T. Howard Duckett Dam: Completely made simple

The Washington Suburban Sanitary Commission (WSSC) supplies water and wastewater service to over 1.6 million customers in Montgomery and Prince George's Counties, Maryland. An important component of the supply system is T. Howard Duckett Dam that creates Rocky Gorge Reservoir. Based on current hydrologic understanding, the design flood for the dam is significantly greater than the flood event used during the initial design of the project. Therefore, the owner requested that a comprehensive dam assessment be performed on the dam, consisting of hydrologic analysis, structural analysis, and alternative development as necessary. The T. Howard Duckett (Duckett) Dam is located on the Patuxent River, just upstream of Laurel, Maryland, and is owned and operated by the Washington Suburban Sanitary Commission (WSSC). The dam was designed by the Ambursen Engineering Company, New York and constructed in 1954. The dam is a composite concrete slab and buttress dam with embankment wing dams. The concrete dam has a maximum structural height of approximately 147-feet and crest length of 720-feet. The concrete slab and buttress dam is of the Ambursen type. The Ambursen Engineering Company is credited with popularizing the slab and buttress style dams in the early part of the 20th century. The dam is a hollow structure, and consists of reinforced concrete buttresses that support upstream and downstream reinforced concrete slabs. The overflow spillway section is controlled by seven, 15-foot high by 27-foot wide radial gates in bays that are 30 feet wide center-to-center. The comprehensive safety analysis included a three-dimensional structural analysis of the dam to assess the safety of the structure for normal (usual), flooding (unusual), and seismic (extreme) loading conditions. The evaluation of the dam was based on the most probably failure modes typical for this type of structure, and included the structural capacity of the reinforced concrete, sliding stability of the dam, and rock scour (erosion) due to overtopping. This paper will discuss how and why these three failure modes were the primary focus for the safety assessment, but will also present reasons why other failure modes, such as overturning, were not evaluated. 14 pp., 3 references.