Attachment J - Design Summary Report - Comanche Dam Modifications.pdf

DESIGN SUMMARY REPORT

FOR MODIFICATIONS TO

COMANCHE DAM

WICHITA MOUNTAINS WILDLIFE REFUGE

COMANCHE COUNTY, OKLAHOMA

COMANCHE DAM NID: OK30003

Prepared for

U.S. FISH AND WILDLIFE SERVICE

DAM, BRIDGE AND SEISMIC SAFETY BRANCH

INFRASTRUCTURE MANAGEMENT DIVISION

FALLS CHURCH, VIRGINIA

Prepared by

W. W. Wheeler & Associates, Inc.

3700 South Inca Street

Englewood, Colorado 80110

In Association with

Schnabel Engineering 3 Dickinson Drive, Suite 200

Chadds Ford, Pennsylvania 19317

100-percent Submittal July 1, 2020

Page i

EXECUTIVE SUMMARY

W. W. Wheeler & Associates, Inc. (Wheeler) in association with Schnabel Engineering

(Schnabel) prepared the designs for the Modification of Comanche Dam that are documented in this Design Summary Report. This report and its appendices document key design criteria, design analyses, construction drawings, technical specifications, construction considerations, an opinion of probable construction cost, and the construction schedule for the Modifications to Comanche Dam. The dam is owned and operated by U.S. Fish and Wildlife Service (FWS).

When this report was prepared, Comanche Dam was a classified as a high-hazard potential dam by the FWS. The dam impounds Comanche Lake that provides water for wildlife habitat, fire protection, and a water supply to fill downstream lakes on the Wichita Mountains Wildlife

Refuge (Refuge). The project is located approximately 12 miles north of the Town of Cache on the Refuge in Comanche County, Oklahoma.

Comanche Dam is located on Deer Creek in the headwaters of West Cache Creek. The dam was constructed in series with Grama Dam, located approximately 0.9 miles upstream of

Comanche Dam. Grama Dam is an embankment dam and Comanche Dam is a concrete gravity dam with two auxiliary spillways located at the left abutment and center sections of the dam. The dam also includes a combined drop-inlet service spillway and low-level outlet works located right of the center spillway. The maximum normal pool elevation in Comanche Lake is 1811.11 with an active storage capacity of about 45 acre-feet.

The FWS has determined that after the modifications to Comanche Dam are completed, the dam can be reclassified as a low-hazard potential dam based on previous analyses completed for the dam (Wheeler, 2018). The Modifications to Comanche Dam were designed to reduce the dam failure risk by stabilizing the dam with a downstream Roller-Compacted Concrete

(RCC) buttress to meet adequate factors of safety during usual, unusual (hydrologic), and extreme (seismic) loading conditions.

Note 1 – All elevations provided in this report are based on the National Geodetic Vertical Datum 1929

(NGVD29).

Note 1 – All elevations provided in this report are based on the National Geodetic Vertical Datum 1929

(NGVD29).

Page ii

DESIGN SUMMARY REPORT

TABLE OF CONTENTS

EXECUTIVE SUMMARY ......................................................................................................... i

TABLE OF CONTENTS ......................................................................................................... ii

1. INTRODUCTION

1.1. Project Objective

1.2. Authorization

1.3. Scope of Work

1.4. Project Team

2. BACKGROUND

2.1. Project Description

2.2. Summary of Dam Safety Issues

2.3. Project History

3. MAJOR PROJECT FEATURES

3.1. Comanche Dam Modifications

4. DESIGN CRITERIA

4.1. Key Design Criteria

5. HYDRAULIC DESIGN

5.1. Hazard Classification Summary

5.2. Inflow Design Flood and Reservoir Routing

5.3. Capacity Curves

5.4. Center Auxiliary Spillway Design Considerations

5.5. Dam Overtopping Analysis

5.6. Reservoir Drawdown

6. GEOTECHNICAL DATA

6.1. Site Geology and Subsurface Conditions

6.2. Geotechnical Data Report

6.3. Seismicity

7. COMANCHE DAM STRUCTURAL DESIGN

7.1. Comanche Dam RCC Stability Analysis

8. CONSTRUCTION CONSIDERATIONS

8.1. Project Permitting

Page iii

8.2. Construction Bidding and Administration

8.3. Temporary Reservoir Control During Construction

8.4. Construction Access and Staging Areas

8.5. Removal of Existing Features

8.6. Access Roads

8.7. Construction Schedule

8.8. Mix Design

8.9

9. RECOMMENDATIONS

10. REFERENCES

LIST OF FIGURES PG

Figure 1 Expected Range of Center Spillway Trajectories 14

LIST OF TABLES PG

Table 5.2.1 Comanche Dam Reservoir Routing Results 10

Table 5.2.2 Grama Dam Reservoir Routing Results 11

Table 5.2.3 Comanche Dam Construction Reservoir Routing Results 11

Table 7.1.1 Summary of Minimum Required Factors of Safety for Sliding 19

Table 7.1.2 Headwater and Tailwater Analysis 19

LIST OF APPENDICES

Appendix A - Design Criteria Memorandum

Appendix B - Design Drawings

Appendix C - Technical Specifications

Appendix D - Engineering Calculations

Appendix E - Inflow Hydrology Technical Memorandums

Appendix F – Schnabel Structural Design Memorandums

Appendix G – Geotechnical Data Report

Appendix I – Hazard Memorandums

1. INTRODUCTION

1.1. Project Objective

The objective of this Design Summary Report is to document key design analyses and construction considerations prepared for the Modifications to Comanche Dam. The

Comanche Dam modifications were designed to reduce the risk and hazard potential of the dam.

1.2. Authorization

The evaluations documented in this report were authorized as part of Task Order No.

140F05190014 under Contract No. F15C00157 between W. W. Wheeler and Associates, Inc. (Wheeler) and the U.S. Fish and Wildlife Service (FWS).

1.3. Scope of Work

Key elements of Wheeler’s scope of work for this Title III design Task Order were as follows:

1. Element A- Engineering Analysis: Perform and document engineering analysis, assumptions, calculations, and production of the final design package.

2. Element B-- Permitting Assistance: Identify federal, state, and local permits required for construction of the final design and subsequently provide assistance in preparing and submitting permit applications on behalf of the FWS.

3. Element C- Drawings and Technical Specifications: Prepare design drawings and technical specifications for the final design.

4.

5. Element E- Design Summary Report: Prepare this Design Summary Report to summarize the final design including design criteria, basis of design and analysis, calculations, and construction considerations.

6. Element F- Meetings and Project Management: Coordinate and manage the

Project including project tracking, invoicing, reporting, and design review webinars.

1.4. Project Team

Wheeler was the lead firm responsible for completing this design. Schnabel Engineering

(Schnabel) was a key subconsultant on the Wheeler Team and was responsible for performing seismic analysis, structural analysis, preparing the construction drawings, and providing an independent review of the technical specifications and this design summary report.

. The following key personnel were part of

Wheeler’s design team:

• Steve Jamieson, P.E. – Project Manager and Engineer-in-Charge (Wheeler)

• Kit Choi, Ph.D., P.E. -Technical Reviewer (Wheeler)

• Randy Bass, P.E. – Technical Reviewer (Schnabel)

• Greg Paxson, P.E. – Technical Reviewer (Schnabel)

• Jesse Wullenwaber, P.E. –Project Manager (Schnabel)

• John Treacy, P.E. – Project Engineer (Wheeler)

• Christine Mugele, P.E. – Assistant Project Engineer and Hydrologist (Wheeler)

• John Harrison, P.E. – Structural Engineer (Schnabel)

• Emily Gibson, P.E. – Seismologist and Assistant Project Engineer (Schnabel)

• Todd Lewis, P.E. – Hydraulic Engineer (Wheeler)

The Wheeler team gratefully acknowledges the valuable review comments provided by

Brad Iarossi, P.E. and Dave Hibbs of the FWS Dam, Bridge and Seismic Safety Branch of the Infrastructure Management Division. We also greatly appreciate the valuable input and insight provided by David Farmer, Refuge Manager, Wichita Mountains Wildlife

Refuge (Refuge) and the Refuge staff.

2. BACKGROUND

2.1. Project Description

Comanche Dam was constructed for the U.S. Department of Agriculture, Bureau of

Biological Survey (now the U.S. Forest Service) by the Civilian Conservation Corps (CCC) in 1934. Comanche Dam is one of two dams located in series along Deer Creek on the

Wichita Mountains Wildlife Refuge in Oklahoma. Grama Dam is an embankment dam located 0.9 miles upstream of Comanche Dam. Approximately 80 percent of the drainage basin above Comanche Dam is controlled by Grama Dam. Comanche Dam is a concrete gravity dam. Water in Comanche Lake is considered to be a valuable asset to the Refuge for providing water for fire protection, habitat for wildlife, and for replenishing downstream lakes on the Refuge.

At the time this report was prepared, Comanche Dam was classified as a small size, high hazard-potential dam with an active storage capacity of approximately 45 acre-feet at the maximum normal pool elevation of 1811.11. Although the maximum normal pool at

Comanche Dam is reported at Elevation 1811.1 (FWS, 2016), the service spillway crest is sloped and ranges in elevation from 1811.1 to 1811.6. Hydraulic and structural evaluations conducted for this Project assumed a maximum normal pool elevation of

1811.6 to account for minor debris blockage of the service spillway inlet trashrack.

Comanche Dam is approximately 39 feet high with a downstream slope of 0.54H:1V

(horizontal: vertical), an upstream slope of 0.073H:1V, and a crest width of 21 inches. The structural height of the dam, from the dam crest to the bottom of the foundation, is reported to be 58 feet (FWS, 2016). The dam has two auxiliary spillways and a combined drop-inlet service spillway and outlet works. The center auxiliary spillway is an ogee weir with a crest at Elevation 1828.4 and the left auxiliary spillway is an ogee weir with a crest also at

Elevation 1828.4. A combined drop-inlet service spillway and outlet works is located right of the center auxiliary spillway section. Service spillway and outlet works flows are conveyed through the dam by a 36-inch-diameter, corrugated metal pipe (CMP) outlet conduit. Outlet works flows are controlled by a 36-inch-square, slide gate mounted on the right side of the service spillway intake tower. The slide gate is normally left in the closed position to keep the normal maximum surface near the service spillway crest elevation.

The maximum reported combined discharge of the three spillways is 1,736 cubic feet per second (cfs) at the top of dam elevation of 1830.5 (FWS, 2016).

2.2. Summary of Dam Safety Issues

When this report was prepared, Comanche Dam was considered to be in FAIR condition.

The dam was assigned a Dam Safety Priority Ranking (DSPR) of 3, which is considered

Moderate to High priority. Comanche Dam had two safety deficiencies due to recent stability analyses which concluded that the concrete dam does not meet FWS high hazard-

1 The elevations reported in this document are based on the 1929 National Geodetic Vertical Datum (NGVD29).

potential stability criteria for the unusual (flood) or extreme (seismic) loading conditions

(FWS, 2016). The latest Screening Level Risk Assessment (SLRA) value for Comanche

Dam was 11, due to inadequate factors of safety during unusual flood or extreme seismic loading conditions. The consequence of dam failure in the SLRA was identified as Level

1, which is considered to be minor downstream damage with little to no loss of life expected.

2.3. Project History

After Comanche Dam was constructed in 1934, it was generally operated to the top of the auxiliary spillways. The drop-inlet service spillway intake was not part of the original construction and was included as part of the dam modifications completed in 2000. As part of the federally mandated Phase 1 National Inventory of Dams program, the FWS initiated dam safety evaluations at Comanche Dam in 1982.

In 1988, several dam maintenance issues were completed at Comanche Dam that included:

• Removing and replacing deteriorated concrete and filling concrete joints and cracks;

• Placing shotcrete on the upstream face, crest, and downstream face;

• Replacing the 36-inch-square sluice gate and hardware;

• Removal of the gate on the 12-inch-diameter outlet located 11 feet right of the existing 36-inch-diameter outlet works conduit and sealing the 12-inch-diameter conduit with a concrete plug.

Additional dam safety evaluations and field investigations were conducted at Comanche

Dam in the 1990s. In 1995, six vibrating wire piezometers were installed in the dam foundation as part of subsurface investigations in the dam. Two dam safety deficiencies were identified for the original dam with the normal maximum pool at the auxiliary spillway crests, Elevation 1,828.4. These deficiencies were inadequate tensile strength of the dam concrete under seismic loading conditions and inadequate factors of safety for sliding stability at the concrete/rock interface under the IDF and seismic loading conditions.

The FWS initiated work on feasibility-level dam modification designs at Comanche Dam in 1994 to address dam safety issues at both dams as a multiple-dam system. Using information generated from these feasibility evaluations, GEI Consultants, Inc. (GEI) completed a conceptual design evaluation of both Grama Dam and Comanche Dam

(GEI, 1997). Due to concerns associated with the cost of the 1997 conceptual designs, a Value Engineering (VE) Study was performed by Reclamation (USBR, 1997). The VE study resulted in two passive, structural alternatives to minimize the cost of the dam modifications:

• VE Alternative 1A - Reduce the reservoir pools in both reservoirs.

• VE Alternative 2A - Eliminate the reservoir pools behind both reservoirs.

In 1998, a risk assessment was performed for Grama Dam and Comanche Dam by

Reclamation, with input from the FWS and GEI (USBR, 1998). The risk assessment confirmed that there was a strong justification to take action to reduce the overall risk at both Grama Dam and Comanche Dam. Reclamation’s risk assessment documented that either the 1997 Conceptual Design Alternative A or VE Alternative 1A would reduce the risks to acceptable levels in accordance with Reclamation guidelines at that time. The

1997 Conceptual Design Alternative A involved raising Grama Dam by two feet and lowering Comanche Dam by 16 feet, so that it could be classified as a low hazard-potential dam and was estimated to cost $4.3 million in 1997 dollars. The FWS ultimately selected VE Alternative 1A for implementation.

Modification designs to Comanche Dam were prepared by GEI and the construction of these modifications were completed between 1999 and 2000. The 2000 Comanche Dam modifications included the following:

• Construction of a new reinforced concrete service spillway and outlet works drop inlet intake tower on the upstream face of the dam at the location of the 36-inch-diameter, low-level outlet. The new service spillway crest was constructed at about Elevation 1811 to permanently lower the maximum normal pool in

Comanche Lake.

The Comanche Dam modifications addressed the structural inadequacies associated with extreme seismic loading conditions by permanently lowering the water loads on the upstream face of the dam. The Comanche Dam modifications also provided additional flood storage to store water that could be released if a sunny-day failure of Grama Dam were to occur. The 2000 Comanche Dam modifications were based on an Inflow Design

Flood (IDF) that was equivalent to the 50 percent of the Probable Maximum Flood (PMF) based on the results of an Incremental Damage Assessment. The structural stability analysis of Comanche Dam was based on the approved IDF and a horizontal bedrock acceleration of 0.8g.

Several structural evaluations of Comanche Dam were conducted from 2014 through

2017 based on increased seismic loading or increased unusual loading associated with

100 percent of the PMF (SM&RC, 2014; URS, 2016; Schnabel, 2016; Schnabel 2017).

These structural evaluations were based on required loading conditions for a high hazard-potential dam, which was generally the 10,000-year seismic event and full PMF loading. Each of these structural evaluations assumed slightly different failure conditions at several representative cross-sections. Some of the failure conditions that were modeled included global stability keyed into bedrock with passive resistance; global stability with no passive resistance from a foundation key into bedrock; failure at a lift joint within Comanche Dam; or failure at the concrete dam/rock interface. Regardless, each stability evaluation indicated inadequate factors of safety for extreme and/or unusual loading conditions.

In 2016, Wheeler completed an updated hydrology and hazard classification for seven dams on the Wichita Mountains Wildlife Refuge that included Comanche Dam and Grama

Dam. Recommendations from that study identified Comanche Dam as potential low hazard-potential dam if it was modified to be stable under high hazard-potential dam loadings (Wheeler, 2016). In 2018, Wheeler and Schnabel worked together to complete a Conceptual Modifications Systems evaluation of both Grama Dam and Comanche Dam to reduce the downstream dam safety risk for both dams based on detailed two-dimensional hydraulic modeling of the seven dams previously evaluated (Wheeler, 2018).

The preferred recommendation from this systems analysis approach was to stabilize

Comanche Dam at its current dam height and normal maximum pool and to reduce normal maximum pool in Grama Dam by 4.1 feet. This selected G1-C1 alternative would reduce the hazard classification at Grama Dam and Comanche Dam from high hazard-potential to low hazard-potential, even when future development in the Town of Cache was considered. The G1-C1 system was carried forward into this final design project for both dams. The preferred alternative for stabilizing Comanche Dam for high hazard-potential loading conditions was to use high capacity, post-tensioned anchors to stabilize

Comanche Dam.

The Title II final design was initially based on reducing the maximum normal pool at Grama

Dam by 4.1 feet and stabilizing Comanche Dam with high capacity, post-tensioned anchors. However, the Opinion of Probable Construction Cost (OPCC) for the combination of these two projects at the 30-percent and 65-percent design level indicated that the dam anchoring concept could be very expensive due to access concerns to install the anchors on the very thin Comanche Dam crest. As required by FWS policy for projects over one million dollars, an independent Value Engineering study was completed by Stanley

Consultants in 2019 (Stanley, 2019). After further evaluation of potential construction costs associated with some of the Value Engineering alternatives, the FWS decided to change the final design concept to a buttress design that would be constructed from either structural concrete or Roller Compacted Concrete (RCC). In early February of 2020, the

FWS made the decision to proceed with the design of an RCC buttress that needed to be complete by the end of May 2020. The new RCC buttress concept and tight design schedule required a fast-track design and additional stability and hydraulic evaluations.

The FWS also decided to suspend all work for the modification of Grama Dam due to cost concerns.

Supplemental downstream hazard classification information requested by the FWS indicated that a good case could be made at this time to reclassify Grama Dam as low hazard-potential dam without further modifications at Grama Dam after Comanche Dam was stabilized. This supplemental analysis of the Grama Dam hazard issue would need to be further evaluated by a new FWS hazard classification panel.

3. MAJOR PROJECT FEATURES

3.1. Comanche Dam Modifications

A downstream Roller-Compacted Concrete (RCC) buttress was designed at Comanche

Dam to stabilize the dam under the anticipated usual, unusual (flood), and extreme

(seismic) loads. Key components of the Comanche Dam modifications include:

1. Improve and maintain access roads during construction between Highway 49 and

Comanche Dam to allow adequate construction access.

2. Site demolition at Comanche Dam including existing handrails, upstream face ladder access to the service spillway trashrack, the existing vibrating wire piezometers, the downstream outlet works retaining wall, and the top 2.9 feet of the center auxiliary spillway crest.

3. Existing features at Comanche Dam that are to be protected during construction include the reservoir water level sensor located on the outlet works gate tower; the service spillway inlet structure and outlet works slide gate and operator; and the left auxiliary spillway section.

4. Perform dewatering and water control at Comanche Dam during construction.

5. Excavate to bedrock downstream of Comanche Dam as shown on the drawings.

6. Complete cleaning and preparation of the downstream dam face and foundation.

7. Extend the low-level outlet works conduit through the new RCC buttress by slip-ling a new 36-foot-long, 32-inch-diameter, High Density Polyethylene (HDPE) conduit into the existing 21-foot-long, 36-inch-diameter HDPE conduit. The annular space between the existing 36-inch-diameter and new 32-inch-diameter HDPE outlet pipe will be grouted and the downstream section of the new outlet conduit will be encased in reinforced concrete and extended one foot downstream of the reinforced concrete encasement.

8. Construct the downstream RCC apron using a combination of mass concrete and

RCC. The initial lifts of the RCC will be considered the RCC Test Section.

9. Construct the downstream RCC buttress from the foundation to the crest of

Comanche Dam.

10. Reconstruct the center auxiliary spillway crest.

11. Install new galvanized handrails and access ladders on the non-overflow crest of

Comanche Dam.

4. DESIGN CRITERIA

General design criteria for the Comanche Dam Modifications are summarized below.

Additional information is provided in the Design Criteria Memorandum in Appendix A.

4.1. Key Design Criteria

The Comanche Dam modifications were designed in accordance with the following key design criteria:

1. Comanche Dam was designed in accordance with the FWS dam safety standards as a low hazard-potential, concrete gravity dam (FWS, 2012-2). The exceptions to these standards are that the dam was designed for a 1,000-year seismic event and the spillway was also designed for a 1,000-year flood event.

2. The modified dam design was not required to be approved by the Oklahoma Water Resource Board (OWRB) because the dam is a federally owned facility.

3. The left auxiliary spillway section was designed to be protected during construction and was not designed to be modified by this Project.

4. A new center auxiliary spillway ogee crest was designed to be reconstructed as a reinforced concrete structure on top of the RCC. The new center auxiliary spillway crest length and control elevation were designed to match the length and crest elevation of the existing center auxiliary spillway.

5. No dam foundation grouting, foundation drainage, or internal dam drainage within the existing dam was included in the design analysis because significant seepage through the dam and foundation had not been identified as a previous dam safety deficiency. Monolith drains were included in the design between the existing dam and the new RCC buttress, but this drainage system was not assumed to be functional in the dam stability analysis. As a result, new RCC buttress was designed for full uplift pressures.

6. Galvanized steel handrails were designed to be installed around the two non-overflow sections of the dam and access was provided across the center auxiliary spillway section via ladders installed on the left and right side of the center auxiliary spillway approach section.

7. The design was completed in anticipation of an open bidding process that was scheduled to occur in the second or third quarter of calendar year 2020.

Construction was scheduled to start by November 1, 2020.

8. The specifications allowed the contractor to control the reservoir water surface in Grama Lake and Comanche Lake with the existing outlet works to minimize the flood damage during construction. The reservoir water surface in Comanche Lake and Grama Lake was intended to be maintained between Elevation 1810.1 and

1811.1 and 1859.0 and 1860.0, respectively during construction.

5. HYDRAULIC DESIGN

5.1. Hazard Classification Summary

Hazard classification analyses were completed for six different combinations or system modifications to Grama Dam and Comanche Dam as part of the 2018 conceptual design evaluations completed by Wheeler and Schnabel (Wheeler, 2018). The FWS originally selected System G1-C1 for implementation. This system involved lowering the spillway and normal maximum pool in Grama Dam by 4.1 feet and stabilizing Comanche Dam at its current height. In accordance with FWS dam safety policy (FWS, 2012), a hazard classification panel was selected to review the hazard classification of both dams. On July

9, 2018, the FWS hazard classification panel unanimously approved a change in hazard classification for both dams from high hazard-potential to low hazard-potential after the

G1-C3 modifications were completed. Although modifications to Grama Dam have been deferred at this time, Comanche Dam can be reclassified as a low hazard-potential dam after the modifications at Comanche Dam are complete.

Supplemental hazard assessment of Grama Dam was completed as part of this report.

The supplemental data is provided in Appendix I2. The preliminary results of the Grama

Dam hazard assessment with the modifications at Comanche Dam support a change in hazard classification from high hazard-potential to low hazard-potential with no further modifications to Grama Dam.

5.2. Inflow Design Flood and Reservoir Routing

For consistency with the seismic design criteria, the 1,000-year storm event was selected as the Inflow Design Flood (IDF) for the design modifications to Comanche Dam. The IDF exceeds FWS standards for low hazard-potential dams. The higher IDF was also selected to reduce the downstream risk associated with both Comanche Dam and Grama Dam by ensuring that Comanche Dam was stable for larger floods than the 100-year flood.

Wheeler also developed the PMF and flood frequency inflow to update the hydrologic information associated with the Comanche Dam modifications. The updated inflow design parameters and modeling assumptions are documented in Appendix E.

Two rainfall-runoff reservoir routing models were developed using the U.S. Army Corps

Hydrologic Modeling System (HEC-HMS) to simulate the design inflows. The PMF, HEC-

HMS model was initially developed in the conceptual systems design report (Wheeler, 2018) and updated as part of this document to reflect the modifications to Comanche Dam documented herein. The PMF model includes the entire West Cache Creek watershed. A brief list of updates to the PMF model include:

• Updated combined spillway and low-level outlet works capacity curve at

Comanche Dam, as described in Section 5.3;

• Updated the auxiliary spillway capacity curves at Comanche Dam, as described in

Section 5.3;

• Updated Comanche Dam crest elevation and dam crest length to match design drawings; and

• Updated impervious area percent for Grama Dam and Comanche Dam sub-basins to reflect no further modifications at Grama Dam.

A flood frequency HEC-HMS model was also developed for just the Comanche Dam watershed. The above-listed updates for Comanche Dam and Grama Dam were also included in the flood frequency model. No modifications were included at Grama Dam, which controls 80 percent of the watershed above Comanche Dam. Rainfall-runoff simulations were modeled for the 1000-year through 2-year storm events in the flood frequency model. An initial reservoir water surface elevation at Grama Dam and

Comanche Dam was assumed as 1864.1 and 1811.6, respectively. Table 5.2.1 summarizes the results for the updated inflow hydrology at Comanche Dam. Table 5.2.2 summarizes the results of the updated inflow hydrology at Grama Dam for informational purposes only based on the assumption that there would be no further modifications to

Grama Dam.

Table 5.2.1: Comanche Dam Reservoir Routing Results1

Parameter

100%

PMF

1000-year

100-year

50-year

25-year

10-year

5-year

2-year

Maximum Water Surface Elevation (feet)

1833.3 1831.7 1830.7 1830.3 1829.8 1829.1 1827.9 1824.6

Inflow Volume (acre-feet)

7,767 2,457 1,465 1,252 1,054 820 666 500

Peak Inflow Discharge (cfs)

9,992 4,753 2,790 2,348 1,925 1,421 1,110 782

Peak Outflow Discharge (cfs)

9,931 4,576 2,223 1,626 1,079 456 150 143

Freeboard at Dam Crest 1830.5 (feet)

-2.8 -1.2 -0.2 0.2 0.7 1.4 2.6 5.9

1 – Results are based on Grama Dam initial water surface elevation at 1864.1, Comanche Dam initial water surface elevation at 1811.6, and both low-level outlet works are closed. Elevation 1811.6 was chosen as the maximum elevation of the sloped drop-inlet service spillway crest as the more conservative elevation for the reservoir routing and hydraulic calculations of Comanche Dam.

Additional reservoir routing was also performed to provide information for construction water control purposes under the assumption that Grama Lake would be lowered 4.1 feet, to Elevation 1860 during construction to provide construction water control at Comanche

Dam. Table No. 5.2.3 summarizes the results at Comanche Dam for the 2-year through

25-year storm event under the lowered Grama Lake water surface scenario.

Table 5.2.2: Grama Dam Reservoir Routing Results1

Parameter 100%

PMF

1000-year

100-year

50-year

25-year

10-year

5-year 2-year

Maximum Water Surface Elevation (feet)

1877.9 1874.1 1871.1 1870.4 1869.7 1868.8 1868.2 1867.4

Inflow Volume (acre-feet)

6,286 1,959 1,166 996 839 652 530 398

Peak Inflow Discharge (cfs)

8,461 7,809 4,792 4,121 3,505 2,763 2,281 1,743

Peak Outflow Discharge (cfs)

7,938 3,860 2,245 1,888 1,550 1,139 889 625

Freeboard at Dam Crest 1878 (feet)

0.1 3.9 6.9 7.6 8.3 9.2 9.8 10.6

1 – Note No. 1 from Table 5.2.1 is also applicable to Grama Dam.

Table 5.2.3: Comanche Dam Construction Reservoir Routing Results1

Parameter 25-year 10-year 5-year 2-year

Maximum Water Surface Elevation (feet)

1829.7 1829.0 1826.1 1822.7

Inflow Volume (acre-feet) 1,006 809 575 421

Peak Inflow Discharge (cfs) 1,304 822 659 510

Peak Outflow Discharge (cfs)

409 146 138 127

Freeboard at controlling overtopping elevation during construction 1826.5 (feet)

-3.2 -2.5 0.4 3.8

1 – Results are based on Grama Dam initial water surface elevation at 1860, Comanche Dam initial water surface elevation at 1811.6, and both low-level outlet works are closed.

5.3. Capacity Curves

Four capacity curves were used in the previously described model and final design hydraulic and analysis for Comanche Dam. The reservoir volume capacity and spillway discharge capacity curves used in the hydrologic modeling of Grama Dam were used as documented in (Wheeler, 2018). The documentation of the Comanche Dam capacity curves is located as follows:

• Comanche Lake volume capacity curve Appendix A1 of (Wheeler, 2018),

• Updated Comanche Dam service spillway/outlet capacity curve (Appendix D1),

• Updated Comanche Dam auxiliary spillway capacity curve (Appendix D2)

• Tailwater rating curve (Appendix D3)

The reservoir volume capacity curves for both Grama Dam and Comanche Dam were updated based on 2017 bathymetric surveys and 2014 LiDAR data (Wheeler, 2018). The

Comanche Dam reservoir volume and updated service spillway and auxiliary spillway discharge rating curves and data tables are provided on Sheet No. 3 of the Drawings provided in Appendix C.

With the exception of about the first two feet of weir flow over the service spillway crest, the majority of the service spillway discharges were conduit controlled through the new

36-foot-long, 32-inch-diameter, SDR 21 HDPE discharge pipe. Conduit flow was calculated using the Bernoulli equation and the updated service spillway discharge curve calculations are documented in Appendix D1 of this report. The maximum discharge through the service spillway at the non-overflow dam crest elevation of 1830.5 was calculated as approximately 155 cfs.

The discharge curve for the combined left and center auxiliary spillways were developed based on the procedures and nomographs for designing ogee spillway crests documented in Reclamation’s Design of Small Dams (USBR, 1987). Wheeler assumed the ideal ogee crest shape rating curve for both auxiliary spillway curves. These assumptions were justified and documented in Appendix D2. The auxiliary spillway calculations were based on depth (P) of 1.9 feet and a design head (Ho) of 3.5 feet and are provided in Appendix

D2. Additional design considerations for the center auxiliary spillway design are documented in Section 5.4. The auxiliary spillway capacity curves are also documented on Drawing Sheet G-3 provided in Appendix B. The maximum discharge capacity of the combined auxiliary spillways was calculated as about 1,703 cfs at the non-overflow dam crest elevation of 1830.5.

The tailwater rating curve was previously developed based on a one-dimensional, steady flow HEC-RAS model in the 2018 conceptual systems design report (Wheeler, 2018). No changes in the downstream channel have occurred between the 2018 report and this design report; therefore, Wheeler used the previously developed model to estimate the tailwater curve downstream of Comanche Dam. As documented in Appendix D3, the tailwater rating curve was developed based on simulating flows at increments of 500 cfs through a 1,500-foot-long reach downstream of Comanche Dam.

5.4. Center Auxiliary Spillway Design Considerations

The design of the center spillway section was an iterative process that occurred concurrently with the fast-track design of the RCC buttress. In order to simplify the design and construction, it was determined that the RCC buttress and center auxiliary spillway steps would be formed as one continuous, combined stepped system with two-foot high steps. No center spillway training walls were included in the chute section of the center spillway because of the small difference between the auxiliary spillway crest and non-overflow dam crest of just 2.1 feet. Center spillway flows are expected to spread out laterally in the center auxiliary spillway chute section, but because of the location of this spillway in the center of the dam and the consistent RCC steps across the downstream slope of the buttress, no detrimental effects of this spreading flow was expected. Other issues that were considered in the hydraulic design of the center auxiliary spillway described in this section include:

• Establishing the ideal location of the center ogee section in the upstream and downstream direction;

• After the location of the ogee section was established, the approach losses upstream of the ogee section were evaluated;

• Then energy dissipation of the flows over the center spillway stepped chute section were evaluated;

• Finally, riprap protection in the discharge channel downstream of Comanche Dam was designed.

The ogee crest structure was design to be constructed on top of an approximately one-foot-thick, cast-in-place concrete slab constructed over the RCC lifts in the center auxiliary spillway section as shown on Sheet 15 of the drawings provided in Appendix B. A simplified geometric analysis approach was used to establish the optimum location of the ogee section on this 14-foot-long section of RCC. Two potential jet profiles of the center auxiliary spillway were evaluated to illustrate the expected best case and worst case flow profiles at the IDF discharge. The discharge profiles for these two cases are documented in Appendix D-6 and are illustrated on Figure No. 1. The best profile was based on the

“nappe” jet profile that corresponds to the geometry of the ideal ogee weir trajectory shape calculated in Appendix D2. In this case, flows over the ogee during the IDF would fully engage the spillway chute steps, which would dissipate some of the energy as water flows over the downstream steps.

The “best case scenario” does not consider aeration or spreading of the jet, which was considered in the calculation of the “free” jet profile calculations documented in Appendix

D6. The “free” jet profile was considered as the “worst case” scenario for the jet issuing from the ogee weir under IDF discharge conditions and was calculated based on internal

Wheeler conservation of energy program. In this case, the jet would not interact with the downstream RCC steps but would be fully contained within the designed stilling basin as shown on Figure No. 1. More complex modeling, such as three-dimensional computational fluid dynamics modeling, could be performed for this analysis, but this effort was beyond the schedule and budget constraints of the design.

This flow bracketing approach was considered adequate for the spillway design of this low hazard-potential dam. The apex of the ogee center spillway section was located about nine feet downstream of the upstream edge of the dam crest based on this simplified flow bracketing approach. For spillway discharges less than the IDF the center auxiliary spillway discharges are expected to more fully interact with the downstream steps of the

RCC buttress, which would dissipate more of the energy of the discharge. Another simplifying design approach was that it was assumed that none of the energy would be dissipated on the downstream RCC steps and that spillway flows would fully impinge within the 12-foot-long stilling basin.

Another key consideration for the design of the auxiliary spillway was the calculation of approach losses as water flows over the approach section of the spillway. These approach losses were calculated in Appendix D2 based on an approach velocity of five feet per second using procedures documented in Design of Small Dams (USBR, 2017). The total approach losses were calculated to be about 0.04 feet, which was considered to be inconsequential. As a result, approach losses were ignored in the auxiliary spillway capacity calculations.

Figure 1 – Expected Range of Center Spillway Flow Trajectories

Riprap sizing for the center auxiliary spillway discharge channel, located immediately downstream of the stilling basin, was evaluated using two alternate methods: one from the

Urban Drainage and Flood Control District (UDFCD, 2016) and the other from the U.S.

Federal Highway Administration (FHWA, 2006). These calculations were based on downstream discharge depths and velocities during the IDF as documented in Appendix

D7. Both methods indicated that a minimum mean rock diameter (d50) of four inches would be sufficient to protect the designated section of stream bed between the stilling basin and the natural downstream rock channel. Wheeler conservatively specified the riprap with a d50 = 12 inches because this was considered to easier to produce from the foundation excavation. This size was also considered to be consistent with the remaining riprap stockpile located near the Grama Dam spillway.

5.5. Dam Overtopping Analysis

To minimize construction costs, the auxiliary spillway widths were not increased beyond the original design widths to prevent overtopping of the non-overflow sections of the dam.

As documented in Appendix E and Table 5.2.1, the non-overflow sections of the dam will be overtopped by up to 1.2 feet for about six hours during the IDF. Previous evaluations concluded that rock scour associated with dam overtopping was not a significant dam safety concern as summarized by the following:

“… extensive plucking of rocks or scour during large overtopping flows is unlikely because the joints are tight and clean.” (GEI, 1990)

Wheeler performed additional rock scour analysis to confirm that this conclusion remained valid and this analysis is documented in Appendix D4. The erodibility Index methodology as described in (USBR, 2015), (Wibowo et al, 2005), and (Annandale, 1985) was used to perform the rock scour analysis. Four non-overflow cross-sections of Comanche Dam were evaluated, along with a fifth cross-section taken in the center uncontrolled auxiliary spillway section, were assessed to calculate the expected overflowing jet trajectories, impact stream power values, and erodibility indices of the natural ground surface at the point of impact for each jet to determine the probability of rock scour initiation. The analysis indicated that the probability of scour initiation downstream of the non-overflow sections of the dam during the IDF was significantly less than one percent, which was considered highly unlikely to occur. As a result, Wheeler does not recommend providing additional overtopping protection, such as additional concrete, immediately downstream of the non-overtopping sections of the dam. Overtopping stream power values in these locations were quite low, ranging between 0.10 and 0.16 kilowatts per square meter (kW/m2). If the center auxiliary spillway discharges were to impact natural rock the stream power at the center of the dam would be significantly higher during the IDF at approximately 5.44 kW/m2. If the center auxiliary spillway bedrock were not protected with more than 11 feet of RCC and mass concrete, the probability of rock scour initiating would be less than 50 percent. The mass concrete and RCC in the center spillway stilling basin is considered to be adequate to prevent rock scour from occurring at the maximum section of Comanche

Dam.

5.6. Reservoir Drawdown

The reservoir evacuation criterion was re-evaluated using the updated low-level outlet works curve developed as part of the combined service spillway capacity curve documented in Appendix D1. The reservoir drawdown capacity calculations were developed using a HEC-HMS reservoir routing model of Comanche Dam with minimal inflow and the discharge capacity associated with only the outlet works rating curve.

FWS guidelines for drawdown criteria for either high hazard-potential dam or low hazard-potential dam is based on guidance from ACER Technical Memorandum No. 3 (USBR, 1982). These guidelines state that the reservoir needs to be drawdown to target depths between 10 days and 100 days, depending on the reservoir storage and depths and hazard classification. Comanche Dam exceeds all of these drawdown requirements and can be completely drained in approximately 5.6 hours, if the outlet works gate is fully opened.

6. GEOTECHNICAL DATA

6.1. Site Geology and Subsurface Conditions

Comanche Dam is located in the Wichita Mountains Uplift geological province of southwestern Oklahoma. The Wichita Mountains are a major structural feature of the Uplift and are about 75 miles long and 30 miles wide, along a general N60°W axis. The mountains rise about 700 to 1,200 feet above the surrounding plains to a maximum elevation of 2,464 at the summit of Mount Scott. The Wichita Mountains are bounded on the north and south by high-angle fault systems. The northern boundary of the Wichita

Mountains Uplift is called the Wichita Frontal Fault which includes a series of major faults that strike parallel to the axis of the mountains. One of the faults, the Meers Fault, is located about 7.5 miles from the dam and is considered active.

The project site is underlain by Mount Scott Granite, which is the most abundant granite in the Wichita Uplift. The granite is medium to fine-grained quartz-perthite granite containing primarily plagioclase, hornblende, and some magnetite, biotite, apatite, zircon and sphene.

6.2. Geotechnical Data Report

There have been three previous subsurface investigations conducted at Comanche Dam since the 1980s (Hanson, 1986; GEI, 1995; GEI, 1998). Geologic data, boring logs, and field and laboratory testing from these investigations are summarized in a Geotechnical

Data Report provide in Appendix G. This data report can be used by contractors bidding on the project.

Key conclusions from the previous subsurface investigations at Comanche Dam were summarized as follows:

• Numerous structural fractions are present. One primary and three secondary lineament orientations were identified. The upstream-downstream alignment of these joints creates an adverse geologic setting with respect to seepage and piping; however, further investigation indicated that the joints are tight or clay-filled with low expected permeability;

• A shear zone was present in the foundation near Station 11+55; however, no known fault activity was identified along this feature;

• The foundation was considered to be water tight despite the discontinuities in the bedrock. Field packer tests indicated hydrologic conductivity rates between zero and 350 feet per year;

• The bond between the concrete dam and the bedrock foundation was considered to be in poor condition; and

• The strength of the concrete and bedrock was high with unconfined compressive strengths in the concrete that ranged from 3,350 to 9,666 psi, with an average value of about 6,400 psi. Unconfined compressive tests of the bedrock ranged from

2,610 to 49,730 psi, with an average value of 17,100 psi.

6.3. Seismicity

The peak ground acceleration (PGA) associated with a seismic event with an annual exceedance of 0.001 was conservatively estimated to be 0.07g by Schnabel based on the

USGS Unified Hazard Tool. This analysis is documented in Schnabel’s Technical

Memorandum in Appendix F (Schnabel, 2020). Although the dam is founded on rock that could be considered to be representative of a Site Class A site, the lower Site Class B/C

PGA was used to account for uncertainty in the site conditions. Schnabel used the full

PGA loading conditions for conservatism compared to the recommended 2/3 of the PGA in EM1110-2-2100.

7. COMANCHE DAM STRUCTURAL DESIGN

7.1. Comanche Dam RCC Stability Analysis

Schnabel used a two-dimensional, global sliding analysis of Comanche Dam at the maximum non-overflow and overflow section of the dam. Schnabel’s stability analysis is documented in Appendix F (Schnabel, 2020). The stability analysis was performed in general conformance with the U.S. Army Corps of Engineers concrete dam stability analysis procedures documented in EM 1110-2-2100 (USACE, 2005). The loading conditions and required factors of safety are summarized in Table No. 7.1.1.

Table 7.1.1: Summary of Minimum Required Factors of Safety for Sliding

Inflow Minimum Required FS Notes

Usual 2.0 Maximum Normal Pool Level

Unusual Flood 1.1 1,000 Year Flood

Extreme (Seismic)1 1.3 1,000 Year Peak Ground Acceleration 1 – EM 110-2-2100 requires an FS of 1.1 for extreme seismic loading for critical structures when detailed site-specific ground motions are available, and FS of 1.3 for critical structures using the coefficient method.

Key inputs and analysis results for the maximum non-overflow and overflow sections at

Comanche Dam are documented in Table 7.1.2. Full uplift pressure was assumed in each analysis case. Due to the limited amount of overtopping depths and expected energy dissipation from the RCC steps, Schnabel neglected nappe forces and tailwater retrogression for the IDF analysis with the 10-foot-wide RCC buttress. A conservatively lower unit weight of both the RCC and the existing Comanche Dam of 145 pounds per cubic foot (pcf) was used to account for the potential for lower RCC unit weights.

Table 7.1.2: Summary of Sliding Analysis

Load Case

Normal Pool 1,000 Year Flood 1,000 Year Seismic

Non-overflow

Overflow Non-overflow Overflow

Non-overflow

Overflow

Headwater EL 1811.6 1811.6 1831.6 1831.6 1811.6 1811.6

Tailwater EL 1788 (dam base)

1794.4 (top apron)

1800.9 1800.9

(dam base)

1794.4 (top apron)

Silt El 1795.5

Fraction Angle 42 deg

Concrete Unit Weight (pcf)

145.0

Horz. Acc. (g’s) 0.0 0.0 0.0 0.0 0.07 0.07

Rock & D/S Soil Passive Resistance

Rock & Soil

Rock & Soil

Rock Only

Rock Only

Rock & Soil

Rock & Soil

Computed Factors of Safety

6.9 4.3 1.6 1.5 4.1 2.9

Required Factors of Safety

2.0 1.1 1.3

Based on our analysis, the 1,000-year flood loading case was the critical load case.

However, the 10-foot-wide RCC lifts provide better construction access, so this 10-foot-width is what really controlled the design of the RCC buttress. The proposed modifications to the dam result in acceptable stability factors of safety for each loading condition. Apron dowels are provided to maintain the connection between the proposed spillway apron and the base of the existing overflow sections and engage monolithic action for overturning stability. Upper dowels were included in the design to reduce the potential for separation from freeze-thaw cycles, seismic loading, or RCC shrinkage.

Schnabel also evaluated the existing Comanche Dam without the RCC buttress for the three loading conditions. For the existing dam case, an 80 percent tailwater regression was used for the non-overflow section and a 60 percent tailwater regression was used for the overflow section at the center auxiliary spillway. The unit weight of the existing concrete in the dam was assumed to be 148.5 pcf. As shown in Table 3 in Appendix F, the normal pool and seismic loading conditions at the existing dam had adequate factors of safety against sliding, but the unusual loading condition resulted in sliding factors of safety of about 0.9. In each case analyzed for the existing dam and the RCC buttress. The

Resultant of the loading forces was located in the middle third of the base of the dam, indicating an adequate overturning design.

8. CONSTRUCTION CONSIDERATIONS

8.1. Project Permitting

Before the Project is bid, the Government will obtain the following permits for this project:

1. A 404 Permit from the U.S. Army Corps of Engineers and a 401 Water Quality

Certification from the Oklahoma Department of Environmental Quality.

2. A National Environmental Policy Act (NEPA) Categorical Exclusion.

3. State Historical Preservation Office (SHPO) Clearance from the Oklahoma

Archaeological Society.

The construction contractor is required to obtain all other permits required for the Work in accordance with federal, state, and other local regulations. As a minimum, other permits may include:

1. A General Construction Permit (OKR10) for Stormwater Discharge from the

Oklahoma Department of Environmental Quality.

2. Any Temporary Construction Permits as required by Comanche County.

8.2. Construction Bidding and Administration

An on-site Market Research/Industry Day was anticipated to occur in the summer of 2020 to present plans for Comanche Dam to…