Hiwassee Operating Guide
What is an operating guide?
TVA uses operating guides for each reservoir to make decisions about moving water through its dams. These guides are based on many decades of operating experience. They are built around the idea of allowing seasonal variations in the use of flood storage space.
Operating guides take into consideration all the features that influence a reservoir’s ability to store and release water—the size and shape of the surrounding watershed, the reservoir’s surface area and the average rainfall and runoff—as well as historical demands for water use and flood storage space at different times of the year.
Observed midnight elevation
Observed midnight elevations are shown for two years: last year and the current year to date. These are the actual elevations of the reservoir immediately upstream of the dam, measured at midnight of each day. The previous night’s elevation is added to the graph by 6 a.m. the next morning.
The balancing guide is used to ensure that water is drawn from all tributary reservoirs equitably when water must be released from the reservoirs during the summer to meet downstream flow requirements. The operating objective is to keep the elevation of all reservoirs similar relative to their positions between their flood guide and their balancing guide.
The flood guide line is a seasonal elevation guide that shows the amount of storage allocated in a reservoir for flood damage reduction. The operating objective is to keep the reservoir level at the dam at or below this line to be ready for flood events.
The reservoir level may rise above the flood guide as a result of large inflows, but the level is lowered to the flood guide as soon as it can be done without increasing downstream flood damage.
From June 1 through Labor Day, elevations are maintained as close as possible to this line to support recreation. During this time, elevations fall below this line only when rainfall and runoff are insufficient to meet system flow requirements.
Expected elevation range
The shaded area represents the reservoir’s expected elevation throughout the year. Based on computer simulations using more than 100 years of historical rainfall and runoff data, the reservoir’s elevation is expected to be in the shaded area an average of eight out of every 10 years on any given date. For this reason, it is also referred to as the 80 percent probability bound.
If you compare the operating guides for different reservoirs, please note that the scale for the vertical “Elevation” axis varies. To keep the size of the charts consistent, a larger scale is used for reservoirs that fluctuate significantly; a smaller scale is used for reservoirs that typically fluctuate only a few feet.