Alternatives Evaluation Report_Willow Creek Dam.pdf

Willow Creek Dam MT DNRC Dam Feasibility Study

Alternatives Evaluation Report

Willow Creek, Montana February 14, 2025

February 14, 2025 | i

Contents

Executive Summary

1 Introduction

1.1 Purpose

1.2 Sedimentation Potential

2 Spillway Replacement

2.1 Potential Failure Modes

2.2 Hydrology

2.3 Freeboard

2.3.1 Normal Freeboard

2.3.2 Minimum Freeboard

2.3.3 Freeboard Checks

2.4 Spillway Crest

2.5 Spillway Chute

2.5.1 Spillway Wall Heights

2.5.2 Spillway Chute Alternatives Summary

2.6 Terminal Structure

2.6.1 Terminal Structure Alternatives Summary

3 Structural Considerations

3.1 Spillway Crest

3.2 Spillway Chute

3.3 Flip Bucket

3.4 Defensive Design Measures

4 Geotechnical Design

4.1 Spillway Foundation

4.2 Embankment Buttress

4.2.1 Potential Failure Modes

4.2.2 Alternatives Description

4.2.3 Analyzed Alternatives

5 Construction Cost Estimates

6 Preliminary Construction Sequence

7 Summary and Recommendations

8 References

Tables

Table ES-1. Recommended alternative costs ........................................................................................... ii

Table 2-1. Summary of PFMs developed during 2022 Simplified PFMA

Table 2-2. Freeboard calculations methodology

Table 2-3. Freeboard calculations methodology applied to Willow Creek Dam

Table 2-4. Spillway crest alternatives

Table 2-5. Spillway crest alternatives summary

Table 2-6. Spillway chute alternatives

Table 2-7. Terminal structure alternatives

Table 4-1. Summary of PFMs developed during 2022 Simplified PFMA ii | February 14, 2025

Table 4-2. Embankment mitigation alternatives summary

Table 4-3. Buttress alternative results

Table 5-1. Estimated construction costs

Table 7-1. Recommended alternative costs

Figures

Figure 2-1. Conceptual flip bucket design

Figure 3-1. Typical sections of the spillway chute (dimensions are in feet)

Figure 3-2. Typical control joint (dimensions are in feet)

Figure 3-3. Flip bucket layout (dimensions are in feet)

Figure 4-1. Plan view of topography and bathymetry from 2022 surveys

Figure 4-2. Comparison of slopes from 2022 topography and merged bathymetry for the Willow Creek Dam (WGM 2022) with original design cross-section

Figure 4-3. Alternative E2 cross-section: Minimum soil buttress

Figure 4-4. Alternative E3 cross-section: Soil buttress with capacity for future raise

Figure 4-5. Alternative E4a cross-section: Crest height increase

Abbreviations

2D two-dimensional 3D three-dimensional AACE Association for the Advancement of Cost Engineering ac-ft acre-foot/feet ACI American Concrete Institute ASCE American Society of Civil Engineers ASTM American Society of Testing and Materials cfs cubic foot/feet per second DNRC (Montana) Department of Natural Resources and Conservation DS Design Standards EL. elevation EM Engineer Manual FHWA Federal Highway Administration Fr Froude number ft foot/feet HDR HDR Engineering, Inc.

HEC-HMS Hydrologic Engineering Center Hydrologic Modeling System HEC-RAS Hydrologic Engineering Center River Analysis System H:V horizontal:vertical (slope) IDF inflow design flood LC load case LOL loss of life MDE maximum design earthquake mph mile(s) per hour MSE mechanically stabilized earth MWS maximum water surface N/A not applicable NAVD88 North American Vertical Datum of 1988 NWS normal water surface OBE operating-based earthquake O&M operation and maintenance PFM probable failure mode

February 14, 2025 | iii

PFMA potential failure mode analysis PMF probable maximum flood PMM Principal Meridian of Montana RCC roller-compacted concrete STA Station USACE United States Army Corps of Engineers Vt velocity USBR United States Bureau of Reclamation WSEL water surface elevation WUA (Willow Creek) Water Users Association

Appendices

Appendix A. Wind-Wave Analysis ............................................................................................................ A

Appendix B. Spillway Sizing Calculation................................................................................................... B

Appendix C. HEC-HMS Outputs ..............................................................................................................C

Appendix D. HEC-RAS Output .................................................................................................................D

Appendix E. Chute and Terminal Structure Sizing .................................................................................... E

Appendix F. Seepage and Slope Stability Analyses ................................................................................. F

Appendix G. Cost Estimate ..................................................................................................................... G

Appendix H. Conceptual Plan Sheets ......................................................................................................H iv | February 14, 2025

This page intentionally left blank

| February 14, 2025

Executive Summary

The Montana Department of Natural Resources and Conservation (DNRC) contracted with HDR Engineering, Inc. (HDR) to evaluate potential improvements to the spillway and dam embankment at Willow Creek Dam east of Harrison, Montana. The purpose of this

Alternatives Evaluation Report is to present the conceptual-level alternatives for reconstructing the Willow Creek Dam spillway and rehabilitating the earthen embankment.

Development of alternatives were largely based on the standards of practice currently set by DNRC’s Dam Safety Program and US Bureau of Reclamation (USBR). More specifically, the DNRC Dam Safety Program’s Technical Notes, which present guidelines for analysis and design of spillways and seismic analysis of dams and for conducting potential failure mode analyses. Similarly, USBR has published industry standards for assessing embankment dams in Design Standards No. 13 as well as Design of Small

Dams. Other publications generally relied on for this effort included Federal Emergency

Management Agency (FEMA) and US Army Corps of Engineers (USACE) guidelines and engineering manuals, respectively.

In a previous task of this contract, HDR and DNRC along with the Willow Creek Water

Users Association (WUA) performed a Simplified Potential Failure Mode Analysis

(PFMA) workshop. In accordance with DNRC’s Technical Note 5, this effort was used to identify and categorize potential failure modes of the Willow Creek facility, including, the spillway, the embankment, and low-level outlet. The results of this effort led to the identification of several failure modes with high-ranking categories and subsequent field investigation efforts and assessments specific to the spillway and embankment. Post assessment efforts have led the team to develop mitigation alternatives for both the spillway and the embankment as presented herein.

The alternatives analysis for the spillway evaluated conceptual designs for the control structure, chute, and stilling basin. The primary goals of the design are to reestablish the original design operating level in the reservoir, increase the spillway capacity to meet the requirements of the DNRC Dam Safety Program, and potentially restore the original design storage volume.

Overall, 12 alternatives were explored for the spillway control structure with 4 variations to the weir geometry and 3 variations to the weir elevations and locations. The geometries assessed include an ogee crest weir, duckbill weir, linear labyrinth weir, and arced labyrinth weir. The alternatives were preliminarily screened and resulted in seven alternatives being carried forward for further consideration. The results of the analysis indicated that the linear labyrinth weir was the preferred alternative. The main benefits of this alternative were that the labyrinth weir maintained a similar footprint to the existing weir and did not require additional excavation into the left abutment. The elevation (EL) of the weir would be raised to EL 4,747.90 feet, which would result in a dam raise of approximately 7.2 feet.

Two spillway width configurations were assessed for the spillway. The first configuration, herein referred to as the tapered configuration, transitioned the spillway chute width from

90 feet at the upstream end of the spillway to 50 feet approximately halfway down the

February 14, 2025 | i embankment. This configuration matches the existing spillway configuration. The second configuration, herein referred to as the straight configuration, consisted of a consistent chute width of 90 feet. Two spillway profile configurations were assessed in conjunction with the spillway width configurations mentioned above. The first spillway profile configuration followed the existing spillway profile, which includes the vertical grade breaks along the profile. The second spillway profile consisted of a uniform slope along the spillway chute. The tapered spillway chute matching the existing spillway profile was selected as the preferred alternative to maintain the spillway within the existing spillway footprint and limit the amount of modification required to the hillside and dam embankment.

Additionally, two terminal structure types were assessed downstream of the spillway chute including a Type II stilling basin and a flip bucket. The flip bucket was selected as the preferred alternative because of its ability to adequately provide energy dissipation while maintaining the existing terminal structure footprint and not encroaching on the

Willow Creek channel downstream.

Preliminary structural design for the spillway components (e.g., crest, chute, and terminal structures) were developed to estimate wall thickness, foundation considerations, and inform the conceptual plan development and cost estimates.

Previous studies, including a Simplified Potential Failure Mode Analyses (PFMA) and a geotechnical assessment performed by HDR, indicated that the downstream slope of the embankment does not meet stability factors of safety for static and seismic load cases

(LC). The primary goals for the embankment dam alternatives were to address identified potential failure modes (PFMs) and calculated low factors of safety relative to industry standards, specifically under static and seismic loads, through the improvement of the embankment’s downstream slope. A secondary concern that DNRC presented to HDR was that of reservoir capacity; at least two alternatives provided concepts to increase the reservoir pool elevation. The following embankment mitigation alternatives were assessed:

1. Roller-compacted concrete (RCC) dam

2. Minimum soil buttress

3. Soil buttress with capacity for future raise

4. Soil buttress with raise:

a. Soil raise

b. T-wall raise

c. Mechanically stabilized earth (MSE) wall raise

For the embankment alternatives, HDR relied on standards set by the DNRC Dam Safety

Program, the United States Army Corps of Engineers (USACE), and the United States

Bureau of Reclamation (USBR). The results of the assessment indicated that providing an engineered-fill buttress downstream of the existing embankment dam would stabilize the embankment under static and seismic loads and also provide the capacity for a future reservoir raise.

ii | February 14, 2025

Preliminary calculations were performed to size the above-mentioned preferred alternatives for the spillway and embankment modifications and inform the conceptual-level cost estimate. Class 4 costs were calculated in accordance with the Association for the Advancement of Cost Engineering (AACE) class level and are summarized for the recommended alternatives in Table ES-1.

Table ES-1. Recommended alternative costs

Alternative Subtotal Class 4 cost estimate

-30% +50%

3B-2: linear labyrinth grade raise $3,651,940 $2,556,358 $5,477,910

Tapered spillwaya $14,903,314 $10,432,320 $22,354,971

E4a: crest height increase $3,709,631 $2,596,741 $5,564,446

Total Costs $22,264,885 $15,585,419 $33,397,327 a Includes flip bucket costs.

February 14, 2025 | 1

1 Introduction

The Willow Creek Dam is located on Willow Creek east of Harrison in Madison County, Montana, approximately 17 miles south of the city of Three Forks. More specifically, the dam is in Sections 26 and 35, Township 1 South, Range 1 West, Principal Meridian of

Montana (PMM). According to the Willow Creek Dam Manual for Operation and

Maintenance (O&M) (reference 22), the earth fill dam is owned by the Montana

Department of Natural Resources and Conservation (DNRC) and operated by the Willow

Creek Water Users Association (WUA). The dam, whose construction was completed in

1938, is 105 feet high and 453 feet long, and has a variable crest elevation (EL) of approximately 4,751.0 feet with a crest width of 28 feet. The watershed drainage area to the dam is estimated to be 155.3 square miles (reference 14). The embankment slopes are 1.5:1 (horizontal [H]:vertical [V]) on the downstream side, the top 10 feet of the upstream side is 2:1, and the remaining upstream slope is 3:1.

The existing spillway, located along the left dam abutment, consists of an uncontrolled

90-foot-wide ogee crest, which transitions to a concrete-lined, 90-foot-wide chute on the upstream side and tapers downstream to a 50-foot-wide chute. The maximum capacity of this spillway at the dam crest (no freeboard) is 8,421 cubic feet per second (cfs)

(reference 14).

WGM Group (WGM) performed a topographic and bathymetric survey. It was determined that there is approximately a 6.5-foot discrepancy between the elevations defined in the

O&M manual and the elevations defined in the WGM survey. Therefore, the elevations from the WGM survey, which are based on the North American Vertical Datum of 1988

(NAVD88), are used in this Alternatives Analysis Report. WGM also completed a storage analysis using the 2023 Topographic and Bathymetric Survey (reference 40). The results of the analysis determined that the dam has a maximum storage capacity with the pool elevation at the dam crest (EL 4,751.0 feet) of 21,287 acre-feet (ac-ft) and 13,073 ac-ft at normal pool elevation at the spillway crest (EL 4,742.5 feet). Originally, per the existing

O&M Manual, the dam was assumed to operate with a total storage capacity of 18,000 ac-ft, which originally assumed the pool elevation at the spillway crest. A review of the

2023 WGM survey identified EL 4,747.9 feet as the elevation required to satisfy the

18,000 ac-ft of storage.

The most recent spillway condition report (reference 39) indicates that the concrete is deteriorating because of weathering. Spalling and delamination areas have also been mapped and categorized during recent inspections by DNRC. A follow-up inspection in

2018 was performed to map unsound concrete, obtain core samples, and map cracking in the concrete. The results of these efforts were documented in a letter to Dam Safety on February 23, 2018, titled Willow Creek Dam Spillway—Operating Permit Conditions

(reference 19). The letter outlined the results of the 2017 and 2018 spillway inspections and the observed conditions of the concrete and joints and recommended a reduced reservoir elevation. Based on this information and in discussions with DNRC, it was decided that the best course of action for the spillway was replacement. The replacement options and conceptual-level design analysis are further described in this report.

HDR Engineering, Inc. (HDR) assessed the two-zoned soil embankment for seepage and stability. Using available geotechnical and geologic data as well as data collected in

2 | February 14, 2025

2024 during a limited exploration program (reference 16), HDR developed mitigation alternatives to stabilize the embankment, bringing the facility to current standards. In addition, HDR developed three embankment alternatives to accommodate a future reservoir pool increase.

1.1 Purpose

DNRC contracted with HDR to develop the evaluation of potential improvements to the spillway and dam embankment. This report summarizes the design criteria and reviewed alternatives, provides a 10 percent conceptual-level design of the recommended alternatives, and supports the identification of construction costs for the spillway and embankment. Potential modifications and assessments of the gatehouse, gates, outlet, and instrumentation are not included in the present report as they were previously provided in separate reports and memos.

1.2 Sedimentation Potential

The storage discrepancy of the 5,000 ac-ft between the original design volume and the

WGM survey is unknown. As such, the potential for storage loss because of sedimentation was approximated utilizing the following steps:

• HDR approximated natural profiles from the surveyed invert of the outlet to the existing channel inverts for Willow Creek and Norwegian Creek at their approximate confluence with the reservoir.

• Cross sections along these profiles were extended using a 20:1 overbank slope to the assumed natural slope at the edges of the reservoirs.

• A surface was created using the assumed channel data and compared it to the

WGM’s topographic and bathymetric surface.

• The volume under WGM’s topobathymetric surface was conservatively assumed to be fill (i.e., sediment).

• The volume of total “sedimentation” was then calculated with a 3D volume surface.

The resulting volume of sediment was determined to be less than 600 ac-ft. Given the order of magnitude difference between the HDR estimate (600 ac-ft) and the approximate 5,000 ac-ft discrepancy noted in the WGM analysis, the storage loss is not likely caused by sedimentation in the reservoir. The design of the spillway and embankment considered the potential of providing the original design volume of 18,000 ac-ft as a design consideration. However, the discrepancy is not likely attributed to sedimentation. As such, future loss of storage over the life of the proposed project was not considered during the design.

2 Spillway Replacement

Because of the undersized capacity and significant deterioration, the existing spillway has been identified for full replacement. The new crest, chute, and terminal structure will

February 14, 2025 | 3 be constructed in approximately the same location as the existing spillway. Preliminary hydraulic calculations were completed to assess the capacity of the proposed spillway alternatives. Results and design considerations are discussed in the following sections.

2.1 Potential Failure Modes

A Simplified Potential Failure Mode Analysis (SPFMA) workshop was performed on

October 26, 2022, via online conference in which Potential Failure Modes (PFMs) for

Willow Creek Dam and its appurtenant structures were identified, developed, and evaluated. The PFMs pertaining to the spillway are summarized by category in

Table 2-1. Based on the results of the SPFMA, existing condition of the spillway, and discussions with DNRC Dam Safety staff, the decision was made to pursue a full replacement of the spillway rather than a rehabilitation or overlay. Hence, all spillway alternatives consist of a full replacement.

Table 2-1. Summary of PFMs developed during 2022 Simplified PFMA

PFM ID PFM Description Loading

Condition

Category I

PFM-S2-H Breach of the spillway crest and uncontrolled release of the reservoir. Hydrologic

PFM-S7-N,

PFM-S7-H

Cascade failure of panels. Not clear if this will fail more than just the spillway based on rock foundation under spillway.

Normal

Hydrologic

PFM-S8-H Removal of the spillway slab exposes the foundation and leads to the failure of additional slabs, walls, and surrounding area. Joints and foliation planes in the rock beneath the spillway may cause failure of foundation materials. Failures may progress upslope and some portion of the reservoir contents may be released.

Hydrologic

PFM-S10-H Cascade failure of panel. Not clear if this will fail more than just the spillway based on rock foundation under spillway.

Hydrologic

PFM-S11-N,

PFM-S11-H

Removal of the spillway slab exposes the foundation and leads to the failure of additional slabs, walls, and surrounding area. Joints and foliation planes in the rock beneath the spillway may cause failure of foundation materials. Failures may progress upslope and some portion of the reservoir contents may be released.

Normal

Hydrologic

Seismic

Category II/III

PFM-S3-H Breach of the spillway crest and uncontrolled release of the reservoir Hydrologic

PFM-S6-H Removal of the spillway slab exposes the foundation and leads to the failure of additional slabs, walls, and surrounding area. Joints and foliation planes in the rock beneath the spillway may cause failure of foundation materials. Failures may progress upslope/across slope and some portion of the reservoir contents may be released.

Hydrologic

PFM-S9-H Erosion leads to failure of the lower portion of the spillway. The full progression of the failure is unknown due to inadequate geologic information. Structural failure of spillway components leads to evacuation of downstream communities.

Hydrologic

Category IV

PFM-S-13-S Spillway walls fail during an earthquake event result in inability to pass flows until repaired

Seismic

4 | February 14, 2025

2.2 Hydrology

The Inflow Design Flood Hydrology Report (reference 14) completed by HDR as part of this project outlines the hydrologic analyses to determine the Inflow Design Flood (IDF) that the Willow Creek Dam spillway will need to convey. The report determined that the peak inflows into the reservoir for the IDF and Probable Maximum Flood (PMF) were

19,328 cfs and 41,743 cfs, respectively. The IDF used a loss of life (LOL) of 18, which was determined in the Hydraulic and LOL Report completed by HDR as part of this project (reference 13). The hydrologic report also analyzed the flood routing through the reservoir and spillway. For the analysis, the 60-inch-diameter concrete primary outlet structure was conservatively assumed to be closed, and the initial water surface elevation (WSEL) was set to the existing spillway crest (EL 4,742.5 feet). Based on the analysis, the existing dam is overtopped in the IDF and PMF events. Peak discharge at the dam and after attenuation is 14,805 cfs and 38,696 cfs for the IDF and PMF, respectively. Hence the proposed spillway will need to be designed to convey the IDF of

14,805 cfs.

2.3 Freeboard

Freeboard identifies the height above the normal or flood reservoir pool elevation needed to protect the embankment from overtopping because of wave action. The freeboard requirements were then used to develop initial crest elevations for multiple spillway configurations. The freeboard requirements were assessed using the design criteria and methods in the United States Bureau of Reclamation’s (USBR’s) Design Standards 13

Embankment Dams, Chapter 6: Freeboard (DS-13(6)) (reference 35). DS-13(6) outlines the methodology for determining the dam crest elevation to prevent wave overtopping.

Table 2-2 summarizes the guidance from DS-13(6).

Table 2-2. Freeboard calculations methodology

Type of freeboard Approach to freeboard analysisa

Minimum

Select a design crest elevation the higher of:

MWS + 3 ft MWS + run-up and setup from a wind velocity exceeded 10% of the time

Normal NWS + run-up and setup from a 100-mph wind velocity

Checks

Design crest elevation is adequate for run-up and setup during the IDF when reservoir is within 2 ft of the MWS

Design crest elevation is adequate for run-up and setup during the IDF when reservoir is within 4 ft of the MWS a This table is a reproduction of Table 6.1.4-1 from USBR Design Standards 13, Chapter 6.

b MWS = maximum water surface, NWS = normal water surface.

2.3.1 Normal Freeboard

Normal freeboard corresponds to the freeboard to protect the dam from overtopping should waves build up from a 100-mile per hour (mph) wind while the reservoir is operating normally. Normal freeboard was first checked based on the guidance provided in DS-13(6). The upstream slope of the embankment face varies. The top 10 feet is at a

February 14, 2025 | 5

2:1 (H:V) slope while the remaining upstream embankment is a 3:1 slope. Reservoir water levels have no meaningful effect on the wind-wave growth analysis but would control the ultimate height of protection required because of run-up. The fetch length was defined as 1,856 feet. Using the previous variables described, the run-up and setup from a 100-mph wind velocity was determined to be 2.5 feet. The calculations for the wind velocities and wave heights are presented in Appendix A.

2.3.2 Minimum Freeboard

The minimum freeboard refers to the height added to the peak reservoir WSEL during the IDF. DS-13(6) recommends using 3 feet above the maximum reservoir water surface or the sum of the wave run-up and setup from a wind velocity exceeded 10 percent of the time. The wind-wave analysis presented in Appendix A showed that the wave run-up and setup from a wind velocity exceeded 10 percent of the time was less than 3 feet. As such, for each spillway scenario the minimum freeboard was defined to be 3 feet.

Appendix A includes the memorandum documenting the results and assumptions used for the wind-wave analysis.

2.3.3 Freeboard Checks

Freeboard checks are additional measures to verify that the crest elevation is adequate for wave run-up and setup during the durations of the IDF when the reservoir is within 2 feet and within 4 feet of the maximum reservoir WSEL. The inverse of these durations can be assumed to be the hourly exceedance probability of the largest wind event that could be expected while the reservoir is within 2 feet and within 4 feet of the MRWS. As noted in Section 2.3.2, the computed minimum freeboard was found to be 3 feet based on the wave run-up and setup from a wind velocity exceeded 10 percent of the time. For the scenario when the reservoir elevation is within 2 feet of the reservoir maximum water surface (MWS) the exceedance probability is approximately 10 percent, the wave run-up and setup height for this was determined to be less than 5 feet. As such, the 3-foot minimum freeboard can be considered adequate to protect the dam. For the scenario when the reservoir elevation is within 7 feet of the reservoir MWS the exceedance probability was determined to be 5 percent. Since the 1 percent wave run-up and setup height was determined to be less than 7 feet, the 3-foot minimum freeboard can be considered adequate to protect the dam from wind events. Therefore, the spillway and embankment designs will incorporate a minimum freeboard of 3 feet. Table 2-3 presents the resulting freeboard depths as they correspond to the methodology outlined in

Table 2-2.

6 | February 14, 2025

Table 2-3. Freeboard calculations methodology applied to Willow Creek Dam

Type of freeboard Approach to freeboard analysisa

Minimum Select a design crest elevation the higher of:

MWSa + 3 ft MWSa + 1.8 ft

Normal NWS + 2.5 ft

Checks

1.9 ft

2.5 ftb a MWS = maximum water surface, NWS = normal water surface.

b 1% exceedance wave run-up and setup height.

2.4 Spillway Crest

The following four crest configurations were evaluated to determine the best potential alternatives to carry forward for analysis:

1. Ogee weir crest

2. Duckbill weir crest

3. Linear labyrinth weir crest

4. Arced labyrinth weir crest

The following three elevation and spatial configurations were assessed for the four crest geometries listed above. It should be noted that these designations are only applicable to the spillway alternatives and not the embankment alternatives.

• “A” alternatives use the existing dam and spillway crest elevations and are located approximately at the same location as the existing crest.

• “B” alternatives include the potential for a present or future raise of the spillway crest to EL 4,747.90 feet and are located at approximately the same location of the existing crest. This elevation (4,747.90 feet) allows for a dam storage volume of

18,000 ac-ft.

• “C” alternatives include the spillway crest raise and include a horizontal shift of the existing crest upstream into the approach channel.

Both “B” and “C” alternatives include dam embankment raises that satisfy the freeboard requirements outlined in the previous section. The initial geometry for each alternative was developed to pass the peak IDF discharge. The ogee crest and duckbill crest alternatives used guidance outlined in USBR’s Design of Small Dams (reference 34).

The sizing calculations for the linear labyrinth weir alternatives were completed using

Brian Crookston’s Labyrinth Weirs PhD dissertation (reference 8). The sizing calculations for the arced labyrinth weirs were completing using the aforementioned dissertation and

Nathan Christensen’s Flow Characteristics of Arced Labyrinth Weirs (reference 6). To maximize efficiency for the labyrinth weir configurations quarter round crest shapes were assumed for the linear and arced labyrinth configurations. Aeration devices were not

February 14, 2025 | 7 considered with the design. Given the feasibility level of this study there is limited data regarding air entrainment and cavitation during spillway operations. These should be investigated during the final design.

Table 2-4 below lists a summary of the initial alternatives considered along with the alternatives selected for further development. The main selection criterion was the conformance with existing site topography. Rating curves were developed for the alternatives selected for further analysis. These rating curves were then input into the

Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) model developed as part of the Inflow Design Flood Hydrology (reference 14) to determine a more precise peak WSEL and discharge during the IDF.

A temporary linear labyrinth alternative was analyzed with the intent of allowing for a reservoir pool raise in the future, thereby allowing for right-of-way and permitting to be completed independent of the dam improvements. Spillway Alternative 3B-2 would consist of the same embankment raise and the same horizontal weir configuration as

Alternative 3B-1. However, Alternative 3B-2 would have a spillway crest elevation of

4,742.50 feet to align with the original operating reservoir level. Once land acquisition and permitting have been completed, the remaining 5.40 feet of the weir crest would be constructed to achieve a storage allocation of 18,000 ac-ft.

Table 2-5 below shows a summary of the resulting spillway crest alternatives along with factors for consideration for each alternative. The sizing calculations for the spillway crest alternatives are included in Appendix B. The HEC-HMS outputs are included in Appendix

C. Preliminary layouts are provided in Appendix H. Given the improved hydraulic performance and conformance with site topography when compared with the other reviewed alternatives, Alternative 3B-2 is recommended for future consideration.

8 | February 14, 2025

Table 2-4. Spillway crest alternatives

Alternative Approximate crest length (ft) Carried forward Reason for dismissal

1: Ogee crest

A: Existing elevations (weir and crest)

200 No

• Approximately 110 ft extension/excavation either into the hillside or out into spillway approach channel

• Required regrading/reconfiguration of the spillway chute and approach channel to align with the extended length and maintain flow efficiency

B: Grade raisea 90 Yes N/A

C: Grade raise and shifta 150 Yes N/A

2: Duckbill

A: Existing elevations 150 (normal to flow) 900 (total length)

No

• Significant extension into the approach channel

• Possible efficiency loss at the edges of the crest

• Required regrading/reconfiguration of the spillway chute and approach channel to align with the extended length and maintain flow efficiency

B: Grade raise 90 (normal to flow) 450 (total length)

No

• Significant extension into the approach channel

• Possible efficiency loss at the edges of the crest

• Requires regrading/reconfiguration of the approach channel to align with the extended length and maintain flow efficiency

C: Grade raise and shift 150 (normal to flow) 400 (total length)

No

• Longitudinal shift into approach

• Likely to require significant regrading in the approach and abutment to account for weirs extension into the reservoir

3: Linear labyrinth

A: Existing elevationsa 165 (normal to flow) 1150 (total length)

Yes N/A

B (1, 2): Grade raisea 90 (normal to flow) 600 (total length)

Yes N/A

C: Grade raise and shifta 150 (normal to flow) 400 (total length)

Yes N/A

4: Arced labyrinth

A: Existing elevations 320 (normal to flow) 1,700 (total length)

No

• Significant extension into the approach channel

• Total crest length increases significantly

• Possible efficiency loss at the edges of the crest

B: Grade raise 90 (normal to flow) 494 (total length)

No

• Significant extension into the approach channel

• Total crest length increases significantly

• Possible efficiency loss at the edges of the crest

C: Grade raise and shifta 150 (normal to flow) 510 (total length)

Yes N/A a Alternative considered for further design.

February 14, 2025 | 9

Table 2-5. Spillway crest alternatives summary

Alternative

Spillway crest elevation

(ft NAVD88)

IDF peak

WSEL

(ft NAVD88)

Dam crest elevation

(ft NAVD88)

IDF peak outflow (cfs)

Factors for consideration

1B: Ogee crest grade raise

4,747.90 4,756.54 4,759.54 14,119

• Does not require width extension into hillside

• Dam raise: 8.54 ft

1C: Ogee crest grade raise and shift

4,747.90 4,754.61 4,757.61 16,432

• Longitudinal shift into approach (i.e., reservoir)

• Some grading required in hillside

• Dam raise: 6.61 ft

3A: Linear labyrinth existing elevations

4,742.50 4,748.62 4,751.62 16,968

• Required regrading/reconfiguration of the spillway chute and approach channel to maintain flow efficiency

• Requires 80 ft extension into the hillside

3B-1: Linear labyrinth grade raise

4,747.90 4,755.21 4,758.21 13,812

• Requires regrading/reconfiguration of the approach channel to maintain flow efficiency

• Does not require width extension into hillside

• Dam raise: 7.21 ft

3B-2: Linear labyrinth grade raise

4,742.50 4,751.03 4,758.21 15,435

• Requires regrading/reconfiguration of the approach channel to maintain flow efficiency

• Does not require width extension into hillside

• Dam raise: 7.21 ft

3C: Linear labyrinth grade raise and shift

4,747.90 4,753.95 4,756.95 15,734

• Longitudinal shift into approach (i.e., reservoir)

• Some grading required in hillside

• Dam raise: 5.95 ft

4C: Arced labyrinth grade raise and shift

4,747.90 4,753.15 4,756.15 16,405

• Longitudinal shift into approach (i.e., reservoir)

• Some grading required in hillside

• Dam raise: 5.15 ft

10 | February 14, 2025

2.5 Spillway Chute

Two spillway width configurations were assessed. The first configuration, herein referred to as the tapered configuration, transitioned the spillway chute width from 90 feet at the upstream end of the spillway to 50 feet in width approximately halfway down the embankment slope. A convergence angle of 11.9 degrees was calculated for this configuration using guidance provided in the USBR’s Design of Small Dams. The second configuration, herein referred to as the straight configuration, consisted of a consistent chute width of 90 feet.

Two spillway profile configurations were assessed in conjunction with the spillway width configurations mentioned above. The first spillway profile configuration followed the existing spillway profile, which includes the vertical grade breaks along the profile. The second spillway profile consisted of a uniform slope along the spillway chute.

Flow through the chute configurations was evaluated using a one-dimensional

Hydrologic Engineering Center River Analysis System (HEC-RAS) model. The model was truncated downstream of the spillway crest as it was determined that the tailwater of the weir would not impact the initial chute sizing calculations. The flow rate selected was

15,000 cfs, as it was the approximate mean of the IDF discharges from the spillway weir analysis. The model incorporated topographic data collected by the WGM Group.

Printouts of the HEC-RAS model are located in Appendix D. The analysis showed that all four configurations would convey the design discharge (IDF). Given the additional costs associated with expanding the footprint of the spillway, it is recommended that the tapered spillway configuration be carried forward for further design. Additionally, using the existing spillway profile is recommended for further consideration as there is likely ample cost associated with the additional excavation of rock associated with the single profile alternative.

2.5.1 Spillway Wall Heights

The USBR Design of Small Dams was used to provide preliminary wall heights for the different alternatives. The wall heights were used to inform the conceptual-level construction costs for each spillway chute alternative. The required freeboard outlined in

Section 9.19 of USBR’s Design of Small Dams was calculated and then added to the flow depth to determine the spillway chute minimum wall height. The spillway chute wall height for the straight configuration was determined to be 8.0 feet, and the spillway chute wall height for the tapered configuration was determined to be 10.0 feet. These calculations are included in Appendix E.

2.5.2 Spillway Chute Alternatives Summary

Table 2-6 presents a summary of the spillway chute alternatives.

February 14, 2025 | 11

Table 2-6. Spillway chute alternatives

Alternative Chute width(ft) Chute wall height(ft)

Factors for consideration

Straight

Existing profile

90 8.0

• Wall height is higher than the tapered configurations

• Requires extension into hillside and embankment

• Lost efficiency caused by water surface separation at grade break

• Would require a transition curve to be installed

Uniform slope

90 8.0

• Wall height is higher than tapered configurations

• Reduced risk of water surface separation from chute surface

• Requires extension into hillside and embankment

• Requires additional excavation of rock materials and increased costs

Tapered

Existing profile

90 upper 50 lower

10.0

• Follows existing extents; limited impact on lateral extents

• Wall heights lower than the straight configurations

• Lost efficiency caused by water surface separation at grade break

• Would require a transition curve to be installed

Uniform slope

90 upper 50 lower

10.0

• Follows existing extents; limited impact on lateral extents

• Reduced risk of water surface separation from chute surface

• Requires additional excavation of rock materials and increased costs

• Wall heights lower than the straight configurations

2.6 Terminal Structure

Two terminal structure types were assessed for use at the downstream end of the spillway chute. The terminal structures are designed to dissipate energy and transfer flows into the natural stream channel. The first configuration analyzed was a USBR stilling basin. The USBR Design of Small Dams was used to select the stilling basin type.

The results from the HEC-RAS model developed for the spillway chute (Section 2.5) were used to define the type of stilling basin. The terminal structure calculations only used the hydraulic results for the tapered spillway configuration with the existing profile as it is recommended for further analysis. The HEC-RAS model produced Froude numbers (Fr) ranging from 5.31 to 8.00 and velocities (Vt) ranging from 60.8 to 82.0 feet per second from the chute analysis. Section 9.21 of the USBR Design of Small Dams lists the following criteria for identifying the appropriate basin type:

• If Fr < 1.7 use a Type I stilling basin

• If 2.5 < Fr < 4.5 use a Type IV stilling basin

• If 4.5 < Fr and Vt > 50 feet per second use a Type II stilling basin

• If 4.5 < Fr and Vt < 50 feet per second use a Type III stilling basin

As such, a USBR Type II stilling basin was determined to be appropriate. The length of the stilling basin for the tapered chute existing profile configuration was determined to be

140 feet. Based on calculated length, it was determined that the stilling basin alternative would extend into the natural channel and adjacent hillside, which would increase

12 | February 14, 2025 excavation costs. In addition to the structure dimensions, special consideration needs to be given to the tailwater elevation of the stilling basin. The tailwater elevation dictates the location of the hydraulic jump and subsequent energy dissipation. The required tailwater depth is equal to approximately 30 feet which would need to extend into the Willow

Creek channel. If the minimum tailwater conditions are not met, the stilling basins will not operate as intended. Based on the results of the HEC-RAS analysis, these tailwater elevations would not be met. As such, a USBR Type II stilling basin is not considered for further design. Full calculations for the USBR Type II stilling basin are included in

Appendix E.

The second terminal structure assessed was a flip bucket. The flip bucket geometry was developed in accordance with the United States Army Corps of Engineers (USACE)

Hydraulic Design Criteria Volume I (Reference 25). The flip bucket dimensions were determined for the tapered chute using the existing profile configuration. The calculation used the velocity head (85.3 ft) at the bottom of the chute from the aforementioned HEC-

RAS model of the tapered spillway and a preferred horizontal landing distance. The distance from the flip bucket to the channel (55 feet) was used as the horizontal landing distance, as this would place the jet in the channel. The resultant 10-degree bucket lip angle was determined to best fit within the constraints of the site while also meeting the hydraulic criteria. The proposed lip elevation was determined to be 4651.53 feet. A conceptual design profile for the flip bucket is shown in Figure 2-1. The proposed lip elevation is 0.03 ft higher than the existing lip elevation. The bucket lip angle is less dramatic than the existing flip bucket, which results in the proposed design being approximately 7 feet longer than the existing. Of note, there will likely be a need for a cutoff wall and riprap armoring in the outlet channel. These design measures will mitigate erosion and assist in protecting the flip bucket, spillway structure, and outlet channel.

February 14, 2025 | 13

Figure 2-1. Conceptual flip bucket design

2.6.1 Terminal Structure Alternatives Summary

Table 2-7 presents a summary of the terminal structure alternatives.

Table 2-7. Terminal structure alternatives

Alternative Appx. Structure length (ft) Factors for consideration

USBR Type II stilling basin 140

• Dissipates energy prior to entering the channel

• Significantly increases the concrete quantities compared to the flip bucket alternative

• Walls and floor encroach into the channel;

does not fit the existing footprint of the spillway

Flip bucket 30

• Fits within the existing footprint of the spillway

• Elevations are similar to existing therefore requires less site work than the stilling basin alternative

• Less concrete than the stilling basin alternative

• Energy is dissipated in channel; erosion could be of concern

14 | February 14, 2025

3 Structural Considerations

This section presents structural considerations for the spillway, including the spillway crest, spillway chute, flip bucket, and defensive design measures. These considerations were developed to largely inform the conceptual plan development and the cost estimate.

3.1 Spillway Crest

Preliminary strength and stability calculations were performed for the spillway crest to determine approximate wall sizing. Several alternatives are proposed for the spillway crest. The design of the labyrinth weir (Alternative 3B-2) was performed for the critical case per the guidelines listed in the USACE Engineer Manual (EM) 1110-2-2104, Strength Design for Reinforced Concrete Hydraulic Structures (reference 27). The following load cases were selected to evaluate the stability:

• Usual: normal operating condition

• Extreme: flood condition (IDF)

• Extreme: coincident pool and maximum design earthquake (MDE)

Based on the 10 percent design, the stem wall of the labyrinth weir is designed to be 1.5 feet thick with reinforcing steel in both faces. The crest was designed to account for the future design condition with a crest height of 9.4 feet.

3.2 Spillway Chute

The spillway chute was designed to meet the stability and strength criteria. Stability analyses of the concrete structures were performed based on USACE EM 1110-2-2100, Stability Analysis of Concrete Structures (reference 26). The main load on the chute training walls is the backfill behind the wall. The following load cases were selected to design the walls:

• Usual: channel empty

• Extreme: water in channel, flood condition (PMF)

• Extreme: coincident pool and MCE

Overturning stability analyses were performed for the load cases stated based on these walls considered as “normal” structures per USACE guidelines. The walls were designed to resist the equivalent fluid pressure from the backfill. For LC1 and LC3, the water table was assumed to be below the footing. For LC2, the water level is taken at 4 feet above the footing. Overturning was the governing factor for setting the width of the footing. The retaining walls rely on the chute slab to brace against sliding.

The strength design of the wall was performed following EM 1110-2-2104, Strength

Design for Reinforced Concrete Hydraulic Structures (reference 27). Based on the 10 percent design, the wall stem thickness tapers from 1.5 feet at the top to 2.0 feet at the bottom of the stem and is placed on a 2-foot-deep footing.

February 14, 2025 | 15

The chute slab is also designed as a 2-foot-thick slab. During the PMF case, rapid drawdown can occur, resulting in uplift pressures acting on the bottom of the slab after the spillway flow has stopped, and the phreatic surface not dropping below the slab. In that case, the uplift pressure is greater than the self-weight of the slab. Therefore, anchors are used to provide resistance to uplift. Rock anchors were designed based on

EM 1110-1-2908, Rock Foundations (reference 31). To counter uplift, rock anchors are installed in the chute slab at 10-foot spacing in each direction. The orientation of the anchors is normal to the slab. The anchors installed normal to the slab are used to mainly resist uplift and buckling of the slab when the slab is used to brace the sidewall lateral loads. Typical control joints at a spacing of 35 feet is also provided in the spillway

(reference 36), as shown in Figure 3-1.

The design of steel reinforcement for the chute walls and slabs was performed in accordance with American Concrete Institute (ACI) 350-20, Code Requirement for

Environmental Engineering Concrete Structures and Commentary (reference 1) and EM

1110-2-2104, Strength Design for Reinforced Concrete Hydraulic Structures (reference

27). Other important code-related considerations include, but are not limited to, the environmental durability factors associated with size and distribution of flexural reinforcement, and minimum temperature and shrinkage steel reinforcement ratios based on the spacing of contraction joints. Reinforcing steel was designed based on conformance with American Society of Testing and Materials (ASTM A615), Grade 60.

Figure 3-1 below shows typical sections of the spillway chute and Figure 3-2 shows a typical control joint.

Figure 3-1. Typical sections of the spillway chute (dimensions are in feet)

16 | February 14, 2025

Figure 3-2. Typical control joint (dimensions are in feet)

3.3 Flip Bucket

The flip bucket starts at EL 4,654.26 (Station [STA] 3+10.96) and ends at EL 4,651.53

(STA 3+37.56) at the flip bucket lip; the invert of the flip bucket is at EL 4,649.53 (2 feet below the lip elevation). The flip bucket is 24.75 feet long and has a radius of 42.78 feet.

The flip bucket foundation consists of a reinforced concrete slab resting directly on rock.

The flip bucket foundation slab extends into the soil for 10 feet (terminating at EL

4,642.26). Figure 3-3 shows the flip bucket layout.

Figure 3-3. Flip bucket layout (dimensions are in feet)

February 14, 2025 | 17

3.4 Defensive Design Measures

Adequate defensive design measures will mitigate adverse hydraulic conditions (e.g., stagnation pressure potential), debris, and ice loads associated with frost heave and minimize crack widths. Defensive design measures are essential in spillways to prevent potential failure modes from initiation or full development. Typical defensive measures for mitigating the PFMs were referenced from USBR’s Design Standards 14: Appurtenant

Structures for Dams (Spillways and Outlet Works) (reference 36) and include the following:

• Waterstops: to obstruct water flow path through slab joints

• Transverse cutoff: to prevent vertical offsets at transverse joints and limit the water path from inside the chute to the foundation

• Longitudinal reinforcement across chute slab joints: to minimize crack width at joints

• Filtered underdrains: to relieve uplift pressures and prevent movement of foundation material into the drainage system and initiation of foundation erosion

• Insulation: to protect the drainage system from freezing and prevent frost heave in susceptible soils

4 Geotechnical Design

This section presents geotechnical design for the project, including the spillway foundation and embankment buttress.

4.1 Spillway Foundation

The spillway foundation was investigated in 2015 by DNRC (reference 39) and most recently by HDR in 2024 (reference 16). The current spillway appears to be generally supported by weathered gneiss and small to moderate areas of fill materials. The fill materials are identified to be silty sand and clayey sand soils. Identifying the materials under the current spillway has resulted in identifying the risk of slab jacking and subsequent failure of the spillway slabs. Therefore, the proposed improvements include a robust approach to a new spillway foundation.

The proposed improvements presented herein should be founded on slightly weathered gneiss bedrock with the incorporation of rock anchors where necessary to resist uplift.

Following anchor design methods presented in USACE EM 1110-1-2908, Rock

Foundations (reference 31) and the Federal Highway Administration’s (FHWA’s) Ground

Anchors and Anchored Systems (reference 10), HDR calculated allowable anchor capacities. Anchor calculations are based on a minimum factor of safety of 3 and cone-shaped failure mode of the rock and unconfined compression tests of sampled rock.

18 | February 14, 2025

4.2 Embankment Buttress

HDR’s…