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About TPU > Services > Power > About Tacoma Power > Dams & Our Power Sources > Cowlitz River Project > Riffe Lake Restoration

Working to safely restore Riffe Lake’s original water level

Lowering Riffe Lake because of seismic safety concerns 

Since learning that Riffe Lake’s original water level posed a risk to the public because of the seismic safety of the spillway piers, Tacoma Power has been working with its federal regulator (the Federal Energy Regulatory Commission, or FERC) to fix these issues. We’ve had to keep the water levels low in the lake as we worked together with FERC to engineer new solutions to the public safety challenge. Keeping water levels low has resulted in community frustration near Riffe Lake, limiting public access to recreation and fishing. It’s also posed an operational challenge to Tacoma Power, because holding water levels low means decreased generation and revenue potential. 

Finding the solution 

We’re pleased to say we’ve investigated how to ensure public safety in a seismic event while keeping the lake at its previous fill level, and we’re going to undertake a variety of projects designed to address these safety concerns. These projects are designed to meet all of FERC’s requirements to raise Riffe Lake’s level while protecting the downstream public. Based on our program timeline estimate and dependent on FERC’s final approval, we believe the lake levels will be restored by 2031. 

Projects to address public safety

Before restoring the Riffe Lake to previous water levels, we must complete complex projects and related efforts. The following projects are currently scheduled, but additional projects are under review and may be added down the road.  

Performance Based Testing 2020-2022: Complete

In 2022, we conducted a performance-based test on the Mossyrock Dam to evaluate its structural response under a low-level simulated earthquake load. The results will inform the computer modeling necessary to understand the dam’s performance for extreme seismic events.

Seismic Hazard Analysis 2017 – TBD: Ongoing

The site-specific Seismic Hazard Analysis (SHA) for Mossyrock Dam involved a complete evaluation of potential earthquake sources in the region, including nearby fault lines and seismic zones. The analysis incorporated “probabilistic and deterministic” approaches to estimate the levels of ground shaking that could impact the dam. We assessed local soil and rock conditions to understand their effects on seismic wave transmission and dampening. We will use these results to inform the dam modeling and determine whether further dam projects are necessary to ensure public safety.

FERC recommended we wait for the results of seismic studies before submitting the analysis. Unfortunately, the seismic studies were delayed in part due to the COVID-19 pandemic.  This was submitted to FERC in 2023 and we are waiting for acceptance.   

Riffe Lake Cultural Resource Study 2018 – 2028: Ongoing

In 2017, because of FERC’s orders to lower the lake and comply with the National Historic Preservation Act through Tacoma’s FERC license, Tacoma Power entered Section 106 consultation with the Yakama Nation, Cowlitz Indian Tribe, and the Department of Archaeology and Historic Preservation (DAHP).  

The goal is to identify and protect any historically or culturally sensitive areas in the Riffe Lake draw-down area.  Everyone involved agreed that Tacoma Power would conduct annual vegetation and soil monitoring, research historical documents and topographic maps, and conduct further archaeological surveys at lower shoreline elevations over 10 years (2018-2028), unless the lake level gets restored earlier. 

Kosmos Mill Oil Cleanup 2020 – 2028: Ongoing 

This project aims to perform environmental cleanup and remediation at the buried historical Kosmos Mill site. In 2019, after we lowered Riffe Lake, an oily sheen was observed in Rainy Creek. Tacoma Power immediately deployed Best Management Practices (BMP) and designed and installed a protective cap to prevent contaminants from going any further while further investigating the site. Under an agreed order with the Department of Ecology, Tacoma Power is conducting an environmental investigation and subsequent remedial action of the Kosmos Mill site area to ensure no contaminated or hazardous material from the buried mill site enters Riffe Lake after the reservoir gets restored to its original 778.5 feet elevation. 

Spillway Gate Seismic Remediation 2024 – 2026: Initiated 

The spillway tainter gates in Mossyrock Dam went into service in 1968 and are used to help regulate the reservoir elevation of Riffe Lake. Over the past 55 years, industry standards have changed, and we better understand how tainter gates perform during seismic events.

This project aims to study and confirm or improve the structural performance of the four Spillway Tainter Gates in Mossyrock Dam in today’s environment.

This project does two important things; the first is to maintain compliance with regulatory requirements and ensure safe operation of the gates. The second is to use this project as a part of the program to safely restore the maximum original reservoir elevation of Riffe Lake to 778.5 feet. 

Spillway Removable Bulkheads Project  – 2024 – 2027: Initiated 

This project addresses the risk posed by a potential seismic failure of the spillway piers. To reduce this risk, the Interim Risk Reduction Measure (IRRM) initially involved lowering the reservoir to 749 feet. This adjustment minimizes the volume of water released during a major earthquake.

Although repairing the spillway piers so they won’t cause a flood during an earthquake goes a long way to ensuring the risk to the public is minimized, there are many other projects that need to be completed before public safety can be assured. 

A permanent solution for repairing the spillway piers requires extensive and complex analysis. This project aims to implement an alternative by installing removable bulkheads or stoplogs in the spillway bays downstream of the piers. This will reduce the water released during a seismic event rather than requiring a lower lake level. This alternative will need review and acceptance by regulators after the design is complete.  

The objective is to use this alternative as a component to safely restore the Riffe Lake reservoir to its original elevation of 778.5 feet. 

Arch Dam Analysis 2024 – 2028: Initiated 

Because of research over the past two decades, we now understand the seismic hazards at Mossyrock Dam more clearly. To analyze seismic performance of the Arch Dam, we will create a state-of-the-art 3D stability model of the dam, spillway, and spillway piers. We will develop the computer model to assess the strength and stability of the structure, which includes the spillway piers, and it will be used to evaluate risk and make informed decisions when it comes to earthquake safety. 

Riffe Lake Shoreline Vegetation Study and Remediation 2024 – (End date TBD): Initiated 

Since we lowered the elevation of Riffe Lake in 2017, vegetation has grown within the reservoir’s drawdown areas.  We will analyze the plants and develop growth models to determine what to do about these areas. Our goal is to address the plants in the draw-down area to ensure community safety, protect the environment, and support our operations once the lake is at its original level. 

Left Embankment Analysis and Repair 2024 – 2030: Initiated 

This project addresses stability concerns of the Mossyrock Dam left embankment, particularly after an earthquake. We are investigating as-built conditions to determine if repairs or a complete replacement of the embankment will be necessary to meet current standards and regulations. Keeping the lake level low reduces the consequences of a failure in an earthquake.  Analysis and mitigation performed throughout this project will help ensure we can meet public safety needs once Riffe Lake’s levels are restored. 

Estimated timeline for Riffe Lake restoration projects

2017

Lake lowered to address public safety during an earthquake; Board of Consultants formed. The board consists of seismicity, arch dam modeling, and geotechnical engineering specialists. The board will continue to provide oversight to the remediation of the seismic performance of the dam as necessary.  

2018-2021

Pacific Earthquake Engineering Research Center (PEER) Ground Motion Report.
The PEER Center, key to our seismic hazard analysis, published ground motion models in March 2021. 

2022

Completed Performance-Based Testing. These field tests helped to estimate how the dam would respond to a seismic event. Seismic Hazard Analysis and “Time History” development, helping us understand the probability and severity of earthquake ground motions at the dam.

2023

Updated Seismic Hazard Analysis completed submitted to FERC for acceptance.
Performed new FERC-required risk analyses to identify potential impacts to dam and secondary structures. 
Initiated the Riffe Lake restoration program, designed to restore Riffe Lake’s original level. 

2024

Complete the Riffe Lake restoration program schedule.
Select designer for the spillway gates and bulkhead projects.
Finalize Riffe Lake restoration program plan
Begin cultural resources study.
Begin vegetation study.
Scope development for the embankment project completed
Select consultant for the dam analysis 

2025

FERC reviews and accepts Existing Conditions Analysis for Spillway Gate Project
FERC reviews and accepts and Existing Conditions Analysis for the Removable Bulkheads Project
Finalize Shoreline Vegetation Study and a recommendation for remediations
Department of Ecology acceptance of Kosmos Mill Cleanup Action Plan. 

2026

FERC authorizes construction of the Spillway Gate Seismic Remediation Project. 
FERC authorizes construction of the Removable Bulkheads Project.

2027

Spillway Gate Seismic Remediation Project is complete 
Cultural Resources Study recommendation for Historic and Cultural Preservation is complete

2028

FERC reviews and accepts the Mossyrock Arch Dam Analysis
FERC reviews and accepts the Existing Conditions Analysis on the Left Embankment Project.
FERC authorizes construction on the Left Embankment Remediation Project (if required)
Kosmos Mill Cleanup Remediation Project is complete. 

2029

Complete Riffe Lake Cultural Resource Study
Update the Cultural Resources Management Plan and Archeological Management Plan. 

2030

Mossyrock Left Embankment Remediation Project is complete. 
Request to refill the reservoir submitted to FERC
Seismic reservoir restriction removed.

2031

Riffe Lake restored to its original “pool” of 778.5 feet.

***Timeline dates are estimates and will be updated as information becomes available*** 

***Many of the milestones and projects completions are dependent upon Federal Energy Regulatory Commission (FERC) review and acceptance*** 

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