ALTERNATIVES FOR UPGRADING AN AMBURSEN DAM WITH GATED SPILLWAY TO SAFELY MANAGE THE PROBABLE MAXIMUM FLOOD

摘要

Over 1.6 million people in eastern Maryland receive their drinking water from surface water sources. An important component of the water supply system is raw water storage to ensure a safe, reliable supply. To that end, a river intake on the Potomac River and three water supply reservoirs have been constructed and maintained. One of these reservoirs is impounded by a 147-foot-high, concrete slab-buttress (Ambursen) dam with a gated spillway, which was built in the late 1940s. The dam is classified by the Maryland Department of the Environment (MDE) Division of Dam Safety as a "high hazard potential" structure. Since the dam's original construction, the standards for what constitutes the design rainfall event have changed. Under the current state dam safety regulations, the Probable Maximum Flood (PMF) is the required storm event that "high hazard potential" structures, must safely pass. Updated hydrologic/hydraulic studies indicate that the dam's spillway is inadequate to safely pass the PMF. In December 2004, MDE directed the owner to conduct a comprehensive dam safety engineering analysis of the dam. URS Corporation (URS) was retained to complete the engineering and safety analyses for the dam. The study examined the spillway capacity, global stability, and structural integrity of the dam and associated spillway and the erosion resistance of abutment and foundation materials. The study also assessed the impact of increased flows on downstream development. The URS study evaluated multiple rehabilitation alternatives: (1.a) parapet walls with scour protection along the toe, (1.b) roller-compacted concrete (RCC) overtopping protection with scour protection along the toe, (2) installation of additional radial gates, (3) construction of a new auxiliary spillway on one or both abutments, and (4) construction of a new dam downstream of the existing structure. The study recommended Alternative 1.a, which consists of a parapet wall and scour protection along the dam's toe, as the preferred alternative. Following their review of the study and alternatives analysis, MDE issued a letter concurring with the recommended alternative and encouraged the owner to proceed with final design. URS performed the final design and prepared the construction drawings and specifications. The project is to be bid during late summer 201 land construction is anticipated to begin in the fall of 2011. The paper describes the hydrologic and hydraulic analyses that increased the PMF for this project, as well as the global stability, structural analysis, and erodibility studies that were conducted for the dam. It also describes the alternatives that were considered for safely passing the PMF and the analysis and evaluation that went into selecting the preferred alternative. Finally, the paper describes various detail design issues addressed during the final design process.