Earthen Spillway Failure Impacts Limited in New Jersey

A dam failure like the one on February 12 at the Oroville Dam in Northern California does not present similar risks to New Jersey’s population because of differences in the State’s dam design, topography, and magnitude. According to media outlets, heavy damage to the Oroville Dam’s main spillway compromised its structural integrity, hindering its safe release of floodwater. Water flowing down the unpaved auxiliary spillway eroded the hillside and roughly 180,000 people in the inundation zone were subsequently evacuated.

  • The Oroville Dam’s auxiliary spillway consists of a concrete control structure and an earthen hillside that has not been used since its construction in 1961. Following heavy rain events in the area, a controlled release of water began eroding the hillside, which prompted dam officials to warn it could fail.
  • Contrary to this design, most of New Jersey’s large dams do not use earthen hillside spillways. Instead, they use concrete overflow or other control structures, which New Jersey Department of Environmental Protection engineers inspect annually. Smaller New Jersey dams make use of earthen auxiliary spillways. Dam safety inspections consist of:
    • Identifying conditions that adversely affect dam safety and functionality;
    • Noting the extent of deterioration as a basis for long-term planning, periodic maintenance or immediate repair;
    • Evaluating conformity with current design and construction practices; and
    • Determining the appropriateness of the dam’s existing hazard classification.
  • Engineers suggest the degradation of the Oroville Dam’s primary spillway was due to its design and foundation on weathered rock, and the auxiliary spillway lacked a mechanism to prevent erosion. By contrast, most of New Jersey’s large dams use a more stable concrete design, and are located in mountainous regions anchored in secure bedrock.
  • The sheer size of the Oroville Dam and its impoundment dwarf those in New Jersey. The Oroville Dam stands 770 feet tall and impounds 3.5 million acre feet of water, whereas New Jersey’s tallest dam stands at 280 feet and its largest impoundment stores 168,000 acre feet of water.

For additional information, please contact NJOHSP’s Preparedness Bureau at