When designing a drainage system, it's important to ensure that the system can handle the peak flow required without incurring unnecessary costs. Whether you're dealing with trench drains, catch basins, french drains, or pavers, it's important to understand the equations behind peak flow, and its implications for your drainage system. We'll explore the factors to ensure that your drainage system is robust enough to endure heavy rainfall, yet affordable enough to be practical.

Recurrence Intervals and X-Year Storms

If you've watched the news in the aftermath of Hurricane Helene, you've probably heard about the concept of a 100-year storm, especially if you live near western North Carolina -- Helene was a 1000-year storm! Does that mean that a storm like Hurricane Helene only happens once every 1000 years? Unfortunately, that's not the case. As Asheville can attest, and the previous article mentions, these once in 1000 year storms have happened twice in the past six years. As the US Geological Survey notes in their fantastic article about 100-year floods, most hydrologists don't love the nomenclature -- a 1000-year storm has a 0.01% chance of occurring in a given year; over the course of a million years, such a storm would be expected to happen only 1000 times. We've graphed the recurrence intervals and their respective probabilities of occurrences below:

The graph illustrates how the probability decreases as the recurrence interval increases. Here's a breakdown of the probabilities:

It's important to remember that these are just probabilities, and as Asheville shows, they can happen more frequently than the data suggests, especially as climate change affects weather patterns. The term "X-year storm" refers to the average long-term recurrence interval, not a guarantee of time between events. So how exactly does it effect us when designing drainage systems?

Impact on Drainage System Design

When designing a drainage system, it's important to protect critical infrastructure. Hospitals and other critical areas, obviously, should plan for more extreme events than non-critical areas. As a rule of thumb, we recommend designing drainage systems for a minimum of 25-year storms, with more important and critical infrastructure being protected from 100-to-500-year storms -- once you get over a storm this size, almost all bets are off: your drainage system won't protect you from a mudslide, and at record precipitation levels, you should be extremely aware of your surroundings. If you're in the United States, the NOAA provides excellent data that's extremely granular about the amount of rainfall that your area may receive in a given timeframe, with a 90% confidence interval. This high quality data serves as the basis for our recommendations, and is used in the calculations for hydrological methods below.