Rain Gardens and Swales in Brownfields

In an effort to improve the city’s resilience in the face of an increasing intensity and frequency of rainfall events due to climate change, the City of Calgary, working with developers, has mandated that the brownfield development of a former Canadian Forces barracks in the city will include substantial use of low impact design methods. Low impact design methods are suite of physical design choices that help to retain stormwater onsite, reducing the load on municipal sewer systems and, therefore, sewer backup flooding. In this particular project, the construction of swales and rain gardens are the primary method of intercepting overland flow. The development is ongoing, but at the moment the site has proven successful at retaining most precipitation onsite.

Understanding and Assessing Impacts

Many cities in Canada are already facing substantial issues of surface water runoff overtaxing local sewer infrastructure, leading to higher maintenance and treatment costs and increasing the risk of basement flooding due to sewer backups. With climate change projected to increase the number of high-precipitation events, which are more likely to cause sewer backups as the pipes are filled to excess extremely quickly, this issue is likely to only increase in importance as the years go on. Implementing low impact development methods would be economically justified by reducing sewer loads in the absence of climate change. However, given that that is not the case, mandating that new developments retain local runoff on-site is an especially forward-looking plan, as climate change is expected to increase the frequency and magnitude of high-intensity precipitation events. In the specific case of the Currie Barracks, the site is also a former military installation and the surface water may carry many additional contaminants from the unique nature of its previous use.

Identifying Actions

Over the course of development, several meetings were held between designers, approvals departments, and city management to discuss how the unique nature of the site affected the relevance of low impact design methods versus more traditional stormwater retention methods. The City of Calgary was also developing a guidebook for low impact design methods concurrent with the design of the site and both project teams communicated frequently to ensure that they could learn from each others’ experiences. The development is scheduled to occur over four phases, three of which are completed, and will ultimately house some 18,000 Calgarians.

Implementation

An important thing to understand is that the implementation of these features did not necessitate the creation of whole new structures, but rather the modification of existing ones for relatively low cost. For example, the rain gardens of Currie Barracks are created with 1m of loosely-packed topsoils and organic mulch, compared to usual 10-15cm, allowing them to retain much more water on site. While costs vary based the contractor and the cost of supplies, making simple modifications such as this – increasing the depth of rain gardens that were being built regardless – is a relatively low-cost method of achieving substantial results. In fact, the Currie Barracks does not posses any greater proportion of permeable space than similar developments, is just uses the space is does have more wisely. One of the sticking points of this project is that Calgary has a rather unique microclimate thanks to the Chinook winds. These are warm air masses that affect the city making it so that even in the middle of a Prairie winter, temperatures can rise dramatically; one such event in 1962 saw temperatures rise over 40C in one hour. This means that any plan must take into account a much greater frequency of freeze-thaw events. In the case of Currie Barracks, this necessitated a raised drainage outlet with a special non-clogging sewer tie in. The two main types of green infrastructure utilized in the plan were rain gardens and grassed swales.

Outcomes and Monitoring Progress

All reports indicate that the existing low impact design methods are performing adequately. One of the strengths of green infrastructure as a water retention method is that it makes the space more livable than traditional design methods. The evapotranspirative processes of the vegetation help to produce a cooler microclimate in the Summer heat, provide habitat for animals of all kinds, and create a more aesthetic environment for the people living there. All of these help to create a healthier and happier community while simultaneously performing the needed stormwater retention role.

Next Steps

As the project is still in development, the next steps are a mixture of ensuring that the future development still implements the low impact design methods while also continuing to monitor the existing emplacements to ensure that they are performing their roles well. Upkeep and maintenance of existing structures is also important as time goes on, letting the rain gardens get filled in with detritus, or the drains of the swales to be clogged with trash, will substantially decrease performance.

Resources

Link to Full Case Study