This December, the Savannah River Basin received above-average rainfall at all three Corps of Engineers reservoirs. Excess rainfall filled the reservoirs past their “summer full pool” levels and into a portion of their flood storage space. A new record was also set for December rainfall in the Thurmond sub-basin.
We recorded 8.5 inches of rain in the Thurmond sub-basin this December. That’s 220 percent of what’s considered “normal” rainfall for the month. This is the wettest December on record for the Thurmond sub-basin, since we started recording data in 1948. The second wettest December on record for Thurmond was in 1953 with 8.35 inches.
This December we recorded 7.9 inches of rain in the Russell sub-basin (197 percent of normal) and 8 inches in the Hartwell sub-basin (156 percent of normal). In the last week of December the entire basin recorded 400 percent of normal rainfall, pushing reservoir levels above summer full-pool into flood storage.
Current Basin Conditions:
As of this writing (Dec. 31), the current elevations are:
- Hartwell 661.7 ft-msl (5.7 ft above winter guide curve, 1.7 ft above summer full pool)
- Russell 476.3 ft-msl (1.3 ft above guide curve/full pool)
- Thurmond 333.3 ft-msl (7.3 ft above winter guide curve, 3.3 ft above summer full pool)
Reservoir levels are now beginning to stabilize. Over the next several weeks, our water managers will increase releases from the Thurmond Dam to bring the Thurmond reservoir out of flood storage. During that time, we expect releases at the Hartwell and Russell dams to remain relatively constant. Starting Jan. 1, daily average outflows from the Thurmond Dam are scheduled for 25,000 cfs.
When the Thurmond reservoir approaches 330 ft-msl, we plan to increase outflows at the Hartwell and Russell dams to evacuate flood storage in those reservoirs. We are targeting an elevation of 658 ft-msl at Hartwell and 328 ft-msl at Thurmond (two feet below full summer pool) by late January so we can reach the winter “guide curve” elevation during the typical winter re-fill period.
However, our water managers and power plant operators carefully manage outflows to make the most efficient use of the water through hydropower production.
Weather forecasts suggest the possibility of receiving up to one inch of rain basin-wide in the next seven days. We are carefully monitoring conditions around the clock and will adjust operations as necessary.
As always, projections are subject to change based on unforeseen weather events. For the most up-to-date data on lake levels, water management, or rainfall, visit our water management site at http://water.sas.usace.army.mil.
Impacts of December Rainfall on Winter Drawdown
December ranked the second wettest month of 2013 for the Thurmond and Russell sub-basins, due to heavy rain in the middle portion of the basin. It was the fourth wettest month of the year at Hartwell.
Above average rainfall resulted in a dramatic rise in lake levels throughout the month. Hartwell reached full summer pool (660 feet per mean sea level) Dec. 9 and gradually increased to 1.7 feet above full summer pool by the end of the month. Thurmond Lake reached full summer pool (330 ft-msl) on Dec. 24 and increased to more than 3 feet above full summer pool by the end of the month.
Most of December’s rainfall occurred during intense isolated rain events that dumped between 1 to 2 inches of rain in less than 24 hours.
For example, on Dec. 22 and 23, a rain event brought 3.2 inches of rain to the Thurmond sub-basin. That rainfall and the subsequent run-off brought up the lake level more than 2 feet. Another rain event occurred Dec. 28 and Dec. 29, which brought an additional 2.3 inches of rain to the Thurmond sub-basin, yielding inflows as high as 50,000 cubic feet per second (cfs) for that period. That additional rain event further increased the lake level by more than two feet. Hartwell and Russell also experienced similar rain events with related spikes in lake levels.
The December rainfall demonstrated how quickly the reservoirs respond to moderate rainfall during this time of year. In colder conditions with saturated soil, nearly all rainfall turns into runoff, yielding approximately one foot of lake-level rise for every inch of rain. This is one reason that justifies the conventional wisdom of a “winter drawdown.” Another few moderate rain events of this type in succession can quickly put water managers in the flood-fighting mode.
While we had originally planned to reach a four-foot winter drawdown this winter, excess rainfall kept reservoir levels higher than guide curve. Rainfall throughout the end of November, combined with December rainfall, contributed to the overall trend of a wet 2013.
We welcome your questions and comments in the “comments” section below.
~ Tracy Robillard and Russell Wicke, Corporate Communications Office