Frequently Asked Questions (FAQ) about the Eyak Lake Weir Project
In 1972 a sheet pile weir was installed at the outlet of Eyak Lake to re-establish the water surface to pre-earthquake (1964 Good Friday earthquake) levels and keep nearshore sockeye spawning beds submerged.
Our primary project goals are to improve fish passage up and downstream of where the weir is located, maintain current water levels to protect important nearshore spawning sites for salmon, in particular Sockeye salmon, maintain current lakeshore property lines, maintain Eyak Lake as an emergency drinking water source, and improve safety around the structure.
The secondary project goals are to continue to have the weir function for the current level of boat passage, provide continued fishing access, and expand fisheries monitoring.
- Characterizing the flow parameters:
- Accurately measuring the flow over the weir is tricky due to its unique shape and degraded condition. The design needs to mimic the performance of the weir to maintain the similar range of water levels in Eyak Lake and Eyak River.
- The Eyak River has an interesting set of flow conditions. The rise of the water levels in the Eyak River to where they over top the weir and matches the elevation of Eyak lake (backwatering) is symptomatic of a hydraulic condition called flow hysteresis. What this means for the project is that we need to collect more data than usual to fully describe the behavior of the Weir and the Eyak River.
- The geometry of the weir and the bridge:
- The angle of the existing weir, the bridge and the Eyak River are currently in a configuration that is not conducive to easy navigation, fish passage or reducing erosion at the bridge abutments. The project partners will be looking at the bridge abutment and weir geometry to select the best configuration that advances the project goals.
A big part of this project is improving the ability for small and juvenile fish to get above and below the weir. A few of the aspects that are considered when designing to improve fish passage are: swimming speeds, slope, roughness, and natural stream function.
- Swimming speeds: Scientists and researchers have studied the swimming abilities of juvenile fish at various sizes to determine swimming performance thresholds or “barrierity”. They found that for juvenile Coho (around 3.5” long) that their sustained swimming speed is 1.3 ft/sec. Thus it is recommended that for this project we shoot for a target water velocity of 1 ft/sec with anything between 1 – 1.3 ft/sec being acceptable. We have a great opportunity, given the slope and site conditions, to achieve pockets of slower velocities that will allow for passage of salmon species as well as the entire suite of native fish species and macroinvertebrates as well.
- Slope: For this project, we are looking at designing a structure with a slope of 3% and with no vertical jumps larger than 0.4 feet.
- Roughness: Other important features that allow for safe passage of many native fish species relate to incorporating big habitat rocks (roughness) and providing multiple flow paths (high and low flow channels). Rivers are complex and dynamic environments. Their complexity allows for different conditions that favor different species and ages of fish to be able to move up and downstream. Roughness breaks up straight “laminar” flow and simulates the natural complexity you might see in a big riffle, for example.
- Function: A design style called rocky ramp may be used in this project as it functions like a natural riffle, providing lots of diverse flow paths, and can require less maintenance. Again, the impetus behind the design is to mimic natural river features as much as possible while also achieving all the other project goals (maintaining lake levels and recreation opportunities with a safe and stable structure).
- Discharge – this is how much water flows over the weir and at what time particular quantities of water flow into Eyak River.
- Water Velocity – We are looking at how fast the water flows over the weir and through the bridge to inform how we approach the fish passage design and mitigate bank erosion.
- Water Levels – The water levels are important. We are measuring the water level in Eyak like and Eyak River to understand the range of lake and river water levels are. The project goal is to replace the weir and not interfere with the water level of Eyak Lake.
- We’ll be supplementing our data with historical data collected by the National Weather Surface (NWS). The NWS collects water surface elevation data at the bridge and Eyak Lake. These data will be tied to the project datum and QC’d before being integrated into our analysis.
- Geotechnical Data – In 2021 we drilled exploratory bore holes to discover what the soil conditions are at the site. This information will be used to design the weir structure and to make sure the weir is constructed to the applicable seismic codes.
- This project is being funded by the Exxon Valdez Oil Spill Trustee Council. Initial planning work was covered by financial support from USFWS, USFWS, and ADOT&FP. Those contributions helped leverage a grant from National Fish and Wildlife Foundation to complete the preliminary investigations during 2021.
- The current project partners are a group of 13 organizations: ADFG, CEC, City of Cordova, DNR, DOT, DOWL, FEMA, NOAA, Native Village of Eyak, PWSSC, The Eyak Corporation, USFS, and USFWS.
In 2020 USFS completed an initial assessment of current weir conditions. Upon evaluation of this assessment, partners agreed that preemptive efforts to upgrade the weir were a priority in order to avoid reacting to an unexpected weir failure. In 2021, we completed preliminary investigations including the site survey, H&H report, geotechnical investigations, and concept designs. In 2022 we began data collection at the project site for discharge and velocity measurements. We also began the design process for a final design. In 2023 we anticipate to cover the 20%, 35%, 65%, 95%, and 100% design and review processes with ample opportunity for community input. In 2024 we hope to begin construction.
Visit the Eyak Lake Weir project webpage and visit the “Planning Phase 2: 2022-2023” tab. Upcoming and past opportunities for input, such as meetings and presentations will be posted there. Use the Eyak Lake Weir Comment Form to submit comments, questions, and photos anytime and also use that form to add your email to our project outreach and updates email list. Contact email@example.com with any questions.
DOWL has a team of engineers working on the project that spans their Drainage, River Systems and Dams groups. They are based out of Anchorage and have worked on multiple fish passage projects here in Cordova and similar weir projects across Alaska and the Mountain West.
Design a weir that is structurally sound, that matches or reduces the risks to critical infrastructure and properties downstream of the weir and to facilitate and encourage fish passage. As a secondary goal we aim to provide windows for recreational boaters to traverse the weir.
- Understand the river:
- Measure the existing conditions at the weir and Eyak River. This includes how much water flows from the weir (discharge), the water depth in the Eyak Lake, the weir and Eyak River and how fast the water is flowing. These parameters are recorded throughout the year to give us a holistic picture of how they change with the season.
- Understand the requirements of fish passage
- We are working with Native, Federal, State, and funding partners to fulfill the requirements of the grant and the best practices for fish passage.
- Understand the need from the community
- It’s important that the technical work is informed by the community based on they you observe and what you need from the weir. We want to capture the thoughts and observations of the community members that live close to the weir and those who use it recreationally throughout the year.
- Develop initial concepts
- We compile the feedback we get from partners and the community into objectives, combined with our initial data, we produce a concept drawing/plan that will act as the basis of the project going forward. We’ll adapt and hone the design as we go, but we use this as our ‘foundation’.
- Feedback and finalize designs
- We conduct our technical analysis and go through iterations of the design to meet the project objectives in line with feedback from stakeholders
- While we’ll be working with regulatory bodies throughout the process, we’ll submit the design to permitters like Cordova Planning Department, ADF&G, USACE, and DNR Dams Safety
- We’ll work with CRWP and the Contractor to oversee that the construction phase meets the design and follows the design intent
- Geotechnical Study
- This is to understand what material lies below the weir to design the new structure to withstand any hydraulic forces and seismic activity
- Initial Hydrologic & Hydraulics Study
- The initial study looked at the existing data available for the site and determined a path forward to collect the data to inform the design.
- Alternatives Analysis
- We looked at the range of options that may work for the site and collected feedback from stakeholders and the community in Summer 2021
- Data Collection (on-going)
- To understand the flow characteristics of the weir (how much water flows over the weir and when it flows) we are taking measurements of water level, how fast the water is flowing and the quantity of water throughout 2024. To make sure we have the quality of data that we need, we follow USGS standard operating procedures.
We have several regulating bodies that are part of the project stakeholders. They are kept informed of the data collection and the design process and have the opportunity to comment at each stage through the project. Ultimately, we’ll submit our designs to the following regulating agencies:
- U.S. Army Corps. of Engineers
- Alaska Department of Natural Resources; Division of Mining, Land and Water
- Alaska Dam Safety Program
- Alaska Department of Fish and Game
- City of Cordova Planning Department, in conjunction with FEMA
- U.S Fish and Wildlife Service
Project partners are very conscious of the homes and critical infrastructure up and downstream of the weir. The bridge and buried utilities are also important for the airport and other residents that live and recreate past 6 Mile. This is one of the reasons why we have a thorough data collection effort to inform the design. As part of our design process, we will work with the Alaska Dams Safety Program (DNR) and the City of Cordova Planning Department/FEMA to model the impact of the structure on the waterbody and to match the existing hazard potential classification and regulatory status of the weir.
Yes, there are no plans by any current partners to change access to fishing in the vicinity of the Eyak Lake weir.
The current plan is to begin construction in 2024. This section will be updated as we get closer to construction and have a better understanding of the timing for this aspect of the project.