Given the long-term and sustained drought in the Savannah River Basin, we are working with state agencies to get support on two deviations in our procedures that we think will help retain more water in the reservoirs. The two changes we are proposing are 1) Allowing the pools to rise above the winter drawdown, and 2) Holding minimum outflows until full pool is achieved. An explanation of these actions is outlined below.
Sustained Drought Management vs. Flood Risk Management
This year, if we get the rain, we plan to allow the pools to rise above the winter drawdown to the summer full pool levels.
Here’s what this means. The typical refill period for lakes and reservoirs takes place in winter months. One of the main reasons for this is because most vegetation is dormant needing little, if any, water. For this reason most of the rain that falls in the winter and early spring becomes runoff. Another reason is that often spring rain can be high in volume and intensity. These combined facts create a potential flood risk every spring. Therefore the water control manual advises that reservoirs be maintained several feet below full pool in winter months to make room for excessive runoff. This is meant to reduce the risk of downstream flooding.
However, the Savannah River Basin is in a period of sustained drought, and forecasts suggest the basin may receive less-than-expected precipitation this winter. Based on this situation we have decided to allow the pools to rise above the winter drawdown up to the summer full pool levels if we get the rain to refill the reservoirs. This will put the basin in a better position to deal with potential rain deficits in the summer and autumn.
Holding Minimum Flows As the Pools Refill
The other action we’re working for is to get resource agency concurrence to hold outflows at 3,800 cubic feet per second (cfs) throughout the refill period. If we can get support for this action we will execute without an environmental assessment.
Our current drought recovery procedures directs water managers to increase outflows once water levels rise two feet above the trigger level that dictated lower outflows. For example, the Savannah River Basin enters Drought Trigger 3 when the reservoirs fall to 14 feet below full pool. At that point outflows are reduced to a specified flow rate, depending on the time of year. If that flow rate is 3,800 cfs the drought recovery plan states that 3,800 cfs will be held until the lake levels rise to 12 feet below full pool levels. Once the levels rise to 12 feet below full, the plan directs release rates to re-target Drought Level 2 outflows, which are higher outflows than Drought Level 3. However, if we can get resource agency concurrence, we will make an exception and hold at the lower flow rate (3,800 cfs) through Levels 2 and 1 until the reservoirs are full.
This temporary deviation takes into consideration the likelihood that we will remain in a period of sustained and long-term drought. Although reservoirs may be refilled just as they were during the brief wet period of 2009, a temporary respite from below average rainfall does not mean we have recovered from drought. Drought recovery takes place not when lake levels are full, but when we begin to receive sustained, long-term precipitation at above normal levels. This temporary deviation from the drought plan addresses the unique circumstances and will allow the reservoirs to refill much faster than it would take otherwise in a period of drought.