Wet and Dry Retention Ponds

Faced with the dilemma of handling stormwater runoff from new development in a cost-effective manner, the City of Boucherville, Quebec, opted to use a mix of wet and dry retention facilities to create a retention system that not only performed its intended hydrologic function well, but improved the beauty and livability of the neighbourhood at the same time. As a natural greenspace is developed for human use, the previously vegetated areas give way to hardened surfaces like concrete and asphalt. These surfaces do not readily absorb water, but instead quickly convey it off the premises, which can cause flooding and sewer backups downstream in addition to polluting receiving waters with the detritus present on streets and sidewalks. Retaining water on-site will slow down this process, but traditional designs are costly and not pleasant parts of a community’s landscape.

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.

Identifying Actions

The city knew that it had to do something about the impacts of stormwater runoff, the Sabrevois River is vulnerable to inflow changes and would quickly see environmental issues arise if overland runoff was left unchecked after the new development. They were also particularly interested in utilizing a low-cost model, so as to not discourage development and real estate sales through exorbitant stormwater fees. Once the combination of wet and dry features was decided upon, the city set to work attempting to incorporate these features into the community in a way that enhanced the livability of the space. It was decided that after the stormwater retention ponds had been constructed, then they would be integrated into the city’s recreation network through bike lanes and walking pathways.

Implementation

There were two wet retention ponds built. These resemble small urban lakes, effectively being a depression that remain partially filled year-round, but possessing the capacity to hold more water during a precipitation event. These two wet ponds were supplemented with the construction of two additional dry ponds, which remain dry most of the year but are similarly capable of retaining large amounts of water during a precipitation event. The two types of ponds were connected with drainage pathways. Finally, once the ponds were completed, they were added to the network of recreational biking and walking paths, allowing people to enjoy the water features with their attendant vegetation and wildlife.

Outcomes and Monitoring Progress

The ponds offer notable improvements in flood reduction, with the wet retention ‘urban lakes’ offering flood protection up the one-in-fifty year events, as opposed to the one-in-ten year event protection provided by previous stormwater infrastructure in the municipality. All combined, the four ponds provide a similar capacity for retaining stormwater as was provided by the greenfield space that existed before development. In this way, development could occur without incurring any unnecessary additional stresses to the municipal sewer system or the natural environment. The wet ponds also retain a layer of vegetation that helps to filter water before it is deposited into the environment, reducing pollution. Furthermore, the recreational aspect of the new development have proven to be very popular with citizens, demonstrating that functional infrastructure need not be an eyesore to the community it serves.

Resources

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