Warren Mill Dam Alternatives

This Class I – High Hazard dam, constructed in the early 1900s, provided a source of water and electricity for a downstream paper mill (now closed). The New Jersey Department of Environmental Protection (NJDEP) owns the portion of the dam located within Holland Township; the portion in Pohatcong Township is privately owned. Regular Dam Inspections conducted 2009-2019 by Civil Dynamics/GZA rated the dam in “poor” condition due to inadequate spillway capacity and other deficiencies.

Challenge

The deteriorated dam is a hazard to downstream life and property and must be made compliant with current state New Jersey Dam Safety Standards by either rehabilitating or decommissioning it. From the environmental perspective, the desire is to decommission the dam since the Musconetcong River is part of the National Wild and Scenic River System. With the removal of the Hughesville Dam in 2016, the Warren Mill Dam is currently the first dam on the Musconetcong River. State and federal agencies and several non-profit organizations support dam removal and the restoration of fish access. However, the cost to manage the large volume of sediment in the reservoir upstream of the dam impacts the feasibility of decommissioning the dam.

Solution

GZA was engaged in early 2020 by the State of New Jersey to develop and evaluate alternatives to either rehabilitate or decommission Warren Mill Dam in accordance with New Jersey Dam Safety Standards. To provide the State with the information necessary to select the appropriate alternative, GZA conducted a comprehensive evaluation that included:

• A geotechnical investigation of the dam and spillway.
• Comprehensive engineering evaluation (spillway capacity and structural stability) of the existing condition of the dam and spillway.
• Comprehensive evaluation of the quantity of sediment in the reservoir (including 35 sediment probes advanced from a barge-mounted tripod drilling rig).
• Evaluation of the sediment quality in the reservoir and within the river.
• Comprehensive hydrologic and hydraulic modeling of the Musconetcong River and the dam.
• Development of preliminary fish passage alternatives for a rehabilitated dam.
• Development of preliminary breach designs and comprehensive hydrologic and hydraulic modeling of possible breach scenarios.
• Development of sediment management alternatives, including dredging, dewatering and final placement of the sediment. 
• Hydraulic modeling of a phased decommissioning alternative that allows the sediment to be mobilized and released downstream. This evaluation included sediment transport modeling.
• Development of cost estimates for the alternatives.

Benefit

The Study was completed in 2021 and well-received by the Division of Dam Safety and Flood Control, the Division of Fish and Wildlife and the Bureau of Freshwater Fisheries. These agencies are working to move forward to decommission the dam.