Perhaps inspiring Murphy’s later pronouncement, mathematician Augustus De Morgan said,
“Whatever can happen, will happen if we make trials enough.”
At some point, your flood infrastructure will be tested by a storm event equal to or larger than the event it was designed to withstand. The chance in any given year that a storm sufficiently strong enough to cause devastating damage is relatively low, however, across a span of decades the probability of such a storm increases exponentially.
Understanding the increasing probability of the “100-year storm” is critical when considering the conditions of aging infrastructure.
What is the 100-Year Storm?
Is the 100-year storm one that occurs every hundred years? Not quite. The “100-year storm” refers to a precipitation event with such intensity that there is only a 1% chance that a storm of such intensity will occur in any given year.
The 100-year storm is used as the design storm for many large regional facilities nationwide because it provides a sufficient level of safety against flooding—99% of storms that occur each year for a particular duration will be less intense than the 100-year storm.
However, the probability of 100-year storm occurrence is more likely than many realize, said Jonis Smith, Dudek senior project manager for stormwater resources. Across a longer time frame, the chance that a 100-year storm could occur in the next 10 years (9.6%) or 25 years (22.2%) increases significantly from the 1% chance of the 100-year storm occurring this year.
If, in a certain city, all of the bridges that span rivers or creeks were designed for a 100-year storm event, then in a 50-year period designers should expect approximately 40% of these bridges to experience a storm event equal to or larger than the 100-year storm.
Probability and Precaution
“I often hear people state that in 25 years they have never seen a certain channel with a flow deeper than a couple of feet, so what need is there to make the channel bigger,” Smith said. “But you can’t just continue to ‘buy time’ if a system is in need of rehabilitation.” In the past 25 years, there is a 22.2% chance that a 100-year storm occurred and, thus, a 77.8% chance that it did not. However, in the next 25 years, the probability of a 100-year storm event occurring nearly doubles to 40%.
Probability of Occurrence During Facility Design Life
Additionally, new data from the National Weather Service reveal that the100-year storm is actually 10−15% bigger than old data indicated. That means that existing old structures are undersized and may not actually provide sufficient flood protection against a100-year storm event.
Across the state this year, powerful storms caused considerable damage including flooding and bridge collapse. As we move into a future of potential long droughts and sudden deluges of rain, it is increasingly important to recognize the jeopardy of the 100-year storm and consider improvements to aging infrastructure that may have been designed according to outmoded, inferior data.
Jonis Smith is a professional engineer with 21 years’ experience in all aspects of stormwater management, flood control engineering, and water resource system design. For more information, contact him at email@example.com or 949.373.8334.