When developing an Intensity-Duration-Frequency (IDF) curve, what is the specific purpose of plotting rainfall intensity against duration for various return periods?

When developing an Intensity-Duration-Frequency (IDF) curve, the specific purpose of plotting rainfall intensity against duration for various return periods is to empirically derive and visually represent the unique statistical relationship between these three critical hydrological parameters for a particular geographic location. This relationship is fundamental for the design and analysis of water resources infrastructure.

Rainfall intensity refers to the rate at which precipitation falls, typically measured in units like millimeters per hour (mm/hr) or inches per hour (in/hr). Duration is the continuous period over which a rainfall event occurs, ranging from minutes to several hours or days. The return period, also known as the recurrence interval, represents the average time interval, in years, over which a rainfall event of a given intensity and duration is expected to be equaled or exceeded once; it quantifies the probability of a specific storm event occurring in any given year. For instance, a 100-year return period storm has a 1% chance of occurring in any single year.

By plotting intensity against duration for multiple specific return periods (e.g., 2-year, 10-year, 50-year, 100-year), a family of curves is generated. Each curve corresponds to a specific return period and shows how the maximum average rainfall intensity decreases as the duration of the storm event increases. This occurs because short-duration storms tend to have higher peak intensities, while longer-duration storms, while accumulating more total rainfall, typically have lower average intensities over their entire duration. Conversely, for a fixed duration, the intensity increases as the return period increases, reflecting rarer and more severe rainfall events.

This plotting process distills complex historical rainfall data into a practical design tool. The resulting IDF curves allow hydrologists and engineers to determine the appropriate design rainfall intensity for a specified storm duration and return period, which is crucial for sizing stormwater drainage systems, culverts, flood control structures, and other infrastructure to adequately manage runoff and mitigate flood risks, ensuring their long-term functionality and safety.