Evolving Municipal Stormwater Management Objectives

Prior to the 1970’s, the approach to flooding control was to convey drainage as quickly as possible to nearby streams and rivers in an effort to protect human life and reduce property damage. This often led to increase costs of infrastructure due to very large storm sewers as well as substantial erosion and degradation of downstream receiving waters. During the 70’s and 80’s, stormwater management ponds were introduced which provided flood storage for less frequent storm events to allow post development flows to mimic predevelopment levels. This approach often reduced base flows in watercourses beyond acceptable levels, did not address downstream erosion concerns for smaller storm events and did not address the pollutants in stormwater which reduced downstream water quality.


In the 1990’s, environmental agencies and municipalities looked for an integrated approach to stormwater management in an effort to achieve water quantity, quality, downstream erosion protection and protection of base flows in rivers and streams. For new planning areas on a larger scale, watershed and subwatershed stormwater planning was introduced and became an important part of the overall planning and approval process. Subwatershed Impact Studies provided the overall stormwater objectives for new developments. Subdivisions or sites within the subwatershed were required to adhere to the objectives of the lager study and as a result, site designs became less onerous as centralized stormwater management facilities achieved quality and quantity objectives. Smaller infill developments in older areas required onsite stormwater management to ensure the capacity of downstream municipal sewers were not exceeded and the quality of the stormwater was addressed before leaving the site.


While the practises of the 1990’s still hold true, stormwater management objectives have evolved as agencies continue to monitor the effectiveness of the stormwater management practises put in place. In recent years, municipalities and conservation authorities are reluctant to accept broad statements on stormwater objectives and now require mathematical quantification of the practises proposed.


In recent years, municipalities and conservation authorities have imposed a water balance component for all infill and new site developments. The objectives vary but typically require the retention of 5mm to 25mm of rainfall to be retained on site through stormwater reuse, groundwater recharge or evapotranspiration. This represents 50% to 90% of all rainfall events for developments within Southern Ontario. In developments with favourable site conditions (low water table, high native soil porosity, available area to store water underground, available storm outlet etc.) the objectives are easier to achieve and considered cost effective. In developments with less favourable site conditions (high water table, low native soil porosity, building footprint encompasses the lot, etc.) the objectives become more difficult to achieve and as a result, construction costs escalate.


To ensure all stormwater objectives are met for a particular development in a cost effective manner, it becomes critical to access site specific constraints and to fully understand the permutations of the available stormwater practises to meet the objectives. Quite often a single practise will meet multiple stormwater objectives. MGM Consulting Inc. recently designed a bio-swale to drain a new parking lot with a below grade infiltration pit located beneath the bio-swale. This feature was located within a single footprint and provided the water quality, quantity and water balance objective for the site.


As technology advances and the science of stormwater management continues to evolve, it is important to identify site specific constraints early in the process in order to provide cost effective options to achieve the evolving municipal approval objectives.


For additional information on this topic, contact MGM directly.