By Jason Widstrom
The recent flooding and resulting damage in Ellicott City, Maryland, made headlines across the country. After seeing images of the devastation, it’s hard not to wonder, “Could this happen in my backyard?” As the City of The City of Falls Church’s stormwater engineer, I’m here to answer that question.
Let’s first look at the history of flooding in Ellicott City, which dates back to 1768, a mere two-years after it was settled in 1766. There have been 15 major floods; 10 of them happening in the last decade, and two of which occurred in the last two years. The majority of these floods were caused by rising floodwaters of the Patapsco River — the very resource that made Ellicott City so attractive for the milling industry that once thrived there. The last two floods were caused by water overtopping the banks of nearby tributaries, an effect of upstream development and the intensity of the storms. The most recent storm on May 27 dropped approximately six inches of rain in just two hours.
The last two storms are of particular interest to the City of Falls Church, because we could also encounter similar circumstances. If you lived in the City of Falls Church in September 2011, you might remember a four-day period when 6.5 inches of rain fell, with one of those days dropping three inches alone. The City predictably experienced flooding along the floodplain corridors of Tripps Run and Four Mile Run, our tributaries to the Potomac River. There was also flooding in a few other surprising locations where our storm infrastructure was challenged to keep up. While that storm was significant, it was not historic like what Ellicott City just experienced, or remotely like Hurricane Agnes that hit the region in 1972, which produced over 10 inches of rain in a single day.
Much of Ellicott City’s and the City of Falls Church’s storm drains and pipe network predate today’s stormwater design standard of a 10-year storm. That means the storm event has a probability of occurring once every 10 years —doesn’t mean you couldn’t have two major storms days apart. A 10-year design storm is the industry standard because it is seen as a cost-effective way to manage stormwater. Nothing prohibits either city from designing new stormwater infrastructure to handle the magnitude of 100-year storms, but it would be outrageously expensive and underutilized most of the time.
So back to the original question: “Could this happen in my backyard?” My answer: “Yes, but it is very unlikely.”
Meteorologists reported last month’s storm that flooded Ellicott City was a 1,000-year storm event, meaning that it has a 0.1 percent chance to occur in a given year. While highly unlikely, a storm of such intensity would also do serious damage in the City of Falls Church, or pretty much anywhere it occurred. Our infrastructure is simply not designed to manage stormwater runoff of that magnitude effectively. That said, our main street is not in danger of washing away as it is not in a floodplain like Ellicott City.
Residences in and along the floodplain would likely experience the worst of the storm, but localized flooding in other areas of the City could also occur. For example, during the 2011 storm, the City had a handful of homes with flooded basements, road closures in the floodplain, and a few blocks of West Broad Street were reduced to two lanes of travel because the outside lanes had a river of rainwater flowing down them. To give you some context, that was only an 80-year storm event.
Rest assured, the City of Falls Church has and continues to invest in maintaining and expanding storm infrastructure, enforce stormwater and floodplain regulations, and promote awareness. What I hope you take away from reading this article is an understanding of flood risk, and that you can make a plan to mitigate that risk, especially if you live within flood-prone areas. Furthermore, I hope readers understand that you do not have to live in a floodplain to get flooded. Storm inlets and pipes get clogged during storm events and that can lead to flooding in places that have never had an issue before.
The anticipated 100-year floodplain tends to get a lot of focus when storms of significant intensity pass over because these are the areas we know are prone to flooding. What many people may not know is that homes within the floodplain that have federally backed mortgages must pay expensive flood insurance. Fortunately, the City is one of only five Virginia communities where homeowners save 20 percent on insurance policies, thanks to our community’s flood risk rating with FEMA.
Do you know if your property is in a 100-year floodplain? The City has an online map available for homeowners to look it up: www.fallschurchva.gov/FloodplainMap. The map provides a quick analysis on whether or not the floodplain is near your home. However, only a survey can certify if your home is within the floodplain. For more information feel free to contact me directly with any stormwater questions by calling 703-248-5026 (TTY 711), or sending an email to firstname.lastname@example.org.
Jason Widstrom is the City Engineer for the City of Falls Church.