The Savannah River Basin sputtered its way through September but somehow still landed close to par. This feat seems even more astonishing – if watching rain gauges can be considered astonishing – when you consider the entire basin collected more than one third of its monthly average rainfall on a single day, Sept. 25.

The rest of the month was a different story.

Hartwell, which received 4 inches last month (84 percent of its average, 4.7 inches), received less than 1/10^th of an inch for 23 of September’s 30 days. One tenth of an inch is like a toddler’s spittle and probably just as annoying if you’re counting on rain.

Russell and Thurmond weren’t much better; they had 8 and 7 days, respectively, out of 30 where the total rainfall exceeded 1/10^th of an inch in a 24-hour period.

Click here to see the hourly data for the sub-basins in September.

Both fared better than Hartwell with respect to their averages (Russell received 3.4 of its 3.7-inch average, while Thurmond got 3.7 inches – just over its average of 3.6).

And after all the hard work of collecting every little drop in September, Hurricane Joaquin and an associated low-pressure system brought a windfall of rain in October.

In fact, the storm delivered an average of more than 5 inches across the sub-basins, which was almost enough to bring the basin out of Drought Level 1. Almost.

When taken in context, two forces came together and in four days ameliorated a deficit for the Savannah River Basin – just in time for the new fiscal year.

– See more at: http://balancingthebasin.armylive.dodlive.mil/2015/10/09/local-weather-systems-in-tune-with-fiscal-calendar/#more-2193

Since our last progress report on the flood storage study published November 2014, we received a few inquiries on when the results would be released. In the above linked post we estimated the study would take approximately six more months to complete, which projected a late spring/early summer completion date.

Off by a few months, the study concluded in July 2015. Although we now have a complete data set and hydrology models, our water managers report that the Corps’ Hydrologic Engineering Center (HEC), the organization conducting most of the study, is in the process of releasing a report of their analysis on the management of the Probable Maximum Flood (PMF).

With this report, HEC will also be releasing a set of design storms which will require further calibration. Once calibrated, we can assess the reallocation of flood storage at Hartwell.

Data analysis on the reallocation of flood storage at Hartwell is scheduled to take a few months, dependent on available funding and resources.

Recommendations will then be forwarded to the District’s Planning Division to determine whether a reduction in flood pool can be considered as a viable alternative in the next phase of the basin’s comprehensive study, said Corps water manager Stan Simpson.

For those needing a recap or overview of the study, our October 2013 announcement detailed why the Corps began the study.

The story explained how the flood pool analysis intended to examine whether the established 4-foot winter drawdown could be reallocated according to new data.

To summarize, HEC developed rainfall models to analyze the ability of the projects to contain:

• the greatest rain event the basin could expect, known as the Probable Maximum Flood (PMF); and
• the 2- to 500-year storm events at the three Savannah River projects for future analysis. The HEC models, once calibrated, can later be used to analyze possible reductions in flood storage at the projects.

Since then, HEC has developed and run several scenarios to test the ability of the projects to pass the Probable Maximum Flood. These same models are being further refined by the Mapping and Modernization Center for incorporation into the Corps Water Management System (CWMS), a tool that offers features that most Corps water managers need to perform operation analyses on a daily basis.

The CWMS suite can be used to test the feasibility of different scenarios and develop estimates of increased damages due to reduced flood storage, according to Simpson.

Water managers and members of the District’s Planning Division will use these results to evaluate the impacts associated with changes in project operations. If decreasing the winter drawdown is feasible, then reallocating the proposed 2 feet of flood storage to conservation storage would become a viable alternative considered in the third and last phase of the Savannah River Basin Comprehensive study.

If the proposed reduction alternative is the chosen alternative for that phase, the district would recommend a reallocation of flood storage which requires congressional approval.

Thanks for reading us, and as always, we welcome your comments and feedback!

~Chelsea Smith, public affairs specialist COE