Assunpink Creek Dam No. 8 is a high-hazard dam located on Pond Run in Hamilton Township, New Jersey. The 2017 condition assessment indicated that the dam did not meet current NRCS criteria for spillway capacity and integrity. The purpose of the project was to provide Rehabilitation Planning Assistance that results in the completion of a Supplemental Watershed Work Plan and Environmental Evaluation (EE).
The Supplemental Watershed Plan–EE was developed using a multidisciplinary approach in accordance with State and Federal requirements. We assisted the sponsors in the development of the purpose and needs statement for this dam and conducted an inventory of the watershed resources. This inventory included the status of operation and maintenance of the dam, sedimentation accumulation rates, potential modes of failure of the dam, recommended hazard classification, consequences of dam failure, and biological, ecological, economic, and social resources of the watershed. The project also included geotechnical investigations to support the development of SITES and WinDAM material properties and evaluation of the seepage and slope stability of the existing dam.
Hydrologic and hydraulic models (HEC-HMS, HEC-RAS, SITES, and WinDAM) were developed to support the benefit-cost analysis, dam breach modeling and inundation mapping, and evaluation of rehabilitation alternatives. Alternatives that meet the identified purposes and needs of the dam, including a no federal action alternative, a decommissioning alternative, and three dam rehabilitation alternatives, were formulated and evaluated. Dam rehabilitation alternatives studied in detail included structural RCC spillway armoring of the existing auxiliary spillway, labyrinth weir spillway, and straight drop spillway. The 110-foot-wide, 4-cycle, 2-stage structural concrete labyrinth spillway was identified as the preferred alternative and was presented during a virtual public meeting with with NRCS, the Sponsors, and the public.
Upon the completion of the public meeting, we developed the Supplement Watershed Work Plan – EE document, which was approved in August 2021.