The Savannah District Water Managers are excited about the drought plan update.
The main element proposed in the current Draft Environmental Assessment (EA) is to determine our water releases from the three-reservoir system during drought based on inflow and pool elevation. The current drought management strategy uses pool elevation as the only drought criteria that designates how much the Corps should release from the reservoirs.
While it’s easy to understand the pool elevation trigger, it does not get initiated until the pools have already fallen to specified trigger levels. We are going to be using the 28-day average flow at the U.S. Geological Survey (USGS) gage on the Broad River at Bell, Ga., as the index site for the inflow-based trigger. The Broad River is an unregulated drainage basin with 1,430 square miles above the gage site.
The Broad River ultimately flows into the Thurmond reservoir downstream of Richard B. Russell Dam. The current drought plan does not require a flow reduction until the pools have fallen to the next drought trigger level.
In the past, the Corps has tended to transition to the next lower drought state as forecasts changed. In the future, inflows will help us make that decision.
Here’s how we—the water managers—see the EA working:
Every Wednesday, as we prepare the water release declaration, the average flow at the Broad River gage for the previous 28 days will be compared to the 28-day average flow, at that same gage, for that same day of the year, for the previous 50+ years of record.
When the current 28-day average is less than the 10 percent of all the years, we would transition flows to the next lower drought level flow. The USGS uses 10 percent as the breaking point between dry and drought. More information on the concept can be found on the USGS Water Watch website.
Here’s a simplified breakdown of our current Drought Contingency Plan compared to the proposed EA update:
The current drought trigger levels and maximum release rates in the Corps’ Drought Contingency Plan are:
Level 1 Trigger – Thurmond elevation reaches 326 ft-msl – Outflows reduced to 4,200 cfs max
Level 2 Trigger – Thurmond elevation reaches 324 ft-msl – Outflows reduced to 4,000 cfs max
Level 3 Trigger – Thurmond elevation reaches 316 ft-msl – Outflows reduced to 3,800 cfs max
Under the proposed interim drought plan EA, if river inflows at the Broad River gage reach a 28-day average of less than or equal to 10 percent of the average historical flow rate, outflows at Thurmond will be further reduced to the following:
Level 1 – Outflows reduced from 4,200 cfs to 4,000 cfs
Level 2 – Outflows reduced from 4,000 cfs to 3,800 cfs
Level 3 – Outflows reduced from 3,800 cfs to 3,600 cfs
The EA also adds new features for wintertime low flow windows. If in Drought Level 2, the Corps will reduce flows to 3,600 cfs between Nov. 1 and Jan. 31. If in Level 3, we will further reduce flows to 3,100 cfs between Nov. 1 and Jan. 31.
While all of these features will help conserve water, they will not end a drought. However, the plan would help store additional water in the reservoirs earlier in drought.
For daily updates on water management in the Savannah River Basin, visit our water management webpage. To learn more about the multiple users of the Savannah River, watch the Corps’ 8-minute video clip “Balancing the Basin.”