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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  • Mirko Bulovic

    Thanks for a great article, I find it very informative.
    I believe reason #1: “(NOAA) forecasts a low probability for drought in the entire southeast through Dec. 31” shouldn’t carry much weight in the decision of how much to draw down the reservoirs since rain/inflow during that period it will be negated by the draw-down itself. Rain forecast primarily matters for January to April when it’s time to fill the reservoirs back up (as also stated in the article: “January we start a slow rise back to summer pool by mid-April”).
    The illustration shows a trend up in the average of the forecasts, especially in 2014. Doesn’t that indicate a greater possibility of La Nina effect in 2014?

    • Thanks, Mirko – really good question. We depend on our
      partners at NOAA who have the expertise in meteorology we don’t have. To help
      you and others understand the graph, you will notice all the lines on the chart
      hover near the center. To indicate La Niña the lines would trend to the top of
      the chart. The lines near the center indicate a neutral pattern. All recent
      droughts have been during a full La Niña pattern. Keep in mind, too,
      meteorological forecasts change constantly as experts observe actual events. We
      communicate with NOAA often and regularly to keep abreast of updates.
      – Billy Birdwell
      Corporate Communications

  • Mark Welborn

    This summer proved that handling extraordinary rainfall is possible even when the lake is full. The guide rule calls for the lake to be back to full by March which is when our highest avg. rain fall occurs so there is no value in drawing it down during lower avg. rainfall months of Nov. and Dec. I am very discouraged at the corps decision to continue to follow an unnecessary and illogical guide rule established over fifty years ago with data accumulated from 1900 to 1950 with very questionable data collection techniques. End result….millions of cubic feet of valuable water irretrievably gone.

    • Hello Mark, please see my response to Jimmy above, re: your remark about handling extraordinary rainfall with full reservoirs.

      Concerning March having the highest annual average rainfall, it is important to note that runoff, and not rainfall is the issue. March rainfall average is not much higher than other months. Most months are comparable in rainfall averages. What is different in January through March is the cold weather, which increases the percentage of runoff per inch of rain. So average winter rainfall is comparable to average summer rainfall, but runoff yields are higher in winter. ~Russell

      • Mark Welborn

        Thanks for the response. If the runoff yields are higher January through March, why does the guide rule call for the lake levels to begin increasing on Jan. 1 and be back to full pool by March?

        • Thanks Mark: the ascending guide curve is in anticipation of the greater runoff expected during that period. ~Russell

  • jimmy

    Nice article but could you please explain what you are going to acomplish with a study and what methods you are going to study that are better than the actual test you have experienced over the past 5 years. We have actual test and data that already show what happens in droughts and excess rainfall. I know it’s just tax payer money we are using to pay someone to do a study but how can that be better than acutual experience? I apoplogize for being sarcastic but as an engineer, it’s hard to imagine how studying something is better than actual data.

    • Hello Jimmy and thank you for your question – it is valid. The assessment is meant to take a look at two things: 1) the probably maximum flood (PMF), which for reallocation purposes this part of the study is irrelevant to the discussion, and 2) the 100- and 500-year rain events. You are correct in pointing out we have data on what we’ve already experienced. But the event this summer was only a 10-year event. But what about a 100-year event? Since there is a one-percent chance every year for an event of this kind it’s not outside the realm of probability. The assessment is necessary because it will model a 100- and 500-year event and compare impacts of our current four-foot drawdown with impacts of lesser drawdowns. The idea is to assess what kind of risk we are taking in limiting the drawdown in the case we get the inevitable 100-year event. If the study demonstrates damages with the lesser drawdown are insignificant compared to the current draw down, we can move to recommend a change in the guide curve.

      In other words, what we’ve experienced isn’t enough to base our operating procedures on because it doesn’t represent all the probabilities. We need to know the impacts of our changes not only on what we’ve seen but what we could reasonably expect to see.

      We are open to change. As stewards of the basin and water operations we are obligated to be prepared before making changes. Thank you for your unput, and as always, we are open to your feedback. ~Russell

      • jimmy

        Russell, Thank you for your reply. First, didn’t they consider the 100 year event when they set the draw down at 4 feet back in 1956 with one pool. Since we haven’t had a 100 year event since then, not sure why it would be different now. Second, what data do you have that supports the 100 year event will occur in the wnter?. All of the data you have collected in the past 60 years doesn’t support an event occurring during the let down period. When your web site was up, I looked for the worst event on record since the lake was built and I believe it was in August of 92 or 93, not during the winter. Your data also indicates the average rainfall per month doesn’t change that much over the year, I think less than 1″. You don’t have to repeat that runoff is worse in the winter. The runoff couldn’t be any worse than what occurred this August, the ground was saturated and the lakes were full. As you stated before, the gates were opened for only the second time in history, again, not during the letdown period, but they did work, just as the engineers designed them to.
        As I stated above, the real disappointing issue is that a group that calls themselves engineers still follows a manual written in the 1950’s. All of this history stored in a computer, that didn’t exist then, and this is the best we can do? Again, sorry for the sarcasm but I still don’t understand. All we are discussing is 2 feet of water.

        • Thanks for continuing the dialogue Jimmy. I’d like to make two points in response to your comment. 1) You are right in pointing out the frequency of storm events is greater in the summer. However, the logic behind the draw down in the winter is that storm events in the winter/spring period (although less-frequent) pose a greater risk. This is because ground saturation is higher and the reservoirs are much more responsive to rainfall at this time. I know you acknowledged this by pointing out the high runoff rate this summer was exceptional. But the logic, at least in principle, still holds that however great the runoff was in July/August, the same event would have demonstrated even greater spikes in runoff during a period where deciduous trees and vegetation and high evaporation rates weren’t claiming a portion of the rainfall. And in the winter it would take much less time to reach the ground-saturation level we experienced this summer. Remember, heavy rains all spring and early summer preceded the July event.

          Now, I think we know each other well enough at this point, Jimmy, so that I can imagine you are perhaps shaking your head at my first point. But please hear my second: 2) The Corps is not opposed to change, and that is why we are conducting the flood-storage assessment. Perhaps this study will indeed show you are right and we don’t need a draw down – or that we could get by with less than four feet. I hope this is the case. If our data suggests this we will move to recommend a change based on the findings. When Thurmond was built they did consider the 100-year event, but there have been enough changes in the basin and in weather patterns since then that a new study is justified, (see today’s post, which explains how the conditions today are different from those in 1954). Again, we’re not saying these new conditions merit the current draw down. We’re only saying conditions have changed and we need to take these changes into account before making a recommendation. We understand the pace of progress can be slow and frustrating. But In order to meet our stewardship responsibilities we must have scientifically defensible data behind us before making recommendations for change. In the meantime, we continue to monitor the constantly changing conditions and will adjust our short-term discharge rate if we start to enter drought. ~Russell