An illustrated guide to risk management: reconciling intensity, duration, and frequency from the watershed’s perspective

Mike Gregory


The acronym for Intensity Duration Frequency (IDF) has long been in the engineer’s vocabulary; in the context of traditional stormwater infrastructure design that assumed the statistical properties of intense rainfall events were similar to those of the resulting runoff from land surfaces. Rather than characterizing either the input function of meteorological events or the response function of hydrologic/hydraulic conditions, a risk management approach can be adopted that considers the resulting impacts of human activity on downstream watercourses and waterbodies. This paper looks at risk management from the perspective of watershed impacts, whether due to development/disturbance of the native hydrology or the construction of engineered collection systems and storage/treatment facilities. The assessment of watershed impacts is highlighted through the lens of the familiar IDF acronym, with Intensity as a measure of severity, Duration as a measure of endurance, and Frequency as a measure of recurrence. The examples presented highlight a range of design functions (flood control and water balance), a range of project locations (Canada and the United States), and a range of structural options (infiltration, bioretention, and rainwater harvesting).

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