We recently published a report on a study that analyzed the ability of our three dams to contain the greatest rain event the basin could expect, with a section that addressed the feasibility of reducing the winter drawdown two feet. Discussion on both follows.
On the worst storm ever
The term we use for the greatest rain event the basin could expect is “Probable Maximum Flood” (PMF). The findings of the study indicate that the Hartwell, Russell and Thurmond dams would survive the PMF intact and operational – that is, water would never top the gates.
However, assurance of dam integrity is where the warm-fuzzy ends on the PMF analysis. Although unlikely, in response to a PMF the dams would need to open wide their spillway gates to prevent an overtopping that would destroy the structures themselves. This means massive outflow. Massive defined as 1.02 million cubic feet per second (cfs) at Augusta.
According to the report, the Augusta Levee is only capable of protecting the city from floods less than 550,000 cfs. Therefore in the face of such a crippling storm event, not the levee – not even Thurmond Dam – could protect the city from a deluge that would dwarf the historic 1929 flood.
For perspective, in the 1929 flood the Savannah River near Augusta peaked around 300,000 cfs. Without the dams the PMF flow at Augusta would peak at 1.26 million cfs. That’s an additional 240,000 cfs – but this difference is meaningless in terms of impact.
I must emphasize that the PMF is a near impossibility. We conducted this analysis because the design criteria recently changed; the dams weren’t designed with Noah in mind. However, if the projects were designed today, a PMF design storm would be used to design the flood capacity. The study enabled us to confirm that Savannah District dams meet today’s criteria.
On changing the winter drawdown
In a previous post we reported there was another purpose to this study that focused on flood-storage reallocation. Using this study we wanted to determine if it would be worth pursuing an alternative in the upcoming Comp Study that included allocating 2 feet of the winter flood storage to conservation storage.
In simpler terms we asked ourselves, is it worthwhile to consider reducing the winter drawdown from 4 feet to 2 feet?
The role of the current study was to create modeling that could process a simulation of the 2- to 500-year storm events at the three Savannah River Basin projects for future analysis. Although the Hydrologic Engineering Center models have not yet been calibrated for these storms, this winter we observed something better than a simulation. The Savannah River Basin experienced a 25-year event in the series of storms that took place in the last half of December 2015.
When this rainfall event began, all three reservoirs were very close to the winter guide curve. Thurmond and Russell were just below their guide curve and Hartwell was little above. This means Hartwell and Thurmond had about nine feet of flood storage capacity.
But with just a 25-year rain event all three reservoirs reached their flood storage capacity. Thurmond’s level rose nearly a foot above capacity (peaking at 336). As a result we needed to open Thurmond’s spillways. To keep Thurmond’s gates from overtopping (at 335) we had to raise the gates to 336.5, leaving a foot-and-a-half gap at the bottom of all 23 gates for water to spill through.
This resulted in an outflow of 45,688 cfs – more than 15,000 cfs above channel capacity. Combined with local inflows at Augusta, the Savannah River peaked at about 54,000 cfs in early January.
The result: flooded properties in Aiken and Richmond counties for weeks, trapping some inside their homes and causing millions of dollars in damages.
Furthermore, models indicate our dams prevented another $13 million in flood damage from this winter storm event. This suggests the potential for even more destruction of property existed; greater amounts of discharge would have increased the amount of damage and perhaps increased risk to human life.
The report concludes that reducing the winter drawdown doesn’t appear to be a feasible (safe) option based on conditions before the storm event (full flood storage available) and what we observed from the intense winter rainfall at the end of 2015. In the words of the report:
“The December 2015 storm revealed that the flood control pools for the Corps’ three multipurpose projects (with the 4-foot winter drawdowns at Hartwell and Thurmond) have the capacity to store slightly less than a 25-year storm event without making use of induced surcharge. The Corps observed that releases from a 25-year storm event will flood portions of Augusta and North Augusta.”
This doesn’t mean we ruled out considering the reallocation of flood storage as a viable alternative in the third phase of the Comp Study. But since flood-risk reduction is a primary purpose of the dams, the best data we have is far from justifying a smaller winter drawdown.
We welcome your comments.
~Russell Wicke, Chief of Corporate Communications