This past week, the region experienced some pretty severe weather. Roads were closed throughout the area due to flooding and downed trees and power lines. Local weather gauges measured as much as 4.43” of rain in New Hanover, but generally speaking, southeastern PA saw somewhere between the 2-year and the 10-year frequency storm event. Additionally, the storms brought strong winds with gusts as fast as 60 MPH.
Now I know what you are thinking – engineers design stormwater management systems for rainfall events up to the 100-year event. If that is the case, why do we see any flooding from something as seemingly minor as a 5-year frequency storm?
The answer is pretty simple, and it is one of the biggest challenges we face in site design. Although new land developments require stormwater management controls to manage up to the 100-year storm event, which in this area is generally about 7.4” of rain in a 24-hour period, the surrounding existing infrastructure is rarely designed to the same standard. Most existing facilities are undersized when compared to modern standards, and that is where the stormwater can back up, pond, and close streets and bridges. The best we can do with new projects is design them to reduce post-development conditions to match pre-development stormwater runoff from our sites, which in turn should reduce some of the burden on the downstream infrastructure. It will take considerable investment from federal, state and local agencies to upgrade all existing stormwater infrastructure, so there is no magic bullet to solve the issue.