HomeMy WebLinkAboutNenana Fatal Flaw MemoKnight Piésold and Co.
Denver, Colorado
DV103.00209.01
Nenana Fatal Flaw Memo
Memorandum
Date: October 31, 2008 DV103.00209.01
To: Mr. Paul M. Park, Golden Valley Electric Association
From: Gilberto Dominguez and Chuck Hutton
Re: Nenana River Hydropower Reconnaissance Study
Technical Fatal Flaw Analysis
Introduction
Knight Piésold and Co. (Knight Piésold) has been retained by Golden Valley Electric
Association (GVEA) to investigate the potential for construction of a small run-of-river
hydropower facility on the Nenana River near the town of Healy, Alaska. The first phase of
the study involved a fatal flaw analysis to determine if the project is technically feasible and
if there are any significant impediments to the project. This phase involved a site visit, a
geologic and geotechnical assessment of the project site, evaluation of flow and head
available for power generation and an estimation of the potential turbine and generator
capacity.
Project Location
The location for the proposed hydropower project is the Nenana River south of Healy,
Alaska. The intake and power plant would likely be located in the stretch of river between
the Alaska Highway 3 bridge crossing and the confluence of Healy Creek.
Site Visit
A site visit was conducted from September 15 to 16, 2008 by Al Gipson and Chuck Hutton
from Knight Piésold and Paul Park from GVEA. The site visit consisted of a raft trip on the
Nenana River from near the Denali National Park entrance down to the confluence with
Healy Creek. In addition, the river and potential intake and power plant site locations were
observed from several vantage points along Alaska Highway 3 and access points above the
railroad.
Hydrology
The stream gaging station nearest the proposed project site on the Nenana River is located
just upstream of the confluence with Healy Creek. The United States Geological Survey
(USGS) monitored the flows at this station (No. 15518000) from October 1, 1950 through
Knight Piésold and Co.
Denver, Colorado
Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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Nenana Fatal Flaw Memo
September 30, 1979. The mean monthly flows for this station are shown in the table below
and Figure 1. The mean flow for the period from May through October is 6,353 cfs. The
mean monthly flow varies from a low of 434 cfs in March to a high of 9,884 cfs in July. The
peak recorded flow for the period of record was 46,800 cfs on July 25, 1967. The flow
pattern is typical for interior Alaska, with high flows from May through October and low
flows from November through April. The 1,910 square mile drainage area includes a number
of glaciers, and therefore the summer flows tend to be sustained at a fairly high rate.
Conversely, most of the winter precipitation falls as snow, and winter flows can become very
low.
Golden Valley Electric Association
Nenana River Hydropower Reconnaissance Study
Nenana River Station 15518000
Mean Daily Flow, cfs
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Avg 558 473 434 514 3,885 9,884 9,516 7,872 4,879 2,149 1,021 688
Max 890 640 610 1,700 17,000 35,900 38,600 29,200 18,200 5,470 2,600 2,600
Min 200 190 190 190 270 3,720 4,060 2,600 1,450 650 290 240
Figure 1
Nenana River - Mean Monthly Streamflow
1950-1979 DATA Sta. 15518000
0
2,000
4,000
6,000
8,000
10,000
12,000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecFlow (cfs)
Mean daily flow records for the Nenana River gaging station 15518000 were downloaded
from the USGS web site and used to develop the flow-duration curve shown in Figure 2
Knight Piésold and Co.
Denver, Colorado
Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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Nenana Fatal Flaw Memo
below. The flow-duration curve is a cumulative-frequency curve that shows the percentage
of time that specified discharge values are met or exceeded during a given period.
The practical power generation period was assumed to be during the summer months from
May through October. Therefore, the flow-duration curve was developed for only those
months. As shown in the table above, the flows during the winter months from November
through April are significantly lower and the river typically freezes over. Based on the
recorded flows for the period from May through October, it appears that there is probably
adequate water available for power generation, depending on the amount of water that needs
to remain in the river for rafting and fish.
As a starting point, normal flows in the 25-percent exceedance range are selected for analysis
of the project installed capacity and energy output. For the flow-duration curve shown in
Figure 2, the 25-percent flow would be about 9,700 cfs. Initially, GVEA assumed a design
flow of 3,000 cfs for the Alternative Energy Grant Proposal for this project. This design flow
would leave significant flow in the river for rafting and fish. Depending on the alternative
design selected for the hydropower facility, it may be possible to increase the design flow
and still leave sufficient water in the river for rafting and fish.
Figure 2
Nenana River Near Healy
Flow Duration Curve (May-October)
0
5,000
10 ,0 0 0
15,0 0 0
20,000
25,000
30,000
35,000
40,000
45,000
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Percent of ExcedenceFlow (cfs)USGS 15518000
Knight Piésold and Co.
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Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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Nenana Fatal Flaw Memo
Project Alternatives
Based on the available flow data and survey information collected by GVEA on the river
gradient, it appears that there are several potentially viable alternatives for hydropower
generation on the Nenana River upstream of the town of Healy. The potential alternatives
are briefly described in the following paragraphs and their locations shown on Figure 3
included at the end of this document.
Alternative No. 1
This alternative would consist of a low impact diversion weir across the river at a sharp left
bend in the river about 3 river miles upstream of the confluence with Healy Creek. This weir
would be common to all alternatives and could be up to 20 feet high. The weir would be
designed to incorporate a bypass channel or whitewater rafting chute. The almost vertical
rock wall on the east side of the river at this bend would provide a good location for a tunnel
portal. The tunnel would extend for about 2 miles from the intake to a point on the east bank
of the Nenana River, just upstream of the confluence with Healy Creek where the river starts
to widen. According to the survey conducted by GVEA, the net drop in the water surface of
the river between these two points is about 62 feet. The diameter of the tunnel would need to
be about 17.5 feet to provide a net head of about 50 feet. Based on a design flow of 3,000 cfs
and net head of about 50 feet, the installed capacity would be about 11.4 MW. The design
flow could be increased, but the tunnel would need to be larger to maintain the net head. If
the project is economically feasible, the optimum tunnel diameter will be established during
detailed design studies. The power plant would contain one or more axial flow Kaplan
turbine and generator units. Access to the weir and tunnel portal would be along the east side
of the Nenana River and would be difficult and costly due to the steep terrain and the
required bridge across Healy Creek.
Alternative No. 2
This alternative would be similar to Alternative No. 1 except that the tunnel would be about
1.2 miles long and daylight further upstream of the Healy Creek confluence. The penstock
would extend for about one mile from the end of the tunnel to the same power plant site as
for Alternative No. 1. The penstock would likely be exposed with steel or concrete supports.
The tunnel and penstock would be the same diameter as the tunnel for Alternative No. 1.
Based on a design flow of 3,000 cfs and net head of about 49 feet, the installed capacity
Knight Piésold and Co.
Denver, Colorado
Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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Nenana Fatal Flaw Memo
would be about 11.2 MW. Access for this alternative would pose a similar challenge as for
Alternative No. 1.
Alternative No. 3
This alternative would consist of a penstock along the east side of the Nenana River, from the
weir to the power plant site near Healy Creek. The penstock would be about 3 miles long
and would likely be exposed with steel or concrete supports. The penstock would be the
same diameter as the tunnel for Alternative No. 1. Based on a design flow of 3,000 cfs and
net head of about 44 feet, the installed capacity would be about 10.0 MW. Access for not
only the weir, but also construction of the penstock, would be difficult due to the steep terrain
and extensive rock outcrops.
Alternative No. 4
This alternative would consist of a low impact concrete weir across the Nenana River with
the hydropower facility integrated into the weir. This would be a low head scheme, but there
would be no penstock or tunnel; as a consequence, the diversion reach of the river will not be
affected and therefore, there would be no requirement to maintain bypass flows (as there is
no diversion reach). These characteristics indicate use of low head, high flow, “pit” type,
axial flow, double regulated turbines incorporated into the weir construction. Based on a
design flow of about 9,700 cfs and a weir height (and net head) of about 15 feet the installed
capacity would be about 10.5 MW. This would require installation of three equal size
turbine and generator units. The weir would include a whitewater rafting chute or bypass.
The weir and hydropower facility could be located anywhere along the river, upstream of the
Healy Creek confluence, where sufficient space is available and a net head of about 15 feet
can be achieved. The location should probably be as near the confluence of Healy Creek as
possible to minimize the length of the access road and transmission lines.
Geology
The following observations, regarding the geology along the river, were made during the site
visit, which included a raft trip and observations from accessible overlooks along the west
side of the river.
The majority of the tunneling would be through schist containing widely spaced basalt dikes
ranging from a few to tens of feet thick. Alluvial fan or glacial deposits may be encountered
Knight Piésold and Co.
Denver, Colorado
Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
DV103.00209.01 6
Nenana Fatal Flaw Memo
north of Sections 4 and 5 where the topography flattens. These deposits would make
tunneling difficult, so a shorter tunnel option may be the most viable. The tunnel would best
be advanced from the downstream end to provide a place to dispose of the tunnel muck. The
schist should have sufficient strength to support the diversion structure.
According to the rafting guide, the river is about 10 to 20 feet deep at the proposed diversion
weir and intake location. Alluvial material on the valley bottom should be thin, on the order
of a few feet thick as this section of the river is characterized by rock generally exposed on
the valley walls and some bedrock knobs exposed in and adjacent to the river. There are
some glacial deposits on the valley walls. It was observed that the railroad fill on the west
side of the river has experienced instability. The glacial deposits may contain clay and
varved glacial soils.
The Healy Fault runs east west parallel to Healy Creek. It is understood to be an active fault.
It appears the fault is north of the proposed construction and therefore, would not cross the
tunnel or penstock. Additional study is required to determine if this fault is a potential source
of ground shaking that could affect the project.
Access to the tunnel inlet would involve constructing a bridge over Healy Creek and
pioneering a road to the inlet site. Large cuts into the steep schist slopes within the half mile
downstream of the inlet would likely need to be stabilized by rock bolting with wire mesh
and gunite.
Environmental Considerations
Identification of environmental issues and permitting is not included in the Knight Piésold
scope of work for this reconnaissance study. However, potential environmental issues were
discussed during the field trip and will need to be addressed for development of hydropower
on the river. These include the following:
• Safe fish passage both upstream and downstream of the weir
• Preservation of whitewater rafting operations
Knight Piésold and Co.
Denver, Colorado
Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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Nenana Fatal Flaw Memo
Safe Fish Passage
The same group that made the field trip met with James Durst from the State of Alaska
Department of Fish and Game to discuss the habitat and fish species associated with the
river. The primary fish species in the Nenana River is grayling, which appear to migrate up
and down the river. The proposed low diversion dams would need to incorporate facilities
for safe passage of these fish. The alternatives involving Kaplan type turbines would
probably need to include fish screens on the tunnel or penstock intake. Providing fish
screens for flows in the order of 3,000 cfs would be expensive, and the use of “fish friendly
turbine” should also be considered. The “pit” type turbines allow relatively free passage of
fish, as they are low head with relatively low speed.
Whitewater Rafting
Maintaining the whitewater rafting operations will likely be required for any hydropower
development on the river. This is a primary tourist attraction for the area and a source of
income for the rafting companies and associated businesses. Currently several rafting
companies operate during the summer season from about mid May to mid September.
According to the rafting guide, about 50,000 tourists participate in whitewater rafting each
summer. The low diversion dam or weir will need to incorporate a hardened bypass channel
or an in-stream flow chute designed for safe passage over the top of the weir.
Summary of Findings and Recommendations
Summary of Findings
Based on data collected and analyzed to date, it appears that there are several potentially
viable alternatives for development of hydropower on the Nenana River upstream of the
town of Healy. For Alternatives No. 1, 2 and 3, the tunnel and/or penstock would be
required, which are large and costly to construct. Further analysis will be required to
determine financial viability. Alternative No. 4 has the lowest impact on the river system, as
there is no diversion reach (i.e. only a low weir with integrated powerhouse and no
penstock/tunnel).
No technical fatal flaws have been identified at this time that would preclude development of
hydropower on the river. It is expected that some members of the community may object to
the potential development of a hydropower scheme on the river. The FERC permitting
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Nenana River Hydropower Reconnaissance Study October 31, 2008
Technical Fatal Flaw Analysis
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process will provide the opportunity for public input and discussion of concerns and potential
solutions to mitigate any negative impacts associated with the development. Community
consultation, environmental and fisheries studies will be required in order to identify and
address all components of the proposed development.
Recommendations
The reconnaissance study should continue to determine the preferred project layout, power
and energy generation potential, in-stream flow requirements (IFRs) for both fisheries and
recreation, identification of community and stakeholder concerns, estimated conceptual costs,
and project economic viability.
Knight Piésold and Co. Denver, Colorado DV103.00209.01 9 October 31, 2008 Nenana Fatal Flaw Memo Alternative No. 1 Tunnel Alternatives No. 1, 2 & 3 Power Plant SiteAlternatives No. 1, 2 & 3 Weir & IntakeAlternative No. 2 Tunnel & Penstock Alternative No. 3 PenstockAlternative No. 4 Weir & Power Plant SiteFigure 3 – Nenana River Hydropower Alternatives