It is widely recognized that extreme precipitation events can have significant impacts on built infrastructure. In Canada, such events have led to some of the highest economic losses relative to any other type of weather peril. Notable examples of heavy precipitation events that have had significant impacts on built infrastructure include the 2013 flooding in Calgary, which resulted in C$1.7 billion ($1.36 billion) in insured damages (Nelson, 2013), or the flooding that occurred in Toronto the same year, which led to nearly $1 billion in insured losses (Flavelle, 2014).

One of the most important elements that can dictate the eventual effects of a heavy precipitation event on built infrastructure is the adequacy of the storm water systems designed, engineered and constructed to manage the large volumes of water commonly associated with such weather events. In this respect, infrastructure practitioners commonly use intensity-duration-frequency (IDF) information to inform the structural details of storm water infrastructure.

IDF information, in brief, describes the frequency,in terms of probability of occurrence, of extreme rainfall events of various rates and durations. This information specifically provides key inputs into a formula used in determining the performance characteristics of built storm water infrastructure. It is also important to recognize that most IDF information is point-specific, and not areal in extent.

This article provides context and a high-level overview of a CSA Group document that provides specific guidance on the derivation, interpretation and use of IDF information: “Development, interpretation and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners (second edition)”, CSA Group, 2010 (v1), 2012 (v2).

The general purpose and driver in the development of this guidance document was to encapsulate the experience and expertise of climate scientists, hydrologists and infrastructure practitioners in the generation and use of IDF information in Canada. It was also noted that there was an increase in demand for IDF information and there was a broadening number and range of entities providing this information. The members of the associated Working Group are acknowledged at the end of this article.

Changing Demand for IDF Information

It is primarily important to recognize that the demand for rainfall IDF information has increased in Canada and throughout many parts of the world in recent years. The exact reasons underlying this increase in demand are diverse, but can be broadly summarized as follows:

  • Climate conditions have changed, particularly in terms of the frequency, intensity and duration of rainfall events across Canada.
  • The incidence of flooding has increased and commensurate built infrastructure impacts and damages.
  • Extreme rainfall events and patterns have become better understood and documented.
  • Urbanization has increased, including that of development in watersheds. This development has resulted in land forms that are generally less permeable to rainfall and run-off.
  • Many storm water systems have deteriorated as they aged or had to service a larger population.

As a result, in addition to a broader user base, IDF values now need to be updated more frequently than in the past and with the need to consider how changes in climatic conditions might impact on the IDF calculations.

From Rainfall Data Collection to IDF Derivation

In Canada, rainfall data is collected by Environment Canada, provincial and territorial ministries, municipalities, and, more recently, other organizations such as universities and other research-based organizations. This data is collected using a variety of measuring instruments andstandards. A key purpose for having data on the occurrence of rainfall events is that this is intended to provide a long-term baseline of observational data for use in characterizing important aspects of climate and weather at regional scales.

Environment Canada’s networks and individual monitoring stations are generally designed, located and operated in accordance with World Meteorological Organization (WMO) guidelines.

A key issue related to the use of point-based IDF information in water infrastructure design and management is how large or small an area is accurately represented by existing IDF values. Various techniques can be used: analyses of historical, site-specific data, analyses of radar data and the use of knowledge of dominant meteorological processes to discern whether distinctive precipitation zones and areas of influence may exist.

The most common approach to derive IDF information is to fit an appropriate frequency distribution to data from observing sites. This allows for an estimation of the parameters of the distribution from which the various return levels are calculated. The Gumbel distribution is the extreme value distribution that has historically been used by Environment Canada. It is fitted to the annual maximum series (AMS) of each rainfall duration and used to calculate the return period for Environment Canada IDF tables and graphs.

Storage and conveyance infrastructure is typically designed and managed by applying a variety of analytical techniques, most of which use IDF information as a key input:

  • The rational method (RM) is the most direct application of IDF curves currently in use in hydrologic engineering, the purpose of which is to estimate the highest peak flow for a given return period;
  • The modified rational method (MRM) is most commonly used in the design of facilities for temporary storage of rainwater, such as detention ponds.

A primary limitation of the RM is that it can only be used to compute an estimate of peak flow. Hence,the MRM was developed to overcome this restriction, and hydrological modelling aims to provide a thorough and comprehensive understanding of hydrologic response throughout an entire watershed.

Factoring in Changes in Climate Conditions: Principles and Approaches

It is widely agreed that with any future increases in atmospheric temperatures, there will be parallel increases in the frequency and intensity of heavy precipitation events. The IDF guidelines provide a number of principles and approaches for considering such changes in climate conditions, including extending the service lives of existing sewers whose design capacity is expected to be exceeded as the result of climate change. Broadly, these include:

  • Removing or diverting loading. For example by: Low Impact Development (LID) or green infrastructure strategies, integrated resource management techniques, rehabilitating sewers to reduce inflow and infiltration, and so on.
  • Expanding or rerouting major flow paths. The solution may not always be in the piped system.
  • Re-evaluating appropriate levels of service under future climate scenarios. For example, is the originally anticipated level of service still going to be practical and economical?

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Acknowledgements

The IDF guidelines were developed with input from an expert Working Group: Heather Auld (Chair), Risk Sciences International (formerly of Environment Canada); John Manson (Vice Chair), Engineers Canada; Brent Burton, Metro Vancouver; Robert Morris (Lead Author), Environment Canada; Joan Klaassen (Lead Author), Environment Canada; Rob Bishop (Contributing Author), MMM Group; Don Haley (Contributing Author), Toronto and Region Conservation Authority; Alain Mailhot (Contributing Author), Institut National de la Recherche Scientifique; Ryan Ness (Contributing Author), Toronto and Region Conservation Authority; Edson Paixao (Contributing Author), Environment Canada; Dennis Westhof (Contributing Author), Westhoff Engineering; Erik Sparling (Special Advisor), Risk Sciences International; Paul Steenhof (Project Manager), CSA Group

Bibliography

CSA Group. (2012). Development, interpretation and use of rainfall intensity-duration-frequency (IDF) information: Guideline for Canadian water resources practitioners (second edition)”. Toronto: CSA Group.

Flavelle, D. (2014, July 5). A year after the Toronto flood. The Toronto Star.

Nelson, J. (2013, January 20). Canadian insurers made record payouts in 2013. The Globe and Mail.