Commander addresses winter drawdown this year

Our continuing efforts to refine our management of the basin have yielded some encouraging results. We secured $240,000 to perform a flood-storage assessment, which will reevaluate how much flood-storage is needed in the reservoirs. The study began this month and we expect it to take about a year. Results of the assessment could demonstrate that less than 4 feet of drawdown is sufficient each winter; or it could confirm that a 4-foot drawdown (or perhaps more) is needed. Stay tuned to this blog by subscribing to us for updates. We will provide more information on the results here when we have them.

As for this year’s winter drawdown, many stakeholders have asked the Corps of Engineers to eliminate or at least reduce the winter drawdown this year. After taking a look at the possibility of deviating from the water control manual this year, we determined the best course of action, based on the scientific data available, is to follow the protocol of current guidance. That is, we will operate to follow the existing winter guide curve.

Below is a video from Savannah District commander Col. Tom Tickner addressing this year’s winter drawdown. Continue reading below the video for more information.

In greater detail, here are the main reasons for a winter drawdown this year:

  1. The National Oceanic and Atmospheric Administration (NOAA) forecasts a low probability for drought in the entire southeast through Dec. 31.
  2. We consulted with our partners at NOAA and they also confirmed that the three-month precipitation outlook shows equal chances for above- and below-average rainfall – the same forecast we received for the summer this year.
  3. The ground throughout the basin remains moderately saturated from above average rains over the spring and summer. This means the reservoirs remain responsive to rain events, and this responsiveness will be more pronounced in the colder winter months.
  4. And perhaps most importantly the El Niño Southern Oscillation (ENSO) forecast is neutral. This means there is no sign of going into a La Niña (dry) pattern. Every major drought we’ve had has been during a La Niña. See the illustration below that shows high probability that conditions will remain neutral:

CurrentENSO_ElNino_LaNina_prediction_01Oct13

However, we continue to monitor the short and long term forecasts. If dry conditions begin to emerge, we will adjust our outflows in an effort to avoid dipping below guide curve in accordance with our drought contingency plan.

The summer rains demonstrated how fast reservoir levels will rise when the soil is saturated. In July, Hartwell Lake rose 4 feet in five days, forcing the dam operators to open the spillway gates for only the second time in the history of the dam. This emergency reduction followed several days of passing water through the dam around the clock. As a result, Hartwell exceeded its storage capacity and filled Russell Lake to the top of its flood storage capacity.

All this came just a few months after the end of a major drought. We went from drought conditions to flood conditions in six months.

Some have said what we experienced was “a 100-year event” with these rains, but this is incorrect. Data taken from NOAA’s precipitation frequency model indicates we experienced a “10-year” event; and this event was isolated to the Hartwell sub-basin only.

The 10-year event occurred over a period of 20 days, from July 2 to 21. The Hartwell sub-basin received 11.7 inches in this period. The 11.7 inches over a 20-day period represented the most condensed rainfall over a defined span of time during the wet period this summer.

In other words, no other combination of consecutive days or rainfall volume this summer demonstrated a greater event than what we experienced at Hartwell July 2-21.

This event used up 4 to 5 feet of flood storage in Hartwell and Russell from July 3 to 9. Thurmond, which by comparison only experienced a 3-year event, had only 2 feet of storage remaining.

Overall, this wasn’t an ideal condition for the basin because with Hartwell and Russell at capacity, all rainfall no matter the sub-basin, would flow into Thurmond. Another 2 inches of rain in the basin would have meant much greater flooding downstream, and perhaps even in areas around the reservoirs.

In ideal conditions we slowly draw down the reservoirs starting in mid-October until mid-December. In January we start a slow rise back to summer pool by mid-April. Historically the upper basin receives the most runoff January through March. We get more runoff at this time partly because rainfall volume is slightly higher, but also because trees and other plants are dormant and evaporation is low.

This lower level in December gives us more room for those normal early-year rains. Rising into the flood storage zone very early in the year puts us in danger of needing to make emergency releases. Emergency discharges can increase the risk of flooding downstream, a reality demonstrated this summer with only a “10-year event.” Even with our controls at the Thurmond Dam, Augusta saw minor flooding along its downtown levee this summer.

Until we have better scientific evidence through further study of the basin, we will continue to follow the established water manual to be prepared for normally increased rains in the upper basin.

~ Russell Wicke and Billy Birdwell, Corporate Communications Office

 

About US Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multi-million dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on Facebook.com/SavannahCorps

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