Green infrastructure (GI) is an umbrella term for infrastructure that delivers benefits to urban development through the ecosystem services that natural systems, like plants and soils, provide. GI’s benefits include reducing the urban heat island effect, improving air quality and beautifying urban spaces, but one of the core design function of GI is stormwater management.
Watershed research over the past 30 years has made a solid case for GI as the best available tool for: preserving and improving water quality, aquatic habitat and fisheries; reducing downstream erosion, flooding and related property damage; and adding capacity and resiliency to aging municipal sewer systems in a changing climate. The most common stormwater GI tools are bioretention, bioswales, green roofs and permeable pavement.
Most Canadian cities now have GI pilot programs but the shift toward making GI a standard practice is well underway. Vancouver plans to capture 90 per cent of runoff with GI in its newly adopted Rainwater Management Strategy; Victoria is incentivizing GI through its new stormwater utility; Calgary and Edmonton are incorporating the latest in green infrastructure design into their stormwater guidance.
Western Canada has world class examples of GI from the absorbent country lanes in Vancouver to Calgary’s parkland bio-retention facilities. However, the industry can, for every success, also point to an example of a failed or dysfunctional GI practice: an eroded bio-retention cell, an infiltration trench that never drains, or uneven permeable pavement.
The problem is not unique to this region. The Center for Watershed Protection conducted a survey of 72 GI projects in Southern Virginia’s James River Watershed. Each of the GI projects as constructed was compared to the municipally approved stormwater design plan. The evaluation found 47 per cent were observed to have one or more deviations from the site plan, many in ways that significantly impaired the required performance. The most common deviations were:
- poor vegetation coverage and health;
- missing or improperly constructed pretreatment devices;
- undersized detention volumes;
- soils clogged with construction sediment, were over-compacted, or did not meet specification; and
- inlets and outlets incorrectly constructed, resulting in flows bypassing or short-circuiting the practice.
GI practices are an integral part of the urban form and require a different approach to construct. GI sites require attentiveness throughout the construction process as to location, intended function, protection from sediment clogging and over-compaction. The stabilization of the contributing subcatchment is critical to the success of GI features. Also critical are proper installation of their unique outlet configurations and use of specified materials. If GI designs continue to be improperly constructed, GI will be stuck with a poor reputation among the public and decision-makers. Therefore, it is critical that designers, contractors and inspectors understand the purpose of GI practices as a new form of stormwater management.
Planning for Success
Designers need to consider construction sequencing in their designs. Erosion and sediment control plans need to include sequencing and sediment management instructions that protect infiltrating practices from compaction and clogging. Often stormwater management features are constructed first, used as sediment traps during construction, then cleaned out when construction is complete. GI should be installed last, or if that is not possible, then sites should be covered and protected during construction.
Tendering for Success
Mandatory pre-bid meetings that include outlining the GI projects’ unique function and requirements have proven to be successful in attenuating non-serious bidders and attracting qualified contractors to the project. Other useful methods include using a pre-qualification process and requesting prime contractors to furnish a list of projects completed with references.
Good GI construction means good on-site erosion and sediment control. Consider including a contract requirement for the resident engineer or site supervisor to have certification from the International Erosion Control Association. Also, a separate line item for emergency erosion and sediment control in the contract allows the contractor to respond quickly to unexpected field conditions with assurance of being paid for their additional work.
Maintenance work under contract warranties is often an afterthought. GI practices require regular maintenance and inspection, particularly during the establishment period for the vegetated practices. The contract needs to explicitly define these requirements and highlight them at the time of bidding. Higher warranty payments or sureties may help ensure the work is done. Another approach is to tender a separate landscape contract that covers planting, establishment, watering as necessary, plant material warranties, inspections and maintenance of the practices.
Build for Success
Communication may be the most important tool for successful GI construction. GI’s purpose, unique materials, special sequencing and potential pitfalls need to be discussed at pre-construction and periodic coordination meetings. The critical GI information must move through all project phases and reach sub-contractors.
Up-front precautions can save contractors from costly delays, fixes and reconstruction. Some materials like bio-retention soil may require special testing to meet specifications. Lab testing should be done well before the material is brought to the site to avoid having to replace installed soil or delay work for new batches to be mixed and tested. Monitoring and maintaining on-site erosion and sediment control will prevent having to replace clogged practices later.
Constructing a bio-retention cell will one day become as standardized and commonplace as constructing a curb ramp. As we work toward that, it is in everyone’s interest to put the extra effort into planning and building better and more functional GI projects. We all want to build projects that will work over the long term to enhance our cities and protect our creeks, rivers, lakes and coasts.
Robb Lukes, P.E., stormwater project manager at Kerr Wood Leidal Associates Ltd., has 12 years of experience in stormwater management policy, planning, design and construction.