LOWER DAM REHABILITATION FROM INCEPTION TO CONSTRUCTION

摘要

The Lower Occoquan Dam (Lower Dam) is owned and operated by Fairfax Water as part of their water supply and delivery system. The dam is also part of the Occoquan River Project, a small hydroelectric facility regulated by the Federal Energy Regulatory Commission (FERC). Fairfax Water is surrendering their FERC license and by doing so, regulatory responsibility for the dam transitions from FERC to the Virginia Department of Conservation and Recreation (DCR). The 60 year old Lower Dam is classified by FERC as a high hazard potential structure. The concrete gravity dam is a typical run-of-river structure with a height ranging from 10 to 30 feet and a length of about 500 feet. The dam exhibited deteriorated concrete; seepage through and beneath the dam; and a non-operational low level outlet. Before completion of the surrender process, FERC required Fairfax Water to address the issues surrounding the Lower Occoquan Dam. Fairfax Water realized dealing with the Lower Dam meant satisfying FERC dam safety guidelines as well as Virginia dam safety regulations. With a looming deadline, Fairfax Water needed to efficiently navigate FERC guidelines and changing Virginia dam safety regulations by evaluating the hazard classification; selecting the appropriate inflow design flood using an incremental damage assessment; evaluate the dam's integrity and stability; assess disposition alternatives; decide how to move forward; and implement the plan. This paper will discuss the following topics: 1. FERC and DCR regulations and inconsistencies discovered along the way; 2. Analyses performed including stability, hazard classification and IDA; 3. Rehabilitation design; 4. Construction including photographs during river diversion, demolition, rock excavation and concrete placement; and 5. Lessons learned.