So you don’t have to be a meteorologist to agree that Hurricane Isaias was one for the record books. While it may not have produced as much total rain or extreme winds as other hurricanes that have traveled through the Delaware Valley like Sandy or Irene, I think everyone can agree that it did produce an incredible amount of rain in a rather short period of time. So after having to come home from a family vacation to pump out a few inches of water from my basement on Tuesday, and hearing stories of many friends and family members who also had more water problems than they’ve ever had before, I decided to do some research to find out how much rain actually fell from Isaias. After spending some time browsing through the area’s rain gauges, it was very clear that rainfall amounts varied widely. Areas in Center City Philadelphia came in generally between 2 to 4”, while the immediate suburbs slightly west of the city such as Delaware and Chester Counties were in the path of the heaviest rainfall. Areas here received anywhere from 5” up to the highest amount recorded of 8.59” in Wynnewood, Delaware County. Now this is a lot of rain considering this amount of water is generally associated with what the engineering community considers more than a 100-year storm. And just a quick note for those of you who don’t deal with this every day as we do, most municipalities in this area and the PA Department of Environmental Protection (DEP) require stormwater management to be designed for storms up to the 100-year storm. The “100-year storm” is simply the estimated likelihood of that storm event happening in any given year. The 100-year event has a one (1) percent chance of occurring in any given year, but statistically that doesn’t mean it couldn’t happen every year or even twice a year. It is just a measurement of the chance of it happening.
With that being said, the 100-year storm event in this area would be considered anywhere from 7.2” to just over 8” of rainfall within a 24-hour time span. Most people would look at this and the rainfall amounts I referenced above and be like, well Isaias must have been a 100-year storm. To be honest, I initially thought so too, however, looking more closely at the storm's timing, this 7” to 8.59” of rain occurred over a time span of approximately 8 hours. It started about 3 AM and tapered off around noon. After reviewing the NOAA National Weather Service charts, I was absolutely blown away to see that 8” of rain within a 9-hour timeframe in the Delaware Valley is a 1,000-year storm. Yes, you heard that right! It would be considered a 1,000-year storm. This storm has a 0.1% chance of happening in any given year, and of course it just happened. Knowing how extreme an event such as that is, I absolutely understand why roads and basements were flooded, and stormwater facilities were spilling over their banks. It was just too much water, way too quickly. Not only were we dealing with the high total volume of rainwater, but also the extreme intensity of the rainfall for a long period.
So for those of you who would like us to start designing the stormwater management facilities for the 1,000-year storm, please give us a call because we’d love to help out; just don’t be surprised by the size and cost associated with controlling that much stormwater.