CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

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  CSA PLUS 4013:19

  September 2019

  
  

  Title: TECHNICAL GUIDE

  Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

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CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

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ICS 13.060.10

ISBN 978-1-4883-2625-7

© 2019 Canadian Standards Association

All rights reserved. No part of this publication may be reproduced in any form whatsoever without the prior permission of the publisher.

CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

Contents

Preface 5

1. INTRODUCTION TO THE GUIDELINE 6

   1. Background and objectives of this Guideline 6

   2. Target audience 8

   3. Process used to develop this Guideline 8

   4. Main elements of this Guideline 9

   5. Limitations of this Guideline 10

   6. Terminology 10

      
      

2. EXTREME RAINFALL — METEOROLOGICAL ASPECTS 10

   1. Overview of atmospheric processes associated with extreme rainfall events 11

      1. Synoptic processes 12

      2. Convective processes 14

      3. Tropical cyclones 15

      4. Topographic and orographic processes 16

   2. Extreme rainfall in Canada 18

      1. Greatest rainfalls in Canada 18

      2. Spatial and temporal characteristics of extreme rainfall 21

      3. Atmospheric processes influencing extreme rainfalls in Canada 23

      4. Natural variability in the climate system and rainfall extremes 25

         
         

3. RAINFALL OBSERVATIONS & NETWORKS 26

   1. Background on rainfall observation networks 26

   2. Instruments, siting, and measurements 27

      1. Instruments 27

      2. Siting 30

      3. Measurements: Data processing and quality checking 30

   3. Network design in support of IDF development 33

      1. Understanding the climatology of the area of interest for IDF application 33

      2. Rain gauge site selection 34

      3. Data collection requirements 34

   4. Supplementary observation methods 35

      1. Radar 35

         
         

4. DERIVATION AND DISSEMINATION OF IDF VALUES 35

   1. Extreme value statistics used in IDF calculations 36

      1. What is a return period? 36

      2. Statistical distributions 36

      3. Difference between return period and average recurrence interval 37

      4. Sampling statistical uncertainty (confidence intervals) 37

      5. Return period calculations used for Environment Canada IDF information 38

      6. World Meteorological Organization (WMO) Guidelines for extreme value statistics for IDF calculation 38

   2. Derivation of IDF curves 38

      1. Overview 38

         
         

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         CSA PLUS 4013:19

         TECHNICAL GUIDE

         Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

         
         

      2. Data collection 39

      3. Data processing and quality control for IDF calculations 39

      4. Description of the Environment Canada rainfall IDF graph 40

      5. Description of Environment Canada IDF Table 1 43

      6. Description of Environment Canada rainfall IDF Table 2 46

      7. Description of Environment Canada rainfall IDF Table 3 48

      8. Main points to remember about Environment Canada IDF information 49

   3. Additional methods and related information 50

      1. Other methods for IDF derivation 50

      2. Related information 52

      3. Design storms 52

      4. Probable maximum precipitation (PMP) 53

      5. Estimating a return period of an event once it happens 53

      6. Reduction for large areas 56

      7. The use of extreme value statistics in a changing climate 57

   4. Dissemination and maintenance of IDF information 57

      1. Dissemination 57

      2. Maintenance and updating of IDF information 58

   5. Summary of Environment Canada IDF information characteristics 58

      
      

5. CLIMATE CHANGE AND RAINFALL PROJECTIONS 59

   1. Framing the climate change and IDF issue 59

   2. Overview of changes in temperature in Canada 60

      1. General 60

      2. Observed changes in temperature 63

      3. Projected changes in temperature 64

   3. Overview of changes in precipitation in Canada 66

      1. General 66

      2. Observed changes in precipitation extremes 67

      3. Projected changes in precipitation extremes from GCMs 67

   4. Strengths, limitations, and key uncertainties associated with different approaches for projecting extreme precipitation 70

      1. General 70

      2. Interpreting projections from GCMs and RCMs 71

      3. Statistical methods 71

      4. Using the physical relationship between temperature and extreme precipitation as a proxy 72

         
         

6. APPLYING IDF INFORMATION FOR CURRENT AND FUTURE CLIMATES: A PRACTITIONERS’ GUIDE 75

   1. Practitioners, IDF information, and infrastructures 75

      1. Common types of infrastructures 76

   2. Common analytical techniques utilizing IDF information 79

      1. Rational Method and its application 79

      2. Hydrologic modelling approach 82

      3. Conclusion 86

   3. Challenges and recommendations relating to the use of IDF information 86

      1. Clarifying theoretical basis for applying IDF Information 86

      2. Uncertainties in application of IDF information 88

         
         

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         CSA PLUS 4013:19

         TECHNICAL GUIDE

         Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

         
         

      3. Seasonality, low frequency climate variability, and changing climates 89

      4. Climate change, IDF values, and design criteria 90

         1. Recognize potential benefits of future advancements in climate models and downscaling techniques 90

         2. Use of statistical downscaling 91

         3. Use of longer duration IDF information 91

         4. Uncertainty in return periods 91

         5. Uncertainty and climate stationarity assumptions 91

         6. Applying a simple “increase” factor using CC relation 92

   4. Fostering forward-looking decision making for existing and new infrastructures under a changing climate 94

      1. General 94

      2. Existing infrastructures 94

      3. New infrastructures 95

Appendix 1 — MEMBERS OF THE IDF WORKING GROUP 97

Appendix 2 — TYPES OF NATURAL VARIABILITY IN THE CLIMATE SYSTEM (ENSO, PDO, NAO,

etc.) 98

Appendix 3 — LIST OF ENVIRONMENT CANADA QUALITY CONTROL CHECKS FOR TBRG DAILY AND HOURLY DATA 100

Appendix 4 — RETURN PERIODS AND THEIR INTERPRETATION 102

Appendix 5 — DISCUSSION OF EXTREME VALUE THEORY AND DISTRIBUTIONS 104

Appendix 6 — GLOSSARY OF TERMS 109

Appendix 7 — LIST OF ACRONYMS 113

Appendix 8 — REFERENCES BY CHAPTER 114

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CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

Preface

CSA PLUS 4013 (2nd Ed. Pub. 2012) — TECHNICAL GUIDE: Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners, has been designed for professionals with a role in the planning, design, management, inspection, and regulation of stormwater, drainage, wastewater, and flood management systems. It is not a design text book, but rather a resource for understanding the derivation, and application in water system planning and design, of rainfall intensity-duration-frequency (IDF) information.

In 2018, CSA was requested to update relevant parts of the document to reflect the latest scientific understanding of climate change and how to incorporate climate change into the formulation and application of IDF information. As such, work was undertaken to update Chapters 5 and Chapter 6 of this document. Most of the same members of the initial Working Group contributed to this (with the addition of a few others). All other material in the document remains the same as the version published in 2012, except for an update of the membership of the Working Group (Appendix 1) and the

references for Chapters 5 and 6 (Appendix 8).

Notes:

1. Use of the singular does not exclude the plural (and vice versa) when the sense allows.

2. Although the intended primary application of this Standard is stated in its Scope, it is important to note that it remains the responsibility of the users of the Standard to judge its suitability for their particular purpose.

3. This Standard was developed by consensus, which is defined by CSA Policy governing standardization — Code of good practice for standardization as “substantial agreement. Consensus implies much more than a simple majority, but not necessarily unanimity”. It is consistent with this definition that a member may be included in the Technical Committee list and yet not be in full agreement with all clauses of this Standard.

4. To submit a request for interpretation of this Standard, please send the following information to inquiries@csagroup.org and include “Request for interpretation” in the subject line:

   1. define the problem, making reference to the specific clause, and, where appropriate, include an illustrative sketch;

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   3. where possible, phrase the request in such a way that a specific “yes” or “no” answer will address the issue.

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   2. relevant clause, table, and/or figure number;

   3. wording of the proposed change; and

   4. rationale for the change.

September 2019 5

CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

CSA PLUS 4013:19

TECHNICAL GUIDE

Development, interpretation, and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners

1 INTRODUCTION TO THE GUIDELINE

CHAPTER SUMMARY

This Guideline has been designed for professionals with a role in the planning, design, management, inspection, and regulation of stormwater, drainage, wastewater, and flood management systems. It is not a design text book, but rather a resource for understanding the derivation, and application in water system planning and design, of rainfall intensity-duration-frequency (IDF) information.

IDF information describes the frequency (probability of occurrence) of extreme rainfall events of various rates and durations. This Guideline is intended to equip the reader with the ability to ensure that rainfall IDF characteristics are properly considered in the planning and design of water infrastructure. In particular, this can be provided through familiarization with the assumptions contained within current IDF information, and clarification regarding any limitations of this information. The effects of climate change on the frequency and intensity of extreme rainfall events are also addressed.

CHAPTER SUMMARY

This Guideline has been designed for professionals with a role in the planning, design, management, inspection, and regulation of stormwater, drainage, wastewater, and flood management systems. It is not a design text book, but rather a resource for understanding the derivation, and application in water system planning and design, of rainfall intensity-duration-frequency (IDF) information.

IDF information describes the frequency (probability of occurrence) of extreme rainfall events of various rates and durations. This Guideline is intended to equip the reader with the ability to ensure that rainfall IDF characteristics are properly considered in the planning and design of water infrastructure. In particular, this can be provided through familiarization with the assumptions contained within current IDF information, and clarification regarding any limitations of this information. The effects of climate change on the frequency and intensity of extreme rainfall events are also addressed.

1.1 Background and objectives of this Guideline

Canada has significant investments in stormwater, drainage, wastewater, and flood management systems. Every day, Canadians rely on this infrastructure to protect lives, property, and natural systems such as creeks, rivers, and lakes. In designing and managing these works, practicing professionals need to be concerned with the probability of occurrence of extreme values of rainfall amounts, often for specific storm durations. Rainfall IDF information commonly forms a critical input when applying the analytical techniques routinely used by practitioners.

IDF information is meant to describe the frequency (in terms of probability of occurrence) of extreme rainfall events of various rates and durations. The demand for rainfall IDF information has increased across Canada over recent years for a number of reasons. First, as the spatial heterogeneity of extreme rainfall patterns becomes better understood and documented, a stronger case is made for the value of “locally relevant” IDF information. Second, Canada continues to become increasingly urbanized. As urban areas expand, making watersheds generally less permeable to rainfall and run-off, many older water systems fall increasingly into deficit, failing to deliver the services for which they were designed. Understanding the full magnitude of this deficit requires information on the maximum inputs (extreme rainfall events) with which drainage works must contend. Finally, research now indicates that climate change will likely result in an increase in the intensity and frequency of extreme precipitation events in most regions in the future. As a result, IDF values will optimally need to be updated more frequently

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