HomeMy WebLinkAboutCordova Hydro Storage Assessment Project_Snyder Falls Creek Feasibility - Oct 2013 - REF Grant 7013013Cordova Electric
Cooperative
Snyder Falls Creep
Hydroelectric Project
Final Feasibility Cost Estimate
�',MDRTON D. McAMLLE)f•; 4
Na, CE12523,
Prepared For: Cordova Electric
Cooperative
Prepared By: McMillen, LLC
October 30, 2013
MCMILLEN, LLC
Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
TABLE OF CONTENTS
Section1
........... .......... ....... ............... ............................................................................................................ I
Introduction...................................................................................................................................................
I
1.0
Introduction...................................................................................................................................1
1.1
Purpose..........................................................................................................................................1
1.2
Scope.......................................................................................,.,...................................................1
1.3
Background...................................................................................................................................1
1.4
Pertinent Data Sources ...............................................................................................................1
1.5
Report Organization......................................................................................................................
2
Section2
.......................................................................................................................................................3
EngineeringConsiderations
..........................................................................................................................3
2.0
Introduction...................................................................................................................................
3
2.1
Engineering Geology and Dam Considerations............................................................................
3
2.1.1
Engineering Geology........................................................... ........
3
2.1.2
Dam Types....................................................................................................................................
6
2.1.3
Construction Materials..................................................................................................................7
2.1.4
Future Investigations.....................................................................................................................8
2.2
Project Description........................................................................................................................9
2.2.1
Location........................................................................................................................................
9
2.2.2
Dam.............................................................................................................................................10
2.2.3
Reservoir.....................................................................................................................................10
2.2.4
Intake...........................................................................................................................................11
2.2.5
Power Conduit ................................................. ...............11
2.2.6
Powerhouse....„...........................................................................................................................11
2.2.7
Transmission Facilities ....................................... ..................11
Section3
..........................................................,..,....,.................,................................................................12
Construction
Approach and Cost Estimate.................................................................................................
12
3.0
Introduction...........................................,.....................,.........,.....................................................12
3.1
General........................................................................................................................................12
3.2
Basis of Cost Estimate... .............................................................................................................
12
3.3
Cost Items...................................................................................................................................
12
3.4
Project Construction....................................................................................................................
13
3.4.1
Site Access..................................................................................................................................
13
3.4.2
Materials.....................................................................................................................................
13
3.4.3
Construction Methodology.........................................................................................................
13
3.4.4
Unusual Conditions (Soil, Water, and Weather).........................................................................
13
3.4.5
Unique Construction Techniques................................................................................................
13
3.5
Construction Approach...............................................................................................................
14
3.5.1
Mobilization................................................................................................................................14
3.5.2
Site Access.............................................................................................................................14
3.5.3
Dam Structure.........................................................................................,.............................14
3.5.4
Penstock......................................................................................................................................15
3.5.5
Electrical and Communication....................................................................................................
15
3.5.6
Powerhouse & Equipment..........................................................................................................
15
3.5.7
Switchyard & Transmission Line...............................................................................................
15
3.6
Project Cost Summary -------------- ................................................................. .................................
15
3.7
Conclusions ........................ .........................................................................................................
15
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
Section4.....................................................................................................................................................17
Conclusions and Recommendations...........................................................................................................17
4.1 Conclusions.................................................................................................................................17
4.2 Recommendations.......................................................................................................................17
TABLES
Table 1-1. Report Organization and Purpose...............................................................................................2
Table 2-1. Preliminary Evaluation of Dam Types ...................................... 6
.................................................
Table 3-1. Estimated Project Costs (October 2013 dollars).......................................................................16
FIGURES
Figure 2-1. This USGS topographic map of the project area presents some of the glacial and tectonic
features present relative to the lower dam site. Note the lineament mapped immediately southeast of the
lowerdam site...............................................................................................................................................4
Figure 2-2. This photograph is an oblique view of the lower dam site during the winter season. Note the
abrupt vegetation line on the west side of the creek and the recent slide activity emanating from the snow
chutes on the eastern ridge. Photograph source is Google Maps.................................................................. 6
Figure 2-3. Area Map of Snyder Falls Creek Project.................................................................................10
APPENDICES
Appendix A — Cost Estimate Assumptions
Appendix B — Bid Totals and Cost Reports
Appendix C — Figures
Appendix D — Photographs
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
SECTION I
INTRODUCTION
1.0 Introduction
Section 1 presents the overall project organization, scope, and background as well as the purpose of the
cost estimate study.
1.1 Purpose
The purpose of this report is to present a feasibility level discussion of the construction approach and
estimated project cost for the proposed Snyder Falls Hydroelectric Project (Project).
1.2 Scope
The scope of this feasibility level cost estimate study included:
• Visit the project site to view the proposed location and site conditions for the proposed Project;
• Obtain available data related to the Project development including photos, previous reports and
studies, and proposed project features;
• Develop options for accessing the project site and constructing the project features;
• Prepare a feasibility level cost estimate for the Project.
• Summarize the analysis and results in a cost estimate report.
1.3 Background
Cordova Electric Cooperative (CEC) submitted a Notice of Intent to file for an original license to
construct and operate the Snyder Falls Creek Hydroelectric Project (Project) located near Cordova,
Alaska in 2009. Within this submittal, a Pre -Application Document (PAD) for the Project was included
which outlined the basic features of the project which included:
• A 100- to 150-foot high concrete dam impounding a 433 to 933 acre reservoir, respectively;
• A power conduit approximately 3600 feet long from the reservoir to the powerhouse;
• A powerhouse, switchyard, and other appurtenant facilities near tidewater; and
• A primary transmission line of submarine and overhead or underground construction.
1.4 Pertinent Data Sources
The following data sources were used in developing the cost estimate study:
• "Site Reconnaissance and Hydrology, Snyder Falls Creek" PowerPoint presentation prepared by
Cordova Electric Cooperative, April 16 2013.
• "Application for Preliminary Permit for the Snyder Falls Creek Hydroelectric Project near
Cordova, Alaska", submitted by Cordova Electric Cooperative to the Federal Energy Regulatory
Commission, November 2008.
• "Notice of Intent, Pre -Application Document, Request to Use Alternative Licensing Procedures,
Snyder Falls Creek Hydroelectric Project, FERC No. 13238-000", prepared by Cordova Electric
Cooperative, October 2009.
• Aerial Images and Site Photos provided by Cordova Electric Cooperative, September 2013.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
1.5 Report Organization
The report presents the feasibility level review and cost estimate preparation for the Project. The report is
organized to provide a logical representation of the cost estimate preparation for the Snyder Creek
Hydroelectric Project. The major report sections and intended purpose are presented in Table 1-1.
Table 1-1. Report Organization and Purpose
Section
Description
'ur ose
1
Introduction
Summarizes the project purpose, background, and scope
Presents the engineering considerations and assumptions
2
Engineering Considerations
which served as the baseline for preparing the project cost
Construction Approach and Cost
estimates
3
Outlines the construction approach and associated cost
Estimate
estimates for the proposed project
4
Conclusions
Summarizes the approach, conclusions, and
recommendations from the study work effort
Appendices
A
Cost Estimate Assumptions
Outlines the assumptions used in developing the cost
estimates
B
Cost Estimates
Contains the cost breakdown and data
C
Access Options
Presents sketches of the proposed access options
considered for project construction
D
Photo ra hs
Consists of relevant photos of the proposed project site
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
SECTION 2
ENGINEERING CONSIDERATIONS
2.0 Introduction
Section 2 presents a brief overview of the engineering considerations associated with the Project. The
information presented within this section was obtained from the reference documents (see paragraph 1.4)
and the project team judgment related to the Project development.
2.1 Engineering Geology and Dam Considerations
2.1.1 Engineering Geology
The project site is located within a glacially -carved cirque valley and the lower dam site is sited at the lip
of this geologic feature (Figure 1). Based on reconnaissance photographs a small cirque nick -point is
present at the lip, creating the incised channel that present today. The sides of this nick -point appear to be
bedrock.
Bedrock geology at the project site is anticipated to consist of thinly to thickly bedded greywacke
sandstone with interbedded fine-grained rock types. The beds are steeply dipping (-80 degrees) to the
south and are readily evident in bedrock exposed along valley walls. Published geologic maps and
reconnaissance lineament mapping indicate there is a prominent lineament that trends north northeast
across the valley immediately upstream of the lower dam site. Bedrock in the vicinity of this lineament is
likely highly fractured due to movement along it.
The majority of major lineaments mapped in the area have similar trends as that shown on Figure 1.
However; there are mapped lineaments that trend to the northwest. This is a similar orientation as the
Snyder Falls Creek drainage. Several northwest trending joints are visible in oblique aerial photographs of
the nick point. The potential for a lineament trending down the drainage access should be evaluated as
part of the site investigation.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
Figure 2-1. This USGS topographic map of the project area presents some of the glacial and tectonic
features present relative to the lower dam site. Note the lineament mapped immediately southeast of the lower
dam site.
Just upstream of the cirque lip, the valley floor is mantled by glacial soils that appear to be a combination
of gravelly morainal and alluvial deposits. The thickness of these soils is unknown but is not likely to
extend significantly below the depth of nick -point. Further up the valley, bedrock outcrop exposures are
predominant in the valley floor.
Based on preliminary evaluations the primary engineering geology uncertainty that could influence
project feasibility and cost include:
• Rock mass quality of the lower dam abutments
• Seepage along faults and fractures
• Slope stability at the dam abutments and slopes surrounding the reservoir
• Snow avalanches
Rock Mass Quality - Rock mass quality at the lower dam abutments will influence foundation support
capacity for the dam and the configuration of how the dam connects into the abutment hill slopes. The
rock lip lacks passive support on the downstream side as the slopes drop steeply off to the north. Aerial
oblique photographs collected at the site indicate the rock appears to be moderately to highly fractured
with thin to thick bedding. The thin interbeds are likely to be significantly weaker than the greywacke.
Rock quality may also be compromised due to the presence of a large lineament immediately south of the
lower dam site. Rock quality will need further evaluation to determine bearing capacity and the potential
for rock block failure. These evaluations will influence the selection of the type of dam that most suitable
for the site.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
Seepage - Seepage is the loss of reservoir water through fractures and openings in a rock mass or
interstitially in soils. Seepage is often worsened by the presence of highly fractured or faulted bedrock. If
not controlled, seepage through rock and soil can lead to local slope instabilities or worst case, dam
foundation failure. In addition, water volume loss through seepage under and around the dam would
represent a significant drop in overall project efficiency.
The through -going lineament mapped near the downstream abutment is likely to have created zones of
higher permeability bedrock. Since the lineament roughly parallels the primary drainage to the north, the
potential for seepage pathways perpendicular to the dam axis may have limited through -going pathways
by the presence of fine-grained bedrock interbeds. However, the potential for a northwest trending
lineament through the nick -point could create a preferential pathway for seepage to the north.
Slope Stability - The primary slope stability hazards within the project area are rockfall and rock slides.
Rock cliffs along the eastern ridge above both dam sites are likely to produce small to large rockfall
events. These are unlikely to significantly impact the lower dam site but could damage the penstock and
ancillary structures within the project area. Therefore, the project infrastructure should be designed to
either avoid or resist rockfall impacts.
Rockslides are typically much less frequent but can be significantly more damaging. A moderate
rockslide may result in minor to moderate damage to project infrastructure. Large rockslide events may
discharge directly to the reservoir, potentially producing a seiche that could overtop the dam when the
reservoir is full. Evidence of large scale rock slope failure was not observed in the reconnaissance
photographs.
Snow Avalanche - There is significant exposure to snow avalanche hazards within the project area. At
the lower dam site there are several avalanche chutes that extend over 900 feet above the dam foundation.
Vegetation and recent snow slide history (Photograph 1) indicate that snow slides regularly reach the
creek channel. Similar hazards are present at the upper dam site in association with the eastern ridgeline.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
Figure 2-2. This photograph is an oblique view of the lower dam site during the winter season. Note the
abrupt vegetation line on the west side of the creek and the recent slide activity emanating from the snow
chutes on the eastern ridge. Photograph source is Google Maps.
2.1.2 Dam Types
A preliminary evaluation of suitable dam types is summarized in Table 2-1. Based on site and seasonal
construction constraints it appears that a concrete arch dam would be the most favorable type of structure
based on current information. Alternative dam types may be considered based on geologic and site
constraints.
Table 2-1. Preliminary Evaluation of Dam Types.
Dam
Structure
Advantages
Key Considerations
• Limited amounts of concrete
• Rock needs to be hard, durable, and resistant to
required
weathering — high quality rock appears to be present in
• Simpler to construct relative to
valley upstream
concrete structures - need
• Structure requires relatively wide footprint — slopes on
earthmoving equipment only
the order of 1.51-1:1 V — 2H:1 V
• Can be constructed in poor
• Requires an impermeable membrane — a natural source
Rockfill Dam
Weather (rainfall does not
(clay) is unlikely so would require artificial barrier such
significantly impact)
as concrete
• Materials almost entirely
• Settlement needs to be limited to maintain impermeable
available on site
membrane distress
• Lowest foundation demands
• Adaptable to a range of
foundation and abutment
conditions
Concrete
• Structure has a smaller
• A high quality rock source for aggregate required for
Gravity/RCC
footprint than a rockfill
RCC or PCC - It appears that local rock quality is
Dam -
structure
suitable for aggregate
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
• Foundation demands less than
• Structure width in between rockfill and concrete arch —
concrete arch
typical width ranges from 2/3x to Ix height of
• Adaptable to a range of
structure.
foundation and abutment
• Limited suitable aggregates source materials <3" in
conditions
size. Aggregate would likely have to be processed on
• Can be curved or straight
site.
• Needs competent rock foundation
• More weather sensitive than rockfill
• Smallest structural footprint
• Structure requires a relatively narrow gorge, typically
• Structure requires lowest
crest length:height ratio Is <10:1 — the site appears to
concrete volume, therefore
meet this criteria
potentially least expensive
• A high quality rock source for aggregate required for
RCC or PCC - It appears that local rock quality is
suitable for aggregate
Concrete Arch
• Limited suitable aggregates source materials <3" in
Dam
size. Aggregate would likely have to be processed on
site.
• Very high foundation and abutment load demands -
need high quality rock in foundation and abutments
Need high quality rock source for concrete aggregate —
rock would be processed on site
• More weather sensitive than rockfill
Dam Site Evaluation. Based on review of the site topography, geomorphology, and aerial photos, it
appears that the site is favorable both geologically and geometrically for the siting of a dam structure.
The rock appears to have both high strength and weathering resistance which indicates that the foundation
bearing capacity is likely sufficient for a concrete arch -type structure. This type of structure generally
requires the highest foundation demands of any dam type, and thus the site may also be feasible for
alternative structures such as a concrete or roller compacted concrete (RCC) gravity dam. The visible
rock conditions and geomorphology also suggest a low to moderate risk of encountering significant
seepage, and that seepage may be adequately controlled with a foundation and abutment grouting
program.
A concrete arch dam is potentially the most economical structure as it requires the least amount materials
to construct. Such a structure also has the most stringent foundation requirements.
Upper Dam Site Options. Potential sites for a second, shorter storage dam exist in the basin above the
lower dam. Based on photos from the area, rock outcrops at the surface in these areas. These structures
would have relatively large crest length to height ratios, and rockfill or gravity structures would be
applicable. These sites would take advantage of favorable foundation and topographic conditions but
would also be more vulnerable to rock fall and avalanche conditions.
2.1.3 Construction Materials
The cost of any dam structure in a remote location is highly influenced on the availability high quality
construction materials for use in fill and for concrete aggregate. The valley upstream of the dam appears
to have a limited volume of fine aggregates due to glaciation; however, rock shallow rock is prevalent,
which could be mined for use as aggregate.
The rock quality in the valley appears to be high. Weaker, more highly fractured rock has likely been
scoured from the valley due to glacial erosion, leaving stronger, more highly resistant rock. There are near
vertical rock outcrops in the east side of the basin that have accumulated very little rock talus. This
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
suggests rock of high strength and durability, as talus has not been produced due to weathering of
fractures and freeze -thaw action. It is anticipated that the native rock quality is sufficient for concrete
aggregates.
It is likely that material processing will be required to generate aggregates and other construction
materials required for any dam structure. Processing will include sorting, screening, crushing, and
potentially mining of rock.
Native rock characteristics, durability, and suitability for use in concrete will be identified during future
site investigations. These investigations will also characterize the thickness of overburden soils, and their
potential for use as construction materials as well.
The sand, gravel and cobble deposits at the lower dam site could also be processed and used for concrete
aggregate. An additional benefit to using this material as an aggregate source is that the unit volume of
material removed results in additional reservoir storage.
2.1.4 Future Investigations
A number of issues need to be investigated further to characterize the suitability of the site for a concrete
arch dam:
Overburden Soils: Based on aerial photos, it appears that there may be 20 feet or more of
overburden soils along the creek channel at the dam site. These materials will need to be
excavated to competent rock for any new structure. Deep soils may require the use of temporary
shoring and dewatering systems.
Abutment Strength: The ridge that the dam abutments will tie into is oriented perpendicular to the
line of thrust generated at these locations. The abutment rock strength will need to be investigated
to determine if sufficient strength/mass of rock is present to support the structure.
Downstream Geometry: The dam site is located near the crest of a significant slope dropping to
the valley floor at sea level. The stability of this slope will need to be evaluated, especially in
light of the large foundation loads for a concrete arch structure. The foundation may also provide
limited passive resistance due to geometry.
Characterization of the site should be conducted in a phased approach due to investigation cost and
logistics associated with the site location. The preliminary investigation phase would be broken down into
the following work phases.
Phase 1 Preliminary Geological Reconnaissance: This effort would focus on collecting geologic
information at the site though non-invasive methods. An initial desktop study would be followed by a
field reconnaissance to map and characterize rock outcrops and soil deposits as well as identification of
geological hazards. Particular attention would be focused on characterizing the rock mass in the vicinity
of mapped lineaments. Potential abutments would be mapped to identify potential rock blocks for stability
analysis. A geophysical program could be included to define the depth of unconsolidated deposits as well
as potentially image the major north northeast lineament that crosses the drainage. A primary objective of
this work would be identification of potential fatal flaws associated with the project location and proposed
structure type.
Samples of the surficial deposits would be collected to characterize onsite aggregate suitability for
potential use as fill or in concrete mixes. These samples would be tested for strength, durability, and
soundness.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
Ground base survey of a remote location can be a considerable project cost at this phase of the project. An
alternative to this is the collection of a high resolution Light Detection and Ranging (LiDAR) survey of
the basin area. With sufficient ground control absolute accuracy of the final DEM would likely be well
below a foot for the entire basin. This data could be obtained at considerable cost savings and used for
preliminary engineering. This date could also be used for basin hydraulics, avalanche hazard analysis and
identification of rock blocks in the vicinity of the dam abutments. Actual ground survey would be limited
to the immediate dam areas as part of final design.
Phase 2 Preliminary Subsurface Investigation. The initial geotechnical investigation would be
designed to collect sufficient subsurface information to inform preliminary design and identify potential
fatal flaws. This would include subsurface explorations along the ridge at the lip of the glacial cirque to
assess their suitability as abutments. Borings would be intercepted to target potential lineaments for
characterization. The proposed foundation locations for both the lower and upper would be investigated
for overburden thickness and to characterize bedrock conditions. Because the bedrock is rotated to a near
vertical orientation, angled borings will be required to better characterize the rock mass.
Both insitu and laboratory testing would be conducted to develop rock mass characteristics. Insitu testing
would include packer testing and the installation of vibrating wire piezometers to characterize rock mass
permeability and groundwater levels. Laboratory testing would include index testing of surficial soils as
well as strength testing of rock samples. This information would be used to characterize the rock mass for
geotechnical engineering analyses.
Future Phases: Based on the results of the preliminary phases of investigation, future phases would
likely entail additional detailed geologic mapping and supplementary subsurface investigations to further
characterize geotechnical conditions and develop dam design.
2.2 Project Description
2.2.1 Location
The Project would be located approximately 5 miles north of Cordova, Alaska (Figure 2-3). The Project
is located on the Alaskan mainland, within the Unorganized Borough of the State of Alaska. The
approximate geographic coordinates for the Project are: N60 deg 39", W145 deg 35".
Feasibility Cost Estimate Page 9 October 30, 2013
Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
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LOCATION MAP
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Figure 2-3. Area Map of Snyder Falls Creek Project
2.2.2 Dam
As outlined in the PAD, the dam would be a concrete arch design located in the narrow canyon
approximately 4 miles from tidewater. The existing canyon appears to support a maximum dam height of
approximately 150 feet based on topographic features. The arch nature of the dam structure would
transfer the loads from the dam section into the rock abutments allowing a thin concrete section to be
utilized. In order to develop estimated quantities to support the cost estimate effort, similar concrete arch
dams located in Alaska were used to determine an estimated dam configuration. The Blue Lake Dam
located near Sitka Alaska is a thin shell concrete arch dam approximately 149 feet tall which is currently
being raised by 83 feet to a height of 269 feet. The dam crest is approximately 256 feet long with a
conventional 140 foot long ogee crest overflow spillway located in the center of the dam. The dam
thickness varies from 25 feet at the base to 8 feet at the crest. Swan Lake Dam located near Ketchikan,
Alaska is a thin shell concrete arch dam with a height of 174 feet and a crest with of 480 feet. Similar to
Blue Lake, the dam has a conventional ogee overflow spillway located in the center of the dam. The dam
thickness varies from 17 feet at the base to 6 feet at the crest. Using these two dam configurations, the
Snyder Creek Dam was assumed to have the following characteristics:
• Height of 150 feet
• Crest length of 250 feet
• Conventional ogee crest overflow spillway 100 feet long
• Dam thickness ranging from 24 feet at the base to 8 feet at the crest with an average of 12 feet
used to determine the estimated concrete volume.
• Dam is keyed into the rock abutment and foundation.
2.2.3 Reservoir
The reservoir created by the 150 foot dam would create a reservoir with a surface area of approximately
25.1 acres. The reservoir volume would be approximately 943 acre-feet.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
2.2.4 Intake
During construction, it is anticipated that Snyder Creek will be diverted through a hard rock tunnel
excavated through the rock abutment. For the purpose of developing the cost estimate, it was assumed
that the powerhouse intake would be constructed on the diversion tunnel with the penstock routed to the
powerhouse. This would allow the spillway to discharge into the canyon downstream from the dam. An
isolation gate would be constructed on the upstream face of the intake to allow dewatering of the intake
structure and penstock for inspection purposes. A low level release would also be provided through the
dam.
2.2.5 Power Conduit
The power conduit would consist of a welded steel penstock approximately 3600 feet in length extending
from the dam to the powerhouse. The penstock would be routed through the diversion tunnel then be
surface mounted along the lower reaches of the penstock alignment.
2.2.6 Powerhouse
The powerhouse would consist of a pre-engineered metal building constructed on a reinforced concrete
foundation. The building would have a footprint of approximately 50 feet by 80 feet and a structure
height of approximately 30 feet. A concrete thrust block would be located on the upstream side of the
powerhouse to resist the thrust loads from the penstock loading. The powerhouse would be placed at
approximately 20 feet with the maximum operating reservoir level of 1464 feet providing a rated head on
the turbine of approximately 1430 feet. Assuming an average discharge of 53 cubic feet per second (cfs)
based on existing hydrology, the powerhouse would have a rated capacity of approximately 3 MW. The
powerhouse would be fitted with a single impulse turbine.
2.2.7 Transmission Facilities
The Project power would be conveyed from the powerhouse switchyard to the existing Humpback Creek
project transmission facilities located approximately 4.5 miles from the Project site. A new 12.5 kilovolt
(W) transmission cable would be constructed between the Snyder Creek and Humpback Creek project
sites. The transmission line would consist of either overhead or submarine lines depending on the site
specific characteristics. The new transmission line would connect with CEC's existing transmission
facilities at the Humpback Creek hydroelectric project located approximately 5.1 miles North of Cordova.
This existing line currently connects the Humpback Creek project with CEC's electrical distribution
network in the Cordova service area.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
SECTION 3
CONSTRUCTION APPROACH AND COST ESTIMATE
3.0 Introduction
This section presents the conceptual cost estimate for the Project. The cost estimate was prepared based
upon recent similar projects and past historical costs. A discussion of the basis for developing the
estimate is presented in the following paragraphs.
3.1 General
The pricing presented within this section is considered a feasibility level cost estimate for the Project.
The presented costs are budgetary estimates based upon reasonable assumptions (see Appendix A) for
work in this location and under the anticipated conditions found in the area. These prices are not intended
to represent the lowest possible cost to perform the work under competitively bid conditions, but instead
are intended for budgetary purposes only. The cost estimate for this project has been broken into the
following cost items:
• General Conditions
• Mobilization
• Site Access
• Dam Structure
• Penstock
• Powerhouse & Equipment
• Switchyard & Transmission Line
Appendix A presents additional clarifications and assumptions used in developing the cost estimate.
3.2 Basis of Cost Estimate
The included cost estimate for the Project is based upon the information provided by Cordova Electric
Cooperative in the Application for Preliminary Permit for the Snyder Falls Creek Hydroelectric Project.
The provided costs are based upon the quantities and dimensions stated in the application document
referenced above. Additionally, several options for site access routes for personnel and equipment to
reach the dam and intake location were developed. Each of these access options have been priced are
included in the provided cost estimate backup.
3.3 Cost Items
The cost estimate prepared and presented in Table 1-1 reflects the anticipated construction cost based
upon current market conditions for similar work. The presented costs are based upon current prices and
should be escalated appropriately to the anticipated mid -point of construction for accurate future costs.
Costs are based upon construction work only the client can expect to pay to the general contractor, and do
not include the costs for permitting, design, or construction inspection and oversight.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
3.4 Project Construction
3.4.1 Site Access
The project site is located 7.4 miles north of Cordova, Alaska on the Snyder Falls Creek. It is assumed
that all construction operations will take place on property owned or operated by CEC. Multiple options
have been identified for access to the upper limits of the project and are presented in the included cost
estimate backup. It is assumed that the necessary amount of space required for project staging, material
laydown, and job office set-up will be available the various work locations.
3.4.2 Materials
Readily available construction materials are assumed to be used on this project including the use of onsite
aggregates for use as sub -base material as well as in onsite batched concrete. The following equipment
and materials required for this project would be considered long lead items and the
submittal/approval/fabrication process should be started as early as feasible and expedited where possible:
• Steel Penstock Pipe
• Gates, Valves, and Operators
• Electrical and Communication Cable
• Turbine/Generator Equipment
• Powerhouse Building
• Bridge Crane
• Switchyard Equipment
• Transmission Line
3.4.3 Construction Methodology
Standard construction practices, materials, and equipment are anticipated on this project with any possible
exceptions being noted in Appendix A.
3.4.4 Unusual Conditions (Soil, Water, and Weather)
The work season near Cordova, Alaska is anticipated to be approximately June through October. With
the short work season and remote location, it is assumed that the selected general contractor will maintain
working operations for multiple shifts in each day as well as a minimum of 6 working days per week.
Wet conditions are anticipated requiring dewatering during below grade activities. Additionally, it is
assumed that diversion of water via cofferdams will likely be required for the construction of the dam
structure as well construction of the tailrace channel.
3.4.5 Unique Construction Techniques
While the techniques are not atypical to the installation of hydropower facilities, careful planning shall be
put into the following work items, with specific thought put towards activity sequencing, worker safety,
and task efficiency. These considerations should include:
• Location and construction of barge landing facilities for mobilization and staging.
• Installation of site access roads to allow safe and efficient transport of personnel and equipment
to all work areas.
• Tunneling for both access purposes as well as possible dual use for penstock routing.
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• Construction of the 100 or 150 foot tall arch dam structure.
• Penstock installation through steep terrain
• Weather protection; especially pertaining to cold weather concreting.
3.5 Construction Approach
3.5.1 Mobilization
Mobilization of equipment and materials will be accomplished via barge transport from major port
locations, likely Seattle/Tacoma or Bellingham, Washington as well as Anchorage, Alaska. It is
anticipated that a barge landing location will be developed either at the Humpback Creek hydro -facility
location or further north in Nelson Bay, closer to Snyder Falls Creek. Early efforts will be required to
develop the barge landing facility prior to arrival of heavy equipment and full material deliveries.
3.5.2 Site Access
Several site access options were identified in an attempt to determine both the most cost effective as well
as the most feasible option for safe and efficient personnel and equipment transport to the dam and intake
location. While the options all vary in scope and cost, all contain varying lengths of a lower access road,
a tunnel section through the steep terrain, and an upper access road. The options explored include the
following:
• Option l: Barge Landing at Humpback Creek — This option involves the development of a barge
landing and offloading facility near to the existing Humpback Creek hydropower facility on
Nelson Bay. Access to the upper work areas will be through the further development of the
existing humpback creek intake road to the location of the lower tunnel portal (see Appendix Q.
• Option 2: Barge Landing at Humpback Creek w/ Roadway at Shoreline — This option involves
the development of a barge landing and offloading facility near to the existing Humpback Creek
hydropower facility on Nelson Bay. An access road will be constructed along the shoreline to the
north until the first suitable location (identified as Option 2 in Appendix C) for an access road up
the hill can be created. It is assumed that multiple "avalanche sheds" will be required for this
option to protect the shoreline road from being buried by avalanching snows.
• Option 3: Barge Landing at Humpback Creek w/ Roadway at Shoreline — This option is similar to
Option 2 as described above, but requires additional road at the shoreline prior to beginning the
access road up to the dam site. The tunnel length required for Option 3 is shorter than that in
Option 2.
• Option 4: Barge Landing near Snyder Falls Creek — This option involves the development of a
barge landing and offloading facility in the northern reach of Nelson Bay near to the proposed
powerhouse location. Dredging near the shoreline may be required to allow for barges to access
the shoreline near the northern reaches of Nelson Bay. This option allows for the shortest tunnel
distance as well as upper access road from the upper tunnel exit portal to the dam site.
3.5.3 Dam Structure
Construction of the dam structure will require the construction of a batch plant facility onsite, likely near
to the dam structure location where gravel deposits are available for use in batching. In addition to the
batchplant, it is also anticipated that a small crusher or screen plant will also be required to process the
gravels for use as structural fill, sub -base, and for use in the structural concrete mix. While equipment
sizes will ultimately be based upon the site access limitations (% grade of roads and tunnel diameter), a
crane will be required at the dam location for the placement and lifting of formwork, lifting of rebar
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Cordova Electric Cooperative
Snyder Falls Creek Hydroelectric Project
cages, and the placement of concrete material. Either a mobile crane or a tower crane has potential for
use on this project assuming the necessary picking heights and capacities can be met. Prior to
construction of the dam, diversion of the existing Snyder Falls Creek will be required through the use of
bulk sandbags, temporary sheetpile, or other flow diversion methods. This may require construction of
the dam in a phased approach, until water can be passed through the intake structure located at the base of
the dam.
3.5.4 Penstock
The 24 inch steel penstock will be installed either above ground with the use of anchor blocks and pipe
supports or direct buried. In either scenario and depending upon the decided site access location, is
possible to use the created access tunnel for penstock routing as well. If it is not possible to route the
penstock through the access tunnel, than steep portions of the project may require the use of a cable crane
or helicopter support to transport, stage, and place penstock segments.
3.5.5 Electrical and Communication
An allowance of $2.5 million was included to account for electrical and communication cables which will
be installed alongside the penstock routing to allow for control of the intake gates from powerhouse
location.
3.5.6 Powerhouse & Equipment
The cost of a 50 foot by 80 foot pre-engineered metal building powerhouse facility, which will include the
complete turbine/generator equipment, switchgear, and overhead bridge crane. Additionally, this item
includes the installation of an assumed cast -in -place concrete tailrace channel.
3.5.7 Switchyard & Transmission Line
This line item is based upon the installation of a pre-engineered switchgear building, a standby generator
with enclosure, and a step up transformer building, all located immediately adjacent to the powerhouse
building. Additionally, the cost for installation of the 4.5 submarine transmission line from the new
powerhouse to the existing humpback powerhouse switchyard.
3.6 Project Cost Summary
This conceptual cost estimate was generated in October of 2013 and is based upon the provided
Application for Preliminary Permit of the Snyder Falls Creek Hydroelectric Project. The cost estimate is
reflective of current market pricing and the best information available for the anticipated project direction
for those areas where complete details or specifications are not currently available. Additional cost
backup for each option is provided in Appendix B.
3.7 Conclusions
Table 3-1 indicates the Total Construction Cost associated with the various project access options.
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Table 3-1. Estimated Project Costs (October 2013 dollars)
Estimate
Low -30%
High (+30%
Access Option 1
$91,032,000
$63,722,000
$118,341,000
Access Option 2
$90,080,000
$63,056,000
$117,104,000
Access Option 3
$83,647,000
$58,553,000
$108,742,000
Access Option 4
$67,713,000
$47,399,000
$88,027,000
See Appendix B for supporting cost estimate data.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
SECTION 4
CONCLUSIONS AND RECOMMENDATIONS
4.1 Conclusions
The available data related to the Project was collected and reviewed to develop a firm understanding of
the site specific characteristics of the proposed hydroelectric project. In reviewing the project features, it
was evident that access for construction equipment, labor, and materials was a driving factor affecting the
overall project construction cost. The remote location coupled with challenging site topography has a
significant impact on the ability to access the dam site. Several options were identified and developed as
part of the cost estimate preparation. Once construction access is established, the short construction
window is the second major challenge. The main construction window of June through October in the
upper basin where the dam is located presents construction sequencing challenges requiring multiple
work shifts to execute the dam construction within the available weather window.
Based on the available topographic mapping and geologic information, a 150 foot concrete thin shell dam
located within the existing natural narrow canyon appears feasible. Extending the dam above a 150 foot
height may be feasible, but additional site specific topographic mapping will be required as well geologic
investigations to determine the extent of suitable rock in which to concrete dam abutments. For the
purpose of the cost estimate preparation, a 150 foot dam height was assumed. Upper storage dams
constructed from local rock were also considered. These dams, thought feasible, present more challenges
related to long term stability due to the harsh environmental conditions.
Overall, the Snyder Creek Hydroelectric Project was determined to be a feasible project with costs
ranging from $88 million to $118 million depending on the selected access route option.
4.2 Recommendations
Based on the feasibility level cost review, the Snyder Creek Hydroelectric Project is considered to be
technically feasible with an estimated cost range of $88 million to $118 million. Additional on -site
investigations and engineering analysis will be required to refine the design layout, dam characteristics,
construction schedule and sequencing, construction access, and overall project development.
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Cordova Electric Cooperative Snyder Falls Creek Hydroelectric Project
APPENDIX A
COST ESTIMATE ASSUMPTIONS
Feasibility Cost Estimate October 30, 2013
Snyder Falls Creek Hydroelectric Project
General Assumptions
• General Conditions have been applied at 12% of the project total costs
• Mobilization has been set at 8% of the project total costs
+ Cost estimate based on current day pricing.
• Construction assumed to be spread over two work seasons.
+ Cost estimate based upon the ability to install barge landing sites on Nelson Bay. Improvements
including dredging may be required to accomplish this.
• Cost estimate based upon access roads and tunnels remaining in place following the completion of
construction.
+ Cost estimate based upon construction of the larger height (150') dam described in the application for
preliminary permit document.
• Cost estimate based upon an average dam thickness of 12'
• An allowance of $1,000,000 has been included for intake gates, trash racks, and other misc metals
required at the intake location.
• Cost estimate based upon a 24" steel penstock as described in the application for preliminary permit.
• An allowance of $2,500,000 has been included for electrical and communication cables running from
the powerhouse to the dam/intake structure location.
• Cost estimate based upon the construction of a 50' x 80' Pre -Engineered Metal Building structure for
use as the powerhouse facility.
• Cost estimate based upon the installation of a 3.0 MW impulse type turbine as described in the
application for preliminary permit.
• Cost estimate based upon the installation of 4.5 miles of submarine transmission line as described in
the application for preliminary permit. Transmission Line to be buried on floor of Nelson Bay,
assuming the presence of loose, sandy/silty materials.