CityFloodMap.Com: IDF Updates for Southern Ontario Show Continuing Decrease in Extreme Rainfall Intensities Since 1990 - Environment and Climate Change Canada's Engineering Climate Datasets Version 3.0

IDF Updates for Southern Ontario Show Continuing Decrease in Extreme Rainfall Intensities Since 1990 - Environment and Climate Change Canada's Engineering Climate Datasets Version 3.0

The Annual Maximum Series (AMS) charts in a recent post show updated trends in observed maximum rainfall volumes over various durations. Design rainfall intensities, equivalent to volumes over the various durations, are derived by fitting a statistical distribution to the observations, resulting in intensity-duration -frequency (IDF) values presented in tables and charts for each climate station. A previous post examined trends in IDF values for long-term record stations in southern Ontario based on 1990 to version 2.3 values (updated to 2001 to 2013 data) - see link - the overall decrease in intensities was 0.2 percent with more frequent, small return period, values decreasing the most.

The extended, updated version of Environment and Climate Change Canada's Engineering Climate Datasets has IDF values based on data up to 2017 and was released in March 2019. Information is available from the Environment and Climate Change Canada's ftp site through this link on their website.

Again we can compare design intensity values from 1990 with the current, updated values and determine if older design standard values are appropriate and conservatively above today's values or if updates to standards are required to reflect more intense rainfall rates. For this review, 8 of the 21 stations have had updates to IDF values since the version 2.3 datasets. The average length of record increased from 42 to over 46 years, averaged across all stations and statistics. The charts below show the average change in intensity for all durations grouped together (top chart Figure 1) and considering variations across durations (bottom chart Figure 2).

Figure 1 - Average Change in Southern Ontario IDF Values for Engineering Design by Return Period - Record-Length Weighted Changes Between 1990 and Version 3.0 Datasets for 21 Climate Stations with Long Term Records

Figure 2 - Average Change in Southern Ontario IDF Values for Engineering Design by Duration and Return Period - Record-Length Weighted Changes Between 1990 and Version 3.0 Datasets for 21 Climate Stations with Long Term Records

Observations are that:

● Rainfall intensities are decreasing even further than in the last review.

● The changes in IDF values based on more recent observations are very small and reflect only minor random ups and downs - changes in IDF values due to assumed statistical distribution selection are greater than observed rain data changes. No “new normal” or “wild weather” due to a changing climate.

● Frequent storm intensities (those used for most storm sewer design) are decreasing for all durations.

● The more frequent the storm the greater the decrease in design intensity.

● Rainfall intensities are decreasing more for short durations than longer ones (see short duration red and orange bars in Figure 2).

● Less frequent, severe storm intensities (25 year to 100 year return periods) are deceasing on average.

● Severe storm intensities are decreasing most for short durations.

The following tables summarize values in the above charts. Note that the chart data is weighted by record length so that longer trends are given proportionately more weight. The tables show both weighted and unweighted values -giving more weight to longer record stations results in a greater overall decrease in IDF rainfall intensity statistics.

Table 1 - Trend in Southern Ontario Intensity Duration Frequency Values for 21 Long-Term Climate Stations, Weighted by Record Length - 0.4 Percent Average Decrease in Intensities

Table 2 - Trend in Southern Ontario Intensity Duration Frequency Values for 21 Long-Term Climate Stations, Not-weighted by Record Length - 0.2 Percent Average Decrease in Intensities

What does this mean for engineering design? In general, older design IDF values or curves are conservative reflecting older, higher observed rainfall intensities. Infrastructure designed to older standards will be slightly more resilient today, having a marginally greater safety factor and higher performance under today's extreme weather conditions. Older infrastructure may be stressed by hydrologic or hydraulic factors, or intrinsically lower design standards - see previous posts here on hydrologic factors including at many southern Ontario cities in this post. How the updated values affect municipal engineering design is shown below on an annotated Table 1.

Table 1 Annotated - What has changed? What are IDF values used for? What does this mean for municipal infrastructure engineering design and resilience of sewer and pond designs?

The implications for municipal infrastructure design based on governing durations and frequencies are annotated around the first table. This shows that:

● storm sewers, designed to convey high frequency, short duration intensities, are facing lower rainfall intensities since 1990;

● major drainage systems designed for low frequency longer durations (because critical conveyance segments are often lower in the system where times of concentration are longer) are facing no change in design rainfall intensity;

● storm water ponds designed to hold low frequency, high return period, long duration storms are facing no change in design rainfall volumes.

This just reflects historical trends in southern Ontario, so how about future changes under climate change that should be considered in design? After all, Bill 138’s Planning Act amendments and O.Reg.588/17 require municipalities to identify how they will accommodate climate change effects in infrastructure policies and plans.

The American Society of Civil Engineers ASCE has created a guide that can be considered and that classifies infrastructure by it's criticality, based on potential loss of life and economic impact as well as the service life of the asset to determine an approach for addressing potential future climate change effects. The guide is "Climate-Resilient Infrastructure: Adaptive Design and Risk Management". One of the principles is that given uncertainty with future climate, one may design with today's climate if the risk class is low, as long as future adaptation is feasible. The guide also promotes an approach called the Observational Method (OM), defined as follows:

"The Observational Method [in ground engineering] is a continuous, managed, integrated, process of design, construction control, monitoring and review that enables previously defined modifications to be incorporated during or after construction as appropriate.All these aspects have to be demonstrably robust. The objective is to achieve greater overall economy without compromising safety."

The OM approach has been adapted by ASCE to designing climate resilient infrastructure and has the following steps:

1. Design is based on the most-probable weather or climate condition(s), not the most unfavorable and the most-credible unfavorable deviations from the most-probable conditions are identified.

2. Actions or design modifications are determined in advance for every foreseeable unfavorable weather or climate deviation from the most-probable ones.

3. The project performance is observed over time using preselected variables and the project response to observed changes is assessed.

4. Design and construction modifications (previously identified) can be implemented in response to observed changes to account for changes in risk.

For new subdivisions, adaptation/modifications noted in the last steps could be implemented in the future if rainfall intensities increase. Some relatively minor local system modifications representing adaptation activities could include:

● adding or modifying storm inlets with control devices to limit capture into the storm sewer (upstream of where future HGL risks are predicted);

● adding plugs to sanitary manhole covers to limit inflows (where significant overland flow spread and depth is predicted);

● modifying the outlet of stormwater ponds to optimize storage for larger storms (e.g., add intermediate-stage relief components to limit over control);

● increasing the capacity of overflow spillways in stormwater ponds to convey larger storms that cannot be stored (e.g., widen or line with erosion protection to a higher stage);

● increase pond storage capacity through grading of side slopes (e.g., steeper slopes or steps/walls) at time of sediment removal/cleaning (NB - slope material may be used to bulk up high moisture content sediment to accelerate cleaning schedule);

● sump pump disconnection of gravity drained foundation drains (weeping tiles) for lowest, at risk basements where insufficient freeboard exists to future higher HGL.

In addition, property owners in any areas of increased risk could be made aware of those and be encouraged to raise insurance coverage limits or consider lot-level flood proofing as well. The benefits of the ASCE's stated OM approach is that it can accommodate future climate change effects without over-designing or over-investing in today’s infrastructure. This is feasible if future adaptation opportunities exist in today's design and if new subdivisions have a relatively high level of resilience already (i.e., safety factors, freeboard values, redundancy, conservative design parameters) such that future changes do not drop effective performance in most areas across a system into a realm where damages will occur. There may be risks in critical sections of the infrastructure system that where designed to the limits of current standards.

Considering an OM approach for southern Ontario climate resilience we are in an observation stage (Step 3) now, having skipped Step 1 and designed most systems for historical IDF characteristics, and not having considered adaptation measures in advance (Step 2). Given that rainfall intensities have not changed, the project performance will not have changed since the system was originally designed with historical IDF values. Therefore no modifications/adaptations are required to account for rainfall trends. It is unlikely that performance variation in a new subdivision could be confidently determined for decades given that the chances of experiencing an event that tests design performance are low. Any performance monitoring may have the co-benefit of informing the baseline performance under historical design standards, as explicit consideration of safety factors is not common, and it is possible that modern systems are exceeding their intended capacity and performance level due to these intrinsic design safety factors.

For retrofitting older infrastructure systems, the IDF data is not as critical in determining risk as is the selection of a design hyetograph that will use this data. Most older systems have level of service gaps for yesterday’s and today's climate and extreme weather, leading to current flood risks.

Looking at the OM approach for retrofitted systems, the noted changes in southern Ontario IDF values since 1990 will have no bearing on performance and flood risks and would not trigger project modifications/adaptation. Some conservative design hyetographs used in retrofit analysis do incorporate a safety factor that could account for future climate effects as well as other hydrologic (e.g. antecedent conditions) or operational uncertainties (e.g. local blockages, clogged grates). For example, some municipalities use a Chicago storm distribution that is conservative in terms of system response - this was examined in detail in this WEAO 2018 Conference Paper and presentation. That type of conservative design hyetograph pattern could limit the project response to future IDF changes experienced under less extreme real storm patterns.

What is more uncertain perhaps, at that requires observations, is the baseline performance of the retrofitted system and how well it mitigates flood risk given the diverse range of failure mechanisms possible. That is, infrastructure upgrades on the public collection system will not alleviate lot-level risks that remain, resulting in baseline performance gaps regardless of changes in IDF values or baseline system design. This should be an area of future research, i.e., to quantify baseline mitigation effectiveness (i.e., performance) - as many factors affect performance and occur together at the same time, it may be difficult to separate out what performance variations are due to weather variations versus other factors. For example, real storms have a significant spatial and temporal variability compared to simplified design assumptions (typically spatially and temporally uniform rainfall) - this was explored at a recent National Research Council workshop on urban flooding (see slides 17-19 for a recent example of real-world temporal and spatial variability compared to design assumptions). Nonetheless, an observed gap in performance regardless of the cause can trigger adaptation/modifications to restore performance of a project to its intended level of service. This would likely be possible only if performance is significantly below expectations.

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