A (Moderately) Deep Dive after Hurricane Helene
By Danvey Walsh- Geologist and Environmental Scientist at Equinox
Fair warning, we don’t frequently publish blog posts with math equations, but this is a technical subject. Fortune favors the bold! I also encourage you to follow the links provided to enhance your understanding. For the less bold among you (or the simply too busy, as is more likely the case) at least scroll to the Conclusions section for a brief summary. We here at Equinox are still trying to wrap our heads around Helene. Many others are doing the same and NOAA is absolutely one of the best synopses of the rainfall and flooding I have seen thus far.
In the wake of Helene, I’m sure most folks have heard the term “1000-year event” or something similar, and I’m sure people kind of wonder what that means or how we could ever know that. To begin with, it’s important to recognize that term applies to two separate things, but the distinction is not always clear in media. Specifically, there is a difference between a rainfall event and a flood event. Rainfall and flood recurrence intervals are crucial concepts in hydrology and water resource management. They help us understand the probability of an extreme rainfall event or flood happening within a certain period. This blog post aims to demystify how these intervals are calculated and interpreted.
Rainfall Recurrence Intervals
Rainfall recurrence intervals are calculated using historical rainfall data. The longer the historical record, the more accurate the calculation. Here’s a simplified breakdown of the process:
- Data Collection: Gather daily or hourly rainfall data for a specific location over a long period (at least 30 years).
- Rank the Data: Sort the rainfall data from highest to lowest.
- Calculate the Recurrence Interval: For each rainfall event, the recurrence interval (RI) is calculated using the Weibull formula, which is:
- RI = (n + 1) / m where:
- n is the total number of years in the record
- m is the rank of the rainfall event
- Plotting the Data: Plot the rainfall depth against the corresponding recurrence interval on a logarithmic scale. This graph is known as a rainfall frequency curve.
If you’re interested in a more detailed look at this process, you can use this link.
Flood Recurrence Intervals
Flood recurrence intervals are calculated in a similar way, but instead of rainfall depth, we use peak streamflow data. Here’s how it’s done:
- Streamflow Data: Collect peak streamflow data for a specific river gauge over a long period. Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Of course, the more years of historical data the better—a hydrologist will typically have more confidence in an analysis using 30 or more years of peak streamflow record than one based on 10 years of record. Because really large events happen less frequently, we are less certain about what magnitude event has exactly what recurrence interval.
- Rank the Data: Sort the peak streamflow data from highest to lowest.
- Calculate the Recurrence Interval: Use the same Weibull formula as above to calculate the recurrence interval for each peak flow.
- Plotting the Data: Plot the peak flow against the corresponding recurrence interval on a logarithmic scale. This graph is known as a flood frequency curve.
If you’re interested in a more detailed look at this process, you can use this link.
Interpreting Recurrence Intervals
A recurrence interval is often referred to as a “return period.” However, it’s important to remember that this doesn’t mean the event will happen exactly once every RI number of years. Instead, it means there’s a 1/RI chance of that event being equaled or exceeded in any given year.
This is often expressed as an Annual Exceedance Probability (AEP), and I personally prefer this concept and term. For example, a 100-year flood has a 1 in 100 or 1% AEP or chance of occurring in any given year. Similarly, a 10-year rainfall event has a 10% AEP or chance of occurring in any given year.
For an event with an AEP of 1%, there is a 26% (roughly 1 in 4) chance of it occurring during the span of 30 years, a common length of a mortgage.
Limitations and Considerations
It’s important to acknowledge that recurrence intervals are based on historical data and assume that the future will behave like the past. However, climate change and other factors are altering rainfall and streamflow patterns, making these calculations less reliable. This means we need to supplement and enhance our approach to determining recurrence intervals with additional data to improve it. In fact, this issue is of such concern that the popular tech real-estate marketplace company, Zillow, has begun featuring detailed climate risk information into their listings.
Zillow’s move to provide this information is an example of why it’s crucial to consider the limitations of recurrence intervals and use them in conjunction with other information, such as climate projections and land-use changes, to make informed decisions about stormwater control, water resource management, and flood mitigation.
Here is a recent article on how that is affecting the Carolinas specifically.
Conclusions regarding the magnitude of Hurricane Helene
So, now that we understand the methodologies and recognize their limitations, we still want to understand where Helene stands in the context of history.
In the days leading up to Helene’s landfall the preceding rainfall event produced 10 to 15 inches of rain across the region, saturating the ground and swelling the rivers. And then came Helene itself.
In total, several locations across Florida, Georgia, and the Carolinas recorded more than a foot of rain from Helene, with the highest rainfall totals across the North Carolina Mountains. Busick, North Carolina, recorded a preliminary rainfall total of 30.78 inches, and Mount Mitchell State Park recorded 24.20 inches. The weather station at Asheville’s airport recorded 13.98 inches of rainfall in a three-day period. Estimated rainfall totals from Helene across the southern Appalachians had an Annual Recurrence Interval greater than 1,000 years over a wide area; meaning there is less than a 0.1% chance (annual exceedance probability) of that happening in any given year, according to NOAA’s National Water Center. But for perspective, there is around a 3% chance of a 1000-year event occurring during the span of a 30-year period. Imagine a raffle with only 33 tickets sold. If you buy one ticket, your chance of winning is roughly 3%.
This extreme amount of rain over already-saturated soils resulted in catastrophic and historic flooding across western North Carolina. In the city of Asheville, North Carolina, some likened this flood event to that of the “Great Flood” of 1916. The river gauge along the French Broad River in Asheville exceeded the previous record of the 1916 flood by over 1.5 feet at the peak of Helene’s flooding. Several other rivers in the surrounding area, such as the Swannanoa River at Biltmore, also set new records. All this rainfall also caused intense stress to the mountainous terrain, and nearly 2,000 total landslides have been observed according to the United States Geological Survey. At one point, all roads in western North Carolina were considered closed to all non-emergency travel.
This event was close to, if not a worse-case scenario meteorologically speaking, for western North Carolina. The full extent of the damage is still being realized and will take months to years to document and recover from.
More important than all the math and ranking is this staggering fact, Helene is the third-deadliest hurricane of the modern era (behind Maria and Katrina) with a death toll of over 200, according to NCEI data. Nearly half of those deaths were in North Carolina.
Rainfall and flood recurrence intervals are valuable tools for understanding the probability of extreme events. By understanding how they are calculated and interpreted, and what the current limitations are in a changing climate, we can better prepare for future floods and prevent catastrophic loss of life, property, and the natural heritage which make our home so special.
