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CSA PLUS 4013-2019

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). 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. 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 than in the past and climate change scenarios might eventually be drawn upon in order to inform IDF calculations. Meanwhile, new approaches are emerging for enhancing knowledge of rainfall IDF characteristics. For example, radar imagery patterns are being investigated to retroactively "fill in the (data) gaps" for regions where conventional rainfall measurement instrumentation, and hence IDF data are missing. New approaches are also being considered for the derivation of regional, rather than single point, IDF curves. Environment Canada is the main provider of IDF information in Canada. Environment Canada IDF information is available for locations covered by Environment Canada’s national rate of rainfall monitoring network, currently comprised of 559 stations (see Figure 1.1) installed and managed in accordance with WMO standards. While Environment Canada’s approach to the collection of rainfall IDF data and the derivation of IDF values is in the public domain, this Guideline represents a first attempt to assemble methodologies, assumptions, and limitations into a single document for reference by non-Environment Canada personnel. Given the heightened demand for IDF information, the new and emerging methods for developing IDF information, and the increasing number of parties outside Environment Canada who have begun to calculate IDF values either in a consulting capacity or for their own use, the current Guideline was developed in order to: a) Provide practitioners with a basic understanding of meteorological drivers of extreme rainfall that need to be understood so that practitioners can determine and meet their local needs; b) Outline how rainfall intensity-duration-frequency (IDF) information has traditionally been developed in Canada, including explanations of the assumptions embedded within current IDF information that are of potential significance for water resources practitioners; c) Provide guidance to determine if additional precipitation information, including IDF values, is required to optimize the local design of infrastructure; d) If new IDF information is required, provide guidance for design and implementation of an enhanced precipitation network, including gauging needs and data requirements; e) Discuss what the implications of climate change might be for the development, interpretation, and use of rainfall IDF information and provide guidance on how these implications could be considered; f) Provide examples of how IDF is currently used in water resources infrastructure planning and design, outline best management practices for management of risks and sustainability and provide guidance on a range of adaptation options that address potential changes in rainfall IDF characteristics that could occur as a result of climate change; and g) Highlight opportunities for expansion in future editions of this Guideline, including treatment of seasonal rainfall events, implications for measuring and monitoring and incorporation of rainfall data that meets variable standards for data quality, as well as additional resources on best practices for factoring in expected climate change implications for IDF information. This Guideline is intended to help ensure that rainfall IDF characteristics are properly considered and used in the planning and design of water infrastructure by making clear the assumptions associated with the current IDF information, including its strengths and weaknesses, and the implications of climate change on IDF. 1.2 Target audience The intended users of the document reflect the objectives as outlined, and include: • Water resource and infrastructure practitioners or other stakeholders who want to better understand the derivation and application of IDF information when generating new IDF or enhanced information (i.e. guidance on establishing monitoring networks and collecting available data, through to the compilation and derivation of IDF values) • Water resource and infrastructure practitioners who need to better understand the derivation and application of IDF information in order to sustainably manage their infrastructure; • Water practitioners who need to better understand the current and future climate drivers of extreme rainfall IDF values and adaptation options to deal with the implications of a changing climate; and • Educational institutions, including engineering and other disciplines requiring an understanding of the IDF derivation, interpretation, and application process. 1.3 Process used to develop this Guideline Development of the second edition of the IDF Guideline was informed by the results of an extensive public review of the first edition of the Guideline. The changes from the first to second editions can be broadly organized around efforts to increase the Guideline’s practicality and applicability to its intended end users, efforts to include more discussion specific to climate change adaptation, and improvements in the technical content of the guidelines. Furthermore, two new chapters have been added to the IDF Guideline. Chapter 7 provides an overview of principles for practitioners to use when considering climate change-driven adaptations for water resources infrastructure. Chapter 8 details a number of areas for possible future advancement of the IDF Guideline. The Guideline was developed by an Expert Working Group convened by CSA. Expert Working Group members were selected based upon their expertise on the science of extreme rainfall, climate change, derivation of IDF values, and application of IDF-related information in the planning and design of stormwater, drainage, wastewater, and flood management systems. Members of the Working Group are listed under Appendix of this Guideline. 1.4 Main elements of this Guideline In order to help water resource practitioners understand how current IDF values can be most appropriately applied, it is important that the assumptions embedded within current IDF information be clarified. This requires that a relatively comprehensive picture be provided describing the development of existing IDF values, including the rate of rainfall data, derivation methodologies, the science behind IDF methodologies, and their limitations. This Guideline is structured to take the reader through each phase of the IDF development and interpretation process to the emerging issues associated with climate change and challenges associated with using IDF values in infrastructure planning and design. It first provides an overview of what the science indicates with respect to the meteorological processes that generate extreme rainfall events at different spatial and temporal scales (Chapter 2). Second, it addresses means used to measure rainfall and the science and assumptions that have influenced these approaches (Chapter 3). Third, this Guideline describes the empirical and statistical approaches, as well as the assumptions used, to produce the IDF values (Chapter 4). Chapter 4 also addresses new approaches being used to develop IDF information. Fourth, the Guideline assesses changes in rainfall extremes in Canada and the ways in which climate change may affect the accuracy and utility of current IDF values (Chapter 5). The Guideline also considers the various applications of current IDF information used in the planning and design of stormwater, drainage, wastewater, and flood management systems (Chapter 6), and the adaptive actions practitioners could undertake to mitigate or adapt to the impacts on infrastructure resulting from climate change (Chapter 7). Chapter 8 then considers possible advancements for the Guideline that could be undertaken in future editions. Eleven appendices are included in order to provide the interested reader with additional information and resources in a variety of areas. Figure 1.2 provides a graphic representation of the structure and content of the Guideline. Figure 1.2 - Structure and content of the Guideline 1.5 Limitations of this Guideline This Guideline is not a design text book for water resource infrastructure nor is it meant to be a technical standard. Rather, it deals exclusively with the development of rainfall IDF information, and key considerations or concerns relating to the application of this information in the planning, design, and management of stormwater, drainage, wastewater, and flood management systems. This Guideline does not provide specific direction for the incorporation of climate change assumptions into IDF information. It could, however, serve as an essential precursor to future guidance focused specifically on the derivation of climate change-adjusted IDF information for use in adaptation planning. 1.6 Terminology Definitions for technical terms used in the Guideline are included in a Glossary, Appendix. Appendix provides a list of acronyms.


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