Washington's Worst Floods

7:41 PM, May 6, 2011   |    comments
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Washington, D.C. -- Flooding is one of the world's most dangerous natural occurrences. The Population Reference Bureau reports that flooding claims more lives in the United States every year than either tornadoes or lightning. The Nation's Capital has three major types of floods: (1) those caused by rapid snowmelt after a large storm; (2) the heavy rain from the remnants of a tropical storm or hurricane; and (3) a prolonged period of rainy weather that saturates the ground.

In January 1996, Washington, D.C. had its 5th largest snowstorm on record. According to the National Weather Service, 17.3" fell at National Airport, with amounts nearly double that in the suburbs. By mid-month, however, there was a major shift in the weather pattern that led to much milder weather with temperatures in the 50s combined with heavy rain. The warmer weather caused rapid snowmelt and the heavy rain on top of the melting snow simply didn't have anywhere to flow. This was due to a combination of snow clogged storm drains and ice jams on local waterways. Many low lying areas saw extensive flooding and the District of Columbia reported $10 million in damages.

The second major type of flooding across the Mid-Atlantic Region is caused by decaying tropical storm systems. A good example of this occurred in 1999 after a hot and dry summer. Water restrictions were issued before the Labor Day weekend of 1999. That's when the remnants of Hurricane Dennis brought significant rainfall to the region and helped alleviate the drought conditions.

But the ground was not ready to absorb additional rainfall when, less than two weeks later, the remnants of Hurricane Floyd inundated the region. Rainfall totals up to 5" were common along and west of I-95, while rainfall two to three times that occurred east of the I-95 corridor. Floyd's remnants not only brought an abrupt end to the water restrictions, but conditions went to the opposite extreme, with significant flooding to local water ways and low lying areas.

Another infamous flood in Washington history occurred in June 1972 with the remnants of Hurricane Agnes. On June 21, 1972, not only were daily rainfall records set at National and Dulles Airports, but rainfall totals for the storm were 7.19" and nearly 11", respectively. Rivers, streams and creeks overflowed their banks resulting in property damage. It was also disastrous for the Chesapeake Bay with all the agricultural runoff that accumulated.

The flooding that resulted from such copious rainfall remains the benchmark for how devastating flooding can be across the Mid-Atlantic Region. The Potomac River peak stream flow measured in cubic feet per second (CFS) in the aftermath of Agnes set a record of 359,000 CFS. That's compared to the January 1996 flooding flow of 347,000 CFS. To put that in perspective, the average stream flow for the month of January along the Potomac is only 13,000 CFS.

The third significant type of flooding occurs more gradually since it doesn't revolve around a singular event as the first two kinds do. As many Washingtonians have noticed, this March and April have had above average rainfall. As a result, local rivers and streams saw a gradual increase in their water levels. On April 16, 1.24" of rain fell at Dulles International Airport while 0.6" fell at National Airport. However, even higher rainfall totals were observed west of the DC Metro Region in the Blue Ridge. Since the heavier rains occurred well west of town, it took time for the rain to flow downstream. That's why the catastrophic flooding along the Potomac in Georgetown didn't occur until April 18.

It's important for the public to be vigilant along bodies of water whenever flooding is a possibility. Flash flooding often occurs with little or no notice, but flood conditions usually have warning signs. Meteorologists and local authorities will warn of possible flood conditions so you can take proper precautions such as moving to higher ground. The weather team at 9 News Now will keep you informed of the latest weather conditions, both on air and online at wusa9.com.

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