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In early February, the Neuse River, swollen by near-record winter rainstorms, swept out of its banks at Smithfield and Goldsboro, flooding homes and roads. For residents of the riverbanks, it was a most unpleasant flashback to the aftermath of Hurricane Fran, in September 1996, when the river went on the rampage down its entire course.
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Wait a minute, many people in the Neuse said to one another. Something is wrong here. We have a dam upstream, the Falls Dam, and one of its major purposes is flood control. Why isn't Falls Dam doing its job? To be more specific, why are the dam managers, the U.S. Army Corps of Engineers, releasing water from Falls Lake, over 5000 cubic feet of it every second, while we're struggling to keep our possessions above water? Don't they care about the people who live downstream? |
Falls Dam (U.S. Army Corps of Engineers photo) |
Here's the tougher question: why was Falls Lake full? We have a comparable facility close by: Jordan Dam, in the Cape Fear River drainage. While Falls Lake was at its spillway level, Jordan Lake remained at least 12 feet below its top flood control capacity. Jordan Dam managers kept their gates closed, and there was no flooding downstream that anyone could blame on the dam.
The Corps of Engineers provides very complete information about the design, capacity, and operation of both dams; the figures are readily available on the Internet (see the link below). So it's possible to take a look at the numbers and compare the performance of the two dams. Here are some figures to consider.
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Dam |
Normal elev. |
Spillway elev. |
Capacity |
Capacity |
Basin |
Basin |
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Falls |
250.1 |
264.0 |
220,880 |
5.38 |
770 |
32.1% at |
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Jordan |
216.0 |
240.0 |
538,430 |
5.97 |
1690 |
38.4% at |
"Capacity" is the amount of floodwater the lake can store between its normal and its spillway elevation. Capacity is measured in "acre feet": a unit equal to the amount of water covering one acre to a depth of one foot. An acre foot contains 43 560 cubic feet, or about 1233.4 cubic meters. Jordan Lake's flood capacity is 2.4 times Falls Lake's capacity. On the other hand, the basin drained by Jordan Lake (1690 square miles) is 2.2 times Falls Lake's basin. Thus Falls Lake is somewhat smaller than Jordan Lake, compared to its drainage basin.
We can compare capacities in other ways. Falls Lake's capacity is equivalent to 5.38 inches of rain over its entire basin, while Jordan Lake can catch all of 5.97 inches of rain over its entire basin. (This assumes ALL the rain runs into the lakes, but, of course, quite of bit of real rain is absorbed by the ground and by vegetation.) In addition, Falls Lake controls 32.1% of the Neuse Basin above Goldsboro, while Jordan Lake controls 38.4% of the Cape Fear drainage above Fayetteville.
By every comparison, Jordan Lake should be a little better able to control floods than Falls Lake. But the differences aren't so dramatic: not dramatic enough to explain the big difference in performance after real storms such as Fran and the rains of late January and early February 1998.
So what's going on? To find out, we can look at the historic record of the two lakes. (The Corps has lake levels for every hour of every day since the dams were finished posted on its web site.) And when we do that, a story emerges.
Before Hurricane Fran, no one had ever seen Falls Lake full. In fact, for the first dozen years of its history the lake rarely rose above 256 feet, leaving plenty of capacity to control flooding. Of course, Fran was a very unusual event. But the rains of early 1998 are not so unusual. What we're seeing now is that Falls Lake fills much faster than in the past, even the recent past.
And why is that? The Falls Lake drainage includes a big chunk of north Raleigh, part of Cary, and the northern half of Durham. Runoff from these urban areas has increased enormously since the dam was built. Furthermore, runoff from the rest of Raleigh, Cary, and east Wake flows directly into the Neuse below the dam, compounding the flood problems downstream.
We can predict that these problems will continue and probably worsen as the Triangle area continues its rapid development. Don't blame the dam managers. As Pogo said, We have met the enemy, and he is us.
Internet Sources
Posted March 2, 1998. Features remain online as long as they remain current; they may be updated if new information becomes available.
Copyright © 1998, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
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