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IRON CREEK PROJECT LEVEL II FEASIBILITY STUDY

INTERIM REPO T TO SHOSHONE AND HEART MOUNT AI IRRIGATION DISTRICTS AND WYOMING WATER DEVELOPMENT COMMISSION

BY HARZA ENGINEERING COMPANY ENGLEWOOD, COLORADO

DECEMBER 1982 ENGINEERING COMPANY CONSULTING ENGINEERS

December 7, 1982

Shoshone and Heart Mountain Irrigation Districts 337 East 1st Street Powell, Wyoming 82435

Subject: Iron Creek Level II Feasibility Study Interim Report

Gentlemen:

We are pleased to present our Interim Report on the Iron Creek Project, prepared in accordance with our contract dated September 28, 1982.

Objective

The studies described in this report are the first phase of the Iron Creek Level II Feasibility Study_ They were undertaken to compare two alternatives that would provide for continued diversion of irrigation water to the Garland and Frannie Di­ visions of the Shoshone Project in a reliable, cost effective manner. The alternatives are:

1. Rehabilitating the existing Corbett Darn, Tunnel, and associated works, to provide for continued reliable operation, and

2. Constructing a new diversion dam across the Shoshone River at Iron Creek and retiring Corbett Tunnel and perhaps Corbett Dam.

Hydroelectric installations, which could generate revenues to partly offset construction costs, were evaluated for both al­ ternatives. The second phase of the Iron Creek Study will be a feasibility study of the selected alternative.

Background

Water for irrigation of more than 50,000 acres in the Gar­ land and Frannie Divisions of the Shoshone Project is diverted from the Shoshone River at Corbett Dam, located about six miles northeast of Cody_ Irrigation flow enters the 3.3-mile-long Corbett Tunnel through an intake immediately above the dam on the left bank. The tunnel discharges to a settling pool formed

150 SOUTH WACKER DRIVE CHICAGO, ILLINOIS 60606 TEL (312) 855-7000 CABLE: HARZENG CHICAGO TELEX 25-3540 Shoshone and Heart Mountain Irrigation Districts Page 2 by a dam across Iron Creek, a tributary of the Shoshone River. Garland Canal (which also feeds the Frannie Canal) begins at a headworks structure on the north bank of this pool.

The Corbett Dam and Tunnel diversion system was placed into service in 1908, and only minor repairs and improvements have been made in the 74 years since then. The Irrigation District has expressed concern over the continued serviceability and reliability of the tunnel, given its age. Tunnel failure could result in loss of irrigation water in the Garland and Frannie Divisions for an extended period of time. Therefore, a study of methods to extend the service life of the existing dam and tun­ nel, and a comparison of the costs of dam and tunnel rehabilita­ tion with the costs of building a new diversion dam to supply water directly to the Garland Canal, was undertaken.

Scope

The study of the existing facilities, including Corbett Dam and Tunnel, included a review of construction records and pre­ vious reports and evaluations~ a field inspection to determine the existing condition of the dam, tunnel, and appurtenant works~ and evaluation of this information to develop repair and rehabilitation needs, methods, and costs.

The location, layout, and cost of the proposed replacement diversion dam, located near the mouth of Iron Creek, were drawn from a preliminary report by Tudor Engineering Company and from a site reconnaissance and limited review made during ~lis study.

Hydroelectric power plant arrangements and estimates of costs and benefits were also developed, and any net henefits were used in cost comparisons to defray part of project construction cost.

More detailed studies of the selected alternative are to be conducted during later phases of work.

Results

A reliable diversion of irrigation water can be assurred either by rehabiliting the existing system or by constructing a new dam. Shoshone and Heart Mountain Irrigation Districts Page 3

Rehabilitation Alternative

The rehabilitation measures recommended for the existing system include:

1. At Corbett Dam, repairing sluice gate seats, constructing a training wall to improve sediment deflection, and installing a trashboom;

2. In Corbett Tunnel, grouting, constructing new drains in the invert and in sidewalls where leakage is serious, relining the tunnel with fibrous shotcrete, and resurfacing the tunnel invert: and

3. At the Iron Creek Pool, installing trashracks at the Garland Canal Headworks, rehabilitating the head gates, and repairing the damaged outlet sluice.

A low-head hydroelectric installation at Corbett Dam was evaluated. Preliminary cost estimates indicate that the power plant addition may not be economically feasible, but more detailed studies and estimates are required to verify this conclusion.

Iron Creek Dam Alternative

~le proposed Iron Creek Dam is based on the project outlined by Tudor, and consists of:

1. A 100-ft-high embankment dam with a crest length of about 1,040 ft;

2. A chute spillway with three 35-ft-high by 40-ft-wide radial crest gates;

3. A diversion, river outlet, and power tunnel:

4. A 6.5 MW hydro power plant; and

5. A new gated headworks structure for the Garland Canal, with a 1,000 cfs capacity.

The costs of developing a new darn and the cost of the rehabilitation alternative are shown in Table 1. The cost of a new dam will be higher. Shoshone and Heart Mountain Irrigation Districts Page 4

Table 1

COST COMPARISON

Repair Exist- Construct ing Corbett New Dam at Dam and Tunnel Iron Creek Capital Cost of Repairing Existing Diversion Works or Constructing New Dam $2,500,000 $23,500,000

Capital Cost of Hydroelectric Power Plant 3,500,000 7,200,000

Present Worth of O&M Cost~/ 300,000 500,000

Present Worth of Hydro Power Benefits1./ (2,100,000) (13,QOO,000)

Net Hydro Power Benefits 0]./ (6,200,000)

Net Capital Cost of Repair or New Construction $2,500,000 $17,300,000

.!.I Estimated annual O&M, interest at 10 percent, project life 50 years.

~/ Power sales at $0.044 per kWh, interest at 10 percent, and a project life of 50 years.

Initial reconnaissance estimates suggest that the Corbett Dam hydro installation may not be economical. More detailed studies may show lower cost or higher benefits, but at this time no net hydro development benefits can be assumed. Shoshone and Heart Mountain Irrigation Districts Page 5

Closing

It has been a pleasure working with the District on the studies presented in this Interim Report. We look forward to assisting the District and the Wyoming Water Development Commission in the upcoming Public Information Meeting, and to discussion of the scope of activities for continuing work in Phase II of these stuoies. TABLE OF CONTENTS

Page SUMMARY LETTER

TABLE OF CONTENTS i

LIST OF TABLES iii

LIST OF EXHIBITS iii

FOREWORD

Objective 1 Scope 1 Contents of Report 2

Chapter I - DESCRIPTION OF PROJECT AREA

Shoshone Project I-I

Buffalo Bill Dam 1-2 Irrigation Development 1-2

Corbett Dam Diversion ~qorks 1-3

Dam 1-4 Tunnel Intake 1-4 Tunnel 1-5 Iron Creek Pond 1-5

Chapter II - CORBETT DAM PROJECT

studies Conducted 11-1 Condition of Project 11-1

Corbett Dam 11-1 Corbett Tunnel Intake 11-2 Corbett Tunnel 11-2 Garland Canal Headworks 11-3 Iron Creek Sluice 11-3

Recommended Improvements 11-4

Corbett Dam 11-4 Corbett Tunnel Intake and Sluice 11-4 Corbett Tunnel 11-4 Garland Canal Headworks and Iron Creek Sluice 11-6 Summary of Costs 11-6

-i- TABLE OF CONTENTS (Cont'd)

Page

Chapter III - CORBETT DAM HYDRO DEVELOPMENT

Water Supply 111-1 Corbett Dam Development 111-2 Iron Creek Pool Development 111-4 Comparison of Developments 111-6

Chapter IV - IRON CREEK DAM PROJECT

Project Review IV-l Hydroelectric Benefits IV-3

Chapter V - PROJECT COMPARISON

Appendix A - REPORT ON CORBETT DAM AND TUNNEL

-ii- TABLE OF CONTENTS (Cont'd)

LIST OF TABLES

Number Title Page

I-I Irrigable Areas, Shoshone Project Vicinity 1-3 11-1 Repairs and Modifications to Corbett Dam and Tunnel, Reconnaissance Cost Estimate 11-7

111-1 Average Monthly Flows 111-3

111-2 Corbett Dam Hydro Development, Reconnaissance Cost Estimate III-4

111-3 Iron Creek Pool Hydro Development, Reconnais- sance Cost Estimate 111-6

IV-l Summary of Project Data, Iron Creek Dam IV-2

IV-2 Iron Creek Dam Project, Reconnaissance Cost Estimate IV-4

V-I Cost Comparison V-2

LIST OF EXHIBITS

Number Title

I-I Project Area Map 1-2 Corbett Dam and Tunnel Intake, Existing Conditions

1-3 Corbett Dam, Plan and Sections

1-4 Corbett Tunnel, Tunnel Sections and Lower Portal 11-1 Proposed Intake Modifications at Corbett Tunnel

11-2 Corbett Tunnel Rehabilitation

111-1 Corbett Dam Hydro Development, Schematic Layout

-iii- FOREWORD

Harza Engineering Company (Harza) has been retained by the Shoshone and Heart Mountain Irrigation Districts (Districts), of Powell, Wyoming, to evaluate replacing the existing Corbett Dam and Tunnel with a diversion dam at Iron Creek. The purpose of Corbett Dam or the replacement dam at Iron Creek is to divert water from the Shoshone River to the Garland and Frannie Divisions of the Bureau of Reclamation's Shoshone Project.

Objective

The objective of the studies described in this preliminary report is to compare the costs and benefits of:

1. Rehabilitating the existing Corbett Dam, Tunnel, and associated works, for continued operation, and

2. Constructing a new diversion dam across the Shoshone River at Iron Creek and retiring Corbett Tunnel and perhaps Corbett Dam.

Hydroelectric installations are to be evaluated for both alternatives to determine the amount of revenue that can offset project construction costs.

Scope

The work that has been completed and is described in this report was outlined in Task B, Analysis of Existing Facilities, and in Task C, Comparison of Costs, in the Agreement for Engineering Consulting Services between Harza and the Districts.

-1- Task A was preparation of a Critical Path schedule for the project studies, and was completed in early November 1982.

Task B included a review of construction records and pre­ vious reports and evaluations: a field inspection to determine the existing condition of Corbett Dam and Tunnel and appurtenant works: and evaluation of the available information to develop repair and rehabilitation needs, methods, and costs.

The Comparison of Cost, Task C, included developing cost estimates for the Iron Creek Dam Project based on previous re­ ports, determining the nature and costs of hydroelectric power installations, and computing preliminary hydroelectric power benefits for the alternatives for use in a cost comparison.

More detailed studies of the selected alternative are to be conducted during later phases of the work.

Contents of Report

This report has been prepared to present the results of the alternative studies at three levels of detail. A broad descrip­ tion of the study and results is presented in the Summary Let­ ter. The Main Report covers the overall study background, pro­ cedures, and results in five chapters: 1) Description of Proj­ ect Area, 2) Corbett Dam Project, 3) Corbett Dam Hydro Develop­ ment, 4) Iron Creek Dam Project, and 5) Project Comparison. Finally, a detailed description of the condition of Corbett Dam and Tunnel, and of recommended rehabilitation measures, is given in Appendix A.

-2- Chapter I DESCRIPTION OF PROJECT AREA

Corbet t Dam and Tunnel were constructed as part of the Shoshone Project of the Bureau of Reclamation (BuRec), to divert water for irrigation of project lands. The Project is located along the Shoshone River between the Bighorn and Absaroka Moun- tain Ranges in northwestern Wyoming. The reg ion is dry, with average annual precipitation at Powell and Cody of 5.67 and 9.38 inches, respectively. The water supply for the agricultural development in the area and for municipal and industrial (M & I) uses comes from runoff, largely in the form of snowmel t, from the mountains to the west.

Shoshone projectll

Along wi th Corbett Dam and Tunnel, the Shoshone Pro j ect includes Buffalo Bill Dam and Reservoir on the Shoshone River, which is the main storage development: two small hydroelectric power plants (one currently out of service) wi th associated transmission lines: and four irrigation divisions: Heart Moun- tain, Garland, Frannie, and Willwood. Project development was started in the early 1900 IS. Irrigation began in the Garland Division in 1908 and Buffalo Bill Dam was completed in 1910.

Other private irrigation systems have been developed in the areas around Cody and Lovell.

~I Bureau of Reclamation, Factual Data, on the Shoshone Pro­ j ect, and Shoshone Proj ect Map No. 26-600-400 (November 1972): Modification of Buffalo Bill Dam, Definite Plan Report (September 1982).

I-I Buffalo Bill Darn

Buffalo Bill Darn is a constant radius arch dam, 328 feet high. It was once the highest arch dam in the world. The re­ servoir has a total capacity of 491,350 acre-feet and an active capacity of 443,150 acre-feet. It has a surface area of 6,710 acres (10.5 square miles) and a length of 7 miles, at elevation 5,370.7 ft,msl. The spillway consists of a rock weir, an intake channel, and an unlined tunnel.

Enlargement of Buffalo Bill Dam and Reservoir by raising the existing dam 25 feet has recently been authorized by the u.s. Congress, based on a cost sharing agreement with the State of Vlyoming. By raising the dam and enlarging and gating the spillway, the active storage will be increased by 271,300 ac­ ft to a total of 714,450 ac-ft.

Irrigation Development

Irrigated areas along the Shoshone River below Buffalo Bill Dam are listed in Table I-I. Diversions for the Heart Mountain and proposed Polecat Bench Divisions are made directly from Buffalo Bill Dam: diversions for the Garland and Frannie Divi­ sions are made at Corbett Dam, through the Corbett Tunnel; and diversions for the Willwood Division are made at willwood Dam. Diversions to the private developments are made further down­ stream, below Willwood Darn.

1-2 Table I-I

IRRIGABLE AREAS ~/ SHOSHONE PROJECT VICINITY

Subarea Irrigable Area (Rounded to nearest 100 ac) Shoshone Project

Heart Mountain Division 33,100 Garland Division 35,900 Frannie Division 15,200 Willwood Division 12,000 Subtotal 96,200 Polecat Bench Area (proposed) 19,300 Subtotal 115,500

Private Developments

Elk-Lovell Canal 12,800 Sidon Canal 15,700 Other 14,900

Subtotal 43,400 Total Irrigable Area 158,900

Bureau of Reclamation, Shoshone River Water Supply Study (September 1979).

Corbett Dam Diversion Works

Water for the 51,100 irrigable acres in the Garland and Frannie Divisions is diverted from the Shoshone River at Corbett Darn, located about six miles northeast of Cody. Irrigation flow enters the 3.3-mi-long Corbett Tunnel through an intake immedi­ ately above the darn on the left bank. The tunnel discharges to a settling pool formed by a darn across Iron Creek, a tributary of the Shoshone River. Garland Canal (which also feeds the Frannie Canal) begins at a headworks structure on the north

I-3 bank of this pool. The overall arrangement of the diversion works is shown on Exhibit I-I.

Dam

Corbet t Di version Dam was buil t across the Shoshone River in 1908. From right to left looking downstream, the dam con­ sists of 1) an earthfill section 440 ft long, 2) a 400-foot long reinforced concrete slab and buttress overflow spillway in the old river channel, and 3) a sluiceway and tunnel intake struc­ ture. The general layout of the dam and appurtenant structures is shown on Exhibit 1-2. Details of the spillway and tunnel intake, with cross sections of the major structures, are shown on Exhibits 1-3 and 1-4.

Tunnel Intake

The intake structure for the Corbett Tunnel is located at the left end of the slab and buttress spillway. The intake and its appurtenances were designed to divert water into the tunnel while deflecting most of the occasionally large volumes of gra­ vel bedload of the Shoshone River into a sluiceway. The intake is isolated from the main river and overflow spillway by a re­ inforced concrete training wall.

There are three 4 ft wide by 5 ft high sluice gates in line with the dam immediately to the left of the training wall. The gates are intended to sluice gravel away from the intake struc­ ture. The intake is separated from the sluiceway by a submerged curved weir with crest 8 feet above the sluiceway floor. Flash­ boards have been added to part of its length to improve deflec­ tion of bedload. Two 5 ft wide by 10 ft high intake gates con­ trol flow into the tunnel.

1-4 Tunnel

The Corbett Tunnel is a concrete-lined 10.75-ft-high horse­ shoe tunnel 17,250 feet long. The tunnel invert falls 24 feet from the intake to the exit portal giving an average slope of 0.00139 ft/ft. The concrete lining is nominally 9 inches thick and unreinforced. When ground conditions required, timber sup­ ports were used during original construction. The timbers were embedded in the concrete lining. Drainage was provided under the invert by a 4-inch sewer pipe beneath the centerline and gravel-filled trenches at either edge of the flow section, as shown on Exhibit 1-4.

Iron Creek Pond

The exit portal for the tunnel is on the right bank of Iron Creek, a tributary of the Shoshone Ri ver. A short homogeneous earthfill dam across Iron Creek impounds water from Iron Creek and the Corbett Tunnel, and forms a headpond for the Garland Canal. There is a low-level outlet for the pond through natural ground to the left of the embankment dam.

The headworks for the Garland Canal are on the north bank of the pond. The canal headworks structure is reinforced con­ crete with six 4 ft wide by 8 ft high vertical lift gates equip­ ped with manual operators.

1-5 Chapter II CORBETT DAM PROJECT

Studies Conducted

Studies of the Corbett Dam and Tunnel were conducted to identify project deficiencies, recommend repairs and improve­ ments, and estimate rehabilitation costs so that continued ser­ viceability of the project would be assured. The work consisted of data collection and review, site inspection, and office anal­ yses. Studies were based on interviews with BuRec and Shoshone Irrigation District personnel: review of construction records, photographs and prior inspection reports: observation of tunnel performance: and a visual inspection of the dewatered tunnel, dam, and appurtenances. No testing, depth sounding, probing, drilling, or underwater inspection was conducted. This more­ detailed work will be required to verify the conclusions pre­ sented in this chapter. Principal concl usions are discussed below, and a more detailed technical discussion is included in Appendix A.

Condition of the Project

Corbett Dam

Corbett Dam appears to be in good condition. There is some exposed aggregate on the crest and the toe of the overflow sec­ tion which may eventually require repair, but it is of little consequence at this time. The condition of the downstream over­ flow apron should be checked by probing and sounding with a rod. A limited program of concrete coring and testing is recommended to further check the condition of the concrete.

11-1 Corbett Tunnel Intake

The exposed portions of the intake and sluiceway structures are in good condi tion. The base slab and riverside training wall should be probed to check for underwater damage. There are three operational problems that need to be corrected: 1) the sluice gates seal poorly, 2) trash collects in the intake area, and 3) gravel is sometimes carried into the tunnel. Also, the water circulation pattern at the tunnel entrance may slightly decrease the amount of water flowing through the tunnel.

Corbett Tunnel

Corbett Tunnel is in good condition. The tunnel bottom is abraded to a depth of several inches, presumably by gravel car­ r ied through the tunnel. Cracking, which has reportedly not changed much over the years, was observed in the sidewalls and crown. Dated concrete patches over the cracks show little on­ going movement. The only major tunnel lining fallout occurred in 1981 in an area of soft, wet shale, which was reportedly an area of trouble 1uring original construction.

Impact hammer tests, which correlate roughly with concrete strength, were made at about SOO-foot intervals during the in­ spection, giving minimum estimated compressive strengths of about 2,SOO psi. Tests of concrete cores from the lining will be necessary to more accurately assess the condition and strength of the concrete.

Two conditions have affected the concrete and will continue to do so unless corrective measures are taken.

First, hydraulic pressure around the lining has resulted in considerable seepage in five major areas. The areas of major

II-2 seepage inflow seem to correlate wi th zones of soft sandstone or areas where streams, canal laterals or stockponds are above the tunnel al ignment (see Appendix A). The action of water passing into the tunnel through cracks, joints, porous zones and other passages appears to have leached the cement from the con­ crete in these areas. This increases the susceptibility of these areas of the lining to erosion or localized failure and fallout. This problem may be aggravated by chemically active seepage water (identified by BuRec tests as high in sulphates) which attacks the cement.

Second, the bottom of the tunnel will continue to erode unless it is resurfaced with a more abrasion-resistant material, or a better means of keeping gravel out of the channel can be devised.

The tunnel inspection for this study was limited to a visual walk-through supplemented with a few impact hammer tests. The inspection provided a good indication of surface condition, but a program of core drilling, pulse-echo and radar sounding will be needed to provide data on the condi tion of the outside of the liner where it is in contact with rock.

Garland Canal Headworks. The headworks is in fair condi- tion. There is some structural cracking and the gates are stuck in the open position. Trash reportedly collects at the struc­ ture and occasionally passes through the gate openings into the canal which causes operational difficulties.

Iron Creek Sluice. The sluice structure is in good condi­ tion except for some erosion damage on the headwall.

11-3 Recommended Improvements

Corbett Dam

No improvements appear to be needed on Corbett Dam. How- ever, the downstream apron should be inspected at low water to verify that its condition is satisfactory.

Corbett Tunnel Intake and Sluice

The sluice gates should be repaired. Cost could range from $10,000 to $40,000 depending on ease of dewatering.

A trashboom should be installed at the upstream end of the riverside training wall, at a cost of about $30,000. More study will be required to select the best arrangement.

Portions of the flashboards on the intake weir have failed. The boards should be replaced. Some improvement in sluicing gravel away from the tunnel intake could be achieved by con­ structing a new training wall upstream of the weir on the shore side of the intake, perhaps as shown on Exhibit II-I. This arrangement would cost about $70,000. In addition, there may be some operational changes that could help solve the gravel prob­ lem. Further study will be required to determine the best solu­ tion.

Corbett Tunnel

Corbett Tunnel is generally in good condition. However, the known areas of seepage should be repaired by drainage, sealing and patching as required. This would solve most of the immediate problems but would provide no margin of safety against

II-4 future tunnel service interruption due to failure of the liner in areas that are not in visible distress now.

Without identifying areas of unseen potential future trouble, the conservative course of action to assume for plan­ ning purposes is to strengthen the entire liner regardless of its present condition. Such a program could include the follow­ ing:

1. Grouting along wet areas of the tunnel to reduce in­ flow and further leaching of original concrete,

2. Cleaning liner surfaces with high pressure water jet,

3. Installing a new drainage system beneath the existing tunnel invert, and up the sidewalls in wet areas,

4. Application of a new 2-inch minimum thickness fibrous shotcrete liner to the cleaned surfaces of the old liner,

5. Application of a thin finish coat over the fibrous shotcrete, and

6. Repair of invert abrasion damage.

The repair work described above and shown on Exhibit 11-2 is estimated to cost about $2,260,000. This work should resul t in a rehabilitated tunnel with a future service life close to that of the original tunnel.

It may be less expensive in the long term to repair only those areas that are in obvious need of repair at this time and

11-5 then set up a program of annual maintenance. This course of action is probably more risky than repairing the entire tunnel at thi s time. Annual repairs of liner damage would be easier and faster if a pre-fabricated set of concrete forms were pre­ purchased.

Garland Canal Headworks and Iron Creek Sluice

The existing structure could be converted into a more ser­ vicable trashrack structure for the canal at a cost of about $60,000. If it is decided that operational gates are desirable, the present gates could probably be rehabilitated for occasional use at a cost of about $20,000.

The headwall at the entrance to the Iron Creek sluice needs repair. Repair cost should be about $20,000.

Summary of Costs

The work i terns and costs for the recommended repairs and modifications to Corbett Dam and Tunnel are summarized on Table II-I.

11-6 Table 11-1

REPAIRS AND MODIFICATIONS TO

CORBETT DAM AND TUNNEL

RECONNAISSANCE COST ESTIMATE

Item Cost!./

Corbett Dam

Sluice gate and seat repair $ 40,000

Gravel-filled precast concrete training wall 70,000

Trashboom 30,000

Subtotal $140,000

Corbett Tunnel

Grout, construct drains, re-line tunnel with shotcrete, and resurface invert $2,260,000

Iron Creek Pool

Garland Headworks trashracks $ 60,000

Headgate rehabilitation 20,000

Iron Creek Sluice (outlet) repair 20,000

Subtotal $100,000

TOTAL $2,500,000

Costs are at December 1982 level, and include allowances for contingencies, engineering, and administration.

11-7 Chapter III CORBETT DAM HYDRO DEVELOPMENT

Two hydroelectric development alternatives appear feasible if Corbett Dam and Tunnel remain in service:

1. A low-head hydroelectric plant at Corbett Dam, gene­ rating power with the flow that is normally discharged downriver, and

2. A power plant developing the head between the Iron Creek Pool and at the Shoshone River, using a year­ round flow through Corbett Tunnel and generating power wi th the tunnel flow that exceeds that required for irrigation.

The two projects are mutually exclusive.

\vater Supply

Ri ver flows and the effects of storage (mainly in Buffalo Bill Reservoir) and irrigation diversions have been extensively studied by the BuRec. The results of BuRec' s recent operation studiesl/ have been used to estimate average monthly river flows and diversions at Corbett Dam for the preliminary evalua­ tion of hydro al ternatives presented in this report. Flows presented by the BuRec in acre-feet per month have been convert­ ed to cubic feet per second by Harza. The return flow from the

Bureau of Reclamation, Polecat Bench Area, Wyoming, Defi­ nite Plan Report, Appendix C-Water (1980, Revised 2/81).

111-1 Heart Mountain Division that reaches the Shoshone River above Corbett Dam was estimated to be 30% of the total Heart Mountain return flow estimated by BuRec, and these figures have been included. The average monthly flows used in this report are summarized on Table III-I.

Corbett Dam Development

A new hydroelectric power plant at Corbett Dam would prob­ ably be constructed in the area of the existing sluice, since access is convenient and some reconstruction in this area is required. A possible layout is shown on Exhibit III-I.

The existing concrete overflow dam section would be raised by about 2 ft by constructing a concrete cap.

The hydroelectric turbine/generator would probably be a standard design tubular-type packaged unit. A runner diameter of 8.2 ft (2.5 m) with adjustable blades and adjustable gates would permit generating through a flow range of about 300 to 1,200 cfs at a head of 14 ft. The maximum unit output would be about 1,200 kW, and average annual energy generation is estimat­ ed to be about 4,900,000 kWh.

Related improvements at the intake area might include a trash boom to deflect surface debris, a new sluice with a single radial gate, and a more-streamlined arrangement of walls to deflect bedload to the sluice. Exhibit 111-1 shows an arrange­ ment that was developed for costing purposes, but further study is required to select the most satisfactory layout.

Estimated costs for the Corbett Dam hydro development are shown in Table 111-2.

111-2 Table 111-1

AVERAGE l'-10NTHLY FLOWS (cubic feet per second)

Month

Description Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec

Flow in Shoshone River below Heart Mountain Power Plant 399 543 308 557 1,262 2,324 2,198 1,204 1,125 723 403 440

Estimated return flow from Heart Mountain Division above Corbett Dam 0 0 0 18 76 122 149 123 107 32 0 0

Estimated flow above Corbett Dam 399 543 308 575 1,338 2,446 2,347 1,327 1,232 755 403 440

Diversion at Corbett Dam (to Garland and Frannie Divisions) 0 from BuRec study 0 0 0 275 844 800 1,046 985 697 425 0 0

0 assuming 800 cfs tunnel capacity a a a 275 800 800 800 800 697 425 a 0

Estimated downriver flows 399 543 308 300 538 1,646 1,547 527 535 330 403 440

Note: Flows were developed by Harza from operation study results for the Shoshone River System with Polecat Bench irrigation and without raised Buffalo Bill Dam, completed by BuRec using 1928 through 1977 records. Study details are contained in the BuRec's Polecat Bench Area, Wyoming, Definite Plan Report, Appendix C-Water (1980, Revised 2/81). The flows shown are preliminary, and should be conservative for the purposes of this study. More-detailed evaluation of flows is to be completed during later stages of the of the Iron Creek Dam study. Table 111-2 CORBETT DAM HYDRO DEVELOPMENT RECONNAISSANCE COST ESTIMATE

Item Cost~/

Raise Dam Crest $150,000 Demolition 40,000 Earthwork 50,000 Concrete Construction 470,000 Sluice Gate, With Heaters 80,000 Powerhouse Civil 400,000 Turbine/Generator Package 1,600,000 Powerhouse & Switchyard Electrical, Misc. Eqpt. 640,000 Transmission 70,000 TOTAL $3,500,000

!:../ Costs are at December 1982 level, and include allowances for contingencies, engineering, and administrative costs. Cofferdam and trash boom costs are not included, since they are required for dam repair and improvement.

Iron Creek Pool Development

The full capacity of Corbett Tunnel is required for irriga­ tion purposes during only about 4 months of the year, May through August. If the tunnel were operated at full capacity, assumed as 800 cfs for this analysis, or at maximum available river flow when less than 800 cfs, the excess water available at the Garland Canal Headworks could be used to generate hydro power. About 45 ft of head can be conveniently developed be-

111-3 tween the Iron Creek Pool (above the Garland Canal Headworks: and the old Iron Creek channel above the Shoshone River.

The power installation would include an intake structure wi th trashracks and gate, a steel or reinforced concrete pipe penstock about 450 ft long, and a conventional powerhouse with vertical tubular-type turbine/generator unit. The unit woul, have a runner diameter of 6.6 ft (2.0 m) with adjustable blade and gates. It would generate over a range of 200 to 800 cfs a a head of 45 ft and have a maximum output of about 2,700 kW Average annual energy generation is estimated to be abou 6,400,000 kWh, with essentially all power produced during th months of October to April when Corbett Tunnel capacity is no needed for irrigation.

A new or completely rehabilitated Garland Canal Headwork is needed for this alternative, since outflow from Iron Cree Pool must be divided between the canal and powerplant.

Estimated costs for the Iron Creek Pool hydro developmer. are shown on Table 11I-3.

I11-4 Table 111-3

IRON CREEK POOL HYDRO DEVELOPMENT RECONNAISSANCE COST ESTIMATE

Item Cost!./

Power Intake $300,000 Penstock 760,000 Powerhouse Civil 360,000 Turbine/Generator Package 1,700,000 Powerhouse & Switchyard Electrical, Misc. Eqpt. 810,000 Transmission 70,000

Garland Canal He~dworks 500,000 TOTAL $4,500,000

~./ Costs are at December 1982 level, and inc 1 ude allowances for contingencies, engineering, and administrative costs.

Comparison of Developments

The project costs per kilowatt-hour of energy generated are $0.714 for the Corbett Dam development and $0.703 for the Iron Creek Pool development, less than 2 percent apart. However, the Corbett Dam development has two advantages:

1. Flows will be maintained at all times in the Shoshone River between Corbett Dam and Iron Creek.

2. Corbett Tunnel, the Iron Creek Pool, and Garland Canal Headworks will not be in year-around operation, per­ mitting more convenient repair and maintenance.

111-5 Therefore, the development at Corbett Dam has been used for the project comparisons presented in Chapter V.

III-6 Chapter IV IRON CREEK DAM PROJECT

A new diversion dam to serve the Garland Canal has been studied previously, at reconnaissance level~/. This dam would be located across the Shoshone River near the mouth of Iron Creek, as shown on Exhibit I-I. It would be about 100 ft high and would divert water directly into the canal, eliminating the need for Corbett Dam and Tunnel. The project was proposed to include a zoned embankment dam, a gated chute spillway, a com­ bined low-level outlet and power tunnel, a power plant with two 3,250 kW Francis turbine/generator uni ts, and a new headworks for the Garland Canal. The summary of Project Data from the previous report is reproduced in Table IV-I.

Project Review

The proposed project has been briefly reviewed as part of this reconnaissance-level study, and the proposed damsi te and several possible al ternatives were visi ted in the field. The concept of the Iron Creek Dam Project seems sound from a physi­ cal standpoint, and the costs developed in the previous report appear to be reasonable. Specific comments resulting from the recent review are as follows:

1. The dam location previously selected seems to be a good choice.

2. A spillway location on the right abutment should be studied. This would reduce interference wi th the

Tudor Engineering Company, Preliminary Report, Iron Creek Project (undated).

IV-l Table IV-l

SUMMARY OF PROJECT DATAl/ IRON CREEK DAM

Dam Type: Zoned embankment Crest Elevation: 4,630 ft Top Width: 30 ft Maximum Height: 100 ft Crest Length: 1,040 ft1./ Enbamkment Volume: 600,000 cu yd Foundation Excavation: 250,000 cu yd

Spillway Type: Gated agee with chute Capacity: 75,000 cfs at W.S. El. 4,620 ft Crest Elevation: 4,590 ft Crest Length: 120 ft Gates: Three, 35 ft high by 40 ft wide

River Outlet Description: A 10 ft diameter concrete lined pressure tunnel, a gate chamber, and a 15 ft diameter concrete lined tunnel housing a 84-inch penstock. Low-level intake, cen­ terline El. 4,530 ft High-level intake, centerline El. 4,585 ft Gates and Valves: One 4 ft by 4 ft high pressure slide gate, one 72-inch butterfly valve, and one 72-inch fixed cone valve.

Canal Gate Structure Type: Gated agee Capacity: 1,000 cfs Crest Elevation: 4,595 ft Crest Length: 60 ft Gates: Three, 20 ft high by 20 ft wide

Power Plant Installed Capacity: 6,500 kW Turbines: Two Francis type turbines, 3,250 kW each. Rated head 75 ft

1/ Includes spillway crest and canal gate structure crest. 2/ Tudor Engineering Company, Preliminary Report, Iron Creek Project (undated).

IV-2 existing canal and proposed outlet works and would reduce the length of contact between the enbankment and the spillway walls. Costs would probably be simi­ lar to those previously developed.

3. The diversion and outlet works will probably have to be larger, both to pass a di version flood and to re­ duce velocities and head losses for power production.

4. A single, custom-design, Kaplan turbine/generator unit may provide a less-expensive power plant installation.

A cost estimate for the Iron Creek Project, considering the above comments and drawing from the previous report, is present­ ed in Table IV-2.

Hydroelectric Benefits

Average annual energy production at the Iron Creek Dam project is estimated to be 31,800,00 kWh. This figure is based on average monthly flows presented in Chapter I I I, and a net head of 90 ft.

IV-3 Table IV-2

IRON CREEK DAM PROJECT RECONNAISSANCE COST ESTIMATE

Cost Escalation, Proposed Contingen- Previous Cost cies, Eqgr, Cost Item Estimatel / Change Admin 2/ Estimate Reservoir Clearing and Relocations 400,000 o 300,000 700,000

Diversion and Care of River 200,000 o 200,000 400,000 Excavation and Dam Foundation Treatment 2,500,000 0 2,000,000 4,500,000

Dam Embankment 2,700,000 0 2,100,000 4,800,000

Spillway 4,000,000 0 3,200,000 7,200,000

River Outlet Works 2,000,000 1,000,000 2,400,000 5,400,000

Canal Gate Structure 500,000 -200,000 200,000 500,000

Power Plant 4,000,000 o 3,200,000 7,200,000

TOTAL 16,300,000 800,000 13,600,000 30,700,000

l! Costs are from: Tudor Engineering Company, Preliminary Report, Iron Creek Project (undated).

~/ Escalation at 20%, Contingencies at 30%, Engineering and Administration at 15%.

IV-4 Chapter V PROJECT COMPARISON

The two principal alternatives examined in studies complet­ ed to date, rehabilitating the existing Corbett Darn and Tunnel and associated works and constructing a new diversion dam at Iron Creek, were compared on a present worth basis. The capital and operation and maintenance (O&M) costs of the projects were reduced by estimated hydro power benefi ts (power sales income) to obtain net capital costs for the projects (the capital costs that will be borne by the project developers). The comparative figures are presented in Table V-I

V-I Table V-I

COST COMPARISON

Corbett Iron Creek Darn Darn Capital Cost of Repairing Existing Diversion Works or Construction New Darn $2,500,000 $23,500,000 Capital Cost of Hydroelectric Power Plant 3,500,000 7,200,000 Present Worth of O&M costl/ 300,000 500,000 Present Worth of Hydro Power Benefits1./ (2,100,000) (13,900,000)

Net Hydro Power Benefits 0:1/ (6,200,000) Net Capital Cost of Repair or New Construction $2,500,000 $17,300,000

!/ Estimated annual O&M, interest at 10 percent project life 50 years.

~/ Present Worth of Hydro Power Benefits was computed assuming power sales at $0.044 per kWh, interest at 10 percent, and a project life of 50 years. l../ Initial reconnaissance estimates suggest that the Corbett Darn hydro installation may not be economical. More detailed studies may show lower cost or higher benefits, but at this time no net hydro development benefits can be assumed.

V-2 SCALE 0 2000 4000 FEET I I I

IRON CREEK DAM LEVEL II FEASIBILITY STUDY

PROJECT AREA MAP ENGINEERING COMPANY · DECEMBER 1982 EXHIBIT 1-2

L CONCRETE PAVED SLUICE EI. 4612.5 ---,

TOP OF WALL EI. 4620.5

TOP OF WALL ----tII~ EI. 4634.5

2 - 5' WIDE x 10' HIGH INTAKE GATES

...... -- ..... -- -~

1\ .. DAM SLAB. 1,1 SLOPE-, ~ .. d. J~ L L 4634.57 'I'- EMBANKMENT DAM CREST EI. 4625.5 DAM SECTION CREST EI. 4634.5 I II II II IJ II II r ,r ~

DAM BUTTRESS (TYPICAL) !

'~ L APRON EI. 4612.0-,

3 - 4' WIDE x 5' HIGH SLUICE GATES

r

NOTE: All elevations are original project datum, SCALE 0 10 20 FEET not current datum. I

IRON CREEK DAM LEVEL 1\ FEASIBI LlTY STUDY CORBETT DAM AND TUNNEL INT AKE EXISTING CONDITIONS I--lARZA ENGINEERING COMPANY· DECEMBER 1982 EXHIBIT 1- 3

(Earth section \~~ ~l \\ A ""\ % \ - \~ ": -'-, \ , \ Earth and gravel fill---- \,------_\

20 60

SCALE OF -E ET

15

SCALE OF ;EET

,Origlnol ground f'"7"~""""~~~~~..J-...:s:.:.:urfo ce SECT ION A-A

'<- • ------51 ' - - - EI. 4634, 5-_"-_r-n--dr1,----,--:-<-'""-

----Weep-holes - 4 each bay .' -----£1. 4616,0

" '\-'Sand, gravel "--EI4607 _ Bottom 'Earth and and boulders of buttresses grovel fill SECTION a-a SECTION C-C

SECTION 0-0

SOURCE: BuRec., Project Data, 1981

IRON CREEK DAM LEVEL II FEASIBILITY STUDY CORBETT DAM PLAN AND SECTIONS WAR.ZA ENGINEERING CClfV1PANY . DECEMBER 1982 \ 'iST DRAWINGS EXHIBIT 1- 4 A:COMPANYING SPECIFICATION'> CORBeTT TUNNEL \ GENEcRAl MAP SHOWING LOCA liON OF TUNNEc FIG. 1 riG. 2 \ SECTIO",S BORINGS

4 PLAII OF SUTL'NG BASIN. SLUICI~G TUNNEL AND EVBANKVENT 'C \ DET."LS OF SLUICINS TUNNEL PCRTALS AND GATE SHAFT

NO 6 GA F ~OUSEc NO 7 DE TAll S OF GA TE HOUSE DOORS

I I I I I II I I I I-~J I- 1-' I I 3'OH-.L.j I n- :1

I I I '~-l I I I I : I I I I I • " 1-T'6X8 I I I I I

J/iir0'ea 8 ellmouthed Sewer pt'pe. SECTION A B

SCALE OF fEU SECTION WHERE TIMBERING IS REQUIRED SECTION WHERE NO TIMBERING is REQUIRED , 0 I ! 1

, \

(--­ \ ;.1 .1;)

Sta /74. SECTION C - D FRONT VIEW OF LOWER PORTAL ~El458a5 . II Centre

SCAlf Of FrH 10 10 20 30 SOURCE: \ I! , If f I I I \ Original contract drawing dated June, 1905 \ \ \ \

\ \, IRON CREEK DAM SECTION E - F LEVEL II FEASIBILITY STUDY PLAN OF LOWER PORTAL CORBETT TUNNEL I--IAR..ZA ENGINEERING COMPANY· DECEMBER 1982 TUNNEL SECTIONS AND LOWER PORTAL EXHIBIT II - 1

6' x 6' PRECAST CONCRETE BOX CULVERTS ON END RESTING ON SLAB FILL UNITS WITH GRAVEL EI. 4628.5+

SPECIALLY CONSTRUCTED UNITS TO FIT

EXISTING FLASHBOARD ...... SLUI CEEL 4612.5 ----i----i~ ______...... ---

...... ~-----i---r--- TOP OF WEIR EI. 4620.5

II

SCALE 0 10 20 FEET I I I

IRON CREEK DAM LEVEL II FEASIBILITY STUDY PROPOSED INTAKE MODIFICATIONS

~ ENGINEERING COMPANY' DECEMBER 1982 AT CORBETT TUNNEL EXHIBIT II - 2

EXISTING CONCRETE TUNNEL LINING 0.75' MINIMUM ---,,;;.~.

LJ..ooIIIII--- PROPOSED 2" FIBROUS SHOTCRETE LINING

PROPOSED INVERT RESURFACING I I

PROPOSED FINGER DRAIN AT 6' CENTERS iN WET AREAS --- PROPOSED 10" DIAMETER FLOOR DRAIN

SCALE 0 1 2 3 4 FEET I I I I

IRON CREEK DAM LEVEL II FEASIBI L1TY STUDY

CORBETT TUNNEL REHABILITATION I--IAR.ZA ENGINEERING COMPANY· DECEMBER 1982 EXHIBIT 11- 2

NEW TRASH BOOM ---~

REMOVE ANGLED SECTION OF WALL HIGH WALL

NEW CONCRETE POURED AGAINST OLD WALL

1"----4--11-- 8' WIDE x 15' HIGH RADIAL SLUICE GATE

,....--- RAISE DAM CREST APPROXIMATEL Y TWO FEET

8.2' RUNNER DIAMETER, 1,200 kW TUBULAR· TYPE TURBINE/GENERATOR UNIT

NOTE: Layout shown was developed for cost estimating puposes only. The arrangement and dimensions of structures have not been studied in detail. SCALE 0 10 20 FEET I I

IRON CREEK DAM LEVEL II FEASIBILITY STUDY CORBETT DAM HYDRO DEVELOPMENT

I--lAR.ZAENG1NEERING COMPANY DECEMBER 1982 SCHEMATIC LAYOUT APPENDIX A

REPORT ON CORBETT Dru~ AND TUNNEL Appendix A REPORT ON CORBETT DAM AND TUNNEL

Description

Water for the 51,100 irrigable acres in the Garland and Frannie Divisions is diverted from the Shoshone River at Corbett Dam, located about six miles northeast of Cody. Irrigation flow enters the 3.3-mi long Corbett Tunnel through an intake immedi­ ately above the dam. The tunnel discharges to a settling pool formed by damming Iron Creek, a tributary of the Shoshone River. Garland Canal (which feeds the Frannie Canal) begins at a head­ works structure on this pool. The overall arrangement of the diversion works is shown on Exhibit I-I in the Main Report.

Corbett Dam

Corbett Di version Dam was buil t across the Shoshone River in 1908. From right to left looking downstream, the dam con­ sists of I} an earthfill section 440 ft long 2) a 400-foot-long reinforced concrete slab and buttress overflow spillway in the old river channel, and 3) a sluiceway and tunnel intake struc­ ture. The general layout of the dam and appurtenant structures is shown on Exhibit 1-2. Details of the spillway and tunnel intake, with cross sections of the major structures, are shown on Exhibits 1-3 and 1-4.

The earth section is a zoned dam 16 feet wide at the crest with 3H to IV slope upstream and 2H to IV slope downsteam. The earth section rarely impounds water. It is basically a free­ board dike for high water. The upstream slope is protected against infrequent wave attack by a layer of coarse gravel and boulders. The upstream half of the dam is a mixture of gravel

-1- and clayey fill. The downstream half is gravel and boulders. The toe of the upstream shell is keyed into the foundation.

The overflow dam crest is 13.5 feet above the surface of the downstream apron. Foundation material consists of about 5 feet of coarse alluvium over shale bedrock of the Willwood Formation. The spillway structure is keyed through the gravel i n to the shale bedrock.

There is very little useable storage in the reservoir. Most of the storage volume below the spillway crest is full of river gravels and sands. There is a 9-foot elevation difference between the crest of the spillway and the crest of the earth dam. Capacity of the existing slab and buttress spillway is estimated to be 20,000 cfs.~/

Tunnel Intake

The intake structure for the Corbett Tunnel is located at the left end of the slab and buttress spillway. The intake and its appurtenances were designed to divert water into the tunnel while deflecting most of the occasionally large volQmes of gra­ vel bedload of the Shoshone River.

The intake is isolated from the overflow crest by a re­ inforced concrete training wall perpendicular to the dam, that extends upstream from the crest about 130 feet. The top of the wall is even with the embankment crest elevation for most of its I ength and is 3 feet wide. \Alater flows around the training wall into the intake area.

!/ Bureau of Reclamation, Project Data (1981).

-2- There are three 4 ft wide by 5 ft high sluice gates in line with the dam immediately to the left of the training wall. The gates are intended to sluice gravel away from the intake struc­ ture which is separated from the sluiceway by a submerged weir with crest 8 feet above the sluiceway floor. 81 uice gate dis- charge capacity is about 1200 cfs.

Two 5 ft wide by 10 ft high intake gates control flow into the tunnel. There is a paved sloping apron between the intake approach weir and the tunnel intake. The tunnel intake and sluice gate sills are at the same elevation.

Despi te the design to excl ude bedload from the intake, gravel has heen a problem. A wedge-like mass of gravel that slopes up towards the weir repeatedly accumulates in the area at the upstream end of the intake approach weir. This slope per­ mits movement of gravel into the tunnel intake. After the high water of 1962, wooden flashboards 5 to 6 feet high were install­ ed on the upstream end of the intake approach weir. The flash­ boards provide an extra margin of safety against gravel reaching the intake.

The sluice and intake gates are all original equipment. They have bronze seals, slides, and operating gears. Gates are operated by a gasoline-engine operator that is manually moved from gate to gate on crane rails. A corrugated metal gatehouse above the tunnel intake protects the gate operators and provides storage space. 8toplog slots are provided for both the intake and sluice gates. There is no log boom or trashrack system.

-3- Tunnel

The Corbett Tunnel is a concrete-lined 10.75 ft high horse­ shoe tunnel, 17,250 feet long. The tunnel slopes 24 feet from the intake to the exit portal. The concrete lining is nominally 9 inches thick and unreinforced. The tunnel heading was ad­ vanced through the Willwood Formation shales and sandstones. Records of tunneling conditions were kept and are on file with the BuRec in Billings, Montana. When ground conditions required support during original construction, the heading was timbered wi th 6-in x 6-in timber bents on 3-foot centers, wi th I-inch wood planking behind the timbers. The timbers (where required) were embedded in the concrete lining. While this would indicate a minimum of 3 inches of concrete over the timbers, an examina­ tion of available construction photographs reveals that due to overbreak in excavation, the thickness of concrete over the timbers is probably in excess of 3 inches at some locations (see Exhibit 1-4 for typical tunnel sections).

Drainage was provided under the invert by a 4-inch sewer pipe beneath the centerline and gravel-filled trenches at either edge of the flow section.

The tunnel route is shown on Exhibit I-I. The land above the tunnel is irrigated by the Heart Mountain Division of the Shoshone Project. There are several intermittent streams, canal laterals and ponds above or close to the tunnel alignment.

The invert was completely resurfaced in 1918. Repairs have been few and infrequent. The only major liner failure occurred in 1981 at station 3+00 where a 2.5 ft by 8 ft section of liner near the crown fell, exposing portions of the timber ribbing and the shale in the tunnel wall at that location.

-4- Iron Creek Pond

The exit portal for the tunnel is on the right bank of Iron Creek, a tributary of the Shoshone River. The invert at the portal is about 60 feet above the Shoshone River level.

A short homogeneous earthfill dam across Iron Creek im­ pounds water from Iron Creek and the Corbett Tunnel, and forms a headpond for the Garland Canal. A low-level outlet for the pond was constructed by tunneling through natural ground to the left of the embankment dam. Discharge capaci ty of the outlet is about 300 cfs. Flows are controlled by a manually operated 3- foot diameter gate valve at the throat of a symmetrical venturi.

Emergency spillway capacity for high flows on Iron Creek is provided by an earth cut to the right of the embankment.

The headworks for the Garland Canal are on the north bank of the pond. The canal headworks structure is reinforced con­ crete wi th six, 4 ft wide by 8 ft high vertical lift gates equipped with manual operators. There is no log boom or trash- rack on the canal headworks.

Studies Conducted

Studies of the Corbett Dam and Tunnel consisted of data collection and review, site inspection, and office analyses.

Data Collection and Review

Study of the dam and tunnel involved interviews and data collection sessions with the BuRec in Billings, Montana, and the Shoshone Irrigation District in Powell, Wyoming. Original con-

-5- struction specifications, contract drawings, tunnel geology records, and construction photographs were examined in Powell. Some of these items were reproduced for further analysis in Harza's offices.

The following items were obtained from the BuRec in Bil­ lings:

Corbett Tunnel, Resurfacing Invert, by G.O. Sanford, 1918.

February 26-27, 1963, Inspection Report, by W.J.H. Nelson. (This report contains copies of original design drawings, water quality sample test reports, and summaries of con­ struction condition records for the tunnel).

February 25-26, 1963, Inspection Report, by J.W. Morris.

Report of Proposed Resurfacing of Invert, by C.J. Hoffman, Civil Engineer, April 1968.

Inspection of Corbett Tunnel, Garland Di vision, Shoshone Project, by BuRec, UM-435, February, 1981.

Proposal for Engineering Evaluation of Corbett Tunnel, by K.D. Schoeman, (USBR), September 4, 1981.

The information gathered in Powell and Billings was summarized so that tunnel geology, support requirements during construc­ tion, groundwater conditions, repair history, and prior inspec­ tion observations, all by tunnel station, could be examined at a glance. Summary sheets are presented on Exhibit A-I, Sheets 1 to 18 of 18.

-6- Inspection

The dam, tunnel and Garland Canal Headworks were inspected during October 26 and 27, 1982, by the following Harza and sub­ contractor personnel:

W.A. Rettberg Project Manager D.R. Baier Lead Rehabilitation Engineer R.A. Paige Lead Geologist lfl • A. Hamilton Senior Hydraulic Engineer G.R. Mass Senior Concrete and Materials Engineers D.A. Simons Senior Hydraulic Engineer, Simons, Li and Associates D.H. McCandless Lead Planning Engineer

On October 26, the inspection party entered the tunnel through the intake gate openings. The sluice gates were open and flow in the river was low as a result of regulation at Buf­ falo Bill Dam upstream. The flashboards and crest of the intake approach weir were above water. The Iron Creek Pond at the tunnel exit portal was drained. The inspection party recorded observations on portable tape recorders. Survey subcontractor Engineering Associates of Cody had been previously retained to paint stationing at 100-foot intervals on the tunnel wall to aid in locating significant features. Numerous photographs were taken and Swiss impact hammer concrete strength tests were con­ ducted at SOO-foot intervals. The entire walk-through of the tunnel took about 4 hours.

The day following the walk-through tunnel inspection, Oc­ tober 27, the dam sluice gates were closed and the tunnel intake gates were partially opened to permit the inspection team to observe flow conditions at the tunnel entrance. Tunnel flow was estimated to be about 300 cfs during this test.

-7- The dam was inspected from the left abutment. No soundings were made near any of the structures. The earth embankment was not inspected.

The Garland Canal Headworks was inspecte~ in-the-dry. The Iron Creek sluice entrance and exit were inspected, but the gate valve was not examined due to flow through the structure.

Condition of the Project

Corbett Darn

Corbett Darn appears to be in good condition.

The earth dam appears to have an uneven crest and portions of the embankment are covered with trees and brush. This is of little consequence, however, since the embankment is basically a freeboard dike. Except for some exposed aggregate on the crest and the toe of the overflow section, the concrete in all the exposed structures appears to be in excellent condition. No repairs are necessary at this time.

There are no signs of settlement or cracking of the over­ flow weir. The overflow apron was submerged at the time of the inspection, with water flowing through the sluiceway. No boils or any signs of distress could be seen at the toe of the dam or in the apron area. The apron reportedly has never been dewater­ ed for inspection or probed and sounded to check its condition. The apron should be checked for signs or cracking or toe ero­ sion. Dewatering is not required.

-8- The capacity of the overflow spillway is about 20,000 cfs.~/ The Probable Maximum Flood (PMF) outflow from the raised Buffalo Bill Reservoir upstream, however, will be about 72,000 cfs.~/ The earth dam would be overtopped by large floods. Failure of the embankment under these conditions would probably not cause much, if any, incremental damage downstream. Increasing the spillway capacity might be considered to protect the owner's investment in the dam. However, the return period for the floods that would severely damage the dam is large. The benefit-cost ratio to the owner for spillway expansion would be very low, therefore no action is recommended.

While stability and structural analyses of the overflow structure were not performed, the general design seems applic­ able to the site conditions and appears to be a typical, conser­ vative slab and buttress design. If desireable for hydraulic reasons or to make the economics of hydro-power at the dam more favorable, the spillway crest could probably be raised by at least two feet.

Corbett Tunnel Intake

The physical condition of the exposed portion of the tunnel intake and gravel sluice structure appears good, although there is no way to conveniently dewater the entire area above the intake and sluice for inspection. The base of the training wall

!/ Bureau of Reclamation, Project Data (1981).

~/ Bureau of Reclamation, Modification of Buffalo Bill Dam, Shoshone Project, Wyoming, Feasibility Report, Appendix A - Water Supply (Revised August 1975).

-9- to the right of the sluices is in good condition but has never been probed to check for undermining. A general sounding and probing of the intake area slab and training wall base should be completed at an early date.

The three sluice gates seal very poorly. The bottom sill may have been badly eroded. While good sealing gates are not necessary for optimum tunnel operation, the deterioration should not be permitted to progress. Eventually the lower gate guides might be worn away, endangering gate operation. Unfortunately, the stoplog slots upstream of the gates may also be badly worn, to the point that a good seal there might also be impossible.

According to the operating staff, trash and sediment accu­ mulate at the tunnel entrance. There is no trash boom or trash­ rack to protect the tunnel intake. During high river flows gravel is reportedly carried around the upstream end of the training wall where it is deposited against the curved, upstream end of the tunnel intake weir. As deposition progresses, a ramp of gravel forms up to the top of the weir so that some gravel then enters the tunnel. High water in 1962 resul ted in exten- sive sediment deposition in the tunnel. The present flashboard system was installed after that flood to help control the gravel problem.

Observations and photographs show a strong counter-clock­ wise circulation (plan view) in the pool between the curved weir and the gates. Thus the flow approaches the gates at about a 45 degree angle. Vortices just upstream from both gates are appar­ ent even when the weir and the gate openings are well submerged. The cross flow may reduce the tunnel capacity slightly. It probably causes little structural damage although it may encour­ age rocks and floating debris to strike the center pier and

-10- curved walls. The damage pattern on the pier and wall s, how- ever, does not show a direction-of-approach character.

The flashboards that prevent flow and gravel movement over the weir may be the chief cause of the circulation. While the poor circulation may decrease tunnel flow capacity slightly, al tering the design does not seem worthwhile for this reason alone. The capacity of the tunnel is reportedly satisfactory at the present time.

Corbett Tunnel

Hydraulics. In its present condition the tunnel probably has a composi te val ue of Manning f s roughness coefficient (II nil) of 0.01 7. This value was chosen after studying the resul ts of measurements in other concrete tunnels. It takes into account the slightly roughened walls, the exposed gravel in the floor, and the two bends. The corresponding capacity is about 710 cfs. It makes no difference whether the depth is uniform with the water surface 1.25 feet below the crown, as indicated by water marks in the tunnel, or Whether the upstream two-thirds runs full due to a small surcharge near the entrance. If the val ue of lin" were 0.016, the capacity would be 760 cfs.

Careful cleaning, patching and placing a smooth coating on the floor and walls of the tunnel would probably increase the capacity to 1,000 cfs. Data from many concrete tunnels, how­ ever, indicate that the improvement most likely would be tempo­ rary.

Physical Condition. The Corbett Tunnel is in remarkably good condition considering the age and environment. This at­ tests to the quality of workmanship incorporated into the ori-

-11- ginal construction. The concrete mixture used, which is com­ parable to a 470 pounds of cement per cubic yard, 1-1/2" maximum size aggregate, non-air-entrained mixture, has performed very well.

Detailed inspection notes are summarized by tunnel station on Exhibit A-I.

The tunnel, when inspected, was clear of any debris except for 2 boulders which were sitting in the invert. Marks from the original form lining can be seen throughout the tunnel length. The sidewalls are covered with a thin film of dark material which can be easily removed by scraping. Thickness of the film is estimated at 1/32 inch. The invert of the tunnel is eroded to a depth of several inches, exposing the coarse aggregate particles. Some coarse aggregate has been undercut and sockets indicate some coarse particles have been plucked out. Nowhere has the erosion exposed the timber supports used at irregular locations in the tunnel. At most locations, the 1918 mortar resurfacing can be seen along the contact with the sidewalls. At several locations, grout pipes are seen protruding from the invert. It is obvious that the coarse aggregate exposed in the invert had a very high abrasion or impact resistance.

None of the cracks observed in the sidewalls or crown indi­ cate any current movement even though their width is as much as 3/8 to 1/2 inch in some locations. Patches over the cracks show no to very slight reflective cracking. Some of these patches date back to 1941.

Impact hammer (Swiss Hammer) readings were made periodical­ ly during the inspection. Based on this method of test, minimum compressive strength should be about 2,500 psi. The test proce-

-12- dure used during the inspection was to scrape the film off in a localized area on the sidewall. Five to eight measurements were then made and a rough average of the readings was selected and converted to compressive strength using the correlation scale supplied with the instrument.

Two conditions in the tunnel have affected the concrete and will continue to do so unless corrective measures, as recommend­ ed, are taken. The conditions affecting the concrete are:

1. Hydraulic pressure around the lining in at least five major areas: Sta. 30 to 35, Sta. 61 to 64, Sta. 90 to 93, Sta. 118 to 120, and Sta. 155 to 157. Minor seep­ age was noted at other locations.

2. A sand, gravel, and cobble bedload passing through the tunnel at rather low velocity (less than 10 feet per second) .

Hydraulic pressure on the lining has caused seepage and inflow through the concrete and into the tunnel. This is the major problem. The gravel abrasion on the invert is tolerable at this time, since it has not yet caused structural problems and the increased hydraulic roughness has apparently not reduced flow capacity to a critical point.

The action of water passing into the tunnel through cracks, joints, porous zones, and other passages has leached the cement from the concrete. Therefore, the concrete in those areas has lost strength and has become more suspectible to erosion, spall­ ing, localized failure, and fallout. It is possible that dete­ rioration in those affected zones may be aggravated by dissolved sal ts in the groundwater which have attacked the cementing ma-

-13- terial (also termed sulfate attack). Previous seepage water samples (taken by BuRec) indicate a sulfate content of 1,358 to 1,584 ppm which can be classified as moderately to severely aggressive. Attempts have been made to alleviate the hydraulic pressure by grouting and installation of weep hol~s. These measures were minimal and do not appear to have been very effec­ tive. Posi tive means are necessary to intercept water behind the lining and quickly drain it in a controlled manner. The areas of major seepage inflow seem to correlate with zones of soft sandstone or areas where streams, canal laterals or stock­ ponds are above the tunnel alignment (see Exhibit A-I). The only major tunnel lining failure occurred in 1981 in an area of soft, wet shale which ws an area of trouble during original construction.

Garland Canal Headworks

The Garland Headworks structure is in fair condition. The gates have reportedly not been exercised in many years and are no longer operable. The structure has set tIed and some struc­ tural cracking can be seen. There is some abrasion and freeze­ thaw damage around the gate sill and slots and piers. Under present operating conditions, there does not appear to be a need for a gated structure at the entrance to the canal. The present structure now serves as a poor trash collector. Trash washed down Iron Creek during floods collects on the headworks, wi th some of the trash passing through the structure into the canal. The layout of the existing structure makes trash removal diffi­ cult.

-14- Recommended Improvements

Corbett Darn

No improvements appear to be needed on Corbett Dam. How­ ever, the downstream apron should be inspected or probed at low water to verify its satisfactory condition.

Corbett Tunnel Intake and Sluice

The existing sluice gates do not seat on the bottom, indi­ cating a need for gate sill repairs. If possible, the gates should be dewatered using the existing stoplog provisions. A diver should be retained to inspect the stoplog slots. Even if the slots are badly worn, it might be possible to pI ug stoplog leaks with sandbags, cinders, and canvas well enough to allow the gates to be fixed. If this cannot be done, a cofferdam would be required. Estimated repair cost without a cofferdam is $10,000. Estimated repair cost with a cofferdam is $40,000.

The tunnel intake gates and the submerged intake approach weir just upstream are in good condition. One of the wooden flashboard sections has failed. No repairs to the weir or gates are necessary at this time.

The existing flashboards should be replaced. Some proto­ type experiments with the operation plan for the tunnel and sluices, plus more substantial flashboards or a simple substi­ tute to help excl ude gravel from the tunnel, are suggested. Al though the present configuration near the tunnel entrance is not completely satisfactory from a hydraulic viewpoint, it ac­ tually has protected the tunnel lining from serious abrasion (see below). Since the invert was last resurfaced in 1918, the

-15- tunnel has operated 44 seasons without flashboards and 20 years with them. In 64 years part of the invert has been worn down only a few inches. No costly changes intended to prevent fur­ ther abrasion seem warranted, but two options to improve opera­ tions have been identified.

The first option would be to replace the flashboards in- kind and al ter the operation plan. Prototype experiments would be required.

The amount of gravel coming in to the intake area probably depends upon the sum of the water discharges through the tunnel and the sluices. Estimated flow through the open sluice gates at normal reservoir pool is 1,200 cfs. This is at least 1.5 times the Corbett Tunnel flow. Including the tunnel flow, about 2,000 cfs would be entering the sluice-way, whereas the figure would be around 800 cfs if the sluice gates were closed. It seems clear that the larger flow would carry more gravel into the sluice than the smaller flow. However it is difficult to say which flow condition would carry more gravel over the weir into the intake. Wi th the sl uice gates shut, any gravel that entered the sluice-way would either be deposited or go through the tunnel. Thus, permanent closing must be ruled out, but there could be an advantage in operating the tunnel and sluices al ternately or leaving the sluice gates partly open all the time. The advisability of such changes in operation should be considered and discussed wi th personnel who have viewed opera­ tions during high river flows.

The second improvement option would be to eliminate the flashboards and replace them with a simple precast concrete caisson training wall as shown on Exhibit II-I. The wall is proposed to guide flow into the sluiceway without allowing

-16- gravels and cobbles to accumulate against the weir. Also, by narrowing the entrance to the sluiceway, gravels should be swept through more efficiently. Estimated construction cost for this option is $70,000. The precast caisson sections could be fabri­ cated in the District I s yard and placed wi thout dewatering the intake area.

While there is a third option, replacing the existing flashboards with a higher, more permanent, structure on top of the weir, the cost is between the cost of the caisson structure described above and the cost of replacing the boards in-kind. The cost for the caisson al ternat:ive should therefore be used for conservative planning.

Trash on the flashboards and in the tunnel has been a minor problem for many years. Constructing a trashboom extending nearly straight upstream from the existing training wall or even angled slightly towards the center of the river, should be con­ sidered. An anchor structure in the river would be required. If the boom is positioned properly, high flows over the spillway should sweep debris off the boom. About $30,000 should be used in planning studies to account for the trashboom.

Corbett Tunnel

Corbett Tunnel is generally in good condition. However, the known areas of seepage should be repaired by drainage, seal­ ing, and patching as required. This would solve most of the immediate problems but would provide no margin of safety against future tunnel service interruption due to fail ure of the liner in areas that are not in visible distress now. More detailed coring, pulse-echo, or radar testing would be required to iden-

-17- tify potential trouble areas that show no visible signs of dis­ tress.

Without testing to identify areas of potential future trouble, the conservative assumption for planning purposes is to strengthen the entire lining regardless of its present condi­ tion. Such a program would include the foI'lowing:

1 . Cement or other type of grouting should be per formed along the wet areas of the tunnel to reduce inflow and further leaching of original concrete. (Current in­ flow is estimated at 0.25 to 0.5 cfs at the tunnel discharge portal). The grouting should help make it easier to perform other work including drainage con­ struction (see below).

2. Tunnel crown and sidewalls (including cracks and fis­ sures) should be thoroughly cleaned by high pressure (6,OOO psi) water blasting.

3. Beg inning at the first wet area, a trench should be excavated through the invert lining. The existing clay tile drain beneath the invert should be removed and replaced wi th a larger plastic drain pipe (10 "¢) • Three-inch-wide slots should be cut in the sidewalls to 12" above springline on 6-foot intervals through wet areas, and finger drains (1-1/2 to 2"¢) installed and connected to the floor drain. Slots should be backfilled with dry pack or concrete. Each wet area should be treated in a similar manner.

4. All long i tudinal cracks in the sidewall s and crown should be caulked or sealed.

-18- 5. A 2-in minimum thickness fibrous shotcrete lining should be applied to the sidewalls and crown to pro­ vide structural support and protection for the origi­ nal concrete lining. A thin (1/8 to 1/4 in) finish coat of shotcrete would be applied over the fibrous shotcrete to achieve surface smoothness. The water jet cleaning described in item 2, above, would serve as sufficient surface preparation for the shotcreting. No rock bolting is envisioned.

6. Invert abrasion-erosion damage should be repaired. The alternative repair materials are as follows:

a. Conventional concrete b. Low water-cement ratio conventional concrete with superplasticizer, c. Low water-cement ratio fibrous concrete with superplasticizer, d. Latex modified portland cement concrete, e. Epoxy mortar, or f. Polyurethane surface coating.

Of the above, "c" is expected to provide the most economical abrasion resistant invert.

Estimated cost for the repair work described above is $2,260,000 including contingencies, engineering, and administra­ tion. This cost covers work that is adequate to restore the tunnel and make the future service life of the lining as good as it was when originally built.

The above estimate assumes that the entire tunnel would have a new drainage system installed with a new shotcrete liner

-19- throughout. However, it may be cheaper in the long term to repair the areas of obvious distress, seal the cracks, and resurface the invert now for an estimated cost of about $1,100,000. The difference in cost between this alternative and complete tunnel rehabil i tation (about $1,200,000) could then be used to finance annual repairs of deteriorated liner on an as­ needed basis. A standard set of forms could be purchased to make any annual repairs quicker, better, and easier. The $1,200,000, above, amortized at 10% over 50 years should yield sufficient funds to repair 50 to 100 linear feet of tunnel with an all new reinforced concrete lining each year.

Making only the obviously necessary repairs mayor may not be less expensive in the long term, but this strategy involves added risk. There is a greater chance of service interruption due to lining failure. The risk can be minimized if additional testing, consisting of core drilling, and pulse-echo, and radar sounding is conducted to identify areas of probable future dis­ tress so tha t they can be fixed now. Approximately $100,000 would be required for a thorough testing program. The testing program should probably be conducted regardless of the adopted repair option selected, to verify the conclusions in this re­ port.

Garland Canal Headworks and Iron Creek Sluice

The existing headworks structure could be converted into a more serviceable trashrack structure for the canal for about $60,000. If it is decided that operational gates are desire­ able, the present gates could probably be rehabilitated for about $20,000.

-20- The headwall at the entrance to the Iron Creek Sluice needs repair; extensive erosion has occurred. Repair cost should be about $20,000. The exit portal is in good condition. The gate valve was not inspected due to high flow. An inspection should be made when flow conditions permit.

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