Bowness Flood Barrier

As of April 2021, the feasibility study for the Bowness flood barrier is complete. The results from the study showed a barrier would be effective at helping reduce basement and main floor damage that may be caused by overland flooding. The project would need to be built on private land, and we worked closely with the community throughout the studies.

Many community members wanted more details on the Government of Alberta’s plans for an upstream reservoir before progressing further with the barrier project. As a result, The City is not moving forward with further work related to a barrier at this time.

For more details about the feasibility study results, read our frequently asked questions.

We’ll continue our active participation in two key initiatives the Government of Alberta is leading: updates to the flood hazard maps for Calgary and the feasibility study for a new Bow River Reservoir. As more information on these becomes available and is shared with the community, The City will re-evaluate how a flood barrier can help achieve 1:200 level flood protection, and community support.

1. Updates to the Flood Hazard Area map for Calgary
   
   Land use development and regulations are a fundamental part of our three-pronged flood resilience plan to manage the risk and help avoid the damages caused by flooding.
   
   The City is working closely with the Government of Alberta on the updated flood hazard map for Calgary, which is anticipated to be released later in 2021. As the existing hazard map was created in 1983, the new map will reflect an updated understanding of how a 1:100 flood would impact Bowness and will include new flood zone classifications for Calgary.
   
   We are reviewing our existing floodplain policies and regulations to explore different options for how to regulate these new zones in our city once new flood hazard maps are released, balancing the safety and resilience of river communities while ensuring they remain great places to live. As part of the process, we’ll be gathering feedback on these different policy options through city-wide public engagement. We will share more details with the community when they are available.
   
   
2. Phase 2 of the Feasibility Study for the Bow River Reservoir Options
   
   Upstream flood mitigation remains a critical component of The City’s flood resilience plan for the Bow River.  We remain in regular contact with the Government of Alberta and will continue to advocate for and support their studies to advance this work.
   
   The Province is examining three reservoir storage options to address both flood and drought risk on the Bow River. Results from this feasibility study are anticipated in 2023. While information released is at the discretion of the Government of Alberta, we anticipate the following information will be part of the study results:
   • Field study and public engagement results for each of the three potential locations being studied.
   • One recommended reservoir site (if any), and whether the Government of Alberta should move forward with further study of the recommended site.
   • An estimated storage capacity of the recommended site for both flood and drought mitigation.

This information will help indicate the extent of flood protection that could be provided by an upstream reservoir and what the residual flood risk could be in Bowness. It will also provide insights on the potential timing of completion for an upstream reservoir (expected to be in the range of 12-15 years), in the event the Government of Alberta continues to move forward on the project.

Visit the Government of Alberta’s website for more details on this project and other ways to stay informed about progress and opportunities to provide feedback.

While the flood barrier is not moving forward at this time, what hasn’t changed is our commitment to the community’s safety and personal preparedness through our annual flood season preparations.

The Government of Alberta’s agreement for modified operations at TransAlta’s Ghost Reservoir is in place until 2026, which will help reduce the risk of smaller floods (up to a 1:20 event) until other mitigation is constructed. In the event of a flood, our emergency response plans are ready to help evacuate residents and keep you safe.

In all river communities, flood resilience is a shared responsibility. There are many actions property owners can take to protect their property and help reduce their flood risk. Find out what steps you can take to reduce flood damage to your home and belongings.

We also encourage residents to sign up for our flood preparedness e-newsletter. It’s sent out every Friday from May to mid-July with information on current river conditions, tips and flood mitigation project updates.

Studies were conducted over 18 months to explore the feasibility of a flood barrier from the early conceptual design. 

Prepared by: KCB Engineering

This study helped us predict how the Bow River behaves at the design flow rate of 1,230 m³/s. The results from the computer river model was used to:

1. Determine the barrier height
2. Understand how the river affects groundwater
3. Understand how the river interacts with the stormwater system
4. Understand whether a barrier would affect neighbouring communities

The study results showed the optimal alignment to protect the greatest number of properties in Bowness was along the riverfront targeting two key low-lying areas on the north and south ends of Bow Crescent. These areas were identified to be the source of overland flooding in the community and that a flood barrier could effectively stop water from overflowing the riverbanks. The recommended barrier heights vary from 0.4 to 1.7 metres, depending on the location on the property.

Results of how the river influences groundwater and interacts with the stormwater system are detailed in the separate groundwater and stormwater studies.

Prepared by: KCB Engineering

We follow a consistent design process for all flood mitigation projects. We conduct studies to understand the existing groundwater conditions and determine how they could change if a flood barrier is constructed.

What did we do?

• We conducted groundwater monitoring and pumping tests to understand how changes in the river level may affect how fast and how high groundwater levels rise in Bowness.
• How quickly and in which direction(s) water may flow underground.
• Geophysical studies to understand the geology of the area in greater detail.
• Groundwater chemistry to interpret groundwater flow patterns, before and after peak river flows were observed on the Bow River.

Groundwater conditions are similar to other riverside communities
Bowness was built on a highly permeable aquifer, like downtown and Sunnyside. This means water can travel more easily in some of the geological layers under the ground surface. 

The groundwater flow changes direction during the year
For most of the year, groundwater travels down from the Paskapoo slopes and out towards the river. Maintaining this unobstructed path for the groundwater to exit is important to prevent the groundwater table from rising.

During the spring high water season, the flow reverses direction and groundwater flows from the river into the soil. 

What is the effect on groundwater when overland river flow is blocked by a barrier?

• Slightly lower groundwater table. Study results showed that removing overland flow typically resulted in lowering of the groundwater table, on average by 0.4 m.

Figure ES.4 Peak Water Table Reduction with Surface Barrier, 1:200 Attenuated Event 

• Localized pockets of groundwater may come to the surface, but it can be addressed by pumping wells. Our studies showed that when the overland flow is held back by a barrier during a flood, we may still see localized pockets of groundwater coming to the surface in small areas. The number and placement of pumping wells would be determined in subsequent design phase, if the barrier was considered feasible.  

Groundwater levels with and without a surface barrier in place are shown below:

Figure ES.2 Depth to Peak Water Table, no mitigation, 1:200 Attenuated Event 

Figure ES.3 Depth to Peak Water Table with Surface Barrier, 1:200 Attenuated Event 

• Hydrogeological Characterization and Groundwater Flow Modelling Report – May 2021
• Appendix I: Hydrogeological Site Investigation and Groundwater Monitoring Summary Report 
  
• Appendix I4: Tetra Tech Geophysical Investigation Report
• Appendix II: Alberta Water Well Records 
• Appendix III: Groundwater Model Design Basis Memorandum
• Appendix IV: Numerical Groundwater Model – Calibration
• Appendix V: Numerical Groundwater Model – Groundwater Flood Mitigation Simulations
• Table of Changes

The City hired an independent third party reviewer for the geotechnical and hydrological studies in Bowness based on feedback from the community.  This review helps ensure the studies meet standards, are comprehensive and that their interpretation of the findings are consistent with those of Klohn Crippen Berger (KCB), who conducted the study. Their letters below are a summary of their review:

• Third party review of hydrogeological characterization and groundwater flow modelling report - Letter
• Review of Hydrogeological Conceptual Model and Groundwater Model Design Basis Memorandum
• Review of Hydrogeological Characterization and Groundwater Flow Modelling Report

As with all riverside communities in Calgary, managing the risk of basement flooding can be best achieved through a combined approach – controlling the river flow through upstream reservoirs to keep groundwater levels low; community flood barriers to prevent community flooding; property-level flood protection to limit risk exposure.

When river flows cannot be maintained below basement levels, or when the water table rises due to heavy rainfall, an effective way to help reduce basement flood risk is by taking the appropriate steps at home.  This includes:

• Install a sump pump with a back-up power source.
• Install sanitary sewer backflow preventer valves.
• Ensure downspouts drain onto the ground at least two metres away from your home.
• Improve your lot grading so that it slopes away from the house foundation.
• Repair cracks in the basement foundation.
• Finish basements with easily washable materials, such as concrete floors and walls.
• Install water alarms in the basement to alert you if water is backing up.
• Avoid building living spaces below grade.

Prepared by: Matrix Solutions

This study was used to determine the potential impacts a permanent overland flood barrier would have on existing drainage infrastructure. Using a comprehensive computer model allowed us to compare existing (no-barrier) conditions to a scenario with a riverfront flood barrier to determine how stormwater drainage changes can be managed.

The study results revealed that some modifications to the stormwater system in Bowness would be needed if a permanent overland flood barrier was built. The study identified which outfall gates would need to be closed during a flood event and what infrastructure (e.g. pumps) would be needed to manage drainage while the gates were closed. The study recommended using temporary pumps only deployed during emergency events in lieu of permanent infrastructure. The study also identified that some lots will require a change to their local drainage to ensure stormwater does not build up on their property with the barrier in place. Site-specific solutions would be explored further as part of a potential future phase of design.

During a flood, who is responsible for the closure of stormwater outfall gates?

During a flood, The City would follow its Emergency Response Plan which includes the closure of outfall gates. The Emergency Response Plan would also cover deployment of temporary pumps to ensure stormwater does not back up behind the barrier.

How was groundwater considered? How are sump pumps considered?

The groundwater infiltration into the storm system, whether through cracks in pipes and manholes, leaky pipe joints and connections, and weeper holes, is expected to be a small fraction (1-2%) of the potential flows in the storm pipes. For more information, refer to Section 2.6 of the Stormwater Assessment and Mitigation Report.

Prepared by: KCB Engineering

In this phase, the purpose of the environmental review was to get a baseline understanding of the environmental conditions in Bowness by building an inventory of what exists (e.g. amphibians, birds, vegetation, rare plants and wetlands, soils, fish and fish habitats). Independent biologists captured feedback provided by residents and also visited the area throughout various times in the year to observe and note the wildlife.

If the project were to proceed to the next phase, a full environmental impact assessment would be completed once an alignment was confirmed. This final assessment would provide information on potential impacts to existing habitat features, fish, wildlife and vegetation and how to mitigate them during construction.

• Related studies and reports
• Frequently asked questions
  
• Flood Barrier Evaluation Criteria