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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?