Hurricane Irene: This Time We Survived So-So.  How About Next Time?

 


The Northeast(NE) Branch of the Anacostia River at Decatur St.
in Edmonston, Prince George's County, Maryland
After approximately 1.5 inch precipitation on 9/7/2011

 


The NE Branch during normal flow

Hurricane Irene dropped a great amount of rainwater and left extensive flood and wind damage along its path through the Caribbean, the United States East Coast and as far north as Atlantic Canada.  According to The Associated Press, as of August 31, at least 42 people were killed in 12 states and more than 2.5 million people from North Carolina to Maine were still without electricity.  A few days after the hurricane, Tropical Storm Lee, again, dropped a large amount of rainwater around Upper Marlboro in Prince George’s County.   Overall in this greater Washington, DC area, it looks that we managed the hurricane and the tropical storm so-so.

However, as we know, the hurricane and the storm did not come to the DC area directly.  If a future hurricane directly hit this region what would happen?  It is hard to predict but it seems that we will have a significant damage.

Illustrated in the graph below shows Annual Peak Stream Flow at the Northwest (NW) Branch of the Anacostia River over years.  According to this graph, the peak stream flow is steadily increasing.  This steady increase is highly likely the result of the increase in impervious surfaces such as roofs, roads, and parking lots.  Be noted that for 20 years from 1939 to 1959, there was no extreme high flow events.  During this time period, the highest peak streamflow was just above 4,000 cfs (cubic feet per second).  There were three hurricanes and one tropical storm in the Baltimore – Washington, DC area in this 20 years.  Among them, Hurricane Connie that came in this region on August 13, 1955 dropped 9.5 inches rainwater in Prince George’s County, Maryland.  Five days later from Connie, Hurricane Dian hit this region again on August 18, 1955.  In Baltimore the combined precipitation of Connie and Diane was 18.35 inches.  Nevertheless, again, the peak stream flow in the NW Branch was about 4,000 cfs.

The highest peak discharge can be seen in 1972 when Hurricane Agnes dropped significant amount of rainwater. 

A question arises to me.  If an Agnes-class hurricane comes to this region, what would happen?  With a significant areas of impervious surfaces added to this region, stormwater runoff will gash into our storm sewer system overwhelming it and creating local flooding at many places.  This has been happening even recently when we have smaller, localized storm events.  The stormwater runoff will be concentrated and be discharged to a nearby stream.  Then, a torrential stream flow will erode stream banks sending tons of sediment far above natural level to the Chesapeake Bay.  Highly exposed sewer pipes may collapse discharging billions of gallons of sewage into waterways.  Of course, there will be some casualties.

On the flip side, because of the increased impervious surfaces, our river’s base flow amount is decreasing.  Impervious surfaces won’t allow rainwater to infiltrate into the ground.  Some ground water will seep out in streams supplying clean water to make a base flow during dry weather.  Because of the impervious surfaces, ground water is not recharged and the seepage amount is decreasing resulting decreased base flow amount. 

Dr. Glenn Moglen made a great graph, which is shown below.  In a nutshell, the graph says that the higher end of stream flow (discharge) became 2.5 times higher in 40 years and the lower end of stream flow (base flow) became half to one third in the same time period.  What does this base flow reduction mean?  I think this is a significant problem for aquatic animals.  Their habitat size became one third.  If I use an analogy of a town to explain this, this means that our town size became one third.  Then the population may have to become, at minimum, one third.


Flow Duration Curve for the Northwest Branch of the Anacostia
Graph from the presentation by Prof. Glenn E. Moglen

 

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