Finding low impact development solutions in Edmonton

Extreme rainfall events are becoming more frequent due to climate change. These recurring events can overwhelm existing drainage systems, flood critical infrastructure and homes, release pollutants to waterways, erode those waterways, and lead to serious financial implications and even death. In 2013, southern and central Alberta experienced heavy rainfall that led to the worst flooding in the province’s history, including one death and an estimated $6 billion CDN in losses.

Several other floods have occurred in Alberta in the last decade. This indicates a need for action.

Let’s focus on some of the efforts taking place in Edmonton. EPCOR, the North American utilities provider, assumed ownership of Edmonton’s drainage utility in September 2017. Since then, EPCOR has developed several programs to protect the North Saskatchewan River quality and reduce the risk of flooding throughout the city. The North Saskatchewan River, which winds northeast through Edmonton, provides residents in the city and nearby communities with drinking water.

Since 2019, the utilities provider has also been pursuing its Stormwater Integrated Resource Plan (SIRP).

A look at the stormwater integrated resource plan

EPCOR’s SIRP process identifies stormwater-related risks according to four perspectives: Health and Safety, Environment, Social, and Financial. EPCOR has linked these risk consequences to approximately 1,300 stormwater sub-basins based on public surveys, engineering studies, stormwater and climate models, and historical flood and maintenance records. Through this assessment, EPCOR created a risk ranking for each of the sub-basins in Edmonton. That means the utility can now more efficiently target how to fund upgrades for flood mitigation.

The SIRP program will invest $1.6 billion CDN over 20 years in five key investment themes of Slow, Move, Secure, Predict and Respond. The Slow, or green theme, invests in drainage improvements, such as dry ponds and low impact development (LID), which is an approach to managing stormwater runoff that uses or mimics natural processes and slows the entry of stormwater into the drainage network by absorbing it and holding it in ponds. By storing rainfall and snow melt, the risk of flooding is reduced, combined sewer capacity is increased, and stormwater quality is improved before it reaches the North Saskatchewan River.

Prior to EPCOR’s five-themed SIRP strategy, the City of Edmonton’s grey infrastructure master plan was going to require $3.2 billion over 50 years for sewer separation and large trunk sewer systems. By leaning toward a greener approach, EPCOR aims to save money while still addressing the stormwater risks associated with climate change. One of the key aspects of the SIRP strategy involves slowing the entry of stormwater into the drainage system using LID.

EPCOR’s procurement approach

Prior to 2019, LID design and construction was not widely used in Alberta. Not many people knew of the concept. In 2021, EPCOR began working with three prequalified engineering consultants, after initiating a Master Service Agreement (MSA), to complete the design and construction management services for LID.

Stantec is headquartered in Edmonton and was one of the successful consultants and has since helped EPCOR implement several LID facilities throughout Edmonton in different locations including residential, parkland, and commercial land, as well as on public and private property.

EPCOR also has an in-house team of LID designers who propose concept-level LID projects that are then circulated through the consultants under the MSA to validate the concepts and complete detailed design. EPCOR has also developed standardized design spreadsheets, templates, checklists, and design guides to ensure consistency and efficiency throughout their LID program.

In 2022, EPCOR initiated another MSA, but now for prequalification of contractors to build the LID facilities. This similarly improved procurement efficiency and streamlined construction. This allowed EPCOR to undertake numerous LID projects in an efficient way throughout the year to meet ambitious targets.

In Edmonton, typically LID is proposed throughout the city following two principles: Prioritization and Opportunistic implementation:

• Prioritization refers to EPCOR’s SIRP risk ranking, which identifies the most at-risk sub-basins for possible LID implementation. In the medium and/or high-risk sub-basins, which are typically in the remaining combined sewer areas in the city, EPCOR identifies public property, open green space, or potential industrial/commercial partners where LID could be feasible.
Opportunistic implementation in Edmonton involves installing LID whenever other construction is occurring in the area (e.g., sewer separation and dry ponds, neighborhood renewal, or outfall rehabilitation).

Case study: Outfall abandonment and replacement with LID

Look at an LID case study in Edmonton to see EPCOR’s work with Stantec in the city, starting in Rundle Park, which overlooks the North Saskatchewan River.

Storm Outfall 80 in the park was severely damaged by ice jams along the river. EPCOR’s typical Storm Outfall Rehabilitation Program recommended repairing the outfall structure to continue operation. This outfall structure, however, was very challenging to access to complete repairs from an environmental perspective. It was close to steep, unstable riverbanks that contained sensitive bank swallow habitat and several mature trees. Construction access to repair the outfall structure for long-term performance would require mature tree removal, major regrading along the already unstable riverbank, and destruction of the sensitive bank swallow habitat.

In this case, EPCOR did not prefer the typical approach. Given that Outfall 80 serviced a small catchment area, EPCOR explored if the outfall structure could be abandoned in place. Could the flows be managed by LID instead?

EPCOR retained Stantec to develop LID options to manage parking lot flows currently draining to the storm outfall. As the parking lot storm sewer was a couple meters below ground, the flows would normally have to surcharge to come to surface and drain into an LID facility. Because the existing topography sloped greatly toward the river, however, we took advantage of this slope to install two, tiered bioretention gardens in series on the slope. By excavating the soil over the existing underground storm sewer, we daylighted the sewer to drain into the bioretention garden, which meant that minimal surcharging in the upstream storm sewer was needed.

The design also incorporated a perforated catch basin base slab to infiltrate any water retained in the grey infrastructure due to the elevation differences to the surrounding underlying soils. This design allowed the entire 1:5 year rainfall even — which means a rainfall event of this size has a one-in-five chance of taking place in any given year — from the catchment area to be managed through two large, tiered bioretention gardens. These gardens used the existing underground storm sewer to serve as the inlet to the facility. We all preferred this approach over re-grading the entire parking lot, which would have been much more costly.

Additionally, parkland and green space exist downstream of the bioretention gardens. So, flows greater than the 1:5 year rainfall event can harmlessly overflow, pond, and infiltrate to the underlying soils through the gravel infiltration wells installed as part of this project.

This project included the abandonment of another outfall, Storm Outfall 154, in the city’s Forest Heights neighbourhood. Erosion and unstable slopes along the riverbank where the outfall discharged led to deterioration of the outlet, which would eventually block its functionality. Due to the safety risks of accessing the outfall location for repairs and the small catchment area, we explored whether bioretention gardens could manage the 1:5 year rainfall flows.

Ultimately, due to lack of available space in the neighbourhood, bioretention gardens alone would not offer sufficient storage volume for the 1:5 year event. Instead, a large diameter storage pipe, approximately 180 meters long, was installed to attenuate the stormwater.

This storage pipe led to a flow control structure with an orifice that slowly drains to the combined sewer. Through hydraulic modelling, this solution was identified as the most feasible LID approach required to abandon the storm outfall while slowing the entry of stormwater into the drainage system.

Stantec completed the project on behalf of EPCOR in 2023.