Stormwater management has become an increasingly challenging issue for urban communities. In many cases, rainwater can no longer follow its natural patterns, which increases the volumes of runoff and its associated problems.
In a natural environment, runoff is minimized since rainfall for the most part is dealt with naturally through infiltration, evaporation or plant transpiration. As a result of urbanization, however, we’re seeing added stress on natural watercourses and related environments, simply because the increased imperviousness reduces the ability for rainfall to be absorbed into the soil, detained or treated. This effect is changing the natural water balance.
The water balance is an expression of the flow of water in and out of the system and represents the proportion of precipitation that infiltrates, evapotranspirates, or runs off. If one or more of the elements goes out of its predevelopment balance, it generates consequences to the natural system such as increases in volumes, peak flows and durations, temperatures and sediment. Excessive runoff in particular can lead to flooding and altered stream flow, erosion, algae growth, compromised water quality, threats to aquatic habitat and changes to groundwater resources. Managing the water balance therefore is important in terms of flood protection, sustaining water quality, erosion control and improving infiltration.
There’s a growing need to implement stormwater management systems that control the volume and quality of runoff by other than traditional pipe-based means. The low impact approach, which uses landscaping and other onsite techniques to mimic pre-development conditions, will be especially critical to sustaining the surface and groundwater regime.
The evolution of stormwater practices
Prior to the 1990s, the major focus was simply controlling quantity. This prompted widespread installation of piped drainage systems that were designed to convey the runoff to downstream watercourses as quickly as possible.
In the early 1990s, the focus expanded to include water quality and erosion. At this juncture, we saw the early iterations of ponds as a stormwater measure. Today, as concern grows about protecting fisheries, preserving stream morphology, and protecting groundwater resources, stormwater treatment has come under the spotlight.
Management objectives are now being accomplished through a variety of source and conveyance control measures that more closely simulate conditions prior to development. There are a number of these measures in use today, including
These source and conveyance measures do not preclude the use of traditional end-of-pipe approaches, which continue to be a factor in stormwater planning. Rather, the objective of these measures is to minimize and treat runoff before it reaches the end of pipe elements.
The treatment train
None of the sustainable stormwater measures working independently will address all the runoff issues we face. Rather, the ideal way to address increasingly complex stormwater objectives is the implementation of a series of solutions, often called the “treatment train” approach. Selecting the right combination of solutions for a specific application is very much dependent on location, project parameters and costs.
When assessing choices, one of the first things to look at is site conditions, such as the soil, the topography, and the proximity to fisheries habitats, wells and hazardous land uses. Climatic conditions – including rainfall and winter conditions – must be considered as well. The Water Balance Model powered by QUALHYMO provides a decision support tool that can assist the analysis process.
It’s also important to understand the pre- and post- development conditions and the relative change to the imperviousness or characteristics of the runoff. For instance, not all runoff is the same—different sources can experience different levels of contamination.
Techniques
Samplings of techniques that can be used in stormwater systems include bioretention, green roofs, soakaway pits, swales, filter strips and permeable pavement. These methods provide an indication of the various options that can be used to address specific objectives.
There are many good sources of information about the techniques such as the Low Impact Development Stormwater Management Manual from Credit Valley Conservation Authority and Toronto and Region Conservation Authority. Another good source is the Metro Vancouver Stormwater Source Design Guidelines. Additionally, CSA has developed a series of stormwater management courses that are designed to close the knowledge gap and provide greater insight into the issues, options and application of appropriate solutions.
Collaboration first
While the low impact stormwater approach appears to be an effective solution, implementation in Canada has been slow and sporadic. This is in large part because those in charge of water management services tend to adhere to more familiar and entrenched practices.
Adding to the challenge is that low impact measures require an integrated decentralized approach, where sustainable techniques may have to be installed in a combination of residential properties, parking lots, roads, parks, etc. under both public and private ownership and/or control. This is unlike traditional stormwater management which is primarily a single system owned and maintained by the municipality.
Even within municipalities, solutions need to traverse organizational boundaries. For example, a set of measures may require the approval or involvement of multiple departments, including planning, water services, building, environmental, transportation, parks and communications—not to mention external agencies. Each plays a role, and each requires a level of knowledge, expertise and commitment.
Despite the challenges, studies have proven that costs can be lower with a properly designed and constructed system. By closing the knowledge gap in terms of understanding the available techniques and applications, as well as individual roles and job functions, we can expect to gain real and sustainable benefits from working towards common goals.
Jeff Walker is the project manager of built environment and structures at the CSA Standards.