Figuring the dollar value of flood damage averted
$5.8 billion. That’s a lot of cash — more than most of us can even imagine. But that’s how much damage would likely have occurred in the Tennessee Valley in the past 70 years if the dams and reservoirs along the Tennessee River and its tributaries weren’t available to hold back potential floodwaters.
“When we say the operation of the TVA reservoir system has averted $5.8 billion in flood damage, we are not exaggerating, and we are not just guessing,” says Karen Ford, a civil engineer in TVA River Operations. “That number is based on computer models which compare the number and value of houses and other structures which would have been flooded without the TVA reservoir system with the actual flood damage, if any, associated with specific flood events.
“In 1936, TVA began tracking flood damage at 15 damage centers — locations that are statistically likely to suffer flood damage based on historical flood data. Since then, the TVA reservoir system has helped to eliminate or reduce the damage that would have occurred during 64 separate flood events.”
A floodplain inventory is one of the most important components in calculating flood damage averted, says Ford. “Every five years or so, we conduct field surveys at flood damage centers. At each location, we survey all of the structures in the 500-year floodplain valued at more than $200.”
A 500-year floodplain is the area likely to be inundated by a 500-year flood—that is, a flood event with a 1-in-500 chance of happening in any given year.
Calculating the dollar value of flood damage averted is an exacting process, Ford says.
“First, we determine the number and type of structures in the floodplain and obtain the ground-level and first-floor elevations of each structure — the elevations at which water will begin to surround and then enter the structure. Each structure is listed by type, subtype, construction, address, river mile, and elevation. For example, a single-family dwelling is not just a house. It is a two-story, split-level, brick house built on a slab with a first-floor elevation of 806.2 at Clinch River mile 59.
“Then the next step is to estimate each structure’s value based on tax or real estate records. We also estimate the value of the contents — 50 percent of the structure’s value for homes — and add another 20 percent to account for other losses that may be associated with flood events, such as loss of work time, transportation difficulties, and loss of power. If the structure is a business, the value of equipment and inventory is calculated or obtained from the owner.”
The survey is not limited to residential and commercial structures, Ford says. “Churches, schools, barns, carports, storage sheds — if it’s in the floodplain and valued at more than $200, it’s included.”
All of this information is then entered into a computer program called Potential Flood Damage Analysis, or PFDA. This program generates data that is used to produce curves showing the flood damage likely to occur at specific locations along the river at various flood levels.
Then, Ford says, it is a relatively simple matter to calculate averted flood damage. “We compare two points on the curve: the point showing the damage that occurred at the observed flood level and the point showing the predicted damage at the flood level that would have occurred without regulation.”
Unregulated flood levels are determined using data from TVA’s network of rainfall and stream-flow gages, says Ford. “Gage data is entered into a computer model that tells us how much water is associated with a particular flood event. We use this information to calculate how high the water would have risen at specific locations if we hadn’t been able to store it in upstream reservoirs.”
TVA estimates flood damage averted for several reasons. “It’s a good performance measure,” says Ford, “and it’s helpful both in making decisions about how to schedule water releases during a flood event and in improving future flood operations. Plus, communities along the river system can use the information in siting new development, emergency preparedness planning, and public education and awareness.”
Ford is quick to note that TVA dams and reservoirs cannot prevent all flooding. “Flooding can and does occur on smaller rivers and streams before they empty into our system. The fact is, TVA’s reservoir system regulates less than 10 percent of the total stream miles in the Tennessee Valley.”
In areas where the water flow can be regulated by upstream dams, TVA is often able to reduce flood damage. But even these areas can experience significant flooding, Ford says.
“The May 2003 flood is a good example. The rain fell so hard and so quickly that the river rose more than 15 feet in one day at Chattanooga. And, to make matters worse, the brunt of the storm was centered in the area between Knoxville and Chattanooga — the worst possible location, since there isn’t as much flood storage in Fort Loudoun, Watts Bar, and Chickamauga reservoirs as there is in the deep tributary reservoirs above Knoxville.”
The May 2003 flood resulted in about $18 million in flood damage at Chattanooga. But it could have been much worse, Ford says.
“Without the TVA system of dams and reservoirs, the river would have risen much higher and caused more than $409 million in damage at Chattanooga.”
The flood of May 2003 is a good reminder of why it’s so important to use flood-prone land wisely, says Ford. “A system of dams, even as sophisticated as TVA’s, can’t prevent all flooding. So it’s important to find out your flood risk before you purchase or rent land, houses, or businesses.”
The 2003 event also is a good reminder of why the TVA reservoir system was built. "Even though most of us have no recollection of the time when floods regularly devastated homes and businesses along the river, we can certainly appreciate the damage averted by TVA dams and reservoirs,” says Ford. “The numbers really bring it home: $5.8 billion since construction of TVA’s reservoir system began. Even factoring in the dry years when flooding wasn't a concern, that works out to an average of over $230 million a year.
"Without a doubt, $5.8 billion is a staggering and hard-to-grasp figure. But it would be even more difficult to imagine the physical destruction and loss of life and livelihood that would have occurred if not for TVA's system of dams and reservoirs."
Did you know?
In the legislation creating TVA, Congress charged the new agency with controlling destructive flood waters not only along the Tennessee River, but also in the Mississippi River drainage basin.
Kentucky Dam, located 22 miles upstream from where the Tennessee River joins the Ohio River at Paducah, Kentucky, is particularly important for protecting land in the lower Ohio and Mississippi River valleys. It provides more storage capacity than any other TVA reservoir — a total of four million acre-feet. (That’s a volume equal to one foot of water covering four million acres of land.) This helps to protect the Mississippi River levee system, guarding six million acres of land in Kentucky, Illinois, Missouri, Mississippi, Arkansas, and Louisiana, and helps reduce the frequency of flooding on four million acres of land not protected by the levee system.