The 100-year storm sounds like a storm you may only need to worry about once in a lifetime unless you are really “lucky” and get to see it twice. Lately, it seems we get to witness one nearly every year or so with its little brothers, the 25 yr and 50 yr storms popping up every summer. And with each big flooding event comes the phone calls about neighbors flooding out neighbors because someone put in a pool or added a shed. Our local politicians slide into their all too comfortable “reactive” (rarely proactive) roles and start trying to do something “so this never happens again.” Truly my all-time favorite statement to hear them say is, “we are going to come together in a bipartisan way to enact legislation to ensure this never happens again.” #hugeyawn And when they follow that up saying they are going to “raise awareness” to an issue we all deal with on a daily basis I get all tingly. Sprinkled, however, into the phone calls about flooding are calls ridiculing us engineers on why all of our fancy-dancy and expensive stormwater designs aren’t “working”. This is easily taken in stride if you understand stormwater runoff and how things are modeled. In short, other than the toddler of storm events, the two-year storm, the other storms have ZERO volume reduction occurring when they pass through the previously aforementioned fancy-dancy and expensive stormwater management facilities. You heard that right; VOLUME IS NOT CONTROLLED!
Therefore, you are right. If your neighbor added a pool or a shed or a 100 lot subdivision upstream of you, then you, in fact, are seeing MORE runoff every time it rains. Period end of story. More impervious equals more volume of runoff. The more it rains, the more water there is to runoff. Now, theoretically, the RATE of runoff is less, but that is for another newsletter one day. I think we can all agree that when you are part of the misfortunate that live/work at the bottom of a large watershed, that volume of runoff becomes the main concern. There is just nowhere for the water to go except up, and hence flooding occurs. None of this should really be news to any of us. However, one thing Tropical Depression Ida has done is turned up the candle power on the spotlight shining on these rainfall amounts. Engineers model for the 24-hour storm event, meaning we calculate runoff assuming a certain number of inches of rain falls during a 24 hour period. But the problem is we are getting these rainfall amounts over a much shorter time period, and when you get more rain in a shorter time period, that is bad. For example…7.2 inches of rain in a 24 hour period has a return frequency of 100 years; however, 7.2 inches of rain in a 6 hour period has a return frequency of 1,000 years!! The West Branch of the Brandywine Creek received just that during Ida and flooded large sections of Coatesville. The question now is….how will our local governments react to “make sure this never happens again”? Will the 100-year storm be redefined? Will we be required to design for the 1,000-year storm? That all remains to be seen but rest assured when it changes, D.L. Howell will be ready to design a system to “make sure this never happens again!!!”