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25, 100, or 500?

On September 28th, Hurricane Ian made landfall on the southwestern coast of Florida. The  Federal Emergency Management Agency (FEMA) has classified this storm as a 500-year storm  event, making it one of the worst storms that the United States has seen in recent years. To most  people, the term “500-year storm” is just a phrase, and the expression fails to shed light on the  storm’s impact. To truly understand the significance of this hurricane, it is important to understand  how our current stormwater infrastructure stands up to a storm of this magnitude.  

 A 500-year storm is a storm that has a 1-in-500, or 0.2% chance of occurring in any given  year. The intensity of these storms is determined by FEMA, based on historic rainfall data in a  given area. When designing drainage systems, engineers take this rainfall data into consideration.  Locations with higher annual rainfall rates call for larger stormwater retention systems than  locations with lower annual rainfall rates.  

In New Jersey, the Department of Environmental Protection (NJDEP) requires that we  design our stormwater conveyance systems for a 25-year storm event, which has a 4.0% chance of  occurring in any given year. The purpose of these systems is to convey stormwater runoff from  one location to another, often to a detention system. Stormwater detention systems on the other  hand, which contain a specific volume of water and release it slowly over time, must be designed  for the 100-year storm, which have a 1.0% chance of occurring in any given year. So, if a  stormwater drainage system is designed to NJDEP’s standards, the chances of it failing in any  given year are low.  

As civil engineers, it is our responsibility to design systems that can minimize the effects  of major storm events when they occur. Here at WJH Engineering, our team of professionals is  dedicated to creating designs that ensure the long-team safety of our clients. If you are looking for  a company that specializes in stormwater management design, contact WJH Engineering today!

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