Eightmile Dam rebuild & restoration

After the 2017 Jack Creek Fire, the Dam Safty Office (DSO) raised the hazard classification of the Eightmile Dam. Because the dam had already had maintenance issues, Icicle and Peshastin Irrigation Districts began developing alternatives to replace it. Initially, IPID developed several conceptual alternative design configurations for consideration by DSO. Through several discussions with DSO, IPID carried two alternatives forward: the narrow spillway with gates (Alternative 1) and the wide spillway without gates (Alternative 2). As a result of the EIS scoping process, a third action alternative was added for analysis in the EIS: the narrow spillway without gates (Alternative 3).

The three action alternatives and the No Action Alternative were evaluated in the EIS. The alternatives are described below.

The preferred alternative, Alternative 2: Wide spillway without gates

Alternative 2 includes replacement of the existing dam with an earthen embankment and reinforced concrete dam with a primary spillway length of 180 feet. The primary spillway will be fixed and completely passive. No gates or automated equipment will control the spillway or adjust the spillway crest elevation. This will result in a wider spillway and a larger footprint than the Narrow Spillway Alternative (Alternative 1). There will be no intermediate spillways. The primary spillway will have a hard spillway crest at an elevation of 4,671.0 feet.

During extreme storm events, the lake will flow over the entire length of the primary spillway. A secondary spillway, the same as the Narrow Spillway Alternative, will be created in a low spot south of the main dam structure by hardening an existing channel with rock and riprap. The secondary spillway will have a crest elevation of 4,673.0 feet. The spillways will provide enough capacity to pass the design storm event while maintaining the freeboard in the lake required by DSO.

As with the Narrow Spillway Alternative, water will be released from the lake through a new 30-inch diameter low-level outlet pipe/siphon. The operation and configuration of the low-level outlet pipe will be the same described for the Narrow Spillway Alternative, with the low-level outlet pipe located entirely within the Special Warranty Deed Area.

The low-level outlet valve at the lake will be powered by batteries charged by a solar panel. Lake levels, valve positions, and other controls will be monitored remotely, and the equipment will be operated via radio signal requiring an antenna, which will be located at the dam site. The controls and monitoring equipment will be concealed as much as possible.

Other alternatives that were considered

No Action Alternative

The No Action Alternative is the baseline against which the action alternatives are evaluated and compared. It illustrates the most likely scenario if the project is not implemented. Analysis of the No Action Alternative is required under SEPA (WAC 197-11-440 (5)(b)(ii)).

Under the No Action Alternative, the existing dam would be left as is, and it would continue to operate in its current state and manner, with a primary spillway elevation of 4,667 feet and an outlet pipe that allows drawdown of the lake to a water surface elevation (WSEL) of approximately 4,640 feet without pumping. The invert elevation of the low-level outlet pipe is 4,648.7 feet, but additional drawdown occurs after the lake WSEL has reached the elevation of the low-level outlet pipe due to seepage through the landslide deposits that underlie the dam. Seepage can draw the lake down to a WSEL of 4,640 feet without pumping. The DSO considers the dam vulnerable in the event of a large storm due to changed conditions in the watershed both upstream and downstream of the dam, as well as to the condition of the dam itself. The Jack Creek Fire in 2017 burned a significant forested area in the watershed, creating conditions that generate higher peak runoff rates to the lake.

Operation of the dam under existing conditions is not consistent with DSO regulations and does not meet the DSO’s safety requirements for a High Hazard Dam. The DSO would eventually exercise enforcement actions in accordance with WAC 173-175-620 (3). However, it is not possible to predict with certainty what that action or its effects would be. DSO currently requires IPID to leave the low-level outlet open during the winter and early spring to reduce the risk of a dam failure. Consequently, for purposes of this EIS analysis, it is assumed that the existing state of the dam and its operation remain unchanged.

The No Action Alternative does not meet IPID objectives for water storage capacity for operations and irrigation water delivery. It would not contribute to the IWG Guiding Principle 1 related to streamflow improvements. The dam is currently operating in a deteriorating and unsatisfactory condition. An estimated 150 downstream residences are at risk if the existing dam were to fail, resulting in a High Hazard Classification.

Alternative 1: Narrow spillway with gates

Alternative 1 includes replacement of the existing dam with an earthen embankment and reinforced concrete dam structure equipped with automated control gates over the primary spillway. Three 4-foot-high, 20-foot-long automatic level control gates would be installed on top of the primary spillway, which would have a hard crest elevation of 4,667 feet. The gates would allow IPID to control the water level within the top 4 feet of the lake. When additional water supply is needed, IPID would raise the gates in the late spring or early summer to raise the lake to elevation 4,671 feet prior to releasing the water in the late summer. The gates would automatically lower if the lake level gets too high to protect the dam and prevent overtopping. For example, if a storm occurs when the gates are up and the lake is full, the gates would automatically lower to pass peak flows generated by the storm. This design would allow for a narrow primary spillway (60 feet wide) and therefore a smaller dam footprint compared to the Wide Spillway Alternative (Alternative 2).

During extreme storm events, the lake would continue to rise above the primary spillway. Two 15-foot-wide intermediate spillways on either side of the primary spillway would provide 30 feet of additional spillway width at an elevation of 4,671.5 feet. A secondary spillway would be created in a low spot south of the main dam structure by using rock and riprap to harden an existing channel. The secondary spillway would have a crest elevation of 4,673 feet. The spillways would provide capacity to pass the design storm event required by DSO (a storm that has the probability of occurring once in 1,000,000 years) while maintaining the freeboard (the vertical distance of the crest of the dam above the maximum lake water level) in the lake required by DSO.

Water would be released from the lake through a new 30-inch diameter low-level outlet pipe/siphon. The low-level outlet pipe/siphon would extend from an inlet submerged in the lake approximately 150 feet west of the new dam structure to an outlet in the Eightmile Creek channel approximately 314 feet downstream of the new dam structure. This would allow the lake to be drawn down to a low-water surface elevation of 4,636 feet, which would allow access to stored water without pumping. The low-level outlet pipe would be located entirely within the Special Warranty Deed Area. IPID would release water during the late summer to maintain the water supply available for irrigation use and instream flows in Icicle Creek. Releases through the low-level outlet pipe would be controlled by an automated plug valve at the downstream end of the pipe. IPID would have the ability to adjust the valve remotely to release the flows needed to meet downstream IPID water supply needs and instream flow needs.

The primary spillway gates and low-level outlet valve at the lake would be powered by batteries charged by a solar panel. Lake levels, gate and valve positions, and other controls would be monitored remotely, and the equipment would be operated via radio signal requiring an antenna, which would be located at the dam site. The controls and monitoring equipment would be concealed as much as possible.

Construction methods – applicable to all action alternatives

Constructing the dam would require transporting equipment and materials into and out of the Special Warranty Deed lands within the Alpine Lakes Wilderness. There are no roads that directly access Eightmile Lake. The lake can be accessed on-foot via the Eightmile Lake Trail (Forest Service Trail #1552), which IPID uses for routine maintenance at the dam. The trailhead is accessible from Leavenworth by vehicle following Icicle Road and FSR 7601-116. The distance from the trailhead to the lake is about 4 miles. An estimated 4 to 6 construction personnel per week will likely use the upper portion of this trail for access to the site on foot, although some may choose to hike from the trailhead. IPID proposes to improve and reopen a portion of a currently closed road located outside of the Alpine Lakes Wilderness for administrative use to bring vehicles closer to the project site. 

Helicopters would be used to move all equipment and the majority of materials to and from the site on Special Warranty Deed land. The primary determinant of the type of helicopter to be used is the maximum weight that needs to be transported. The largest payload would be the excavator, which could be partially disassembled to aid in transport and reassembled on-site.

Two approaches to helicopter use were considered.

Preferred - Option 1 would use the larger helicopter to transport nearly all equipment and material to the site at the beginning of the project, followed by a limited number of additional trips using the smaller helicopter to bring materials that were not anticipated initially.

Option 2 would involve the limited use of the large helicopter to move only the heavy equipment and a portion of the material, followed by periodic delivery of materials as needed throughout the construction, using the smaller helicopter. Under both options, the large helicopter would be used for 1 to 2 days to remove equipment and any remaining materials at the end of construction. It is anticipated that the majority of the helicopter trips would occur on weekdays; however, some weekend flights may be necessary.

Please note that option 1 is preferred for helicopter use in order to minimize the total number of helicopter trips, but ultimately this will be dependent on large helicopter availability during wildfire season that will also overlap with the construction period.   

• EIS Volume 1 - EIS and Appendices A-E
• EIS Volume 2 - Appendix F: Response to Comments
• EIS Executive Summary
• Legal Notice
• SEPA Register
• Alternatives Summary Table
• Alternatives Description
• Design Figures for the Alternatives
• Special Warranty Deed Parcels

With the completion of the EIS, Icicle and Peshastin Irrigation Districts (IPID) will now begin engineering design of the new dam.

Ecology is the lead agency under the State Environmental Policy Act (SEPA,) the law requiring agencies to assess the environmental effects of proposed actions prior to making decisions. We determined that the project could result in potentially significant adverse environmental impacts and developed an Environmental Impact Statement (EIS) that identified a preferred alternative for implementation.  Ecology's Dam Safety Office determined that the existing Eightmile Dam is a high-risk facility and must be rebuilt to protect public safety and maintain reliable irrigation supply. As lead agency, we were responsible for ensuring that the EIS was developed in accordance with SEPA requirements. The proponent for the project is the Icicle and Peshastin Irrigation Districts, the owner of the dam.  

We led the EIS preparation, working with an independent consultant team. The EIS team reviewed materials prepared by Ecology, IPID and their technical consultants, the U.S. Forest Service, and other sources to build upon previous analyses. All information was independently evaluated for accuracy, completeness, and relevance to this evaluation. Where information needed to be updated or expanded upon, the EIS team worked with us to determine the best approach to fill those information needs.

Ecology and Chelan County issued a programmatic EIS in 2019. It evaluated the Icicle Creek Water Resource Management Strategy. That programmatic EIS evaluated five programmatic strategies to improve instream flows, improve sustainability of the Leavenworth National Fish Hatchery, protect tribal and non-tribal fish harvest, improve domestic water supply and agricultural reliability, enhance Icicle Creek habitat, and comply with state and federal law including the Wilderness Act within the Icicle Creek Subbasin.  The culmination of that process was the adoption of the program called the Icicle Strategy, intended to provide a program of integrated long-term water resource management and habitat restoration actions. The Eightmile Dam Rebuild and Restoration Project is one of the first projects proposed to be implemented as part of the Icicle Creek Strategy, and is the first project-level EIS undertaken in this phased review process under SEPA. A project-level EIS looks at a specific project action, typically in greater detail than a programmatic EIS.

The Icicle Creek Work Group is composed of local, state, and federal agencies, tribes, irrigation and agricultural interests, municipal/domestic water managers, and environmental organizations. They are working together on a facilitated process to develop a plan to become drought resilient and climate responsive to meet the wide range of Icicle Creek subbasin water needs now and in the future. The Eightmile Dam EIS is a separate but parallel process from the Icicle Workgroup. The EIS is intended to inform Ecology as part of our decision making for approvals of the Eightmile Dam Rebuild and Restoration Project. Input from the Icicle Creek Work Group will be considered along with input from other stakeholders.  

Role of USFS
While Ecology is the lead SEPA agency for this EIS, the U.S. Forest Service (USFS) has provided input on relevant site information and participated in several public engagement opportunities. 

Ecology and the Okanogan-Wenatchee National Forest are jointly reviewing the proposed action to determine the responsibilities of each agency. The USFS will use agency regulations at 36 CFR 220.4(a) to determine which elements of the proposal require environmental analysis under the National Environmental Policy Act (NEPA,) the federal counterpart of SEPA, and will issue an independent NEPA decision for actions or approvals under USFS authority.

As authorized by the Council on Environmental Quality regulations, the USFS will cooperate with Washington on the state’s environmental analysis (40 CFR 1506.2(a)) and may use elements of the environmental review prepared under the State Environmental Policy Act (SEPA) process for any required NEPA analysis (40 CFR 1506.2(b)).

The USFS will continue to work with Ecology as the project moves through the environmental analysis process to ensure any responsibilities under the USFS authority will be met.

Tracking updates in the National Environmental Policy act (NEPA) process
The proposal presented by Ecology during Scoping is being refined through the SEPA process. Ecology is working with the USFS and the IPID to identify any actions which require USFS authorization. 

Once a finalized proposal is developed, the USFS will begin the appropriate NEPA analysis, including a post on their Schedule of Proposed Actions. Proposals subject to the USFS’s NEPA procedures will follow the agency’s applicable public involvement processes.

Lands were conveyed from the Icicle and Peshastin Irrigation Districts (IPID) to the U.S. government as National Forest System lands through a special warranty deed in 1990. Parcels and specific rights associated with the facilities, operations, maintenance, etc. at Eightmile Lake were retained by the IPID as described in the special warranty deed, and are therefore outside the jurisdiction of the U.S. Forest Service. As stated in the scoping documents, the alternatives considered do not extend outside the irrigation districts' deeded lands area, however, associated actions do extend outside the deeded area. 

Map displaying parcels

The No Action Alternative serves as the baseline condition against which the Action Alternatives are compared and is intended to illustrate the most likely scenario if the project is not implemented, consistent with SEPA guidance. Under the No Action Alternative, the dam would be left as is, with a primary spillway elevation of 4,667 feet, and would continue to operate in its current state and manner. This would leave the dam vulnerable to failure which could threaten human lives downstream, and create economic hardship for the IPID. Should a dam failure occur, residences, public infrastructure, and wilderness habitat could be damaged or destroyed. Ecology's Dam Safety Office (DSO) currently requires IPID to leave the low-level outlet gate open during the winter and early spring to reduce the risk of a dam failure. The operation of the dam in this manner is not consistent with DSO regulations, does not meet the DSO’s safety requirements for a high hazard dam, and would ultimately result in enforcement action by DSO. The No Action Alternative does not meet IPID objectives for water storage capacity for operational and irrigation water delivery, however, as noted above, this pathway outlines the most likely scenario if an action alternative is not implemented.

A hydrologic and hydraulic (H&H) study was done by IPID’s technical consultant, Anchor QEA, to assess the ability of the existing dam to pass storm flows. It was determined that the dam would not safely pass the required flow. During this process, Ecology’s Dam Safety Office (DSO) raised the safety rating of the dam to a high hazard level due to the human induced development downstream of the dam. This high hazard rating resulted in the need for the dam to meet higher storm thresholds.  

The dam alternatives were designed to withstand a 1-in-1,000,000 year storm event.  The lowest probability that DSO designs any dam is to a 500-year storm. The current configuration of the Eightmile Dam is estimated to withstand approximately a 30-year storm.

The area impacted by the Jack Creek fire is above the existing and proposed lake water line, and will not be affected by dam operations. IPID is concerned about sediment entering the lake from the burned area but raising and lowering the lake will not affect the sedimentation rate from the burn scar.

While the IPID is responsible to pay for their proposed action and project construction, they are hoping to receive grant(s) to help defer the costs. The IPID has applied for WATERsmart federal grants, FEMA grants, and the Office of Columbia River grants. 

Two approaches to helicopter use were considered. 

Option 2 would involve the limited use of the large helicopter to move only the heavy equipment and a portion of the material, followed by periodic delivery of materials as needed throughout the construction, using the smaller helicopter. Under both options, the large helicopter would be used for 1 to 2 days to remove equipment and any remaining materials at the end of construction. It is anticipated that the majority of the helicopter trips would occur on weekdays; however, some weekend flights may be necessary.

Please note that option 1 is preferred for helicopter use in order to minimize the total number of helicopter trips, but ultimately this will be dependent on large helicopter availability during wildfire season that will also overlap with the construction period.   

Overland Vehicle Transport was initially considered but due to comments received during scoping, this transportation method was dropped from consideration. Transportation using pack animals was also considered but determined infeasible due to the amount, weight, and elevation gain to the dam site. This would also require the excavator on site to be the only excavator used during construction which would likely extend construction to two seasons and longer trail closures.

Every effort would be made to complete the dam construction in one season. If construction could not be completed in one season, actions would be taken to secure the dam for overwintering. Areas that could be overtopped would be secured and stabilized (hardened) with rock. All equipment would be stored on-site or removed if feasible. The outlet pipe would be in working order, and the lake would be held at the lowest level (elevation 4,632) for the winter. Please check out the EIS for more details about overwintering contingency planning.

Scoping is the first step in the Environmental Impact Statement (EIS) process. The purpose of scoping is to receive input on the range, or “scope” of issues to be studied in the EIS. This process is required under the State Environmental Policy Act (SEPA).  Scoping was conducted in December 2020 & January 2021, with the scoping comment period closing on February 1, 2021.

All comments submitted will be considered carefully as the scope of the EIS is being developed. The EIS team, working with Ecology's SEPA Responsible Official, the person overseeing the process, will incorporate those comments that are within the scope of the evaluation for this project. Comments received that address issues outside of this project, or beyond the jurisdiction of Ecology, will not be addressed in the EIS. A Scoping Summary has been prepared that outlines the range of issues identified, and how these issues will be addressed in the EIS. Comments received that will not be addressed in the EIS are also outlined. Individual scoping comments are not addressed in the Scoping Summary, but major themes are described.

Scoping Summary Report