The actual quantity of precipitation that composes a “1 in 1,000-years-storm” will vary from region to region, but different thresholds are used by engineers and policy makers to do everything from pricing flood insurance to designing stormwater infrastructure. When Ellicott City flooded in 2016, meteorologists said it was a “1 in 1,000-years-storm.” Using historical weather data, it’s possible to calculate the probability of different rainfall quantities. If this is a new flood pattern in Ellicott City, what has changed? However, historically most Ellicott City floods have been caused by surges from the Patapsco River, not by overflows and backups from the Tiber Branch uphill. It is a historic mill town that was built to take advantage of the high-volume, fast-moving water. Ellicott City has a long history of flooding, which makes sense given its topography. The recent floods in Ellicott City were caused by both man-made and natural drainage systems being overwhelmed. Hurricanes also come with a tremendous amount of precipitation, so they can overwhelm man-made drainage systems (as with Hurricane Katrina in New Orleans) and overwhelm man-made and natural drainage systems (as with Hurricane Harvey in Houston). Surges can also be caused by hurricane winds, like what happened in New York City during Superstorm Sandy. The Potomac is a tidal water body, and so the surge caused by high tide was a contributing factor.įlooding in Old Town Alexandria on June 5, 2018. Old Town Alexandria flooded on Tuesday, June 5 because the Potomac broke its banks. However, even the natural drainage systems around us can get overwhelmed during big storms or multiple storms. The Combined Sewer Overflow (CSO) watershed in DC. The EPA told the city to stop dumping sewage into surface waterways during storms, and part of DC Water’s solution is a very expensive underground tunnel to temporarily store overflow. To prevent that, the combined sewer has overflows into Rock Creek, the Potomac, and the Anacostia at various locations - and that’s why District residents now have to pay an impervious surface fee on their water bills. In DC, where the storm sewer system is combined with the waste water system serving part of the city, that would mean a mix of rain and sewage backing up into people’s homes. When these systems reach capacity, they can surge at the input points unless there are exit points (called overflows) to prevent that. Man-made drainage systems can get overwhelmed during a storm. Obviously, water, but for the purposes of discussing infrastructure and planning it's helpful to distinguish three different kinds of flooding: from overwhelmed made-made drainage systems, from high tides, and from hurricane winds. Why are these floods happening, and can anything be done about it? The gravity was compounded by the tragic loss of Sargeant Eddison Hermond, a National Guardsman who drowned while trying to help rescue a local business owner. Image by Baltimore Magazine licensed under Creative Commons.Īll eyes are on stormwater management in Howard County following Memorial Day weekend's stunning images of floods rampaging through historic Ellicott City, Maryland for the second time in two years. Screenshot from a Baltimore Magazine video of flooding in Ellicott City over Memorial Day weekend of 2018.
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