NRC National Guidelines on Flood Control Cost-Benefit Analysis Indicate 100-Year Storm Intensities Not Increasing

The National Research Council of Canada (NRC) has released the National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate.

The full guidelines are available here to download: https://nrc-publications.canada.ca/eng/view/object/?id=27058e87-e928-4151-8946-b9e08b44d8f7

These guidelines delve into many topics to support comprehensive benefit cost analysis. As the effects of a changing climate on flood damages (and adaptation benefits of reducing them) are considered, trends in extreme rainfall across Canada are reviewed in the foundation research. The review shows that on average extreme rainfall intensities have not increased when the most recently observed data is considered.

Extreme rainfall intensities can be characterized by the high return period values in Intensity-Duration-Frequency (IDF) statistics. For example the 100-year rainfall intensity would be the rate of rainfall over a range of durations that has a 1% chance (i.e., 1/100) of occurring in a single year, based on the statistical analysis of past observations.   Similarly 50-year storm rainfall rates have a 2% change of occurring during any year (i.e., 1/50), 25-year rainfall has a 4% chance, 10-year has a 10% chance, etc..

The guidelines present the following table showing 2-year to 100-year design rainfall intensity trends at 226 climate stations across Canada for durations of 5 minutes to 24 hours (Appendix E, Table 17):


As shown, 100-year storm intensities have decreased on average by 0.5% from the v2.00 to the v3.10 Engineering Climate Datasets. On average the derived 10-year to 50-year rainfall rates have decreased as well.

IDF intensity values are derived from observed annual maximum rainfall values, called an annual maximum series (AMS). One can look at the trends in AMS values to understand how derived 'return period' intensities are changing over time. The NRC guidelines summarized Environment and Climate Change Canada's analysis of these trends across Canada in the following table (Appendix E, Table 11):


The table counts instances where observed rainfall has decreased significantly (i.e., a statistically strong trend), has decreased (a weaker trend), has not changed, has increased (a weak trend), or has increased significantly (a strong trend).  Across Canada the totals show decreasing trends in 131+2235=2366 data points and increasing trends in 2526+251=2777 data points. And 266 data points showed no change. This summary indicates that both decreases and increases are occurring, with slightly more increases.

The NRC guidelines present additional analysis of more regional trends as well. This can help practitioners understand recent changes in extreme rainfall affecting local flood damages. For example the AMS trends in southern and northern Ontario are presented in the following two tables (Appendix E, Annex E1, Table A Part 3 & Part 4):

Ontario is shown to have increasing and decreasing AMS values (i.e., annual maximum observed rainfall) that varies by location (climate station) and duration. In some cases annual maximum rainfall is decreasing/unchanged or increasing/unchanged for all durations while in others short duration values have increased while long duration values have decreased - or vise versa. For example:  

Stations with all decreasing/unchanged trends for all durations:

  • Elora RCS
  • Hamilton A
  • Toronto City
Stations with all increasing/unchanged trends for all durations:
  • Toronto North York
  • Sioux Lookouk
  • Geralton A
  • Thunderbay CS
  • Belleville
Overall there are more decreasing AMS trends in southern Ontario (top table) than in northern Ontario. (Note the divide for southern and northern climate stations was taken as latitude of 44 degrees so Brockville, Kingston, Kemptville and Ottawa stations are in the northern group.)

The changes in IDF design intensity values for southern Ontario follow the regional trends in AMS values. This is what is expected since IDF statistics are derived from the observed AMS data. While Table 17 above showed changes in IDF values between the v2.00 and 3.10 datasets, essentially the difference resulting from accounting for the last 10 years of observations, southern Ontario IDF changes over a longer period were assessed in the NRC guidelines. The following figure shows the range in design intensities at long term stations since 1990 (Appendix E, Figure 2): 


Overall 2-year rainfall intensities, representing smaller storm severity, have decreased. Meanhile 100-year intensities have increased and decreased. This analysis aggregates all durations. Given the variability from station to station shown earlier (e.g., Toronto City has decreasing trends while Toronto North York has increasing trends, trends in long and short durations are inconsistent at individual stations), there is a benefit in aggregating larger sets of data (across durations and geography) to estimate overall regional trends.

(Aside - the analysis of southern Ontario IDF trends in the NRC guideline presented above were recently updated to reflect the v3.20 datasets and average trends for durations within each return period were analyzed as well. Results are presented in this post: https://www.cityfloodmap.com/2021/10/rainfall-design-intensities-in-southern.html


The figure above shows more consistent decreases across durations for the small storm intensities (i.e., 2 to 5-year return periods.)


6 comments:

  1. This comment has been removed by a blog administrator.

    ReplyDelete
    Replies
    1. Goodnews to everyone out there with different health challenges as I know there are still a lot of people suffering from different health issues and are therefore looking for solutions. I bring you Good news. There is a man called Dr Osato, a herbal practitioner who cures me from HSV2, i have suffered from this virus for the past 5 years. I got my healing by taking the herbal medicine Dr Osato sent to me to drink for two weeks and after the completion of the dosage, I went for a medical checkup and I tested Negative and my doctor confirmed with me that I was completely free from HSV2. All thanks to God for leading me to Dr Osato who was able to cure me completely from this horrible virus, I’m sharing this so that other people can know of this great healer called Dr Osato because I got to know him through Martha Jecorick post who he cured from HSV1&2 . I was made to understand that he can cure several other deadly diseases and viruses. Don’t die in ignorance or silence thinking that there is no cure for HSV1&2 or HIV and don’t let that illness take your life. Contact Dr Osato and get the cure from him. Dr Osato email is osatoherbalcure@gmail.com or WhatsApp him on +2347051705853 and His website is https://osatoherbalcure.wordpress.com . He cure all forms of diseases/virus {1}HIV/AIDS {2}DIABETES {3}EPILEPSY {4} CANCER or all kind {5} HPV {6} BRAIN TUMOR {7} HEPATITIS {8}COPD{9} SICKLE AND ANAEMIA {9} HSV1&2. Be kind enough to share as you read for others to know and get cured.

      Delete
  2. wow this impacting regardless ,I love your enter offering little appreciation to overpowering pics may be part personss controlling alliance being defrent mind full scale poeple , Benefit Beverages NY

    ReplyDelete
  3. I have been suffering from herpes virus for over 7 years now, it was really hard on me and heartbreaking for me because the symptoms were very terrible and painful, i search for cure, went to different hospital but still no hope until a friend of mine introduce me to the great Dr. Williams from west Africa who helped her with some spiritual problem she was having then, i took the chance to contact the herbal doctor, he assured me that he will prepare his herbal medicine and deliver it to me through the ups delivery service and I will receive my package under 3 to 4 days, he said I will take the herbal medication for 21 days following the prescription which will be listed to me, i listened to everything he told me and purchased the herbal medicine having the fear that i might loose my money but to God be the glory everything went well, in a month time i went to the hospital for check
    up and my medical doctor confirmed it several time Negative.You Can contact Dr. Williams through his Email: (drwilliamscure@gmail.com,) or WhatsApp number +2347040461868, Website: https://drwilliamscure.wixsite.com/herbalhom

    ReplyDelete
  4. Goodnews to everyone out there with different health challenges as I know there are still a lot of people suffering from different health issues and are therefore looking for solutions. I bring you Good news. There is a man called Dr Osato, a herbal practitioner who cures me from HSV2, i have suffered from this virus for the past 5 years. I got my healing by taking the herbal medicine Dr Osato sent to me to drink for two weeks and after the completion of the dosage, I went for a medical checkup and I tested Negative and my doctor confirmed with me that I was completely free from HSV2. All thanks to God for leading me to Dr Osato who was able to cure me completely from this horrible virus, I’m sharing this so that other people can know of this great healer called Dr Osato because I got to know him through Martha Jecorick post who he cured from HSV1&2 . I was made to understand that he can cure several other deadly diseases and viruses. Don’t die in ignorance or silence thinking that there is no cure for HSV1&2 or HIV and don’t let that illness take your life. Contact Dr Osato and get the cure from him. Dr Osato email is osatoherbalcure@gmail.com or WhatsApp him on +2347051705853 and His website is https://osatoherbalcure.wordpress.com . He cure all forms of diseases/virus {1}HIV/AIDS {2}DIABETES {3}EPILEPSY {4} CANCER or all kind {5} HPV {6} BRAIN TUMOR {7} HEPATITIS {8}COPD{9} SICKLE AND ANAEMIA {9} HSV1&2. Be kind enough to share as you read for others to know and get cured.

    ReplyDelete
  5. I want to give a testimony on how I got cured from Herpes Virus. Few months back I was having some symptoms . I went to see My doctor and many blood tests was done on me, later on I was told I had Herpes. My doctor told me that there’s no cure for Herpes I felt bad, I went online searching for a possible cure for Herpes virus, I saw a post of Dr. Williams a herbal doctor that’s helping people get cured from Herpes, HIV, HEPATITIS B, CANCER e.t.c. I contacted him and I told him how I’m feeling he said his herbal medicine can cure me. He sent me the medicine and I received the medicine some days after he sent it, i took the medicine as prescribed by him. Before the completion of the medication the symptoms stopped. I went to the doctor and carried out another blood test, surprisingly I was negative. I haven’t had any symptoms anymore. Dr. Williams is great man and his herbal medicine has no side effects. You can reach him on his Email: (drwilliamscure@gmail.com,) or Whats App number +2347040461868, Website: https://drwilliamscure.wixsite.com/herbalhom

    ReplyDelete

Note: Only a member of this blog may post a comment.