USACE MILLRACE-FISH LADDER DECISION DOCUMENT-1-16-26_RFP 2026-01_Appendix C.pdf

January 2026

DESIGN DECISION DOCUMENT

CONTINUING AUTHORITIES PROGRAM, SECTION 1135:

PROJECT MODIFICATIONS FOR IMPROVEMENTS TO THE ENVIRONMENT

LONG TOM RIVER

MONROE, BENTON COUNTY, OREGON

SECTION 1 - INTRODUCTION

The purpose of this Decision Document is to develop the Recommended Course of Action for the millrace and fish ladder during the Plans and Specs phase and provide this information to the Non-Federal Sponsor (NFS) for review. The scope of the Recommended Course of Action includes the complete removal of the millrace and fish ladder in addition to total removal of the Monroe Drop Structure, which would restore approximately 43 acres of aquatic and floodplain habitat, enhancing connectivity to a historic meander through installation of a larger culvert downstream of the drop structure, and grading a pilot channel upstream of the drop structure to the Highway 99 bridge to encourage the river to flow in this channel.

Three options were analyzed for adherence to project authority, scope, reasonable maximization of ecosystem restoration benefits compared to costs, consistent with federal objectives, and engineering considerations. The Recommended Course of Action stays within project scope, CAP authorities, maximizes ecosystem restoration net benefits, and is the least costly COA. The selected plan has been shown to be cost effective and justified to achieve the desired level of output.

1.1 BACKGROUND

The Long Tom River is a 57-mile tributary of the Willamette River. The lower mainstem Long Tom River flows north from Fern Ridge Reservoir (river mile 23.6) to join the Willamette River about 16 miles upstream of Corvallis, Oregon.

The Long Tom River in the vicinity of Monroe was largely shaped by the U.S. Army Corps of Engineers (USACE) Long Tom River channel rectification project that was completed in 1950 (USACE 1943). The channel dredging and widening, along with construction of the side embankments and drop structures were done to improve the flow conveyance capacity of the Long Tom River downstream of Fern Ridge Dam. The channel modifications included the installation of 126 culverts with diameters ranging between 1 and 4 feet being installed through the embankments. The culverts connected relic oxbow channels, small tributaries, and allowed water to enter in and out of the floodplain during high flows, providing hydrodynamic stability on both sides of the embankments. The two-way flow allowed by the embankment culverts during high flows means that they do not act as levees and do not provide incremental flood protection, nor were they originally designed for fish passage.

To increase fish passage and for other ecosystem restoration purposes at this USACE project, an Integrated Feasibility Report and Environmental Assessment was developed by USACE, Portland District in partnership with the City of Monroe and the Confederated Tribes of Siletz Indians for the Long Tom River Project in Monroe, Oregon. This study was completed under Section 1135 of the Continuing Authorities Program to describe the planning process undertaken to formulate and evaluate alternatives for aquatic and floodplain ecosystem restoration activities within the project area.

USACE and the City of Monroe entered into a Project Partnership Agreement (PPA), signed in November 2024, to undertake design and construction of the Long Tom River Ecosystem Restoration Project. USACE designers took a deeper dive into the Recommended Plan as proposed by the Feasibility Study and developed initial designs.

Upon investigation, the designers realized the retaining wall, millrace, and fish ladder are very likely not stable, or at least in a more vulnerable state, after the removal of the drop structure, if no further action were to take place. USACE informed the City of Monroe of their findings while also proposing three potential courses of action to remedy the situation. The proposed actions are outlined in Section 2.

1.2 PROJECT AUTHORITY

The Feasibility Study outlines the project’s authority in section 1.1. That text is copied below:

“This study is authorized under Section 1135 of the Water Resources Development Act of 1986, as amended:

When construction of a water resources project by the U.S. Army Corps of Engineers (USACE) has contributed to the degradation of the environmental quality of an area, Section 1135 of the Continuing Authorities Program allows for implementation of measures for enhancement of the environmental quality associated with the restoration, through modifications either at the project site or at other locations that have been affected by the construction or operation of the project, if such measures do not conflict with the authorized project purposes.

The Long Tom Channel Improvement Project was included as a part of the construction of the Fern Ridge Dam authorized in the Rivers and Harbors Act of 1938 (PL 75-761).”

1.3 PROJECT SCOPE

The Feasibility Study determined the Recommended Plan in section 7.4. That text is copied below (figures have been omitted for clarity and brevity):

“The objective of an ecosystem restoration project is to contribute to [the National Ecosystem Restoration] NER. Contributions to these benefits are increases in the net quantity and/or quality of desired ecosystem resources. The NER Plan must reasonably maximize ecosystem restoration benefits compared to costs, consistent with the federal objective. The selected plan must be shown to be cost effective and justified to achieve the desired level of output. After analysis of all relevant environmental benefits and impacts, the Corps has identified the Tentatively Selected Plan (TSP) as the recommended plan and the preferred alternative per NEPA regulations.

The recommended plan is Total Removal of the Drop Structure (Alternative T), design for this project includes the removal of the drop structure, replacing a culvert to the existing wetland, and adding riprap protection around the piers of the ODOT Highway 99W bridge in Monroe, OR (Figure 18 [of Feasibility Study]). The removal of the drop structure will include the spillway structure and its foundation. At the request of the City of Monroe, the fish ladder and headrace will be left in place on the western side of the river along with the concrete retaining wall on the eastern side for interpretation.

The culvert replacement is for the downstream culvert located at river mile 6.6 on the north end of the Monroe Park. The culvert design will be a 42-inch culvert through the embankment that connects to an engineered riffle on the river side along the embankment to allow hydrologic connectivity and spawning salmon to access the natural breeding areas on the wetland side of the culvert (Figure 19 [of Feasibility Study]). The engineered riffle was preliminarily designed to have each pool be 2 feet deep by almost 12 feet long by almost 12 feet wide. This was used to provide preliminary estimates for construction, but this engineered riffle will need to be further designed by the Project Delivery Team during the planning stage to ensure proper fish passage to this wetland area. See Figure 19 [of Feasibility Study] for the location of the engineered riffle and the 42-inch culvert.

Additional riprap protection was proposed to the existing piers of the ODOT Highway 99W bridge, to ensure their protection. Due to the increased flow of the river passing these piers once the drop structure is removed, additional scour can occur at the base of this bridge. Based on conversations with ODOT, these piers currently have shown scour since the time they were built in 1953. This Corps will provide additional protection around the piers to help ensure that future scour will be minimized around these piers. See Figure 20 [of Feasibility Study] for the ODOT detail used to design the riprap protection around the bridge piers. Appendix I describes the scour analysis performed to estimate the riprap sizing and design.”

The PPA defines the scope of the project in Article I, Part A, text copied below:

“The term “Project” means removal of the Monroe Drop Structure, modifications to an existing embankment culvert, construction of an engineered pool and riffle structure, and rip rap armoring of the OR-99 bridge piers upstream, as generally described in the Long Tom Ecosystem Restoration Integrated Feasibility Study and Environmental Assessment dated March 2024 and approved by the Commander for the Northwestern Division on March 9, 2024 (hereinafter the “Decision Document”).”

SECTION 2 - PROPOSED COURSES OF ACTION

2.1 COA 1: TOTAL REMOVAL

This action includes the complete removal of the Monroe drop structure, eastern retaining wall, fish ladder, and millrace, as well as regrading and/or removal of fill on the western slope and filling in a 3:1 sloped earthen embankment in the location of the eastern retaining wall. The intent of this action is to remove all concrete structures in the river to minimize scope, total project footprint, cost, and operations/maintenance requirements for the NFS.

2.1.1 Engineering Considerations

2.1.1.1 Hydraulics

The total removal of the Monroe drop structure, eastern retaining wall, fish ladder, and millrace structure was assessed using a two-dimensional (2D) hydraulic model of the Long Tom River channel and adjacent floodplain. The modeled water depths and velocities in the vicinity of the Monroe drop structure with all structures removed and assumed embankment slopes resulted in conditions that do not impact flow conveyance, fish passage, or sediment transport.

Figure 1 depicts the modeled velocities and water depths of the Long Tom River in the vicinity of the Monroe drop structure assuming total removal of the structures, as well as features such as the pilot channel that will be constructed through the sediments stored behind the drop structure and the earth embankment on the eastern side of the river that replaces the retaining wall. The maps of velocities and water depths are shown for flows of 100, 800, and 4,650 cubic feet per second that represent typical low, moderate, and high flows during winter months (period relevant to fish passage).

The pilot channel is meant to allow low flows to pass in a defined channel after the removal of the Monroe drop structure providing continuity of fish passage conditions.

The pilot channel was assumed to have a 20-foot bottom width, 2:1 side slopes, and connects existing low point elevations of the channel for the reach between the Highway 99 bridge and the Monroe drop structure location. It is assumed that higher flows in the Long Tom River will reshape the pilot channel and deposited sediments behind the drop structure will be transported downstream after the removal. It is anticipated that as the pilot channel is reshaped and deposited sediments are moved downstream that the effect will be to widen out the outlet region of the pilot channel with a resulting lowering of water velocities and a slight lowering of water depths.

The hydraulic model results do not incorporate the reshaping of the pilot channel, so the results are representative of conditions immediately after the removal of the drop structure and with no changes to the pilot channel. At a flow of 800 cubic feet per second, the velocities in the pilot channel exceed 7 feet per second (Figure 1, upper middle plot) and these velocities are expected to start transporting sediments downstream with the eventual reshaping of the pilot channel. During low and moderate winter flow conditions (100 and 800 cubic feet per second, the flow depths range between 1 and 3 feet (Figure 1, lower left and lower middle plots). At the regulation target flow of 4,650 cubic feet per second the flow depths are on the order of 10 feet with velocities decreasing to 5 feet per second as the flow is conveyed over the entire cross section of the Long Tom River channel (Figure 1, upper and lower right plots).

Habitat suitability index (HSI) curves for water depth and velocities, for both adult and juvenile salmon were used to quantify ecosystem benefits during feasibility (Figure 2).

Adult salmon prefer water depths of less than 3 feet and velocities less than 2.5 feet per second, while juveniles prefer water depths less than 5 feet and velocities less than 3 feet per second. The velocity and water depth maps at 800 cubic feet per second (Figure 1, middle top and middle bottom plots) depict a constriction point at the outlet of the pilot channel that will likely get reworked by initial flows post-removal of the drop structure. It is assumed that water depth and velocity values will lower once the pilot channel is reshaped and resemble values shown in the modeling results immediately up and downstream of the constriction and be on the order of 3 feet in water depth and 3 feet per second in velocity. For the regulation target flow of 4,650 cubic feet per second during floods, the water depth and velocity are not suitable for both adult and juvenile salmon. This was expected as it was noted in feasibility that fish passage conditions were not suitable during flood events.

Flow duration curves for the winter period (15-Sep through 15-Apr) depict the percent of time daily averaged flows are exceeded (Figure 3) that indicate that winter flows exceed 100 cubic feet per second approximately 80 percent of the time and exceed 4,650 cubic feet per second approximately 1 percent of the time. Considering the HSI curve values for associated flows and the percent of time each flow is exceeded during the winter period, the results from this analysis are consistent with those from the feasibility study such that the flow depths and velocities are supportive for fish passage for both adult and juvenile salmon for the typical, non-flood event flows that Long Tom River exhibits during the winter period.

Figure 1. Modeled velocity and depth plots in the vicinity of the Monroe drop structure assuming total removal course of action.

Figure 2. Habitat suitability index (HIS) curves for adult (top) and juvenile (bottom) salmon based on water depth (left) and velocity (right).

Figure 3. Flow duration curve based on daily averaged flows at Monroe for water years 2001 through 2025.

2.1.1.2 Geotechnical

There is no geotechnical risk associated with this course of action. The removal of the structures eliminates risk to right bank levee scour. Scour of the left bank is not possible where bedrock is exposed next to and downstream of the mill race.

2.1.1.3 Structural

There is no structural risk associated with this course of action. The total removal of the structures eliminates any risks.

2.1.2 Non-Engineering Considerations

2.1.2.1 Real Estate

The NFS and the Corps currently own all the necessary real estate rights for COA 1. As stated in the Real Estate Plan (REP), access to the private lands on the West of the riverbank will be utilized through an existing perpetual easement from 1944 on Corps Tract FR -212 which allows the Corps to “construct, maintain, repair, operate, patrol, replace and/or remove a channel improvement and rectification project including all appurtenance thereof;…” amongst other dominant easement rights such as the “right of access thereto over and across the grantor’s private roads, if any.” Lands on the east side of the drop structure to be used for access and staging are owned by the City of Monroe and will be provided for the project. The Corps has the rights to remove the existing mill race and fish ladder as well since they were retained and incorporated into the new structure, making both appurtenances to the drop structure.

2.1.2.2 Cultural Resources

USACE has determined the millrace and fish ladder are not eligible for listing on the National Register of Historic Places (NRHP). The structures lack the necessary integrity and significance required for NRHP eligibility (36 CFR Part 63). Consultation with SHPO is forthcoming, however USACE anticipates SHPO concurring with this determination of not eligible. If SHPO concurs that the millrace and fish ladder are not eligible for inclusion on the NRHP, no additional mitigation will be required for the removal of the millrace and fish ladder.

2.1.2.3 Environmental/Ecological

There are no negative environmental or ecological impacts for this course of action.

Total removal of the drop structure, millrace, fish ladder, and retaining wall will meet all the objectives of the Recommended Plan as proposed in the Feasibility Study section

7.3. Total removal will decrease total impervious surface volumes in-water which will benefit fish and wildlife species that may be in the area. The additional removal of concrete structures would result in indirect benefits that would stem from the enhanced water quality and vegetation leading to better habitat for foraging, nesting, rearing, and rest for many different fish and wildlife species

2.1.2.4 Project Authority and Scope

This action fits within project authority as the environmental quality benefits of removing the millrace and fish ladder are the same as the Recommended Plan as outlined in the Feasibility Study section 7.5.3.

This course of action is a modification to the project scope as identified as the Recommended Plan in the Feasibility Study. However, it still meets the intent of the project to restore habitat upstream of the drop structure. Additionally, the millrace and fish ladder are considered appurtenances of the drop structure so this course of action is considered part of the “removal of the Monroe Drop Structure” as identified in the

PPA.

2.1.2.5 Liability

There is no additional liability associated with this course of action.

2.1.2.6 Cost to Ecosystem Restoration Benefits

A “back of the napkin” estimate has been developed for this action. Once a recommended action is selected and agreed upon by all parties, a formal estimate developed by the USACE cost engineer will be developed and disseminated. The “back of the napkin” estimate was $100k additional to the project. Approximately $50k is allocated for construction. $40k was allocated to Real Estate costs, however since both USACE and NFS have own all the necessary rights to complete this work. This estimate does not include the cost of acquiring the private property of the fish ladder and millrace. With a minimal total project cost increase, the cost/benefit ratio from the Recommended Plan from the Feasibility Study can be most closely maintained.

2.1.2.7 Project Schedule

There is no substantial additional time needed to perform design or construction.

2.2 COA 2: ENGINEERING SOLUTION

This course of action proposes to remove the eastern retaining wall and most of the Monroe Drop Structure leaving in approximately 8-10 feet of the drop structure and the millrace and fish ladder. To provide a lateral buttress that will keep the millrace and fish ladder in place, vertical rock anchors will be installed into the foundation bedrock through the remaining 8-10 feet of drop structure. Rip rap will be added to “encase” the structures. The only piece of concrete visible would be the top of the millrace. The buttress would replace the lateral support provided by the existing structure spanning the river. Using engineering judgement and conservative assumptions lateral fixity would be achieved with tensioned rock anchors in the drop structure.

2.2.1 Engineering Considerations

2.2.1.1 Hydraulics

The hydraulic impacts of the engineered solution were simulated with the 2D hydraulic model described in Section 2.1.1.1. The model’s geometry and terrain were modified to include a riprap encased barrier of the partially removed drop structure, fish ladder, and millrace, as well as a riprap revetment on the eastern side of the river that replaces the retaining wall of the drop structure. The riprap structures on both sides of the river result in an approximate 40-foot reduction in the Long Tom River channel’s width relative to the total removal course of action.

The partially removed drop structure retained 20-feet of its spillway at an elevation of

277.5 feet (elevation with respect to North American Vertical Datum of 1988, NAVD88).

The partial spillway, fish ladder, and millrace structures were encased with a riprap revetment sloping outward with a 2:1 side slope and a top elevation of 275 feet (NAVD88). The eastern bank’s retaining wall was simulated first using a 3:1 earth embankment (as was assumed for the total removal case) but the resulting velocities along the embankment would likely cause scour such that a 2:1 riprap revetment along the eastern embankment was selected. Figure 4 depicts the cross-sectional elevation profile comparing the engineering solution and total removal courses of action.

The same pilot channel described for the total removal course of action (Section 2.1.1.1) was also applied to the engineering solution model to allow for fish passage immediately after the partial removal of the Monroe drop structure. As described for the total removal course of action, it is assumed that once the drop structure is partially removed that sediments deposited behind the drop structure would eventually transport downstream and the pilot channel would be reshaped. However, the reshaping of the pilot channel for the engineering solution would be hindered between the riprap revetment of the encased structures on the west and the riprap revetment on the eastern bank. For the total removal course of action, it was assumed that velocities and water depths would decrease as the pilot channel was reshaped, but this assumption would likely not hold for the region between the revetments for the engineering solution course of action. The hardened banks, constricted width and the fact that the sediment bed at this location is near the underlying bedrock means that there will be minimal changes to the cross-section geometry after the partial removal of the drop structure.

Figure 4. Comparison of cross sections of the total removal and engineering solutions at the Monroe drop structure represented in the hydraulic model.

Figure 5 depicts the modeled velocities and water depths of the Long Tom River in the vicinity of the Monroe drop structure with the engineering solution in place. The plots of velocities and water depths are shown for flows of 100, 800, and 4,650 cubic feet per second as was shown in Figure 1 for the total removal course of action described in 2.1.1.1.

There are only small changes in velocities in depths for flows of 100 and 800 cubic feet per second between the total removal (Figure 1) and engineering solution (Figure 5) courses of action. At lower flows, the water is primarily contained in the pilot channel with velocities that exceed 8 feet per second at the pilot channel outlet at a flow of 800 cubic feet per second (Figure 5, upper middle plot).

When the flow reaches the regulation target of 4,650 cubic feet per second the water surface elevations overtop the riprap encasement of the engineering solution elements and there is flow over the partial spillway of the Monroe drop structure left in place (Figure 5, upper and lower right plots). It is important to note that the 2D hydraulic model quantifies depth-average velocity values, and as the elements of the engineering solution are inundated, there will be significant three-dimensional (3D) flow considerations. The complexity of the 3D flow patterns would need to be assessed using a computational fluid dynamic (CFD) modeling approach that is beyond the scope of the modeling performed for this project. Specifically, the modeling would need to ensure the structural aspects of potential 3D jets in the flow arising from complex flow paths as the encased structures are inundated at higher river flows, as well as potential boating and life safety concerns caused by the complex flow paths.

The modeled water surface elevation at a range of Long Tom River flows was assessed to identify at what flows the water would reach the top of the riprap encasement of the engineering solution, as well as the spillway crest of the remaining Monroe drop structure. The riprap encasement top elevation of 275 feet (NAVD88) is reached with Long Tom River flows of 2,810 cubic feet per second that corresponds to a winter period flow duration of a daily averaged flow that gets exceeded 10 percent of the time (Figure 3). The spillway crest of the remaining Monroe drop structure is at an elevation of 277.5 feet (NAVD88) that is reached with a Long Tom River flow of 4,260 cubic feet per second that is slightly less than the regulation target. The partially removed drop structure was modeled using the one-dimensional weir equation suggesting that there would be a flow of up to 50 cubic feet per second of flow over the spillway at the regulation target flow of 4,650 cubic feet in the Long Tom River.

Figure 6 shows the velocity values and flow paths at a flow of 3,200 cubic feet per second for the engineering solution. The particle tracking tracts show the accelerated flow paths at the junction of the end of the partially removed Monroe drop structure, as well as recirculating zones on either side of the riprap encasement. The recirculating eddy zones are problematic as they will likely result in deposition, especially of the larger sediments that were trapped behind the drop structure that will be jetted out through the construction and deposit just downstream of the fish ladder and millrace.

The effect of sediment deposition here will elongate the constriction region causing the higher velocities observed in the modeled results (Figure 5).

The river channel constriction between the riprap embankments of the engineering solution course of action will decrease the fish passage HSI values (Figure 2) in the immediate vicinity of the Monroe drop structure location because of the higher velocities. The hydraulic model simulations suggested an increase of 0.5 feet per second increase in velocities caused by the engineering solution; however, the hydraulic model does not consider the reworking of the pilot channel that will occur for the total removal case resulting in lower velocities and larger HSI values overall for the in comparison to the engineering solution course of action.

Figure 5. Modeled velocity and depth plots in the vicinity of the Monroe drop structure assuming engineering solution course of action.

Figure 6. Plot of velocity magnitude (color) and flow paths (white traces) for the engineering solution course of action at a flow of 3,200 cubic feet per second.

2.2.1.2 Geotechnical

Design documents and construction as-builts do not exist for the millrace and fish ladder structures. The left bank of the mill race is constructed into bedrock but the foundation conditions of the mill race and drop structure are unknown. These conditions can only be determined with an inspection of the millrace and fish ladder. Such an inspection would require total dewatering of both structures to enter the confined space in the mill race. This inspection would document existing conditions of the fish ladder and mill race prior to USACE installing vertical ground anchors to provide lateral fixity of the structure left in place.

Ground anchors are proposed as stability mitigation to replace the drop structure to be removed. The intent of the ground anchors is to provide a combined vertical loading and shear resistance on the concrete left in place. No investigation is necessary for an empirical design of high load capacity ground anchors. Preliminary recommendations are to assume #10 (1.25-in) Grade 75 all-thread bar with 20-ft embodiment into bedrock beneath the concrete. Each #10 bar can provide up to 57,000-lbf (57 kips) of vertical load and 34 kips of shear resistance. Additional variables such as grout-ground strength and ultimate bar length can be modified depending on the strength of the rock and the drill hole diameter.

2.2.1.3 Structural

There are no as-builts or documentation on how the millrace and fish ladder were built.

The concrete properties including the compressive strength, internal dimensions of the structures, reinforcement, the backfill information, and foundation information are unknown and need to be investigated. There are concerns that the seismic lateral forces can create sliding and overturning stability issues. Additionally, both structures are difficult to inspect fully. The issue of the concrete design life of 100 years will remain. These structures are well past their intended design life.

2.2.2 Non-Engineering Considerations

2.2.2.1 Real Estate

The NFS and the Corps currently own all real estate rights for construction of COA 2.

The NFS will likely need to acquire a permanent easement to perform future O&M on the remaining structures.

2.2.2.2 Cultural Resources

Should USACE’s conclusion of the ineligibility of the millrace and fish ladder to be listed on the NRHP be accepted by SHPO, then this course of action will have no adverse impacts. Should SHPO disagree, then these modifications may or may not be accepted by SHPO. Additionally, mitigation would be required which would likely take the form of submitting the final NRHP nomination documentation to SHPO.

2.2.2.3 Environmental/Ecological

This course of action would constrict the waterway resulting in effects not previously evaluated in the Feasibility Study which could require additional environmental analysis.

This course of action may require additional Endangered Species Act consultation. This course of action will lead to increased water velocities which could have adverse impacts on fish and wildlife species compared to Course of Action 1.

2.2.2.4 Project Authority and Scope

This action will not fit within the bounds of the Recommended Plan as outlined in the Feasibility Study section 7.5.3 because the environmental quality benefits are reduced as described in section 2.2.1.1 and 2.2.2.3.

This course of action is a modification to the project scope as identified as the Recommended Plan in the Feasibility Study. It meets the intent of the project, to restore habitat upstream of the drop structure, to a lesser extent because of the hydraulic barrier created by the constriction of the river in this section.

2.2.2.5 Liability

This course of action will increase the liability of USACE and potentially the NFS should this project cause damage to private property during or after construction. Further, the NFS would be fully responsible for O&M of the new structural components added to protect the fish ladder and millrace.

2.2.2.6 Cost to Ecosystem Restoration Benefits

A “back of the napkin” estimate has been developed for this action. Once a recommended action is selected and agreed upon by all parties, a formal estimate developed by the USACE cost engineer will be developed and disseminated. The “back of the napkin” estimate is approximately $350k-$500k.

The additional project increase plus reduced benefit of removing the drop structure by creating a hydraulic barrier for fish passage creates a significantly less favorable cost/benefit ratio.

2.2.2.7 Project Schedule

The project schedule is impacted by additional plan set and specification development time, pre-construction time for submittal review and approval, additional construction length, and time for obtaining a Permanent Easement for access to the site. The permanent easement would be required for the NFS to access the site to operate and maintain the project after project completion, but it does not need to be acquired before contract award because it is only for O&M use, not construction. There is a 1-2 year threat to the schedule due to heavy rainfall during the 2025-2026 winter season causing slides and floods. Specialty contractors for rock bolts will be busy with recovery efforts for WSDOT/ODOT. Depending on the effects of leaving a partial structure in place, new NEPA analysis and ESA consultation may need to be completed which could delay the project schedule.

2.3 COA 3: FIELD INVESTIGATION AND REPORT

This course of action proposes to perform a full-scale investigation into the structural and geotechnical stability of the fish ladder and millrace after the removal of the drop structure. The field investigation would involve drilling, work over water, using crane and/or barge, turbidity monitoring, specialty equipment access, dewatering, inspection of structure, nondestructive testing, and magnetic testing. This would result in a complete understanding of the two structures and would provide design criteria. The report would analyze the data, determine if the fish ladder and millrace require any modifications with the removal of the drop structure, and if needed, provide a recommended design for fish ladder/millrace stability. This option would still require either the removal of the fish ladder/millrace or another engineering solution to make the fish ladder/millrace structurally and geotechnically stable.

This COA would determine as much design criteria as possible to reduce the conservative assumptions needing to be made and would limit an engineering solution to essentials, should the report determine the fish ladder/millrace are unstable upon the removal of the drop structure. The report could also be used to justify the removal of the fish ladder/mill race or determine there is no need for any structural modification.

2.3.1 Engineering Considerations

2.3.1.1 Hydraulics

There are no direct hydraulic considerations of the field investigations course of action.

It is recommended that if this course of action is taken that it also includes a three-dimensional computational fluid dynamic (CFD) modeling assessment for the flow paths generated by the structures left in place that include the partially removed Monroe drop structure, fish ladder, millrace, and riprap encasement. As noted in section 2.2.1.1, Long Tom River flows greater than 2,810 cubic feet per second will reach the top of the riprap encasement and flows greater than 4,260 cubic feet per second will result in flows over the spillway of the remaining Monroe drop structure. The irregular shapes and immobility of these structures will likely generate complex, three-dimensional flow paths that should be assessed for potential issues relating to structure erosion (e.g., fast velocity jets along riprap or concrete) and potential flow hazards relating to boating and life safety.

2.3.1.2 Geotechnical

Since there is no documentation on how the millrace and fish ladder were designed or constructed, this COA would gather the necessary geotechnical design criteria such as geophysics, bedrock condition, void scour, and the strength of the concrete. This will require testing from a specialty contractor who will likely use a barge and a cofferdam to perform tests such as drilling, coring, non-destructive testing of the concrete, and confined access inside the millrace. Having this design criteria will lead to a more refined geotechnical design that will very likely be similar to what is proposed in COA 2.

2.3.1.3 Structural

It is anticipated that the fish ladder and millrace structures, after a full-scale investigation into the structural and geotechnical stability and structural analysis, will require tensioned anchors and placement of rip rap for seismic events. The issue of the concrete design life of 100 years will remain. These structures are well past their intended design life.

2.3.2 Non-Engineering Considerations

2.3.2.1 Real Estate

The NFS and the Corps currently own all real estate rights for COA 3. All lands are being provided by the NFS and any access to the drop structure that may be on private lands (west side) is covered by a perpetual easement for FR-212 from 1944 when the structure was erected (see also COA 1, Real Estate).

2.3.2.2 Cultural Resources

The considerations for this course of action are the same as mentioned in COA 1 and 2.

2.3.2.3 Environmental/Ecological

There are no negative environmental or ecological impacts for this course of action unless an alternative is proposed similar to COA 2. Course of action 3 will meet all the objectives of the Recommended Plan as proposed in the Feasibility Study section 7.3.

2.3.2.4 Project Authority and Scope

This action fits within project scope as this helps the project team determine the design requirements around the millrace and fish ladder which could ultimately lead to COA 1 or 2. However, this action does not fit within the bounds of the Recommended Plan as outlined in the Feasibility Study section 7.5.3 because of the significant cost increase and therefore does not fit within project authority.

2.3.2.5 Liability

This course of action has no additional liability and maintains the same liability as identified in COA 1 and COA 2, depending on what the investigation and report recommends as the preferred solution.

2.3.2.6 Cost to Ecosystem Restoration Benefits

A “back of the napkin” estimate has been developed for this action. Once a recommended action is selected and agreed upon by all parties, a formal estimate developed by the USACE cost engineer will be developed and disseminated. The “back of the napkin” estimate is approximately $1 mil - $1.5 mil. This results in a much less favorable cost/benefit ratio which would require a revaluation of the Recommended Plan as proposed in the Feasibility Study.

2.3.2.7 Project Schedule

Total impact of 2-3 years of schedule impact. This includes time for field investigation, analysis of data, report writing, the additional time for an engineering solution (see option 2 schedule impacts) or removal (see option 1 schedule impacts), hiring specialty contractors for rock bolts, most will be busy with slide and flood recovery efforts for WSDOT/ODOT, and time securing additional funding.

SECTION 3 - RECOMMENDED COURSE OF ACTION

After a thorough comparison, Course of Action 1 (COA 1), Total Removal, emerges as the clear and recommended path forward. This approach not only meets all project objectives as defined in section 1.3 but does so with significant advantages in cost, technical simplicity, and speed of execution over the alternative COAs.

COA 1 is the most prudent financial choice, as both COA 2 and COA 3 are more expensive. From a risk perspective, COA 1 is also superior. It eliminates the engineering, structural, and geotechnical risks inherent in relying on the fish ladder and millrace, which have exceeded their design life. COA 1 also mitigates the hydraulic and ecological risks seen in the other options.

Furthermore, COA 1 is the only course of action that aligns with the authorized plan from the original Feasibility Study. Selecting any other option would trigger a requirement to reassess the study's recommendations, impacting the project budget and schedule. While a pending SHPO decision on cultural resources affects all three options, COA 1 presents the most straightforward path, offering the highest ratio of ecosystem restoration benefits to cost and effectively eliminating future liability for both USACE and the NFS.

SECTION 3 - RECOMMENDED COURSE OF ACTION