Understanding and Assessing Impacts
Rainfall landing in a naturalized area will be retained and filtered by the existing vegetation and soils. When a greenfield is developed, that vegetation is cleared and a natural soils and capped with a hardened, impermeable surface like concrete or asphalt. Rather than being retained on site, these impermeable surfaces will quickly convey water offsite, either into the municipal sewer system or the natural hydrologic system, depending on how the site is designed. This can lead to a stresses on the sewer system as the majority of rainfall in deposited immediately into the pipes, rather than flowing in in a more controlled manner overt time. As a result, increasing development without taking stormwater retention into account can increase the risk of basement flooding. Additionally, the risk of overland flooding caused by a swelling river is also increased for the same reason. The City of Boucherville recognized the problem that this kind of development could cause and set out to counter it, but mindful of their limited budget and hoping to achieve some co-benefits, opted for a system that incorporated both wet and dry retaining ponds. The City hired consulting engineers who created hydrologic simulations of stormwater flows to help determine the most effective means of retaining water on site. The decision to integrate the recommended structure into the City’s park system came after that.