As a Staff Engineer at Howell Engineering, I design stormwater management systems every day to safely capture and convey stormwater away from proposed impervious features.
Chester County, Pennsylvania experiences a mix of weather patterns due to its proximity to the Atlantic Ocean, including thunderstorms, winter storms and high winds. However, the weather event that most concerns us here at Howell is flash flooding.
As many of you know, Townships throughout Pennsylvania now have stormwater management ordinances for all new construction projects which compare pre-development to post-development conditions. These ordinances outline the required volume to be infiltrated, and the required peak rate reduction needed for each X-year storm event. There may be additional requirements by Township, but these two are common in Pennsylvania to reduce the amount of flooding that occurs during storm events.
My interest in storm events got more personal in 2021. I lost my Jeep Grand Cherokee to a flash flood event stemming from Hurricane Ida. Floodwaters swept my car from the roadway and drifted me into a nearby overloaded basin, where my car sank to its fate. And yes, I was in the vehicle when this all happened, but I was able to exit through my sunroof before my car was completely submerged.
Last week, Hurricane Erin ripped through the Atlantic Ocean with maximum sustained winds of 160 mph and put the entire Outer Banks of North Carolina at risk with storm surges reaching up to 4 feet and swells up to 17 feet.
Comparing my horrible experience during Hurricane Ida to the storm surge from Hurricane Erin activated my PTSD when I realized I would be dead if I were on a road in the Outer Banks when the storm surge was at its highest. Once my engineering brain reengaged, I began to remember how houses and roadways in the Outer Banks are designed for hurricane forces that only Mother Nature knows are coming.
The natural topography of the Outer Banks and the seasonally high water table are driven by the proximity of the Atlantic Ocean. So, traditional drainage systems are ineffective for stormwater management facilities there. Roadways in coastal towns are designed to be elevated just above the 100-year flood zone, allowing them to remain accessible during minor flood events.
However, civil engineers have acknowledged that some flooding is inevitable and have shifted their focus toward making road maintenance easier and more effective during major weather events.
Since manmade stormwater controls can only handle so much until a storm surge comes through, natural defenses like coastal dunes are implemented. By design, they limit a storm surge as much as possible before it overtops and erodes the dunes. After a storm, the North Carolina Department of Transportation deploys crews to restore the dunes and be ready for the next wave of Mother Nature.
A lot of homes in the Outer Banks are built very close to the shoreline and most of them are the first physical improvement a storm surge barges through before it floods the roadway. To prevent separation from the foundation, houses are elevated on pilings driven deep into the sand. They sit above the storm surge levels with open spaces underneath, where the storm surge passes through.
For houses further from the shoreline but still in areas of seasonally high groundwater or softer soil, foundations are built with massive footings that increase the overall strength and stability from erosion. Flood vents and breakaway walls are installed on the lowest levels. These measures allow rushing floodwaters to pass through the house rather than ponding inside and creating hydrostatic pressure that could potentially cause the walls to collapse.
Mother Nature will always keep us on our toes. The more we prepare for her true colors, the better we can prepare for her turmoil.
