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Friday, Jul 27, 2018

Climate change adaptation in urban drainage engineering

Flooding simulation in an urban environment

Climate change is a reality and a risk that must be addressed to improve the resilience of our infrastructure. Changes to our climate will significantly affect the water cycle. They will have a substantial effect on the way we manage water resources and plan our future cities around both water shortages and hazards such as flooding and sea level rise.  The predicted hydrological changes provide us with a unique opportunity to rethink the current state of our water management practices, otherwise known as, climate change adaptation. 

Climate Change Adaptation: A Case Study

A few years ago, the City of Surrey undertook a study to develop future Intensity Duration Frequency (IDF) rainfall curves based on local climate change projections.  What they found was startling.  Based on the detailed local assessment, Surrey can expect an increase of 68% to the peak rainfall intensity by the year 2080. In simpler terms, today’s 100-year storm could become the 2080 5-year design rainfall event. 

As the design of their current urban drainage infrastructure network did not consider the impacts of climate change, the city realized there was an immediate need to reassess their systems.  In 2016, Surrey engaged Associated Engineering to participate in a unique study to assess the impacts of these increased rainfall events on the existing drainage system and evaluate possible adaptation options.  The City of Surrey selected three case study areas to represent the broader drainage and land-use conditions in the urban parts of the community.  We then built a detailed hydraulic model, as well as a flood damage model for estimating economic losses, for each of the three study areas.  

Modelling Challenges

Traditionally, stormwater models consist solely of the underground drainage infrastructure, or minor system.  However, during precipitation events which exceed the design capacity of the pipe network, the uncollected runoff, as well as surcharged stormwater result in overland flooding and the possibility of flood damage. Traditional minor system models provide no insight regarding the surface flooding for events which exceed the system capacity.  Given that the current drainage infrastructure was designed for much smaller rainfall events, mapping the flooding events due to a surcharged system was absolutely critical in light of the significant impacts of climate change to local rainfall. 

To address this deficiency in traditional stormwater modelling, we developed a detailed dual drainage model. This dual drainage model connects the outflow points of the underground system to a detailed 2-dimensional hydraulic model based on LiDAR topographic data. This configuration allows for dynamic flow exchange throughout the simulation.  This model configuration allowed the City to view the maximum flooding extents due to these future precipitation events. This hydraulic modelling approach provides a defensible and transparent basis for estimating economic losses due to surface flooding. 

Model results were produced using this type of hydraulic modelling in the various study areas.  Continuous overland drainage paths exist which results in flow through private property.  This indicates that extensive flood damage could be caused by altered climatic normals.  However, these urban flooding maps only serve to identify the magnitude and extents of their problem.  The lingering question remained of how to deal with these newly identified flooding problems.

Adaptation Considerations

Once the system deficiencies are known, climate change adaptation planning provides a framework to begin asking some of the following questions:

- Should we rebuild our minor system infrastructure?

- Can we fortify our houses and properties against floods?

- Can we reroute flooding extents?

- Are some areas in the City more vulnerable than others?

- Can some of our planned infrastructure upgrades provide an opportunity to fix our drainage issues?

Ultimately, this all boils down to one fundamental question:  Can we rethink our urban drainage planning to safeguard our communities from climate change impacts?  We think so.
Working with the city, we brainstormed 17 possible adaptation solutions, and of those,
modelled and assessed four in detail:

- Installing sewer backflow valves to prevent basement flooding via storm sewer backup.

- Implementing a Minimum-Building-Elevation strategy to raise buildings above potential flood levels.

- Upgrading the pipe system to capture larger design storms.

- Creating new overland drainage paths and storage systems to safely route flood waters away from vulnerable infrastructure.

To assess the relative effectiveness of potential solutions, we considered environmental, societal, and economic implications of each. In addition, we created a “business case” for implementing some of these improvements. Using an in-house GIS method, we calculated damages to buildings in each study area based on flooding extents for various scenarios. 

We then calculated benefit-cost ratios when implementing the various strategies discussed above to identify which of the adaptation strategies were most defensible. We found that each of the four options would be useful given different flooding conditions, and likely a combination of multiple options would form the most effective overall adaptation strategy. 

We hope that this study will serve as a launch point for discussions for adaptation planning within the City and also in other municipalities experiencing climate change impacts and urban drainage problems. We can start envisioning solutions, such as considering how urban rivers and lakes may double as storage protection and recreational opportunities. Adaptation can influence long-term drainage engineering and urban planning decisions for the creation of resilient future communities.