Our Sources Say
Tennessee Valley experiencing unusual summer
T
he Tennessee Valley region has experienced a much cooler and wetter summer than normal in 2013. As a matter of fact, rainfall in the region is running approximately 35 percent above normal so far this year. In addition, this summer will be ranked as one of the coolest in more than 130 years. The increased rainfall has provided TVA with additional lowcost “fuel” for our hydroelectric dams, which has resulted in an increase in hydro generation of almost 45 percent this year. The cooler weather coupled with the increased hydroelectric generation should have resulted in lower utility bills for most customers this summer. Due to the increased rainfall that we have experienced, I thought it would be a good time to review some facts about TVA’s hydroelectric facilities and then discuss how we prepare for the winter flood season. Hydroelectric facilities Hydropower is America’s leading renewable energy resource. Of all the renewable power sources, it’s the most reliable, efficient and economical. TVA maintains 29 conventional hydroelectric dams throughout the Tennessee River system and one pumped-storage facility for the production of electricity. In addition, four Alcoa dams on the Little Tennessee River and eight U.S. Army Corps of Engineers dams on the Cumberland River contribute to the TVA power system. Water is needed to run a hydroelectric generating unit. It’s held in a lake behind the dam, and the force of the water being released from the lake through the dam spins the blades of a turbine. The turbine is connected to the generator that produces electricity. After passing through the turbine, the water reenters the river on the downstream side of the dam. Unlike a conventional hydroelectric unit, a pumped-storage plant uses two reservoirs, one located at a much higher elevation than the other. During periods of low demand for electricity, such as nights and weekends, energy is stored by reversing the turbines and pumping water from the lower to the upper reservoir. The stored water can later be released to turn the turbines and generate electricity as it flows back into the lower reservoir.
Kevin Chandler is general manager, Alabama District Customer Service, for the Tennessee Valley Authority.
44 OCTOBER 2013
Reducing flood damage TVA prepares for the winter flood season by lowering the level of flood-storage reservoirs to make room to hold the runoff produced by winter storms. When a storm hits, TVA holds the water back by reducing releases from the dams in areas where it is raining. When the rain stops and the danger of flooding is over, TVA gradually lets the water out to get ready for the next storm. In the summer, when flood risk is lower, TVA keeps lake levels higher to support recreation. To get ready for winter, TVA begins releasing water from tributary storage reservoirs at a faster rate following Labor Day weekend. This allows TVA to put the stored water to good use during September and October - which are typically hot, dry months - by generating electricity to power air conditioners and supplementing flows for water quality and navigation. Main-river reservoirs don’t fluctuate nearly as much as the tributaries because of their original design and navigation requirements. Their drawdowns are staggered from July through the end of the year to ensure the released water can be used efficiently, generating electricity as it runs through the turbines at as many as nine dams downstream. The seasonal drawdown begins after the 4th of July weekend on Kentucky Reservoir; following the Labor Day weekend on Chickamauga, Guntersville, Wheeler, and Pickwick; and on Nov. 1 on Fort Loudoun and Watts Bar. Reservoirs are typically lowered at least to winter flood-guide levels by Jan. 1 each year. During the flood season, these levels may fall below flood guide levels by several feet to satisfy other operating objectives, but flood guide levels will only be exceeded during flood control operations. As soon as the downstream floodwaters begin to recede, the reservoirs are lowered at a controlled rate to recover flood storage space for future storms. If enough water can’t be released through the turbines, it is sometimes necessary to let additional water flow through sluiceways or over spillways to speed up the drawdown and regain the storage space needed for future rains. Aggressive filling of tributary reservoirs to summer levels begins in mid-March, when the chance of flood-producing storms, prolonged wet periods and multi-storm sequences begins to decline. Main-river reservoirs are kept at lower levels until near the end of the flood season - late April or early May - because flood storage space in these reservoirs is so limited. (For this same reason, however, main-river For more information reservoirs fill more quickly than You can always find more tributary reservoirs.) A small amount of flood storage information on TVA hycapacity is reserved in all reservoirs droelectric facilities and through the summer months as a our river management protection against flood-producing practices by visiting our website at tva.com. storms over limited areas. A
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