We repeatedly use the term “water management” on this blog—but what exactly do we look at when we are “managing” the reservoirs? We have a number of tangible responsibilities related to managing reservoirs, but nearly all of them are centered on a single resource – what we term conservation storage.
Conservation storage is defined depth in the reservoirs allotted to store water used to meet project purposes during normal and drought conditions. Water in the conservation pool fulfills six of the seven Congressionally-authorized purposes, and for this reason we also refer to it as our “active pool” or “active storage.” Flood risk management is the only purpose not met using conservation storage.
The top of conservation storage marks the bottom of the “flood storage.” Flood storage is the depth allocated to capture surplus water during intense rainfall events. Also, the bottom of conservation storage marks the top of the inactive storage pool—the lowest layer of water in the reservoir, designed for collecting sediment.
Each of the three reservoirs was designed with different amounts of conservation, or “active” storage: Hartwell has 35 feet, Russell has 5 feet, and Thurmond has 18 feet.
The Thurmond Dam, the first major storage project on the Savannah River, was completed in 1954. Its primary purpose focused on flood risk management and rural electrification. Thurmond’s conservation pool ranges from 312 feet per mean sea level (ft-msl) to 330 ft-msl (summer full pool).
The completion of the Hartwell Dam in 1962 lessened conservation storage demands on the Thurmond reservoir, adding an additional 35 feet of conservation storage to the basin. The Hartwell conservation pool begins at 625 ft-msl and goes up to 660 ft-msl, which is summer full pool.
The Russell Dam was designed after Thurmond and Hartwell had essentially satisfied the need for conservation storage on the Savannah River. Engineers designed it as a pump-back hydropower facility, which can operate more efficiently by minimizing its drawdown. Project designers determined that the Russell Project satisfied the national cost-benefit analysis with only a 5 foot total normal fluctuation. Therefore based on Russell’s design and purpose, its active storage is only a fraction of Thurmond’s and Hartwell’s active storage.
We are often asked why we can draw Hartwell down so far but only draw Thurmond down by half that amount. While the Hartwell and Thurmond pools have roughly the same capacity, there is more depth and less surface area at Hartwell. This is because Hartwell is located further upstream in steeper terrain.
To meet downstream needs during drought, the Corps initially brings Hartwell Lake and Thurmond Lake down equally (foot-by-foot) for the top 15 feet of conservation storage. However, when Thurmond falls below 315 ft-msl, water managers can no longer match the pool level foot-by-foot. Instead the Corps changes to an equal percentage of remaining water in their respective conservation pools. Therefore, during the rare occasions when drought draws the pools down below 15 feet, Hartwell Lake’s greater depth of conservation storage must provide more of the downstream water supply needs once Thurmond Lake falls below 315 ft-msl.
As always, we welcome your questions and feedback in the comments section below. Thanks for reading us!
~ Tracy Robillard and Russell Wicke, corporate communications office