Location and Purpose: Center Hill is a multi-purpose dam and reservoir on the Caney Fork River, a major tributary of the Cumberland River, about 60 miles east of and 100 river miles upstream of Metropolitan Nashville. Center Hill reservoir is the Nashville District’s second largest flood storage reservoir. Originally constructed for flood damage reduction and hydropower, the dam and reservoir also provide water supply, recreation, and water quality benefits to the region. The purpose of the ongoing Dam Safety project is to lower risk to life, property and the environment to within standard USACE tolerable risk guidelines.
Project Description and Background: Center Hill consists of a 248’ high by 1,400’ long combination earthen embankment and concrete main dam and a 125’ high by 800’ long earthen embankment auxiliary dam referred to as the ‘saddle dam’. To comply with updated inflow design flood guidance, a self-eroding fuse plug section was retrofit into the top of the saddle dam in 1992 as the optimum means for to safely pass additional flood flows. The fuse plug provides emergency project spillway capacity.
Original dam design and construction took place in the 1930’s and 1940’s when designers had limited understanding of adequate foundation preparation techniques for earthen embankments in karst geology. The two large main dam and saddle dam embankments were placed on highly fractured limestone with open and clay-filled features. Evidence of potential internal erosion issues included abnormal water pressure, settlement, wet areas and springs. In 2005, Center Hill was classified among the initial array of USACE Dam Safety Action Classification (DSAC) 1 category projects--the highest urgency for dam remediation. The embankment foundations were the primary focus of the initial Dam Safety construction.
The Dam Safety project was approved in late 2006 and began with design of the first of three major construction contracts. Construction began in 2008 with a significant foundation grouting effort at the main dam embankment and left rim. Foundation grouting is pumping a flowable concrete into the foundation to stabilize the area. The intent of grouting was to make the dam safer by filling voids in the foundation and to prepare the ground to allow a deep foundation barrier wall to be safely built with the standard slurry method. The grouting program also provided valuable information on the extent of rock solution features to facilitate the design depth and extent of the subsequent barrier wall. The grouting contract totaled approximately $107M and was completed in 2010 by Advanced Construction Techniques. The main dam embankment concrete barrier wall contract was subsequently begun in 2012. Constructed by Bauer Foundation Corporation for $122M, the foundation barrier wall was substantially complete in July 2015. The 2.5’ thick concrete barrier extends as much as 307’ below the top of the dam through the embankment and the karst foundation, ending in more competent, solid rock. The concrete barrier wall is the permanent seepage barrier protection for the main embankment.
The third major construction effort reduces risk of both an internal erosion foundation failure and a potential overtopping failure mode at the saddle dam. The contract was awarded in 2016 and includes a concrete reinforcing berm downstream of the saddle dam embankment. The contract also includes excavation and stabilization of a prior rock cut made for grouting the dam’s left rim. The berm structure will be built primarily of Roller Compacted Concrete (RCC) with conventional concrete facing. Rock fill from the left rim stabilization will be placed between the existing saddle dam embankment and the upstream face of the concrete berm. The RCC berm contractor is Thalle Construction Corporation and the current cost of the berm and left rim stabilization is approximately $44M. The overall, current certified total project cost estimate is $353M.
Current Project Status
Construction: The RCC berm foundation rock excavation and batch plant erection are complete. Subsequent work items include foundation mapping, dental concrete, leveling concrete placement and consolidation grouting. A concrete test section, to trial mixes and methods of placement prior to the RCC berm start, was completed in April. A small business contract a site restoration is scheduled for a May 2018 award.
Per Dam Safety regulation, a post-construction evaluation is required to evaluate the implementation of the risk reduction measures in the context of the significant potential failure modes. The evaluation should confirm that the failure modes have been properly addressed, evaluate the dam’s compliance with essential USACE guidelines, and estimate the post implementation risk. The evaluation also determines if any new significant potential failure modes exist and incorporates them into the risk estimate. The Center Hill PIE analysis took place in 2016 and findings were presented to the USACE Dam Safety Senior Oversight Group (DSOG) in February of 2017. Considering the completed remediation efforts, main dam risks were judged to be within USACE tolerable risk guidelines. At the saddle dam, the risk associated with overtopping erosion and internal erosion were both estimated to be above the tolerable societal risk limit until the RCC berm is complete in early 2019. Previously unidentified failure modes, however, were noted concerning the main dam spillway tainter gates. Electrical, mechanical and structural operability issues documented with the 70-year old main dam spillway gates affect the reliability of controlled spillway releases and drive residual project risk. If the spillway gates don’t operate dependably during an extreme flood event, the probability and associated consequences of a premature fuse plug operation are estimated to be above the USACE tolerable societal risk limit. The DSOG recommended that the overall Center Hill project risk rating be lowered to a DSAC 2 and furthermore, that the District conduct a supplemental Dam Safety Modification Study to address gate reliability issues. The study process has begun and public and agency input is being sought for all measures under consideration.
2015 Aerial View of Center Hill Dam and Reservoir. Eight (8) main dam gates rotate upward to pass flood flow. A fuse plug in the top of the saddle dam provides additional emergency spillway capacity to keep the main dam from overtopping during an extreme flood event.
Dam Safety Study: The Dam Safety Modification Study (DSMS) will evaluate spillway gate repair alternatives and potential changes to standard gate operations during a large flood. Pursuant to the National Environmental Policy Act of 1969, as amended, the District is preparing a draft Environmental Impact Statement to support the DSMS. We want public input on the array of alternatives being considered. Examples of measures considered, either individually or in combination, may include: a) Replacement of the current gate machinery with hydraulic machinery that can operate under water; b) Addition of equipment to the current spillway gates to keep them open if the operating machinery is underwater; c) Modification of the spillway gates or gate machinery to allow operation from the top of the dam; d) Relocation of the gate operating machinery to the road level, which would require raising or relocating Highway 96 that crosses over the dam; e) Modification of the emergency operations plan in the water control manual that determines how to manage floods at Center Hill Dam; and f) Other measures as identified by on-going engineering studies, the public, and agencies. All interested parties are encouraged to submit comments along with their name and e-mail address to: email@example.com ; and/or to request to be placed on the project mailing list to receive fact sheets, newsletters, and related public notices. See the “Dam Safety Mod Study May 2018 Public Meeting Presentation Slides” at the tab to the left of this webpage.
Lake Levels: The district has targeted Center Hill Lake levels about 15 to 20-feet lower than normal since 2008 as an interim risk reduction measure while risk reduction construction is completed. After the saddle dam RCC berm is constructed, risk is driven by main dam spillway gate issues. The project will be reassessed when enough of the construction is complete to lower risk and the study risk results are established; both requirements estimated by early 2019. At that time we will determine the appropriate lake levels for the future. While we fully anticipate lake levels will return to normal, there is always a chance of continued lowered lake levels.
(Updated June 2018)