management Planning and Design Guide.

Implementation of Stormwater Management Practices should utilize a "Treatment Train Approach".

Treatment Train Approach

Treatment Train Approach

Low Impact Development (LID) is a stormwater management strategy that seeks to mitigate the impacts of increased runoff and stormwater pollution.

LID comprises a set of site design strategies and distributed structural best management practices that harvest, filter, evapo-transpire, detain and infiltrate stormwater

Low Imapct Development Principles

  • Integrate Stormwater into Planning (Multi Disciplinary Approach)
  • Focus on runoff prevention (innovative planning, LID practices)
  • Treat stormwater at the source as a resources (respect natural flow paths)
  • Create multifunctional landscapes (site aesthetics, energy, conserve potable water, wildlife, parks)
  • Educate and maintain (legal agreements, maintenance plans)

Conventional vs. LID

Erosion and Sediment Control on Construction Sites

Rapid urban expansion and associated construction activities are a significant source of stress to the natural environment.  Construction activities accelerate natural processes of erosion and sedimentation as vegetation is stripped away and soils are left exposed.  Consequently, stormwater runoff from these sites contains high levels of sediment and associated contaminants.  Implementation of adequate water quality controls at construction sites is crucial to ensuring downstream receiving waters and aquatic habitats are adequately protected.

The Erosion and Sediment Control Guideline for Urban Construction (December 2006), produced by the Greater Golden Horseshoe Area Conservation Authorities, provides a detailed description of the various measures recommended on construction sites for erosion and sediment control. These are classified as:

ESC Guideline Document

  • Erosion Prevention Practices
  • Sediment Control Practices
  • In-Stream Control Practices