Using the Progressive Design Build Model to Balance Risk and Costs when Sizing a Diversion for a Small Dam on a Flashy Watershed

ABSTRACT ONLY - Kiewit is responsible for the design and construction of Lake Conestee Dam, a new roller compacted concrete (RCC) dam immediately downstream of the existing masonry dam. To construct the dam across the Reedy River, the design and construction teams collaborated in a progressive design-build approach to develop several diversion concepts including side channels around the work area, siphons, and culverts through the work area. The selected diversion concept balanced the construction and dam safety risks with the construction costs. This paper will describe the analyses performed to support design decisions, risk considerations, the process for selecting the preferred alternative, and provide a summary of the performance of the diversion through the early stages of construction. The diversion is sized based on a statistical analysis of river gage data used to assess the number of overtopping events and duration of overtopping expected while the cofferdam is in place. The strategy was to balance the potential down time due to flooding and cleanup with capital costs for additional diversion capacity. This resulted in a bypass capacity 500 cfs that would be routed through culverts while larger floods would overtop the cofferdam and flood the work area. After flood events, water will be pumped from the work area and the RCC surface will be cleaned and prepared before RCC placement continues. Given the flashy nature of the storm events, the construction team will monitor precipitation and stream flows to identify potential incoming floods for removing staff and equipment from work areas prone to flooding. Lake Conestee Dam was originally constructed in 1892 as a hydropower dam for the Conestee Mill near Greenville. The mill is no longer operational, and the reservoir has largely sedimented in, some of which is heavily contaminated. The existing dam is owned and operated by the Lake Conestee Dam Restoration Project, LLC (LCDRP), and is classified as a Small, Significant Hazard (Class II) Dam in accordance with South Carolina Department of Environmental Services (SCDES) Dams and Reservoirs Safety Act Regulations.