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
Hi Tracy and Russell, I enjoy reading the educational information posted on this blog and appreciate that it takes time to make it concise and comprehensible, so thanks! I haven’t given much thought to sediment, but would like to learn more if you could provide that information at some point. I imagine there are concerns with hazardous waste contaminants in the sediment like PCB’s that may limit or prevent using the sluice gates to discharge sediment downstream. Hopefully the sluice gates are tested once in a while. I expect there are also problems related to flushing through the series of reservoirs. There is the overriding question of how long before sediment is expected to become enough of a problem at each of the reservoirs to require a solution. I expect that USACE has considered these issues and more, and has developed a long term plan which is hopefully something that can be shared. ~Ferris
Hi Ferris, my apologies for the delayed response. You pose some interesting questions that required some research on my part. After speaking with several subject matter experts both here in the District and at the dams, I will attempt to address each of the topics you presented.
First, the possible presence of hazardous waste contaminates such as PCBs are taken into consideration, but we typically don’t see those contaminates in the sediment directly above the dam. Those contaminants are usually found in the outer reaches of the reservoirs near the tributaries. There have been incidents of PCBs at the Twelve Mile Creek, which flows into Hartwell Lake, and the Corps has been an active part of the committee responsible for remediation. For this reason, we focus most of our sediment monitoring on the upper ends of the reservoirs near the tributaries.
As for sluice gate testing, we don’t actively test them—as in, we don’t open them and allow the inactive storage waters to flow through the sluiceways. Doing so would cause “dead water” to pass downstream that would impact water quality and wildlife habitat. But we do inspect them. Sluice gates would only be opened in the remote unlikelihood that we would need to drain the entire reservoir.
The inactive storage amounts in the reservoirs are equal to or greater than the conservation pool. Inactive storage is intended to be just that—non-usable storage due to its very low water quality. Hartwell has ~1.1 million acre-feet (ac-ft) of inactive storage, compared to ~1.4 million ac-ft of usable conservation storage. Thurmond actually has a greater inactive storage (~1.4 million ac-ft) than its conservation storage (~1 million ac-ft).
We occasionally monitor sediment build-up behind the dams, but the quantity and quality of the sediment has no impact on our operations. Just to give you an idea of the depth we are talking about, the bottom of the sluice gates are located about 50 feet above the original river bed. Sediment build-up doesn’t come close to reaching the sluice gates and does not impede our ability to operate them.
~Tracy Robillard, public affairs specialist
Hi Tracy, I thought it might take a bit of research, and your response was sooner than anticipated. Thank you for the effort, those who helped, and the informative post. It appears that sediment will not likely be a concern regarding the sluice gates for at least another century, and I am relieved to learn that pre-EPA era contaminants are not anticipated to be a problem to downstream stakeholders if something ever requires use of the sluice gates. ~Ferris
Ferris has some good questions. Also, I second his applause for the communication work you are doing with us stakeholders.