Maryland dam faces sedimentation threat

In September 2011, Tropical Storm Lee caused flooding in the mid-Atlantic region.  The Susquehanna River rose far above its banks, causing disruptive floods in Pennsylvania and Maryland.  Near the river’s mouth into Chesapeake Bay, massive flooding threatened to breach the 572-megawatt Conowingo Dam.

With its flood gates wide open, the dam survived the flooding.  At peak flows, about 7 million gallons flowed through the dam every minute.  That water transported millions of tons of sediment from the Susquehanna watershed out into the bay, along with large amounts of trash and debris.

The impacts of the flood are still being assessed.  Under typical operations, the dam builds up about 2 million tons of sediment every year, or about two-thirds of the Susquehanna River's total sediment burden.  (Compare the dams currently being removed from the Elwha River in Washington, which had trapped an estimated 24 million cubic yards of sediment.)  Overall, four dams on the Susquehanna might hold up to 280 million tons of sediment.

While Lee removed several years' worth of sediment from the Conowingo Dam, more sediment builds up every year.  The Army Corps is concerned that the Susquehanna River dams have nearly reached their full capacity to hold sediment, and is launching a project to study what could be done, such as sediment dredging or remediation.

Susquehanna River flooding threatens dam, communities

Close on Hurricane Irene’s heels, the remnants of Tropical Storm Lee are dropping up to 10 inches of rain across the northeastern United States.  As we saw when Irene hit Vermont, storms like this can cause not only widespread flooding as streams rise above their banks, but even risk dam failure and more catastrophic flooding.

Now, the rains caused by Tropical Storm Lee have led authorities from Maryland to New York to order the evacuation of nearly 100,000 people.

Flooding along the Susquehanna River is responsible for a large portion of this risk.  Arising out of branches in upstate New York and western Pennsylvania, the 464-mile long Susquehanna is the longest river on the east coast to drain into the Atlantic Ocean.  The river overtopped retaining walls in Binghamton, NY earlier today, leading to road closures that effectively isolate the city.  Downstream, deepening floods have caused the evacuation of the entire city of Wilkes-Barre, PA.

The Susquehanna flooding has also impacted the Conowingo hydroelectric dam about 10 miles above the river’s mouth in Maryland.  With 11 turbines providing a nameplate capacity of 572 megawatts, the Conowingo dam is one of the nation's largest non-federal hydroelectric facilities.  Now operated by Exelon subsidiary Susquehanna Electric Company, the Conowingo Dam is threatened by the Susquehanna floodwaters.  As a result, 50 of the dam’s 53 flood control gates have been opened, causing authorities to evacuate people from the downstream communities of Havre de Grace and Port Deposit.  Area residents remember 1972's Hurricane Agnes, whose rains and flooding caused all 53 flood gates to be opened and the dam operator to prepare for a controlled breach of part of the dam.

As Lee’s rains move out of the area, water levels in the Susquehanna River will peak and then recede.  Time will tell how damaging the flood waters will be.