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September 8, 2011

Port Deposit ordered to evacuate

The mayor of Port Deposit, Md. has ordered a mandatory evacuation of the town starting at 8 p.m. Thursday as waters continue to rise on the Susquehanna.

"All residents of Main Street should prepare to evacuate," the order said. "A shelter has been established at Perryville High School on Perrylawn Avenue.  You are encouraaged to take an emergency kit with you.  Some suggested items are non-perishable food, water, a battery powered or hand-crank radio, extra flashlights and batteries, a change of clothes, and medications."

The dam's owners, the Exelon Corp., said 32 of the dam's 53 flood gates were open at noon Thursday. "We are preparing to open up to 50 gates within the next 48 hours when the dam crests at about Saturday morning at 6 a.m.," said company spokesman Bob Judge.

It would be the first time since Tropical Storm Agnes in 1972 that Conowingo has opened 50 crest gates, he said.

The gates are likely to remain open "at least through Monday" as waters begin to fall, he said.

NOAA Conowingo

Posted by Frank Roylance at 12:35 PM | | Comments (8)
Categories: Flooding


It's going to get really bad in Port Deposit 36 gates are open now and the rest will be open at 7pm tonight and i live in Port Deposit

i understand an overfull dam is more likely to fail, but WHY is the decision of when and how much to open the floodgates left up to Exelon Corp? why is the only action considered- releasing lots of water in a short amount of time- one that does damage to the property of Port Deposit residents (they had to cancel their festival too!)? couldn't they have released slightly more water- but less than what would flood Port Deposit- starting earlier in the week?

to Kathy-The primary reason that more flood gates need to be opened is because a greater volume of water (peak flood stage) will be coming down the river. It must pass downstream or it will overwhelm the capacity of the dam to handle it safely.

Exelon owns and operates the dam, but it is very likely that the operational curves they use to decide how many gates to open are developed in conjunction with other political entities, and are mandated by state regulations to take into account a number factors beyond Exelon's power generating requirements.

Conowingo Dam is not creating the flood, but the way it is operated will tend to knock down the peak flood stage and reduce the effects of the flood downstream.

If there were no Conowingo Dam, Port Deposit would be underwater in a flood like this, and possibly under deeper water than occurs when the dam holds some of the water back.

To follow up on Kathy's question ... When it is apparent that our region is going to get significant rain (as was the case last weekend when it seemed apparent that Lee would be an issue), why can't a modest number of additional gates be opened? Why wait until the last moment to provide some measure of relief and release? It seems like the dam's operators could be more proactive in such situations.

While I sympathize with those in Port Deposit, I don't know that it is reasonable to try to prevent the flooding by releasing water early. It has got to be extremely difficult to predict the future impacts of a storm system over an entire watershed. The storm totals have varied day to day just around the Baltimore region. One day a certain area gets hit hard, another day, a different location. I would think they would not want to run the risk of releasing water early and having the water levels drop to low. Keep in mind, flooding of this magnitude has not happened since 1972, so we are not talking something that occurs with great regularity.

I don't think you folks really understand how BIG the Susquehanna really is. Based on some quick napkin math, enough water is CURRENTLY flowing over the dam to fill its reservoir every two hours IF it was empty. Your simple not dealing with a small stream feeding a large reservoir, your dealing with a VERY large river feeding a comparatively small reservoir...

It's not just the rain here... Its the rain here, in PA, in NY and most of NorthEast all flowing into the Susquehanna to our bay and out to the ocean. The river hasn't even crest in Harrisburg yet and all that water is flowing this way. The dams along the way have to open their gates or the water will spill over. Time to remember that everything that makes its way into the Susquehanna finds its way to the Bay...

to just curious- I am curious too, and do not have any specific knowledge of how the dam is actually operated, so what I have to offer is educated speculation.

Because the dam's primary purpose is for power generation, and not for flood control, the first priority it to maintain a minimum pool level upstream of the dam which would be necessary to generate electricity. This keeps our energy costs down, and in normal times, people would complain if their costs went up.

If Exelon were to open the flood gates every time a big storm is forecast, they would be wasting all that potential energy. And we all know well enough how often a big event is forecast that fizzles out. Add to that, the water flowing by Conowingo is the cumulative effect of what is happening in central Pennsylvania and part of NY state. So it could be raining cats and dogs here, but would have little effect on flow at the dam if the storm tracked to the east or the west, out of the Susquehanna watershed. You can be certain that the dam operators are tracking all that information that helps predict the flood stage, and when the crest will occur where, and it plays into their decision about gates.

That said, I would be fairly certain that the operating rules require that the dam be operated in such a way so as not to increase the risk of flooding to any downstream towns or properties.

Once they know the crest is coming and have an idea of when and how high, then they can start drawing down the pool in advance, to be able to handle the crest when it arrives at Conowingo. They will have to pass most of that peak flow through the dam, because the reservoir just does not have the capacity to hold it. So the draw down is to prepare for the crest.

Opening the gates early or more gradually, as has been suggested, would slow down the rate of rising on the leading edge, but would not have much effect on the ultimate flood crest elevation when it passes through, because the available storage is small relative to the huge volume of water coming down the river. And it is primarily the crest elevation that determines who gets flooded when the crest passes by.

It may be that Exelon is able to provide some flood control for smaller events, and helps keep Port Deposit from having to evacuate as frequently. But when you hear that they are opening all the gates, you can be sure this is a huge flood, and the primary concern of Exelon is to not overwhelm the dam and pass the flood safely. If the dam were to fail, that would be catastrophic.

The flood crest will reach Port Deposit in any case.

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About Frank Roylance
This site is the Maryland Weather archive. The current Maryland Weather blog can be found here.
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page. Frank also answers readers’ weather queries for the newspaper and the blog. Frank Roylance retired in October 2011. Maryland Weather is now being updated by members of The Baltimore Sun staff

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