The U.S. Army Corps of Engineers has completed a comprehensive Environmental Assessment (EA) required to update the Savannah River Basin Drought Contingency Plan.
Data from the EA gave us the opportunity to initiate two changes to the plan. The first sets lower outflows in the winter months if conditions meet Drought Levels 2 and 3. Additionally, we will now use stream flows as a secondary drought indicator during Drought Levels 1 and 2.
Wintertime Outflow Reductions
Under the revised Drought Contingency Plan, we will reduce outflows from the J. Strom Thurmond Dam to 3,600 cubic feet per second (cfs) from Nov. 1 through Jan. 31 while in Drought Level 2; and 3,100 cfs from Nov. 1 through Jan. 31 while in Drought Level 3. This seasonal flow reduction could be extended through February with approval from the National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries.
Broad River Stream Flows and Drought
In the past, we only used pool elevation to initiate flow reduction. When either the Hartwell or Thurmond pools had fallen to a drought trigger, a flow restriction would be placed on Thurmond Dam. Hartwell Dam would in turn reduce its outflows to stay in balance with Thurmond. The revised Drought Contingency Plan now adds stream flow as an additional indicator for drought trigger levels.
The water management team uses the U.S. Geological Survey gauge at the Broad River, located near Bell, Ga. to check stream flow. Because the Broad River is a large, unregulated tributary that flows into the Thurmond reservoir, it provides an accurate representation of natural inflow to the Savannah River Basin. This tributary has a long-recorded history, with more than 70 years of data. Use of streamflow as an index allows us to reduce flows before the reservoirs enter the next drought trigger level.
Again, stream flow will be used as a secondary indicator during Drought Levels 1 and 2. In the plan, if stream flows at the Broad River gauge are less than or equal to 10 percent of the historical flow rate (calculated over a 28-day average), we will reduce outflows from 4,200 cfs to 4,000 cfs in Level 1; and from 4,000 cfs to 3,800 cfs in Level 2. If Broad River flows are higher than the 10-percent historical flow rate, we’ll set outflows at 4,200 cfs in Level 1 and 4,000 cfs in Level 2.
The reservoir system has remained in Drought Level 2 since Aug. 29, 2011. At that time, outflows were set at 4,000 cfs, according to the Drought Contingency Plan. In October 2011, we further reduced outflows to 3,800 cfs under the authority of the District Commander.
With input from the National Weather Service, we anticipate the reservoirs to enter Drought Level 3 in mid-October this year. If the system remains in Level 2 status on Nov. 1, outflows from Thurmond will be reduced to 3600 cfs. However, if the system reaches level three status, outflows will be further reduced to 3100 cfs. Historically, the reservoirs refill during the wetter months from December to April.