Spin, Spill and Sluice

Forget about a white Christmas; in the Valley, we had a wet one. In fact, the entire month of December was record-breaking in terms of rain. Thankfully, the River Forecasting Center was on hand to manage the raging river and prevent flooding.

DECEMBER 31, 2018 — Here are some jaw-dropping facts:

  • December rainfall was 8.61 inches in the Valley, which was 184% of normal. It was the wettest December in the Valley since 1991 and 6th wettest December in the 126 years on record.
  • December runoff was 6.07 inches in the Valley (defined as the total above Kentucky reservoir), which was 288% of normal. We set a new record for highest December runoff ever. Compare that to the previous record of 5.82 inches set in 1990.

As a consequence, TVA reservoirs rose, ahem, precipitously:

rise in lake levels for select lakes

As always, TVA’s River Forecasting Center was on the job throughout the holidays, making sure that flooding was minimized or eliminated altogether, averting millions of dollars in damage by running the water through the system of dams in the most intelligent way possible—spinning as much as possible through the hydroelectric turbines, and releasing the rest through two other processes: sluicing, running the water though gated underwater tunnels built into the dams; or spilling, the dramatic release of water through gates above water as shown in the picture at the top of the page, taken at Chickamauga Dam near Chattanooga Tennessee.

The River Forecasting Center is on the job 24/7/365 monitoring the system, but whereas they normally run forecasts and assessments twice daily, over these holidays they bumped operations up to four times a day—issuing new river forecasts every six hours, according to Tom Barnett, Senior Manager. Not only were the usual River Forecasting Center staffers on the job, extra workers were brought in to help manage and direct the raging waters as well as disseminate information to those who needed it most.

“Our mode of operation for the winter months is flood risk reduction,” he explains. “We had already had a big rain event near the end of November, and had just recovered from that when the late December rains came. We used a great deal of that recovered flood storage to help minimize river flooding impacts across the entire Tennessee Valley.” For context, consider that the volume at the end of the line (Kentucky reservoir) averaged a staggering 1,188,712 gallons of water per second for the entire month of December, and on Dec. 29 reached 2,252,482 gallons per second!

Hydroelectric generators ran around the clock—a good thing for affordable, clean energy. “But that’s just a secondary benefit in the effort to recover flood storage,” Barnett says. Flow from the generators often had to be supplemented with sluicing and spilling to move water to where it needed to go to recover flood capacity, and Barnett estimates that the River Forecasting Center has issued upwards of 250 gate-change orders from December first to today to put the reservoir system back into balance. For the most part, flooding was avoided. Though there was some small amount of flooding on the lower end of the Tennessee River in North Alabama and West Tennessee—closing roads, inundating some farmland, affecting a few homes—the River Forecasting Center once again averted millions of dollars in damage. This time, estimates are close to $140 million dollars saved.

If there’s one thing Barnett wants people to learn from this rainy holiday season it’s this: “The reservoir system is of paramount importance in managing floods. People look at the lakes and think of them as playgrounds or panoramic scenes, and they do provide these benefits. But they need to keep in the back of their minds that these reservoirs were designed and built to provide for flood control, navigation and the production of power. That’s why we draw them down in the fall, to provide flood storage—and you can see that we need it now.”