The 2011 Long Term Control Plan for the Nashville Metro Water Services Combined Sewer System indicated that the capacity of the Driftwood Equalization Basin needed to be expanded from approximately 3.3 million gallons to 6.4 million gallons in order to reduce the number of expected overflow events in a typical year from approximately six to nearly zero.

The original conceptual design by another consultant for the expansion of the Driftwood Equalization Basin involved the addition of several large pumps to convey combined sewage from one compartment into the other. This approach would have required extensive modifications to the structure. Operation of the pumps would have also incurred significant electrical expenditures as well as increased operations and maintenance requirements on Metro Water’s Route Services staff. SSR’s approach alleviated each of these issues, resulting in a project that not only cost 50% less than the original concept, but also one that is more reliable, and has less impact on the environment. This approach improved the social, environmental, and financial viability of this structure for the Clean Water Nashville Program and the citizens of Nashville.

SSR’s project team was selected by Clean Water Nashville through a Qualifications Based Selection (QBS) process. The team consisted of both national and local experts as well as assistance through Nashville’s MBE/WBE program. The team worked collaboratively with Metro Water’s Clean Water Nashville staff as well as their program managers to determine the feasibility of the alternative approach through a “Concept Confirmation” process that we recommended. Once the approach was confirmed by all parties, the design and construction of the improvements instituted the first such process under the newly formed Clean Water Nashville program.

Since the expanded facility went online in 2013, there has not been a single combined sewer overflow from the Driftwood Equalization Basin. This is despite two very wet summers and several storms that exceeded the 2-year, 3-hour design storm adopted as a reference by Clean Water Nashville. This equates to the prevention of approximately nine million gallons of combined sewer from entering the Cumberland River in a typical year. This endeavor was a total project success for the SSR team, for Clean Water Nashville, and for the citizens of Nashville.  The original construction budget for the project was $3.5 million, but actual construction cost was reduced to $1.8 million through innovative engineering.