Final Design Report Net River Dam Modernization and Restoration Michigan Dam ID No. 131 Baraga County, Michigan

Submitted to:

Mr. Tom Pawlowski Department of Technology, Management & Budget State Facilities Administration Design and Construction Division 3111 West St. Joseph Street Lansing, MI 48909

Submitted by:

GEI Consultants of Michigan, P.C.

109 W. Baraga Avenue Marquette, Michigan 49855

November 24, 2025 Project 2304013

Jacob E. Pieti P.E.

Project Engineer

Kevin A. Roell, P.E.

Vice President/Senior Engineer

Consulting

Engineers and

Scientists

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Table of Contents

1 Introduction 1

1.1 Introduction and Project Scope 1

1.2 Project Description 2

1.3 Project Personnel 4

1.4 Survey and Datum Elevation 4

1.5 Design Criteria 4

2 Existing Site Conditions 5

2.1 General Project Description 5

2.2 Basemap Development 5

2.3 Geotechnical Explorations 5

3 Summary of Dam Modifications 7

3.1 Inlet Structure Modifications 7

3.2 Spillway 8

3.3 Outlet Structure 8

3.4 Embankment Reconstruction 9

4 Hydrology and Hydraulics 10

4.1 Hydrology 10

4.2 Hydraulic Design 10

4.2.1 Inlet 10

4.2.2 Culvert11

4.2.3 Tailwater Evaluation 11

4.2.4 Flood Routing Results 12

4.2.5 Freeboard Evaluation 12

5 Geotechnical Analyses and Design 14

5.1 Embankment Design 14

5.1.1 Slope Stability Evaluation 14

5.1.2 Filter Design 16

5.2 Riprap Design 17

6 Structural Design 18

6.1 Existing Structure Evaluations 18

6.2 Stainless Steel Vertical Slide Gate 18

6.3 Box Culvert 19

6.4 Outlet Structure 19

6.4.1 Outlet Structure Apron Slab 19

6.4.2 Outlet Wing Wall Stability 20

6.5 Trash Rack Modifications 20

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6.6 Operator’s Walkway 20

7 Construction Considerations 22

7.1 Site Access 22

7.2 Suggested Construction Sequence 22

7.3 Reservoir Operations 24

7.4 Erosion Control 24

7.5 Temporary Cofferdams 24

7.6 Dewatering 25

7.7 Removal of Temporary Bypass Channel 25

7.8 Anticipated Construction Schedule 25

8 References 26

Tables

Table 2-1. Key Operational Control Elevations and Structural Measurements Table 3-1. Key Project Data (Proposed Construction) Table 4-1. Summary of Tailwater Elevations Table 4-2. Wave Runup and Setup Table 5-1. Embankment Material Properties Table 5-2. Embankment Stability Analyses Outputs Table 6-1. Intake Structure Flotation Calculations Table 6-2. Box Culvert Flotation Calculations

Appendices

Appendix A – Historical Information Appendix B – Design Calculations

Exhibit B.1 – Hydraulics Evaluation Exhibit B.2 - Riprap Sizing Exhibit B.3 – Wind Wave Freeboard Calculations Exhibit B.4 – Inlet and Box Culvert Flotation Check Exhibit B.5 – Embankment Slope Stability Evaluation Exhibit B.6 – Embankment Filter Compatibility Check Exhibit B.7 – Outlet Wing Wall Stability Check Exhibit B.8 – Operators Access Platform Design Calculations

Appendix C – Final Design Drawings Appendix D – Project Manual

B:\Working\MDNR\2304013 Net River Dam Modernization and Restoration\06_ENG\Phase 500 - Final Design\100% Final Design\3. Design Report\Net River Dam Final Design Report_FINAL.docx

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1 Introduction

GEI Consultants of Michigan, P.C. (GEI) has developed this Final Design Report to summarize the design criteria used to develop the Net River Dam Modernization and Restoration design documents. This report is intended to provide:

• A brief description and history of the site,

• A description of the intended design features, and

• Design criteria used to develop the reconstruction design.

1.1 Introduction and Project Scope

This Design Report summarizes the preparation of analyses, calculations, design drawings, and technical specifications for the Net River Dam Modernization and Restoration Project. The goal of this Project is to replace the failed dual-barrel spillway structure, remove the temporary bypass channel, and reconstruct the Net River Dam embankment to maintain the historical impoundment footprint. As a result of the engineering studies, inspections, and interim stabilization designs completed to date, the following fundamental dam safety issues must be addressed before the lake level can be restored:

• Spillway Capacity – The existing spillway capacity was estimated at approximately 850 cubic feet of flow per second (cfs), passing the 100-year recurrence flood event with a peak reservoir elevation (El.) of 1,542.8 feet. The flood would result in 2.2 feet of freeboard remaining (embankment crest elevation of 1,545.0 feet), which reportedly overtops a low area in the reservoir rim evaluated at elevation 1,541.5 feet (reference SCA survey drawings). This low spot is in a heavily wooded area, and as such, wind-driven waves and freeboard for this area is not part of the design criteria. The new structure must incorporate a total spillway capacity to pass the 100-year flood event with

3.5 feet (minimum freeboard at embankment) or peak flood elevation 1,541.5 feet.

• Inlet Structure – The inlet structure has concrete deterioration of varying severity and several operational deficiencies noted. The inlet structure will be repaired, where needed, and modified to allow operator access to both slots for gate manipulation. The modifications will also include replacement of the existing stoplogs with two vertical slide gates to better facilitate active reservoir manipulation, serving the interests of both the DNR and the Keweenaw Bay Indian Community (wild rice crops).

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• Culvert damage – The damaged and deteriorated corrugated metal pipe (CMP) culverts need to be removed and replaced to safely discharge waters downstream through the spillway structure.

• Outlet Structure – There is some uncertainty with respect to the effectiveness of the historical grouting of the outlet structure foundation. The outlet structure needs to be removed, the foundation recompacted, and a new structure constructed to safely release flood water downstream to the river.

• Temporary Bypass Channel – The temporary bypass channel needs to be removed and the earth embankment reconstructed to refill the impoundment to the historical footprint and elevations.

Construction of these modifications and restoration efforts at the Net River Dam are anticipated for summer 2025.

1.2 Project Description

The Net River Dam (MI Dam Inventory No. 131) is classified as a low hazard dam under Part 315 and is located on the Net River in Baraga County, Michigan. The Net River Dam is owned and operated by the Michigan Department of Natural Resources (MDNR) Wildlife Division. The dam provides environmental and recreational opportunities for the public by the creation of the reservoir impoundment, the Net River flooding, and roadway access to the opposite side of the river. The dam was originally constructed in 1953 but failed in 1985 when the embankment dam overtopped due to debris blockage of the inlet structure. The dam was reconstructed in 1989 as part of a wetland mitigation project sponsored by the Tilden Magnetite Partnership.

The dam was reconstructed as a reinforced concrete inlet structure transmitting flow via two 6-foot-diameter CMP culverts through an earthen embankment to a reinforced concrete outlet structure. Historical drawings depicting the dam are included in Appendix A. The inlet structure included a single notch in the upstream face with steel stoplogs for drawing down the reservoir.

The structure is fully encased by a steel-framed trash rack with an operators access platform providing access to the structure crest at the stoplog notch. The embankment is approximately 435 feet long with a 20-foot-wide crest and 3H:1V (horizontal to vertical) upstream and downstream slopes. The embankment extends 255 feet left and 180 feet right of the intake structure1. A crushed aggregate toe drain header, wrapped in geotextile with Schedule 40 PVC outlet pipes, was constructed at the downstream toe of the embankment, immediately adjacent and left of the outlet structure for approximately 40 feet. This corresponds with the location of

1 Left and right are referenced looking downstream.

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GEI Consultants of Michigan, P.C. 3 the 1985 embankment failure. The concrete outlet structure foundation was also reportedly grouted during the 1989 reconstruction work.

In 2014, the Keweenaw Bay Indian Community (KBIC) began planting and managing a wild rice crop at Net River, in partnership with the MDNR. Since 2014, over 10,000 pounds of wild rice seed has been introduced to Net River.

In April 2022, the crest of the right CMP barrel culvert failed, eroding embankment materials downstream and creating a sinkhole in the crest of the embankment. A member of the public notified the DNR, who worked with a local contractor to temporarily stabilize the structure by filling the hole and effectively clogging the culvert with large rockfill and aggregate. During the summer of 2022, GEI assisted the DNR with drawing down the reservoir, performing a dewatered inspection, and designing a temporary bypass channel to provide adequate capacity for accommodating spring runoff flows. The inspection identified severe corrosion of both CMPs, spalling and reinforcement exposure within the concrete inlet structure, and several other minor flaws. A second notch (8 feet long by 5 feet tall) was cut into the upstream wall of the inlet structure, and new timber stoplogs were installed in both notches. The temporary riprap lined bypass channel was constructed through the embankment to the right of the historic structure in late fall 2022. The upstream invert of the channel was constructed at approximate elevation 1,534.0 feet. The channel was constructed with a 30-foot-wide bottom width and 2H:1V side slopes. The channel was lined with MDOT Heavy Riprap, 3 feet thick, overlying coarse aggregate and geotextile. The channel alignment hinged slightly to direct flows from the right side of the structure back to the downstream river channel.

Since 2022, the reinforced inlet structure has been operated with both inlet notches ‘closed’ with timber stoplogs with only minor leakage flows transmitted through the remaining CMP culvert to the tailrace. The right CMP has remained clogged by the riprap repairs. The drawn down reservoir is maintained by flows through the temporary bypass channel. The KBIC has reported the temporary drawdown to be beneficial for the development and resiliency of the wild rice crop. Having the ability to actively control water levels may benefit the further establishment of the wild rice crop.

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1.3 Project Personnel

The following GEI personnel are primarily responsible for performing engineering analyses and developing the design documents for the project:

Project Manager: Jacob Pieti, P.E.

Hydraulics Lead: Geoffrey Kruger, P.E.

Geotechnical Lead: Kevin Roell, P.E., PMP Structural Lead: Richard Price, P.E.

Engineer of Record: Kevin Roell, P.E., PMP

This work has been coordinated with Mr. Bill Scullon and Mr. John DePue of the MDNR, Mr. Tom Pawlowski and Ms. Danielle Newman of the Department of Technology, Management & Budget (DTMB), and Ms. Erin Johnston of the KBIC, respectively.

1.4 Survey and Datum Elevation

Elevations listed herein are referenced to the North American Vertical Datum of 1988 (NAVD88). Horizontal spatial datum referenced to the North American Datum of 1983

(NAD83).

1.5 Design Criteria

The Net River impoundment is classified as a low hazard dam under the regulatory jurisdiction of EGLE. The EGLE dam safety criteria are contained in Part 315, Dam Safety of the Natural Resources and Environmental Protection Act, 1994 PA 451 (NREPA). Per the current EGLE guidelines, low hazard potential dams shall be capable of passing the 100-year flood, or the flood of record, whichever is greater. The proposed inlet and spillway design was analyzed and compared to existing and historic conditions against floods up to the 100-year event. The 2-year event is the design storm for the temporary cofferdam design based on the anticipated construction duration of less than 20 weeks in the summer of 2025. The EGLE guidelines also state that freeboard shall be considered when determining spillway capacity.

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2 Existing Site Conditions

2.1 General Project Description

Net River Dam is operated as a run-of-river structure. The DNR historically has set stoplogs within the slots to the desired reservoir elevation and any inflows pass over top of the stoplogs and concrete walls into the concrete inlet structure. Flows are transmitted to the tailwaters through the CMP barrel culverts. The key elevations of the features at Net River Dam are found in Table 2-1 below.

Table 2-1. Key Operational Control Elevations and Structural Measurements

2.2 Basemap Development

The site topography was developed using a combination of traditional ground-based survey data and bathymetric soundings. GEI utilized publicly available LiDAR data from the U.S. Geologic Survey, one-meter x38y515 MI Baraga 2015 (USGS, 2015) to assist in development of the base map topography at the site. In July 2022, GEI performed a supplemental ground-based survey of the dam embankment and corresponding features using Trimble R10 GNSS with Trimble Access.

GEI supplemented the data with a bathymetric survey of the reservoir and downstream channel performed by TriMedia Environmental & Engineering (TriMedia) using single frequency echo sounder (Sonar). The bathymetric data was coupled with periodic probing of the lake bottom with a 3/8-inch-diameter Marsh probe to assess the depth of sediment accumulation. In September 2022, GEI also performed wetland boundary delineations. GEI integrated the data in AutoCAD Civil 3D to develop the topographic basemap of the dam used in the design drawings.

2.3 Geotechnical Explorations

Due to the absence of historical subsurface information, GEI performed a shallow subsurface exploration program to aid in the analysis and design of the 2022 temporary stabilization work.

GEI performed three hand augers in the downstream embankment slope to understand the composition of the existing embankment shell on July 27, 2022. GEI performed visual engineering classifications on the recovered soil samples in general accordance with the USCS Classification System by estimating the plasticity, moisture condition, color, and presence of

Item Structural Measurement or

Surveyed Elevation

(NAVD88)

Concrete Inlet Top of Wall Elevation 1,536.90 feet Bottom of Stoplog Slot Elevation 1,531.90 feet

Upstream Barrel Inverts 1,528.75 feet Downstream Barrel Inverts 1,526.49 feet

Embankment Design Crest Elevation 1,545.00 feet

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Additional samples were collected from the excavated embankment during the 2022 construction. The samples collected had gradations performed to further understand the grain size distributions of soils within the embankment. Compared to the hand auger samples, the samples collected from the excavated embankment were coarser. In general, the soils were gravelly fine to medium sand, some silt, and trace coarse sand. A modified proctor was performed on one of the obtained samples to determine the maximum dry density.

During construction of the temporary bypass channel in 2022, a large excavation was performed through the embankment to the right of the existing structure. Several ‘grab samples’ of the embankment material were obtained with gradations performed to further understand the existing embankment materials. Visual observations of the embankment and foundation were routinely made by onsite personnel and documented in photos in the daily field reports. The excavated soils were stockpiled in the cleared laydown area immediately east of the parking lot. The embankment fill was observed to be fine to medium sand with trace to some silt and fine gravel (Sample S-5) and gravelly fine to coarse sand with some silt (Samples S-6 and S-7). Onsite, occasional cobbles and boulders were observed within the materials as well. A modified proctor was performed on the S-5 sample to obtain an understanding of maximum dry density and optimum moisture content.

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3 Summary of Dam Modifications

The following sections summarize the proposed dam modifications. The proposed structure elevations are summarized in the following Table 3-1.

Table 3-1. Key Project Data (Proposed Construction)

Feature Feet

Embankment Crest El. 1,545.0 Operator Walkway Floor El. 1,545.0 Top of Inlet Wall El. 1,536.9 Gate Sill El. 1,531.9 Culvert Upstream Invert El. 1,528.75 Culvert Downstream Invert El. 1,528.48 100-year HW El. 1,541.5 Normal Pool El. (Target El.) 1,537.0 100-year TW El. 1,534.0 Normal Tailwater El. 1,530.0

3.1 Inlet Structure Modifications

The existing inlet structure will remain in place, repaired and modified to meet the structural, operational, and maintenance requirements defined by the design team during the Phase 400 assessment. Deteriorated concrete including structural cracking, scaling, delamination, and spalling, will be repaired with reinforced structural concrete. The downstream wall of the structure will be sawcut and partially demolished to expand the opening from the two former CMP culverts to match the proposed precast box culvert inlet opening. A reinforced concrete wall will be constructed immediately downstream of the inlet wall to provide structural support to the modified downstream headwall and a transition to the precast box culvert. Some structural concrete will also be required to be placed within the existing wall to close any former opening wider than the necessary opening for the proposed precast box culvert. The center box beam-column feature will be maintained in place and protected during construction to prevent damage.

The existing steel trash rack will remain in place and will receive some minor field modifications to provide proper clearance for the vertical slide gates to operate within the right and left slots.

Woody and organic debris that has accumulated at the upstream face of the trash rack will also be removed and disposed of.

A new operator access walkway will be constructed above the existing structure, elevated to accommodate the functionality and safe operation of the two new vertical slide gates. The walkway will include access to both gates. The existing steel components (trash rack and

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GEI Consultants of Michigan, P.C. 8 operators access walkway) on top of the intake structure will remain in place to keep large debris from entering the inlet structure. Handrails will be installed around the perimeter of the walkway to ensure operators safety. Elevating the walkway will allow for operation of gates, especially during high flow events. The walkway will extend from the embankment crest to the upstream face of the inlet, spanning across the entire upstream face.

Two new 8-foot-wide by 5-foot-tall stainless steel vertical slide gates will be installed in the left and right stoplog slots. The left notch will be widened to accommodate the installation of the left slide gate. Gates will be specified to have Ultra High Molecular Weight Plastic (UHMW) side seals and a neoprene invert seal at the sill. The gates will be operated by dual stems, with a gearbox to allow for manual hand-crank operation. Additionally, the gate manufacturer will install a drive nut to the gearbox, allowing for gate operation using a drill. The gates will allow for flood pool flows to pass without gate interference. Installation of the gates will also allow for more active water level control, benefiting the wild rice crop and wildlife habitat.

3.2 Spillway

The two CMP barrel spillway culverts are proposed to be removed and replaced with a single 14-foot-wide by 8-foot-tall (internal dimensions) precast concrete box culvert, optimized to pass the 100-year outflow. The precast concrete box culvert will be manufactured in multiple sections and assembled onsite. The connection to the inlet structure will be constructed by a cast-in-place reinforced concrete segment, doweled into, and reinforcing the modified downstream inlet structure head wall and 12-inch-thick (minimum) precast box culvert walls. The spillway will otherwise be constructed of precast box culvert segments with hydrophilic mastic sealant filled joints, stainless steel straps on exterior joints wrapped in geotextile. The spillway will be constructed on top of a compacted aggregate or “mud mat” subbase on the upstream extents, tying into the chimney filter drain system beneath the embankment crest.

The precast manufacturer will design the box culvert to accommodate HL-93 modified live loads. However, routine vehicle traffic is not anticipated over the crest at this time as a locked gate will prevent access to the embankment crest.

3.3 Outlet Structure

The furthest downstream precast concrete box culvert section will have a 1-foot-tall precast concrete headwall. The headwall will be placed on top of the box culvert section, on the downstream edge. Incorporating the headwall will help retain embankment soils, discouraging embankment erosion. The outlet to the spillway will consist of precast tee-shaped wing walls with a cast-in-place reinforced concrete apron. The precast tee-shaped wing walls will flare outward at each side. The wing walls will be installed on both the left and right side of the outlet to protect the embankment soils and riverbank from the river’s discharge. Handrails will be installed atop the wing walls and headwall to protect the public against the fall hazard.

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The cast-in-place concrete apron will protect the streambed against erosion while also limiting the impact of seepage beneath through the embankment beneath the spillway structure. The reinforced concrete apron will terminate with a minimum 12-inch-thick vertical concrete toe wall. The vertical toe wall will protect the subbase soils beneath the apron slab from erosion or washout.

Immediately downstream of the vertical toe wall, the existing riprap along the channel bottom will be supplemented with salvaged riprap from the adjacent temporary bypass channel. The riprap will serve as an energy dissipater, acting to limit the velocity of flows. The riprap will protect the stream bed from the potential of high flows that cause erosion and scour. In the event of a significant discharge event, the riprap shall be evaluated for movement. If stones have been dislodged or moved from high flows, the riprap shall be reset or replaced.

3.4 Embankment Reconstruction

The embankment will be re-constructed at both the existing structure and the temporary bypass channel with a minimum 20-foot crest width (18 feet at the culvert location), 3H:1V upstream and 3H:1V (2.5H:1V at the culvert location) downstream slopes to match existing conditions and provide adequate slope stability. The crest elevation of 1,545.0 feet will maintain the 3.5 feet of freeboard at the dam. The upstream slope will be armored with surplus riprap from the temporary bypass channel to protect against wave-induced erosion. The crest will be overlain with aggregate to provide increased resiliency against any potential vehicle traffic.

The embankment fill will consist of materials salvaged from on-site excavations or imported from an approved off-site source, as required to meet project drawings and specifications. Less permeable soils with a high fines content will be used for the upstream portions of the embankment fill, with a vertical aggregate chimney filter beneath the crest. The filter will encompass the box culvert and cut off seepage flows through the embankment, discharging through the filter at the downstream toe of the embankment. The chimney filter will be constructed beneath the downstream extents of the embankment crest. The top of the filter will match the 100-year flood event at 1,541.5 feet and extend vertically down to subgrade soils. The vertical chimney will be 3 feet of filter sand. Filter blanket materials will be 3 feet thick, with 12 inches of filter sand against subgrade soils, 12 inches of filter gravel, and 12 inches of filter sand above the filter gravel. The filter gravel will always be encompassed by a minimum of 12 inches of filter sand on all sides. The filter will reduce phreatic levels through the embankment and protect against internal erosion failure mechanisms. Downstream portions of the embankment fill will be more permeable than the upstream layers, with a lesser fines content specified. Embankment fill has been specified to be compacted to 95% of the maximum dry density, determined by the modified proctor method.

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4 Hydrology and Hydraulics

4.1 Hydrology

As described in the Hydraulics Evaluation Report dated October 14, 2022, GEI completed unsteady hydraulic modeling of the various conditions of the stream channel using HEC-RAS

4.1.0. Modeling conditions included the historic operating condition (spillway with two-barrel pipes), the current existing structure condition (spillway with one barrel pipe), current temporary bypass channel condition (assuming no flow through spillway), and proposed condition of rehabilitated structure with new box culvert.

The Michigan Department of the Environment, Great Lakes, and Energy (EGLE) provided GEI with a previous HEC-1 watershed model for the Net River Dam. GEI imported the model into HEC-HMS and updated the meteorological model to include additional storm events. The peak outflows provided by EGLE were found to generally agree with the HEC-HMS modeling except for the flood flows at recurrence intervals of 100, 200, and 500 years. The HEC-HMS 100-year inflow is approximately 3,200 cfs.

The historical spillway structure with two functioning barrel culverts, reportedly has a hydraulic capacity of 1,230 cfs (EGLE, 2020). EGLE performed a flood evaluation for the site and provided an inflow peak discharge of 1,200 cfs for a flood with a 100-year recurrence interval

(EGLE, 2020).

The EGLE estimated spillway capacity of 1,230 cfs is higher than the GEI estimated zero freeboard spillway capacity of approximately 850 cfs with dam in previous condition. The design will provide the same 100-year outflow as the previous condition with goal to provide no difference between 100-year flood levels downstream of the dam.

4.2 Hydraulic Design

This section establishes the design criteria to be used for preparation of hydraulic analyses to design the project spillways, and low-level outlets.

4.2.1 Inlet

The project spillways will be designed to achieve the following design goals:

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• The reconstruction/restoration of the project anticipated to provide a 50+ year design service life.

• The reconstruction/restoration of the project will be designed to meet the current industry standards of engineering practice and design standards for low hazard dams in accordance with EGLE.

• The spillways should have sufficient capacity to pass the IDF equal to the 100-year recurrence flood with minimum freeboard.

• Reconstruct the embankments to provide freeboard for floods up to the IDF.

• The structural integrity of the earthen dam and its foundation should not be jeopardized by spillway operations.

• Operation of the slide gates will be the primary means of release to the river under both normal and flood conditions.

• The spillway should fit within the footprint of the existing embankments to minimize the impact to downstream wetlands.

4.2.2 Culvert

The hydraulic analysis and design work will be completed in phases to appropriately size the primary spillway. A one-dimensional (1D) HEC-RAS model will be developed to optimize the size of the spillway. The intent of the HEC-RAS model will be used to optimize the size of the spillways, stilling basin and training walls and refine the preferred geometry to meet the project objectives.

The HEC-RAS model will be used to determine the discharge, flow depths and velocities within the spillway and downstream discharge channel. The design intent of the stilling basin is to minimize impacts that flow velocities can have on the surrounding terrain. The USBR guidelines will be used to design the spillway to transfer flow to reduce scour and erosion in the discharge channel.

4.2.3 Tailwater Evaluation

A tailwater rating curve comparing elevations and discharges downstream of the Net River Dam was developed for all flows up to the IDF. The 1D HEC-RAS model described above was used to estimate the tailwater rating curve downstream of the spillway. The results of the HEC-RAS models indicate that the IDF tailwater of approximately El. 1,534 feet. These results indicate that the tailwater is not high enough to impact the upstream spillway discharge capacity. The tailwater rating curves are included in Appendix B and select tailwater elevations are summarized in Table 4-1 below.

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Table 4-1. Summary of Tailwater Elevations Normal Tailwater

(feet) IDF or 100-year Tailwater (feet)

1,530 1,534

4.2.4 Flood Routing Results

The proposed spillway rating curves described above were input into the HEC-HMS model to estimate the inflow, outflow, and peak headwater for several flood events. Based on the proposed spillway configuration for Net River Dam, the IDF results in a peak inflow of 3,200 cfs, a maximum reservoir water surface at El. 1,541.5 feet, and a peak discharge of 1,880 cfs. The IDF inflow, outflow, and stage hydrographs are included in Appendix B. Based on the configuration described above, the proposed Net River Dam spillway configuration would have sufficient discharge capacity to safely pass the IDF with 3.5 feet of available dam crest freeboard. Input and output data from the HEC-HMS model are included in Appendix B shows the details of the model development.

4.2.5 Freeboard Evaluation

Freeboard was analyzed to evaluate the ability of the existing dam crest to protect the structure from overtopping waves generated by wind. The analysis was performed using recommendations and design standards of the USBR, “Design Standards No. 13 Embankment Dams, Chapter 6:

Freeboard” (Ref. USBR, 2011) for an embankment dam. Wind speed data from the National Climatic Data Center recorded at Iron Mountain, Sawyer, and Escanaba airports were reviewed to determine probable wind speed over the reservoir. Two checks were performed to assess the ability of the dam crest and parapet wall to prevent overtopping waves.

The first check performed evaluates the dam at the normal reservoir elevation. A 100 miles per hour (mph) wind speed was used to evaluate wind wave buildup along the measured fetch offset to the perpendicular face of the dam. The second check assesses the height of the waves generated by winds that would typically occur during the duration of the IDF maximum reservoir water surface elevation. The USBR guidelines recognize that it is highly unlikely that the maximum winds will occur when the reservoir water surface is at its maximum elevation because the maximum level is only present for a short amount of time. Therefore, the wind speed selected for the analysis was determined based on the most likely wind speed to occur for the duration of the event based on historical data. The selected windspeed was 68 mph.

The maximum wave setup plus runup reached at Net River Dam when there is 100 mph sustained wind is 1.25 feet. The distance between the reservoir water surface and the top of the embankment crest during normal conditions is 8 feet; therefore, the freeboard is sufficient to contain the wind waves. At the maximum reservoir elevation and using a 68-mph wind speed during the IDF, the maximum wave setup plus runup is estimated to reach 1.7 feet. The distance

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As a result of this analysis, the proposed spillway capacity upgrades and dam crest is estimated to provide sufficient freeboard for Net River Dam to prevent waves from overtopping the embankment crest during normal and IDF conditions. Wave runup and setup information is summarized in Table 4-2 and freeboard calculations are included in Appendix B.

Table 4-2. Wave Runup and Setup

Normal Reservoir WSEL

(NRWS)

NRWS +

Runup and

Setup

Maximum Reservoir WSEL

(MRWS)

MRWS + Runup and Setup under IDF conditions

Crest Elevation

(feet)

Governing Condition

1,537 1.25 1,541.5 1,543.2 1,545 IDF

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5 Geotechnical Analyses and Design

This section establishes the design criteria to be used for preparation of geotechnical analyses to design the project embankments, filters, and riprap.

5.1 Embankment Design

The reconstructed embankment sections will be constructed as zoned fill structures. The chimney filter and mineral toe drain will manage the seepage through the structure and limit potential for piping or internal erosion failure modes. The chimney filter will surround the box culvert, serving as a diaphragm to control and mitigate potential seepage and contact leak erosion. Given the limited seismicity of the region, seismic analyses were not performed on the embankment structure.

The embankments will be reconstructed with granular structural fills, generally less permeable upstream and more permeable downstream with an aggregate chimney filter and mineral toe drain system. Structural fills will be compacted in 12-inch lifts to provide sufficient internal friction angles to promote stability. The upstream slopes will be protected with riprap revetment.

5.1.1 Slope Stability Evaluation

An embankment slope stability evaluation was conducted utilizing GeoStudio’s SLOPE/W modeling software. The model evaluated the stability of the upstream and downstream faces of the embankment under both normal and flood pool conditions at the critical existing embankment location which will remain post-construction as well as the proposed embankment. The critical location of the existing (to remain) embankment section modeled is located between the intake structure and the temporary bypass channel as defined by having the steepest downstream slope and tallest section of the embankment (to remain). The critical proposed embankment section modeled is located immediately left of the intake structure as defined by having the steepest downstream slope and tallest section of the embankment.

The geometry of the Net River Dam embankment was modeled based on survey data collected by GEI and TriMedia Environmental & Engineering in 2022. Based on the survey data of the existing to remain embankment section, an 18-foot-wide crest was designed with approximately ~6.1H:1V slopes upstream and ~3.1H:1V downstream. The proposed embankment sections are to have ~3H:1V upstream slopes and ~2.5H:1V downstream slopes. The existing crest elevation is at ~1,545.0 feet and the proposed crest elevation is designed at 1,545.0 feet.

Conservative material properties were based on gradations and soil samples from hand auger samples, laboratory testing, and through engineering judgement. The existing embankment section was modeled as two separate layers: the foundation soils and embankment fill. The proposed

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GEI Consultants of Michigan, P.C. 15 embankment was modeled as three separate layers: the foundation soils, embankment fill, and chimney filter soils. The material properties utilized are shown below in Table 5-1. Cohesion for all materials was assumed to be 0. It is assumed that the unit weight of water is 62.4 pcf.

Table 5-1. Embankment Material Properties

Soil Strata USCS Classification

Phi

Cohesion (psf)

Unit Weight (pcf)

Constant Unit Weight above Piezometric

Surface (pcf) Chimney Filter - 30 0 120 125

Embankment Fill SP-SM 32 0 110 117 Foundation GP 34 0 120 -

For all conditions the phreatic conditions were assumed to vary linearly from headwater to tailwater. Under normal pool conditions, headwater was assumed at 1,537.0 feet with tailwater at 1,530.0 feet. For flood pool conditions, the reservoir was assumed at El. 1,541.5 feet with the tailwater at El. 1,534.0 feet.

The Spencer method was used with an assumption that the critical slip surface must be at least 5 feet in depth and either compromise the crest or intersect the phreatic surface. The evaluated conditions for normal and flood pool conditions meet or exceed the required factors of safety, both on the upstream and downstream slopes.

Table 5-2. Embankment Stability Analyses Outputs

Structure Loading Slope HW El.

(Feet)

TW El.

(Feet)

FS

Calculated

Min FS Req’d

Existing (to Remain) Embankment Normal Pool Downstream 1,537 1,530 1.9 1.5

Existing (to Remain) Embankment Normal Pool Upstream 1,537 1,530 3.4 1.5

Existing (to Remain) Embankment Flood Pool Downstream 1,541.5 1,534 1.7 1.4

Existing (to Remain) Embankment Flood Pool Upstream 1,541.5 1,534 3.3 1.4

Existing (to Remain) Embankment Rapid Drawdown Upstream 1,537 1,530 3.2 1.2

Proposed Embankment Normal Pool Downstream 1,537 1,530 1.7 1.5

Proposed Embankment Normal Pool Upstream 1,537 1,530 1.8 1.5

Proposed Embankment Flood Pool Downstream 1,541.5 1,534 1.5 1.4

Proposed Embankment Flood Pool Upstream 1,541.5 1,534 1.9 1.4

Proposed Embankment Rapid Drawdown Upstream 1,537 1,530 1.6 1.2

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As indicated in Table 5-2, the modeled existing embankment section to remain has factors of safety greater than the FERC Chapter 4 requirements.

The chimney filter will lower the phreatic levels through the embankment. When modeling, the analyses for proposed sections assume a rapid phreatic drop through the filter, until matching the tailwater elevation. The embankment will not receive the chimney filter through its entirety, rather the filter will only be constructed within the excavated portions of the embankment. When the reservoir is refilled, phreatic conditions will be slightly below that modeled for proposed conditions but will quickly reach modeled proposed conditions due to the granular nature of the embankment fill materials. The proposed conditions serve as a conservative representation of conditions during reservoir refilling. Construction loading on the embankment was also considered but is not anticipated to be a large concern due to the limited short-term loading duration, similar to 2022 construction with lesser hydrostatic loading than both existing and proposed conditions.

As indicated in Table 5-2, the modeled proposed embankment section has factors of safety greater than the FERC Chapter 4 requirements.

5.1.2 Filter Design

An aggregate chimney filter and filter blanket will be constructed at both embankment sections being excavated and reconstructed. The chimney filter and filter blanket will inhibit piping and internal erosion by filtering fine particles from any seepage flows, therefore decreasing the risk of piping through the embankment. In the unimproved area of the embankment between the two work areas, an aggregate filter blanket will be constructed on the downstream slope. The filter blanket will provide a filtered exit for potential seepage flows between the proposed embankment filtered zones.

Samples of the existing site soil were obtained by GEI from an excavated stockpile onsite (S-5, S-6, and S-7) during the 2022 temporary construction. Sieve analyses were performed on the obtained samples to classify the in-situ soils. In general, the samples were found to be fine to medium gravelly sand with less than 17% passing the #200 sieve. Using the particle size distributions, GEI selected available aggregates specified by the Michigan Department of Transportation (MDOT) construction specifications that will be compatible with each other as well as the in-situ soils. Upon reviewing options, MDOT 2NS natural sand was selected as the filter sand layer, MDOT 29A coarse aggregate was selected as the fine gravel filter layer, and MDOT 4AA coarse aggregate was selected as the coarse gravel filter layer. Compatibility checks for the selected materials and in-situ soils are found in Appendix B. Compatibility of the materials was checked using the guidance from the Federal Emergency Management Agency (FEMA) “Filters for Embankment Dams: Best Practices for Design and Construction” (Ref. FEMA, 2011). This guidance references FEMA, the National Resource Conservation Service (NRCS), and the United States Army Corps of Engineering (USACE).

It should be noted that the permeability check between in-situ soils and fine filter sand (MDOT 2NS) was not met. This implies that the chosen materials may limit the flow rate of seepage water

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GEI Consultants of Michigan, P.C. 17 through the filter under certain conditions. There are no historic issues with seepage breakout or piping through the embankment; therefore, large seepage volumes are not expected to pass through the filter and the critical function of the filter is particle retention during elevated phreatic conditions. It is not expected that excessive groundwater pressures will build behind the filter. The chosen filter sand and gravel drain are still considered acceptable based on the wide use of these materials in similar locations, and the filter and drainage layers are at low risk of failure.

The aggregate chimney filter will also serve as a diaphragm around the precast box culvert. The FEMA guidance assisted the understanding and incorporation of a chimney filter in the excavated embankment sections. The diaphragm will capture potential seepage flows along the precast concrete box culvert sections and outlet the potential seepage through the filter layers. The filter layers will allow seepage to exit downstream of the embankment and laterally through the filter layers to surrounding embankment fill materials. The aggregate filter blanket will be constructed with MDOT 2NS sandwiching a minimum 12-inch layer of MDOT 29A filter gravel. The chimney filter will consist of a vertical column 3 feet thick of MDOT 2NS sand. Considering the phreatic levels through the embankment, the chimney filter will dramatically lower the phreatic elevation when encountering the filter.

5.2 Riprap Design

The riprap for upstream slope revetment will be salvaged from the temporary bypass channel on upstream slopes. Additionally, downstream of the outlet structure the riprap energy dissipator will also be constructed by using riprap salvaged from temporary bypass channel. During the 2022 construction, GEI performed a field gradation of the riprap installed to line the temporary bypass channel. The field gradation found the riprap to have an average D50 of 24.5 inches.

Required riprap sizes were calculated by the USACE, Isbach Equation, and the NRCS Rock-Lined Chute method. The riprap energy dissipator has been designed for a discharge velocity of 1,770.5 cfs and an average supercritical velocity of 20.0 fps, in accordance with the supporting hydraulics evaluation. The unit weight of the rock has been assumed as 165 pcf. For the Isbach Equation, a coefficient of 1.2 was used, as the Net River experiences low turbulence. The Isbach Equation requires a minimum D50 of 33.0 inches, and the NRCS method requires a minimum D50 of 15.0 inches. A weighted average was taken between the two methods to find the minimum required D50 of 24.0 inches. Since the riprap onsite has an average D50 of 24.5 inches, the riprap onsite has been determined acceptable for reuse as upstream revetment and to serve as an energy dissipater.

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6 Structural Design

6.1 Existing Structure Evaluations

A flotation stability analysis was performed to assess the inlet structure under dewatered, normal, and flood pool conditions. The analysis indicated the structure has capacity to resist uplift pressures, with all analyses meeting the FERC required factor of safety. The reinforced concrete connection to the box culvert was conservatively omitted from the analyses. The structure has acceptable factors of safety for dewatered, normal, and flood pool conditions. See Table 6-1 below for a summary of results.

Table 6-1. Intake Structure Flotation Calculations Loading

Condition FS Min FS Req’d Result

Dewatered Conditions 1.4 1.3 Okay per FERC Ch. 10

Normal Conditions 1.5 1.5 Okay per FERC Ch. 10

Flood Conditions 1.3 1.3 Okay per FERC Ch. 10

The modifications to the structure are not anticipated to negatively impact the structural integrity of the structure as the downstream wall modifications will be supported by an integral stiffening reinforced concrete wall immediately downstream as part of the connection to the precast box culvert to the inlet structure. In 2022, the right notch (as viewed looking downstream) was sawcut to assist with passing Net River flows. Analyses were performed against loading conditions to show the remaining concrete sections are adequate to resist the loading conditions.

The modifications to the left notch of the intake will geometrically match the modifications to the right notch in 2022. As such, no additional supporting calculations were deemed necessary for the left notch opening modification.

6.2 Stainless Steel Vertical Slide Gate

The vertical slide gates and operational components will be a self-contained premanufactured gate. The gate is to be constructed from stainless steel, with a mill finish. The gates will be 8 feet wide by 5 feet tall, with a maximum design head of 9.5 feet. The gates will be able to clear the peak flood pool elevation of 1,541.5 feet when fully opened. The gate guides will be embedded into the inlet structure notches with no anchor bolts or welding anticipated for installation. The gates will be designed to accommodate ice loading. Side seal leakage rates will be less than

0.05 gpm/ft.

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6.3 Box Culvert

The precast concrete structure will be structurally designed by the precast manufacturer and stamped by a registered Professional Engineer in the state of Michigan. The precast structures will be designed to accommodate HL-93 modified live loads in addition to the embankment soils and structures at the crest. The precast concrete structures will be sized to accommodate a 100-year design flood event as described in Section 4.2 above.

A flotation stability analysis was performed to assess the box culvert structure under dewatered, normal, and flood pool conditions. The structure was conservatively analyzed at the upstream slope with hydrostatic uplift pressures assumed to match the reservoir head pressure. The structure has acceptable factors of safety under each evaluated loading condition. See Table 6-2 below for a summary of results.

Table 6-2. Box Culvert Flotation Calculations

Loading Condition FS Min FS Req’d Result

Dewatered Conditions 1.4 1.3 Okay per FERC Ch. 10 Normal Conditions 1.5 1.5 Okay per FERC Ch. 10 Flood Conditions 1.3 1.3 Okay per FERC Ch. 10

At the outlet side of the box culvert, a headwall section will be placed on top of the furthest downstream section. The header section will serve to retain embankment soils, discouraging sluffing or erosion off the top of the structure.

6.4 Outlet Structure

Two cast-in-place concrete mixes will be utilized for project works; a ready mix from a batch plant and a bag mix batched locally onsite. The ready-mix will be used for the inlet-box culvert connection wall and the downstream apron slab; the bag mix for the smaller, local concrete repairs within the inlet structure. Both will require reinforcement and structural concrete mix strengths (4500+ psi).

6.4.1 Outlet Structure Apron Slab

A reinforced concrete spillway apron slab will be placed at the downstream outlet of the precast box culvert. The slab will be dowelled into the precast box culvert with a minimum of 12 inches of embedment. The apron slab will be reinforced and utilize a 4500+ psi concrete mix supplied by a batch plant. The apron slab will be a slab-on-grade loaded in compression and is not anticipated to be subject to differential loading, therefore no explicit analyses were performed.

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The downstream end of the apron slab will terminate with a minimum 12-inch-thick reinforced vertical concrete toe wall. The toe wall will serve to protect against undermining of the apron slab. A layer of granular filter materials will be placed beneath the apron slab.

6.4.2 Outlet Wing Wall Stability

The outlet will consist of precast tee-shaped concrete wing wall sections to protect the adjacent embankment soils and riverbanks from discharges. The structural design will be performed by the precast manufacturer to resist lateral earth pressures. An analysis was performed to evaluate the sliding, overturning, and bearing forces the wing walls may experience. The conducted analyses conservatively omit the connection to the box culvert and the reinforced concrete apron slab. The analysis was performed for the critical sections of the wing wall. The tallest section was determined to be the critical section based on a brief sensitivity analysis of wall heights. By inspection, all wing wall heights less than the maximum will be conservatively accommodated by the design.