Project Overview

The Bureau of Reclamation is preparing an environmental impact statement (EIS) for the Scoggins Dam Safety Modifications project. Scoggins Dam, located in western Oregon, is owned by Reclamation. While Scoggins Dam is performing as designed, Reclamation has determined that seismic safety deficiencies exist at the dam. Investigations conducted under Reclamation’s Safety of Dams Program determined that the dam is potentially at risk from severe seismic activity associated with the Cascadia Subduction Zone (CSZ) that could result in dam failure, which poses unacceptable risk levels to people, property, and the environment.  The purpose of the project is to improve public safety by reducing risk associated with severe seismic loadings while continuing to meet authorized project purposes. This project is authorized under the Safety of Dams Act (Public Law 95-578).

Background

Scoggins Dam is an earth-fill, embankment dam located on Scoggins Creek, a tributary of the Tualatin River, about 25 miles west of Portland, Ore. Construction of this 151-foot-high, 2,700-foot-long dam was completed in 1975. The dam’s reservoir, Henry Hagg Lake, is the primary source of water for the Tualatin basin, storing nearly 60,000 acre-feet (active 53,600 acre-feet), providing water for municipal and industrial uses, irrigation, water quality, fish and wildlife habitat, recreation, and flood control. The facility is operated and maintained by the Tualatin Valley Irrigation District. There are roughly 11 miles of shoreline around the lake at full pool; recreation facilities and trails in this area are managed by Washington County as Scoggins Valley Park.

The area of Scoggins Dam and its reservoir have high potential for severe loading initiated by an extreme seismic event from identified active faults, primarily the Cascadia Subduction Zone, a 600-mile fault, stretching from northern California to northern Vancouver Island in Canada. At its closest, the CSZ is 118 miles west of the dam. The principal concerns for Scoggins Dam are uncontrolled releases or dam breaches (dam failure) caused by severe loading from a CSZ seismic event. The dam also could experience less severe loading from local crustal fault earthquakes, the closest being the Gales Creek fault zone.

Around 2007, after completing general investigations of potential seismic hazards at the dam, Reclamation recognized the potential impacts of a CSZ seismic event to Scoggins Dam. Reclamation continued field data collection and evaluation and risk analyses updates through 2011 to improve the understanding of the seismic risk. Since 2011, Reclamation has considered various structural and non-structural options to reduce seismic risk, including options that would increase reservoir storage. In 2022, following completion of a Dam Safety Advisory Team review, Reclamation began furthering design of a dam-safety-only structural option that would reduce risk in accordance with Reclamation’s public protection guidelines. This alternative will be evaluated in the EIS.

Purpose and Need for the Proposed Action

The purpose of the project is to improve public safety by reducing risk associated with severe seismic loadings primarily imposed by a CSZ event, while continuing to meet authorized project purposes. The need for action is to prevent the potential loss of life, property, water storage, and other project benefits due to a seismically induced dam failure.

Proposed Action

Reclamation proposes to reduce the risk to Scoggins Dam in the occurrence of a CSZ seismic event by improving its loadings response performance. This would be accomplished by raising the dam crest, constructing a downstream shear key, creating a new spillway, and placing additional berm material over the existing dam. The project would not create additional reservoir storage in Henry Hagg Lake.

Proposed dam structure modifications are as follows:

• Excavate and backfill portions of the crest and existing embankment

• Construct a downstream shear key

• Install a downstream rock filter and drain

• Install a stability berm over the shear key and downstream slope of dam

• Raise the dam crest by approximately 7 feet

• Demolish the existing spillway, bridge, and ancillary features

• Construct a new spillway, bridge, and ancillary features and extend outlet works

• Construct a new two-lane road across the dam

The existing road across the dam would be closed during construction. An alternative road would be constructed to provide safe public transport.

The project also would include modification to structures around the reservoir such as culverts and recreation trails. Materials for construction would be excavated at one or more borrow areas on the east side of the reservoir, requiring some tree removal. Accessing and transporting materials from the borrow areas through construction of a haul route and/or via the existing Scoggins Valley Road will be investigated in the EIS.

Previous Water Supply Studies

In 2001, the Tualatin Basin Water Supply Feasibility Study was initiated to evaluate a range of water supply options in the basin, including raising Scoggins Dam (publication in the Federal Register on December 13, 2001, 66 FR 64454). A Draft EIS was prepared in 2007, but never published, because of the need to further evaluate the seismic risk of the CSZ to the dam. During 2013, some of the partners in the feasibility study began separately pursuing other water supply options that did not include Scoggins Dam or Reclamation participation. In 2017, following receipt of a Joint Project Authority secured in amendments to the Safety of Dams Act in 2015, Reclamation began working jointly with Clean Water Services, analyzing the feasibility of three options (dam safety only modification, dam raise, and new downstream dam); all options would have reduced seismic risk at the dam, and two would have increased water supply in the basin. In 2021, a determination was made to forego further development of increasing reservoir storage and to support development of a dam safety only modification.

Statutory Authority and Anticipated Permits

NEPA [42 U.S.C. 4321 et seq.] requires federal agencies to conduct an environmental analysis of their proposed actions to determine whether the actions may significantly affect the human environment. Reclamation will analyze the environmental effects of implementing the proposed action and alternatives, and a no action alternative. The U.S. Army Corps of Engineers, Tualatin Valley Irrigation District, Washington County, Joint Water Commission (representing the cities of Beaverton, Forest Grove, and Hillsboro), Clean Water Services, and Confederated Tribes of the Grand Ronde Community of Oregon have accepted invitations to participate as cooperating agencies for the EIS. Other entities will be considered, as necessary, during the NEPA process. In addition to NEPA, various other federal, state, and local authorizations may be required for the proposed action. Applicable federal laws include, but are not limited to, the Endangered Species Act, National Historic Preservation Act, and Clean Water Act.

Project Area Overview Map

For further information

For more information on the Scoggins Dam Safety Modifications Project EIS, please visit the project website (Scoggins, CPN Region | Bureau of Reclamation (usbr.gov)) or contact Rebecca Thompson, natural resource specialist (BOR-SHA-SCNEPA@usbr.gov).

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