Why Irma rainfall had marginal effects on levels: inflows remain below average

Many basin stakeholders have expressed interest in why reservoir levels didn’t respond better to the Hurricane Irma rainfall received on Sept. 11 and 12.

The previous post was intended to assure the public there was sufficient storage capacity in our reservoirs to absorb the forecasted rainfall. Based on that forecast which predicted 7 inches of rain, we estimated Hartwell would rise 3.5 feet and Thurmond up to 6 feet. That message was important for the general audience because some were suggesting the reservoirs should be lowered in anticipation of the rain event. Lowering reservoir levels in anticipation of forecasted weather is against Corps policy and deviates from the water manual.

As Hurricane Irma continued to shift westward, actual observed rainfall was much less than NOAA’s forecast of 7 inches. Instead Thurmond received 2.5 inches of rain and Hartwell less than 2.1 inches, definitely not enough to cause flood concerns, so attention shifted to anticipated benefits of the rainfall.

Despite the 2-plus inches of rainfall, Hartwell and Thurmond barely responded. Let me first reassure readers what did not cause unresponsive levels.

We did not increase discharge to make room for rainfall. Nor did we increase discharge after Irma rainfall. In fact, due to persistent drought we have maintained releases at approximately 4,000 cubic feet per second (cfs) or less from Thurmond for more than a year now. This is the minimum release rate allowable for Drought Level 2, and it follows the directives of the drought plan. We didn’t deviate from the water manual throughout Hurricane Irma.

So why did the reservoirs respond so poorly to Irma rainfall?

The answer is limited runoff. The dry ground soaked up most of the rain before it could make its way into the reservoirs. Exceptionally dry conditions have driven inflows down to a minimum level requiring additional rainfall before runoff occurs.  Hartwell has a 12.4 inch rain deficit over the last 12 months.

Rainfall has improved over the last several months, but it is normal for pools to decline this time of year with average rainfall. We need sustained above average rainfall for conditions to improve. Every drought we’ve experienced has been overcome by a return to above average rainfall, and we anticipate a similar recovery with this drought.

Thanks for reading.

~Russell Wicke, Corporate Communications

About U.S. Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multimillion 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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