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Why do we need a study on the winter drawdown?

Posted by on October 15, 2013


Picture1We often hear people suggest the Corps should limit the amount of winter-draw down to 2 feet instead of four. They point out that when Thurmond Dam and Lake were built, a 4-foot winter drawdown was established to make room for excess runoff above Thurmond. But afterward Hartwell and Russell reservoirs were constructed upstream. These reservoirs provide more flood storage space because their collection capacity combines with Thurmond.

Indeed, with the construction of Hartwell and Russell there is more flood storage available in the basin. The question now becomes: is this additional storage capacity enough to decrease the amount we draw down the reservoirs in the winter? The answer may very well be ‘yes,’ but because so much has changed since 1954 we don’t have enough data (yet) to answer that question definitively.

For example, we know human development, especially development of urban areas, changes the rainfall-runoff ratio. That is, in areas where porous soil is replaced with concrete roads, parking lots and buildings, less water soaks into the ground and more water fills the reservoirs. Cultivated lands, such as farm land also changes this ratio.

Another example of change since 1954 is the pendulum swing of the dry-wet periods. Although we are generally receiving about the same annual amount of precipitation from 60 years ago, the wet and dry swings tend to be more extreme. When rainfall intensity is high in short periods, greater amounts of flood storage may be necessary.

We need to examine these kinds of conditions before making the case to reallocate flood storage. Because of all the unknowns, the prudent approach is to assess current conditions and how they affect impacts of probable excessive rainfall.

For these reasons we have taken initiative to implement a flood-storage assessment that will provide the information needed to make an informed decision on the amount of winter drawdown required. The study began October 2013 and we expect it to take about 12 months.

The assessment will look at two things: 1) the Probable Maximum Flood (PMF), which however unlikely, is the greatest rain event the basin could ever expect, and 2) the 100- and 500-year rain events. The assessment is necessary because it will model a 100- and 500-year event in the basin how it exists today and compare impacts of our current four-foot drawdown with impacts of lesser draw downs. 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, the basin’s past rainfall record isn’t enough to base our operating procedures on because past experience doesn’t represent all 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 in the basin as it exists today.

Thanks for reading us, and we welcome your questions.

~Russell Wicke, Corporate Communications Officer

Courtesy the Corp of Engineers Savannah District

US Army Corps of Engineers Savannah District

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