Evolution of SWM

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In the mid 1970s, attempts to control runoff flow rates from urban developments were initiated. By the late 1980s, water quality became an additional focus and in the late 1990s, approaches to mitigate accelerated stream channel erosion were introduced. Lot level stormwater management approaches have been advocated in Ontario since 1995 (OMMAH, 1995), but widespread application has yet to occur. Today, with improvements in our understanding of watershed systems and the potential impacts urbanization can have on aquatic ecosystems, stormwater management addresses a broad suite of issues including fluvial geomorphology (stream channel forming processes), groundwater resources and the protection of aquatic and terrestrial habitats.

Municipalities, with the support of conservation authorities, review stormwater management facilities and plans designed to address this multitude of concerns. This has led to an increasing complexity in stormwater management planning and design including:

  • increasingly complex stormwater management facilities and best management practices;
  • the need to involve more inter-disciplinary expertise in studies to define environmental opportunities and constraints;
  • expanding requirements for multi-purpose stormwater management facilities; and,
  • increased emphasis on the treatment train approach and use of multiple types of controls to address environmental issues.

CVC and TRCA have been extensively involved in integrated watershed-wide environmental monitoring for many years. The results of this monitoring have shown that the environmental health of many watersheds continue to decline as urbanization increases. This environmental deterioration has taken place despite widespread compliance with provincial and conservation authority requirements for stormwater management planning and facility design. Conventional stormwater management, which focuses on controlling peak flow rate and the concentration of suspended solids, has failed to address the widespread and cumulative hydrologic modifications in watersheds that increase the volume of stormwater, increase the runoff rate, and cause excessive erosion and degradation of stream channels. Conventional stormwater management also fails to adequately treat other pollutants of concern, such as nutrients, pathogens and metals.

This is why CVC and TRCA commissioned the development of the 2010 stormwater management guide, to provide guidance on the kind of cutting edge practices that are needed to protect the health of the CVC and TRCA watersheds. The LID SWM Guide draws on published research, literature and local studies to provide planning and design guidance that reflects regional policies, practices and climate. It provides information and guidance on the following:

  • how to integrate stormwater management into the urban planning process;
  • how to design, construct and maintain a range of LID stormwater management practices; and
  • the kinds of environmental and performance monitoring that should be carried out.

Acknowledging that it will not always be possible to maintain the predevelopment water budget of a site, predicted increases in runoff from land development that cannot be mitigated through stormwater infiltration practices should be minimized through practices that either evapotranspire (e.g., green roofs, bioretention), or harvest runoff for non-potable uses (i.e., rainwater harvesting). In areas where development has already taken place, LID can be used as a retrofit practice to reduce runoff volumes, pollutant loadings, and the overall impacts of existing developments on receiving waters. LID practices can include:

  • conservation site design strategies (i.e., non-structural LID practices);
  • infiltration practices;
  • rainwater harvesting;
  • runoff storage and evapotranspiration;
  • runoff conveyance;
  • filtration practices; and landscaping.

Studies show that implementing LID practices can have multiple positive environmental effects including:

  • protection of downstream resources;
  • abatement of pollution;
  • recharge of groundwater;
  • improvement of water quality;
  • improvement of habitat;
  • reduced downstream flooding and erosion;
  • conservation of water and energy; and
  • improved aesthetics in streams and rivers.

These combined benefits help to mitigate potential negative impacts of climate change on groundwater levels, risk of flooding and stream channel erosion.