Sweetheart Dam Access Challenges and Design Considerations Duff - - PowerPoint PPT Presentation

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Sweetheart Dam Access Challenges and Design Considerations Duff - - PowerPoint PPT Presentation

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Sweetheart Dam Access Challenges and Design Considerations Duff Mitchell Juneau Hydropower Inc. Tom Fitzgerald Schnabel Engineering September 21, 2017 schnabel-eng.com Agenda Project Summary and Timeline Permitting Status

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schnabel-eng.com

Sweetheart Dam Access Challenges and Design Considerations

Duff Mitchell – Juneau Hydropower Inc. Tom Fitzgerald – Schnabel Engineering

September 21, 2017

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Agenda

 Project Summary and

Timeline

 Permitting Status  Project Challenges  Design Concepts  Recent Investigations

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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

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Project Features

Dam Powerhouse

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Dam Location

Gilbert Bay Proposed Dam Site Tunnel Alignment

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Lower Sweetheart Lake

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Powerhouse Location

Powerhouse Site Gilbert Bay

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Gilbert Bay

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Gilbert Bay

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Gilbert Bay

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Sweetheart Falls

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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
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Remote Site

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Steep Topography

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Tongass National Forest Roadless Rules

 No permanent or temporary

access roads to the dam site

 No disturbance until the

project was permitted.

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Geology

Site Regional Lineaments Site Area Lineaments Generally consistent with Foliation and Joint Set #1

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Dam Foundation – Right Abutment Area

Stop D-04 Biotite Schist and Quartzite Shear zone at Stop D-04 Stop D-20 - Laminated Quartzite

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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

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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

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Deaggregation Plot

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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)

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Engineering Considerations - Spillway

 PMF outflow of 20,300 cfs with 14.1 feet

  • f head

 100-year outflow 7,000 cfs with 7.4 feet

  • f head
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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

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Conceptual Dam Type Selection

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Dam Type Considerations

 Spillway needs  Site geometry and dam footprint  Foundation conditions  Construction duration  Weather dependencies  Access issues and equipment needs

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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.

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RCC Considerations

 Aggregate source  Mix design  Foundation

Treatment

 Batching and

Conveying

 Equipment  Placement  Penetrations  Facing system

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Recent Investigations

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Questions?