Deerfoot Trail is Calgary, AB’s principal north-south freeway and an important component in the North-South Trade Corridor of the Americas. A major extension is in progress, which will provide a south link to Highway 2. The proximity of the proposed construction to critical fish habitat areas of the Bow River became a significant issue in the approval process and in the layout of the roadway. Earth Tech (Canada) Inc., under its precedent firm of Reid Crowther, is the principal consultant on the 13-km-long portion of the project within the Calgary city limits.
The combination of a tight project timeline and recent regulatory revisions led to the adoption of a variety of stormwater management (SWM) innovations. Many of these innovations arise directly from a multiple-benefit approach to the design.
Regulations
A variety of surface-water guidelines and regulations affect stormwater management in Calgary. Of greatest significance to this project are the Federal Department of Fisheries and Oceans regulations that prohibit habitat impacts to fish-bearing streams. The river reach immediately downstream of Calgary, within the project area, is a world-class trout fishery, and many known spawning and rearing sites are in close proximity to the proposed river crossing and stormwater discharge locations. For the project areas discharging to the Bow River south of Highway 22X, the fisheries requirements became the most limiting factors in the design of the stormwater system.
The other guidelines strongly affecting this project are those of Alberta Environment, the province’s environmental protection ministry, with respect to total suspended solids (TSS). As of January 2000, the provincial government guideline for TSS from a stormwater facility is 80% minimum removal of the total load. It is common practice in Calgary to recognize particles of 150 microns and less as suspended solids. Alberta Environment also publishes guidelines for flow depths and velocities in publicly accessible open channels that affected some of the design work.
Alberta Provincial Parks’s expectations became relevant because the only feasible location for discharge of the major portion of the flows lies within Fish Creek Provincial Park.
The City of Calgary Sewers Division administers the TSS loading guidelines of the Alberta government and also imposes a number of other guidelines concerned with stormwater management and system operations, as summarized in the SWM design objectives section of this article. Simplicity and minimal long-term operating costs underlie many of the city’s guidelines. In particular, the Sewers Division dictated that pumping of stormwater was to be avoided.
The City of Calgary Parks and Recreation Department also expressed a strong preference to have at least a one-to-one replacement of any valued natural habitats with equivalent created habitats.
Public Expectations
Reflecting the high value placed on the Bow River and its fishery, many public-interest groups are vigilant defenders of the river. In particular, Trout Unlimited and the Calgary River Valleys Committee expressed a number of very specific expectations during the process of the planning and detailed design of the freeway. As an example, Trout Unlimited sought and achieved a routing of the freeway that would avoid grading encroachments to a trout fry side channel habitat. This same side channel is to receive water from one of the stormwater facilities.
In another segment of the project, a local Rotary Club chapter actively expressed interest in the appropriate reclamation of the Fish Creek Park South lands. The larger public interest was also expressed by the local alderman and member of the legislative assembly.
Client Expectations
Given the current regulatory climate of increasing emphasis on surface water quality and the regulators’ openness to consideration of credible public-interest proponents, the client expectations included a proactive approach to resolving potential approval-delay issues.
Another client expectation was the creation of cost-effective stormwater management facilities that would require very little reconstruction as long-term roadwork is constructed over the next 20 or more years. The client gave the consultant some latitude in discussing potential SWM solutions with the various regulators and interest groups so that an approvable system could be efficiently designed.
SWM Design and Innovations
In the interest of protecting local natural environments, the overall design criteria established a higher level of urban stormwater quality than previous projects in Alberta. Several of the sites offered particular constraints because of size and natural conditions. To effectively satisfy the design criteria while dealing with site constraints required an openness to new approaches on the part of the client and the reviewing authorities. Canada’s Department of Fisheries and Oceans, Alberta Environment, and the City of Calgary responded positively to the proposed plans, allowing a high degree of innovation while simultaneously facilitating efficient approvals.
Other design criteria, not directly related to stormwater management, were included to encourage a broad base of support for the proposed plans. These specifically relate to environmental, educational, and passive recreational use of public open space.
SWM Design Criteria
The following criteria were incorporated:
- Provide 1:100-year detention for drainage from the tributary area.
- Provide trapped lows and piped drainage for the interchange areas for at least the 1:5-year event and overland flow for the 1:100-year event over the entire right of way, such that inundation of the roadway is avoided.
- Remove a minimum of 80% of TSS for waters discharging to the Bow River without subsequent treatment.
- For release to critical fish habitat areas, treat TSS to achieve maximum concentrations of 10 mg/l of sediments greater than 10-micron particle size.
- For release to critical fish habitat areas, ensure that stormwater does not significantly raise the temperature of the receiving waters.
- Control overland flow velocities to minimize erosion (some of the local soils are highly erodible).
- Avoid direct discharges to critical fish habitat areas except under extreme conditions.
- Avoid resuspension of captured sediments under any conditions up to the 1:100-year flows.
- Provide containment for hazardous spills originating on the roadway or ramps.
- Provide replacement wetland and wooded thicket habitats relating to removals required for the freeway construction.
- Use stormwater facilities as borrow pits in the Highway 22X interchange area.
- Provide for economical removal of accumulated sediments.
- If feasible, mitigate seasonal salt concentration effects.
Overall SWM System Design
The overall drainage area encompasses 312 ha. Total active storage for the 1:100-year event is 106,500 m3. Total passive storage, consisting of permanent pools and wetlands, encompasses 43,500 m3.
The system comprises four stormwater treatment facilities along the length of the road right of way and one infiltration facility in Fish Creek Provincial Park. Piped systems and trapped lows have been designed to efficiently convey storm drainage in the vicinity of the interchanges and other tight site areas. Elsewhere, drainage is conveyed via roadside and median swales. Reflecting the diversity of conditions in the project area, each of the facilities exhibits specific innovative characteristics.
All of the facilities include submerged access routes that ease excavator and truck access into the forebays so that accumulated sediments can be economically removed. All permanent pond areas have a gravel shoreline and a terraced construction to allow safe public access and avoid the need for exclusion fencing.
All of the new SWM facilities include soil media infiltration beds that will provide buffering of seasonal spikes of salt concentrations arising from road salt applications. Although the infiltration beds do not eliminate the effects of salt, they can be effective in maintaining salt levels below the threshold of environmental impact on the receiving waters.
All of the facilities have been designed with appropriate planting terraces above and below the permanent water level so that self-sustaining native vegetation is readily established and maintained. Two different native grass seed mixes have been used to achieve low-maintenance cover under the range of extremes expected on the project (“protected and moist” to “exposed and dry”).SWM Facility Designs
The designs for five different facilities are detailed below: Barlow Trail Interchange extended detention wetland, modifications to Prestwick Pond, Highway 22X Interchange ponds and wetlands, Fish Creek south infiltration area, and Bow River pond.
Barlow Trail Interchange Extended Detention Wetland
The catchment area for this facility is 43 ha. There is active storage of 18,000 m3 and passive storage of 4,500 m3.
This design was prepared in response to an early regulatory directive that stormwater treatment would be required for preexisting sections of Deerfoot Trail that were being upgraded. Thus, the SWM facility had to fit into the available space between the new interchange, the existing freeway, and existing residential development. The design follows an extended detention (ED) model as described by Schueler et al. (1992). The ED model features wet facilities within a larger basin that is partially inundated between storm events. Note the relatively small ratio of passive storage to active storage. This model is particularly well suited to moderate-scale catchments in Calgary’s climate, where water yields in dry years might not support full-scale, constantly inundated facilities.
The design is off-line, discharging stormwater to an existing storm trunk main in the vicinity. Because the interchange is the principal access route to a major industrial and warehousing district, there is an increased risk of hazardous materials spills. The design features a three-cell design with a manual operation sluicegate between the first two cells to permit the isolation of contaminants.
Another innovative feature of the design lies in the central shallow groundwater filtration cell. This cell mimics the hydrology of a natural fen wetland and provides enhanced treatment of small- to medium-volume frequent storm events by slowly filtering water as it is passed from cell 1 to cell 3. A very high degree of water treatment is provided, while supporting a diverse wetland habitat.
Prestwick Pond Modifications
The catchment area for the Prestwick Pond area is 42 ha. Active storage is 19,500 m3 and passive storage 4,800 m3. This existing section of Deerfoot Trail did not offer the space for a separate facility. Instead, the water is to be discharged to an existing wet pond immediately east of the freeway. The discharge line from this pond to the Bow River is nearing its limit of flow under large events. Thus, there was a great deal of scrutiny of the contemplated retrofit to ensure that compromise to existing facilities or residential areas would not occur.
The design features a modification of the existing pond to create a new sedimentation forebay and modifications to the outlet structure to alter the permanent water level and the discharge curve slightly. The creation of a new forebay dike within a large existing pond required dewatering of the pond, the alternative of floating sediment baffles being unacceptable to the City of Calgary.
Highway 22X Interchange Ponds and Wetlands
The catchment area for this facility is 148 ha, with active storage of 45,000 m3 and passive storage of 23,200 m3.
This is the major SWM facility in the project, serving the highway junction interchange that was created in a natural topographic low. There is both a risk of flooding and of groundwater effects on the roadway in this area, because approximately 8 km of the right of way length slopes down to this point. The presence of numerous natural ponds and wetlands in the area attests to the near-permanent wet conditions. A high-pressure gas line and a planned high-voltage transmission line substantially affected the layout of the SWM facility, resulting in a division into two linked facilities.
The design comprises two separate ponds with a 5.5-m difference in elevation. The upper pond has been designed to utilize and maintain the environment of a cluster of natural wooded swamps and pothole wetlands. This design follows the ED model and collects water mainly from grass swales draining the right of way to the south.
The lower pond has been designed according to the pond/marsh/pond model as described by Schueler et al. In addition to receiving throughflows from the upper pond, this pond receives drainage from the main interchange area. As with Barlow Trail Interchange wetland, the heightened risk of hazardous materials spills led to the provision of a segregation sluicegate between two of the cells. The third and fourth cells are primarily emergent bulrush and cattail marshes, providing a high degree of water treatment. Water from this facility is conveyed via pipe to the infiltration area.
The creation of the Highway 22X interchange resulted in the modification or removal of several hectares of natural wetland and wooded swamp habitat.
Fish Creek South Infiltration Area
The stormwater management design accommodates all of the discharge from the Highway 22X ponds as well as direct runoff from a portion of the existing Highway 22X right of way. Both these flows are discharged to a forebay with an integral marsh wetland to remove sediments from the untreated Highway 22X runoff. From here the water is discharged to a system of low-gradient grassed distribution swales and a shrub thicket area over unmined gravel deposits. Under normal runoff conditions, all of the runoff water infiltrates before reaching the gravel quarry pit ponds. After large runoff events (approximately 1:5 year), some of the water will reach these ponds as overland flow. The ponds themselves are not surface-connected to the river. In yet larger events, approximately 1:10 year, the Bow River will inundate the gravel quarry ponds but not the sediment forebay, which is located above the 1:100-year floodplain.
The design of this facility avoided the creation of a new outfall to the Bow River, which greatly aided in the approval process. All of the water reaches the river as subsurface flows that are well mixed with existing groundwater so that minimal effects on water quality and temperature in the receiving water are anticipated.
Bow River Pond
This facility has a catchment area of 78 ha, active storage of 24,000 m3, and passive storage of 11,000 m3. A new crossing of the Bow River is required at the current south-city limits. The crossing location is adjacent a significant trout-spawning and -rearing side channel, leading to a strong regulatory focus on fish habitat protection. Unfortunately, the extent of the escarpment cuts and river valley fills required to build the roadway results in the generation of substantial stormwater runoff.
Site constraints required that the stormwater treatment facility extend beyond the standard right of way, requiring an additional land purchase of 5.25 ha. A 6.5-ha area was assembled for the SWM facility. As a result, there were competing demands for a high degree of water treatment in a minimal footprint. Other design challenges were that the forebay had to be located above the 1:100-year river flood level and that riparian woodland habitat lost to roadway construction had to be replaced.
The SWM facility design combines a pond, marsh wetland, and infiltration bed. The sediment forebay pond creates a spiral flow path to enhance sediment removal within a minimal footprint. The boundary layer effect has been recognized as detrimental to the performance of standard straight-line flow ponds. Under high flow rates, in particular, standard pond designs that assume linear plug flows may perform in a suboptimal fashion because a high proportion of the flow is confined to a central “channel” with high velocities and low retention time. The spiral pond design uses the boundary layer effect as a positive feature so that water velocities leaving the forebay are minimized under a broad range of flow rates.
Another feature of the spiral forebay design is an integral marsh wetland that further slows and dissipates flows so that a high proportion of suspended particles are removed. The marsh also offers some replacement habitat value.
Water is released from the marsh to a flow distribution pool and swale. It then infiltrates through a bed area created with silty sands excavated from the escarpment road cut. The shallow groundwater flows horizontally 100 m through the infiltration bed and is discharged with existing groundwater seepage to the river side channel. A number of shallow “brooks” collect seepage water and create habitat of potential value for small trout. Other valued habitats created in the infiltration bed area include riparian meadow, riparian tree and shrub thickets, and localized small pools.
The creation of the infiltration bed avoided the need for a new outfall to the Bow River or the side channel and thus facilitated approvals by the Department of Fisheries and Oceans and Alberta Environment. Because water reaches the river as subsurface flows well mixed with existing groundwater, minimal effects on water quality and temperature in the receiving water are anticipated.
Multiple Benefit Design Approach
The design procedure Earth Tech employed on this project emphasizes the achievement of multiple benefits. This approach seeks to create value for all affected parties, including the general public. This is in contrast to the more typical “problem-solving” design approach, which seeks to minimize immediate costs, focus on technical issues and work strictly within the land that is in the direct control of the project client.
Other aspects of the multiple benefit design approach illustrated by this project include:
- Seeks a solution that builds community support and minimizes potential confrontation.
- Solves more than the immediate problem.
- Addresses contextual issues and opportunities that might be of great relevance to the local public, land owners, and politicians.
- Faces the fact that “public and residents” are not a homogeneous group. Diverse benefits are needed to appeal to divergent interests.
- May allow limited use of land that is not under the direct control of the project client.
- Expedites regulatory approvals.
- Integrates operational and public safety concerns more effectively.
Predicted Performance
Design objectives included 80% overall removal of TSS loading and concentrations of less than 10 mg/l of particles 10 microns or larger for the Fish Creek South and Bow River Crossing systems. The chart shows predicted performance for each facility in meeting the design objectives.
Sediment loading and predicted performance of the wet-pond components were modeled using QHM watershed modeling software. Wetland loading and predicted performance for TSS reduction was modeled using the procedures described by Kadlec and Knight (1996). This approach was augmented with information from Calgary local wetland performance, in particular the Elbow Valley Constructed Wetland, which is a test system being operated and monitored by the University of Calgary. The performance of infiltration and overland flow components was estimated from the performance of several similar facilities in the Calgary area. Some of these systems are consistently achieving more than 95% TSS removal. Nutrient performance has not been modeled because it is not currently regulated for stormwater in Alberta. Based on the performance of similar systems, however, we are expecting more than a 90% reduction of nitrogen loads to the river and a 70% reduction of phosphorus.
This project builds on local experience of the past decade in the design of natural, self-sustaining systems for the treatment of urban stormwater in northern plains climate conditions. The removal of fine silt sediments, hazardous spill containment, and provision of valued habitat replacement resulted in timely regulatory approval and brought many other stakeholders into support of the stormwater facility and the larger project of which it is a component.
