Recently, the D.C. and surrounding area saw torrential downpours and major flooding, but this is not the first time it has happened, nor will it be the last. The District of Columbia and Maryland are known to have low elevations compared to sea-level, with D.C. having the lowest point at 1 foot where the Potomac River meets the eastern Maryland border. In general, D.C. is only 410 feet above sea-level and the counties surrounding D.C. ( Prince Georges, Fairfax, Montgomery and Charles) range from 235 to 880 feet above sea-level. While the local elevations are not at New Orleans level, D.C. is still particularly susceptible to overbank flooding in Potomac Park, along with tidal basin and over the National Mall area.
When looking at the top natural weather disasters in the world, flooding is rated second, only behind extreme heat, when it comes to human tragedies. When talking about deaths due to flooding, most of them come from drowning, as people try to drive through flooded waters. Remember, just six inches of water can sweep away a grown man, twelve inches a car/SUV off the road, and eighteen inches a large vehicle as in a semi-truck. In mid-July, multiple thunderstorms gave the region 1-3″ of widespread rain in some of the most low-lying areas, with upwards of 5″ as far west as the Shenandoah Valley. When people hear the word flooding, the District is susceptible to four different types of flooding, three of which are caused by excess rainfall or snowmelt, and one by the level of the tide. This list includes: OVERBANK FLOODING, URBAN DRAINAGE FLOODING, LEVEE-CAUSED FLOODING AND TIDAL/STORM SURGE FLOODING. Overbank flooding occurs when the river channels receive more rain than they can handle and does not permit the water to flow through. Urban drainage flooding occurs when the sewer system built to handle water runoff, is overloaded past its design capacity. When this happens, flash flood warnings are issued. Areas with levees can be inundated behind the levees because they are relatively flat, and the levee serves as a block to the water flowing to the river. Channels may be built and/or pumps are installed to move the water past the levee. Lastly, tidal flooding occurs when there is an abnormal rise in water level preceding a storm, usually from the residual impacts of a tropical cyclone. Out of the four different types of flooding that occurs in our neck of the woods, the most frequent types are the consequences of river overflow and urban drainage failures. Overbank flooding is an easier risk to manage because fairly reliable warning systems typically provide longer lead times before the flooding begins. Throughout recorded history, some of the significant flood events in and around the D.C. area have been the Great Flood of 1889, The Great Potomac Flood of 1936, the Records Flood of 1942, Hurricanes Agnes in 1972 and the Federal Triangle Flash Flood in 2006. In the Triangle Flash Flood, persistent onshore winds caused copious amounts of tropical moisture to move over into the region, flooding several federal building, including the National Archives, portions of the Smithsonian Institution and the IRS building.
It was so bad at the IRS building, that employees did not begin returning to their headquarters until nearly 6 months later! That event also brought about a record that stands today at Reagan National Airport, where 9.41″ of water , which is the most ever recorded in a 2-day period locally. Thankfully, no fatalities were reported in the District, but the damage totaled $10 million. So how has the area tried to lower the probabilities of flooding in D.C. and surrounding region? For one, the Federal government has implemented a multitude of floodplain laws, regulations, executive orders, policies and agency guidance. In addition to federal laws and regulations, the District has also initiated its own rules, such as building regulations for construction in a FEMA-designated floodplain, while others, such as the Anacostia Waterfront Initiative’s storm water regulations, are entirely local requirements. It has taken time, but overall, years of planning and work have paid off in reducing the number of widespread flooding problems. Between the U.S. Army Corps of Engineers, FEMA, District Department of the Environment and the National Capital Planning Commission, established by Congress in 1924, through planning and policymaking have made the region better suited for what Mother Nature brings us in the future.