"Is the 100-Year Storm Model in Urban Planning Obsolete?"

On July 16th, 2024, the City of Toronto received 98 mm (almost 4 inches) of rain, more than the average amount for the entire month (Toronto Star). This caused significant flooding, the closure of roads, businesses, and schools, the loss of power, over 1700 emergency calls for assistance to Toronto Fire, and an estimated billion dollars of damage in southern Ontario. This is the third time in 11 years that Toronto has recorded what is usually a once-in-a-100-year storm for the Greater Toronto Area.

Urban Planning Models For Storms

Urban planning for storm resilience typically involves using design cycles based on return period models, which refer to the frequency of storm events, like the "100-year" or "500-year" storm models.

These design cycles help determine how infrastructure, land-use policies, and other elements of urban planning are developed to withstand certain magnitudes of extreme weather. Here's a closer look at the different models and how they are used:

1. 100-Year Storm Model

The 100-year storm model represents a storm event with a 1% chance of occurring in any given year. In urban planning, infrastructure and systems designed using this model aim to manage the kind of severe weather expected once in every 100 years.

“It’s just a really big storm that happens very rarely,” explaining that a 100 year storm is the equivalent of 115 mm of rainfall in 24 hours. Don Goodyear

Applications in Urban Planning:

• Stormwater Infrastructure: Drainage systems, culverts, and storm sewers are often designed to handle flows associated with a 100-year storm. This means the pipes and basins are sized to prevent excessive flooding during the most common but still intense storm events.
• Floodplain Management: Regulations frequently restrict development in areas that would be inundated during a 100-year storm. These zones are identified on floodplain maps, helping guide where buildings are safe and where additional flood risk measures are required.
• Green Infrastructure: Urban planners often integrate features like retention ponds, bioswales, and green roofs to manage and absorb the rainfall associated with a 100-year storm. These features act as additional support to existing grey infrastructure.

Considerations:

• The 100-year storm model is standard for many urban stormwater and flood planning purposes, balancing practicality and risk. However, as extreme weather becomes more frequent due to climate change, a 1% event could occur more often, necessitating updates to design standards.

2. 500-Year Storm Model

The 500-year storm model represents a storm with a 0.2% chance of occurring in any given year. Designing for a 500-year storm generally results in much more resilient infrastructure, capable of withstanding extremely rare but catastrophic events.

Applications in Urban Planning:

• Critical Infrastructure Protection: Planners may use the 500-year model for essential services and critical facilities like hospitals, power stations, and wastewater treatment plants. These facilities must function during extreme events to support emergency response and recovery. For example, flood barriers and elevation requirements might be designed to withstand the storm intensity represented by a 500-year model.
• Flood Protection Projects: Areas with significant economic or strategic importance—such as downtown districts or zones with dense populations—might consider the 500-year model for major flood protection projects. For example, levees, floodwalls, and pumping stations can be designed with the capacity to handle such events to protect critical areas.
• Emergency Shelters and Safe Zones: Buildings designated as community emergency shelters are sometimes required to meet higher flood and storm standards, incorporate elements to withstand a 500-year storm, and ensure they are operational when they are needed most.

Considerations:

• Designing for a 500-year event is more costly, requiring a more extensive and more robust infrastructure. Because of financial constraints, applying this standard universally in urban planning isn’t always feasible. Instead, it’s often reserved for the most critical systems and locations.

Balancing Between 100-Year and 500-Year Models in Urban Planning

In practice, cities adopt a risk-based approach to balance planning for both 100-year and 500-year storm models, depending on the area's vulnerability and the critical nature of the infrastructure:

• Residential vs. Critical Infrastructure: Residential areas and general urban infrastructure are typically built to the 100-year standard, while critical infrastructure may be built to withstand a 500-year storm. This prioritization allows cities to allocate resources more effectively while still safeguarding key services and vulnerable populations.
• Adaptive Design: Urban planners are increasingly considering climate change projections and moving towards more flexible, adaptive approaches to infrastructure. Some cities are updating their stormwater systems to deal with more frequent 100-year storms, or planning to be upgraded progressively towards a 500-year capacity if extreme weather trends continue to worsen.

Integrating Green and Grey Infrastructure

• Hybrid Approaches: Urban areas often combine grey infrastructure (e.g., large stormwater pipes and channels) designed for 100-year events with green infrastructure that provides additional capacity and resilience against higher-magnitude storms. Green infrastructure helps to manage overflow and can serve as a buffer for extraordinary events, potentially approaching 500-year storm intensities.

Examples from Practice

• Flood Barriers and Parks: In cities like Toronto, the Don Mouth Naturalization and Port Lands Flood Protection Project is an example of using elements of both 100-year and 500-year storm planning. The goal is to reduce flooding under regular heavy rainfall (100-year events) and provide resilience for extremely severe events (500-year models) by restoring natural floodplains that absorb excess water.
• Zoning and Building Regulations: In flood-prone areas, buildings may be mandated to elevate their foundations above the estimated 100-year flood level. For buildings housing essential services or highly valuable assets, designers might choose to use the 500-year flood level as the benchmark.

Climate Change Considerations

Climate change has forced urban planners to rethink return period models like the 100-year or 500-year storm because extreme events are becoming more frequent and intense. Cities are increasingly considering:

• Dynamic Updates: Revising and updating flood maps and planning models more frequently to reflect new data and climate projections.
• Higher Standards for New Developments: Many cities now require that new infrastructure or developments go beyond the traditional 100-year standard, incorporating buffers to handle the unpredictability of future weather patterns.

Conclusion

• 100-Year Model: This standard is used for most general urban stormwater infrastructure and building codes, balancing cost and risk to mitigate reasonably foreseeable storm events.
• 500-Year Model: This is used for critical facilities and infrastructure that must remain operational during extreme events, providing a much higher level of resilience.

When deciding which design cycle to apply, we must consider financial constraints, criticality, and changing environmental conditions. Storm sewers need to be designed and funded for our new normal, with a view to the future. The data used for urban planning modelling must be kept up-to-date and relevant.

The goal is to ensure that cities start preparing at the design stage and that our essential infrastructure is protected (by design) to withstand severe weather and is resilient enough to handle the "worst-case" scenarios effectively.

References

1. City of Toronto Flood Protection Initiatives:

- Information on flood protection projects like the Don Mouth Naturalization and Port Lands Flood Protection can be found on the [City of Toronto’s official site](https://www.toronto.ca/services-payments/water-environment/flooding/).

2. Toronto and Region Conservation Authority (TRCA):

- TRCA’s floodplain management and resilience efforts are outlined in various reports and planning documents available at the [TRCA official website](https://trca.ca/conservation/flood-risk-management/).

3. Toronto Wet Weather Flow Master Plan:

- Detailed guidelines on managing stormwater in Toronto are covered in the [Wet Weather Flow Master Plan](https://www.toronto.ca/services-payments/water-environment/).

4. National Infrastructure Standards for Urban Drainage:

- The use of 100-year and 500-year storm models in urban planning and their application can be found in municipal planning guides, such as the Canadian Standards Association (CSA), which provides guidelines for infrastructure resilience.

5. City of Toronto Design Criteria for Sewers and Watermains https://www.toronto.ca/wp-content/uploads/2021/01/8d16-ecs-specs-dcm-design-criteria-sewers-watermains-Jan2021.pdf

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