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Sweetheart Dam
Access Challenges and Design
Considerations
Duff Mitchell – Juneau Hydropower Inc.
Tom Fitzgerald – Schnabel Engineering
September 21, 2017
Agenda
Project Summary and
Timeline
Permitting Status
Project Challenges
Design Concepts
Recent Investigations
Sweetheart Creek first prospected by Frank Cook, discoverer of the
Jualin Mine
1906 Sweetheart Lake first identified as a hydro resource
1915 to 1927 USGS gauged Sweetheart Creek
1929 Project selected by US Government as a Federal Power Site
Classification Site in Public Land Order 221 May 14, 1929.
1952 Interior Secretary reports to Congress on Sweetheart Lake
1958 USGS Plan for Damsite
1983 Alaska Power Authority Plan
1983 Environmental Impact Assessment for Gilbert Bay
2009 FERC Permit filed-development and studies commenced
2014 FERC License Filed
2015 October EIS and License Process
2016 Final EIS issued-May 31, 2016
2016 FERC License issued-September 8, 2016
2016 Army Corps of Engineers 404 Permit issued
2017 ADNR Tideland Lease Permit and Easement issued
2017 ADNR Water Rights issued
Sweetheart Lake Timeline
Project Features
DamPowerhouse
Dam Location
Gilbert Bay
Proposed Dam Site
Tunnel Alignment
Lower Sweetheart Lake
Powerhouse Location
Powerhouse Site
Gilbert Bay
Gilbert Bay
Gilbert Bay
Gilbert Bay
Sweetheart Falls
Alaska Considerations
Average High Temp = 48.1
Average Low Temp = 36.2
Precip Avg. Inches per Year = 62
Snow Avg. Inches per Year = 88
Construction Season
Dam Site – June to October
Powerhouse – May to November
-From U.S. Climate Data – Juneau Alaska
Remote Site
Steep Topography
Tongass National Forest Roadless Rules
No permanent or temporary
access roads to the dam site
No disturbance until the
project was permitted.
Geology
Site Regional Lineaments Site Area Lineaments
Generally consistent with Foliation and Joint Set #1
Dam Foundation –
Right Abutment Area
Stop D-04
Biotite Schist and Quartzite
Shear zone at Stop D-04
Stop D-20 - Laminated Quartzite
Implications for Project
Strong Rock Suitable for Dam and
Powerhouse Foundations and Tunneling
Geologic Hazards can be Mitigated:
Sedimentation,
slope stability
seismicity
Multiple Sources of Rock Borrow Material
Favorable Orientations of Compositional
Layering and Joints
Rock Observed does not Appear Heavily
Faulted or Sheared
Engineering Considerations - Seismicity
Peak Horizontal Ground Acceleration
(PHGA) value used for preliminary
design.
Return Period
(years)
Ground Motion
(g)
500 0.073
1,000 0.094
2,500 0.124
5,000 0.148
10,000 0.177
Deaggregation Plot
Engineering Considerations - Spillway
Drainage area of 35 square miles
Surface area of Lower Sweetheart
Existing – 1,200 acres
Proposed - 1,700 acres
100-year inflow 14,000 cfs
PMF Inflow about 39,000 CFS
Overflow spillway 125 feet wide at El.
636.0 (~76 feet above existing)
Engineering Considerations - Spillway
PMF outflow of 20,300 cfs with 14.1 feet
of head
100-year outflow 7,000 cfs with 7.4 feet
of head
Construction Considerations -Diversion
Fair data set
Seasonal flow
Low flows in winter, high
flow in spring and
summer.
Not unusual to see
peaks 1,000 to 1,200 cfs
Construction duration –
1 or 2 seasons?
Concrete dam
Need for Low-Level
Outlet
Conceptual Dam Type Selection
Dam Type Considerations
Spillway needs
Site geometry and dam footprint
Foundation conditions
Construction duration
Weather dependencies
Access issues and equipment needs
Benefits of RCC Gravity Dam
Simple, robust design that is
adaptable to the terrain.
Less risk related to
foundation parameters.
Can be built quickly (in one
season).
Long term durability and
performance.
Can accommodate spillway
needs.
RCC Considerations
Aggregate source
Mix design
Foundation
Treatment
Batching and
Conveying
Equipment
Placement
Penetrations
Facing system
Recent Investigations
Questions?