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SUSITNA HYDROELECTRIC PROJECT
REPORTS BY POWER AUTHORITY AND
ACRES EXTERNAL REVIEW PANELS
1980 -82 .
Table of Contents
Section A: Reports by Power Authority Review Board Panel
Section B: Reports by Acres External Review Panel
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INTRODUCTION
June 5, 1981
SUSITNA HYDROELECTRIC PROJECT
EXTERNAL REVIEW PANEL
REPORT NO. 2
The Panel met with representatives of the Alaska Power Authority, Acres
American, Terrestrial Environmental Specialists, Inc., and the Fish and
Game Department in Anchorage on June 3-5, 1981 for discussions of on-going
studies for the Susitna Hydroelectric Project. On June 3rd, representctives
of Acres American, TES and AOF&G described the current status of these
studies, after which separate group discussions \'Jere held on geotechnical,
hydraulics and hydrology, and environmental subjects to revievl specific
problem areas in more detail. A site inspection was made by Dr. Merritt on
June 4th and 5th to review the field geotechnical exploration program. Or.
Rohan met with representatives of Battelle on June 2nd, C~ugach Electric
Association on June 4th and Union Oil on June 5th to discuss alternatives
to the Susitna project. This report, which summarizes the Panel's opinions
and recorm1endations, was prepared on June 4th and 5th and discussed with
representatives of the Power Authority staff and Acres. Or. Seed was not
able to attend the meeting.
GEOTECHNICAL INVESTIGATIONS
Following Dr. t~erritt's 1~ day visit to the \~atana and Devil Canyon sites,
discussions were held at High Lake concerning the on-goi~g field program
and preparation of information appropriate for the feasibility design. The
following comments summarize these discussions and are offered to aid .in
the timely-completion of the field program.
General -The preparation of finalized geologic maps and profiles is not
keeping pace with the rapid accumulation of fiela information. This
situation is compounded by the recent acquisition of a large quantity of
field geologic data collected by previous Corp of Engineers work which was
never reduced and presented in final form by the Corps. Moreover, the
original Corps boring logs need to be reviewed (re-1ogged) to assure that
all field information is presented in a consistent manner.
A schedule for completion of the various phases of work for the summer
program has been prepared to assure that the necessary information is
analyzed in time for the next phase of feasibility design. The External
Review Panel will be prepared to review this work during our October
meeting.
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Watana Site
Fie 1 d geo 1 og i c mapping is undenoJay, the resu 1 ts of \'I hi ch wi 11 be used to
best locate the remaining exploratory borings .. Present structure_layouts
indicate that the "Fins 11 shear zone should nqt intersect any tunnels or
open cuts. Special attention is being given to the projection of the
"Fingerbusteru shear zone concerning its possible intersection of the
downstream portion of the tailrace tunnels. Present infonnation suggests
that this zone lies downstream of the proposed underground powerhouse;
however, exploratory borings are planned to confirm this interpretation ..
Additional seismic surveys will be done to better define the geometry of
the buried channel on the right abutment and additional borings and pumping
tests are planned for the next phase of explorafion.
Devil Canyon Site
The geologic mapping is·we11 advanced at this site and no new shear zones
have been identified on the abutments. Boring BH-7 has confirmed the
presence of a shear zone (previously recognized) bene~th the topographic
lineation on the left abutment. This feature wll be receiving careful
attention during the upcoming Task 4 study.
Numerous open stress relief joints have been recognized in the upper por-
tion of both abutments and are apparently more prevalent on the left side .
The field geologists will be mapping these features in detail to assist in
preliminary layouts of the required excavation for the arch dam.
Four borings remain to be drilled at Devil Canyon; 2 \'/ill pass beneath the
river to explore for geologic structures and 2 more drilled into the abut-
ments near the river to determine general rock quality. If the river hole
in progress encounters favorable conditions, then the second hole may not
be required for the feasibility design. Cons·idering the excellent rock
exposures, the two remaining borings may best be drilled at theupper
elevations (on the left side) rather than close to the valley bottom as
presently planned. These holes should be directed. tv cross the stress
relief joints to determine their presence at depth. The drill advance can
be carefully watched to determine the presence of open joints. A borehole
camera would provide the most direct method of assessing the presence and
magnitude of these features and is being considered by Acres' personnel ..
SEISt4IC STUDIES
Seismic studies have evaluated all known and detectable faults and line-
aments in the project area. The l981 field program calls for a study of
thirteen features identified as signifcant in the 1980 investigations,
In order to firm up design for the major structures .in the project, it is
essential that conclusions regarding the significance and impact of each of
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these features be reached as soon as practicable. Delay in completing this
work and evaluating the parameters required for design will have an .i~por
tant effect on meeting the project schedule.
HYDRAULICS AND HYDROLOGY
The field program for surveys and collection of hydrologic data is con-
sidered to be adequate for the current feasibility study. Nodifications to
the original scope of work involve studies of navigation effects. However,
after analyses of available existing data and data to be collected, it may
be found necessary to collect some additional short-term information to
firm up tentative conclusions in one or more areas. Specific comments on
some areas of data collection are presented below.
Flood Flows
Stream flow data are being obtained at a sufficient number of existing,
reactivated and newly installed t ging stations throughout the drainage
area to enable a reliable determination of flood flows. Studies to date
indicate that the Corps PM~ is about 20,000 cfs too low. A report on flood
discharges will be issued for review in a few weeks. Some 80 water level
cross-sections have been taken in the Susitna River. HEC programs are
being developed for free surface and ice covered water levels for various .
size floods. Reports will" be is~ued on free surface water levels in July ·
and ice covered conditions somewhat later. These studies should estab1ish
reliable bases for determining river tailwater levels at the dams and \'later
surface profiles in downstream reaches of the Susitna River.
Sediment Data Collection
The river se..:iment measuring .program has not been started. This program
should be defined and started as soon as possible under the guidance of the
USGS or a private river sediment expert. It is essential that bed load
measurements be made during this runoff season to enable a reasonable
assessment of the effects that depletion of sediment 1 oads by constructi·on
of the dams would have on downstream river conditions. The Panel is con-
cerned that the necessary sediment data may not be available in time for
inclusion into the June 30, 1982 feasibility report.
Reservoir Capacity
Recent reservoir surveys have been completed frc.r.~hich more accurate capa-
city curves have been developed. At yJatana, the revised curve indicates
one to two percent less reservoir capacity between elevations 1700 and
2100, but the .capacity is essentially the same as shown by the·original
curve at maximum pool elevation 2200. This small difference does not re-
quire revisions in the design development studies. HowEver, the revised
capacity curve should be used in final design. ·
At Devil Canyon, the revised reservoir capacity curve based on the latest
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survey indicates significantly greater capacity than the initial capacity
curve,-being approximately 30 percent greater at elevation 15Gb. Since
power operation would be near maximum pool nearly 100 percent of the time,
the revised greater capacity would have little influence on design develop-
ment studies. However, the greater capacity curve should be used in final
design and reservoir filling and drawdown studies.
Energy Outplit
The firm energy output for the Watana/Devil Canyon system has been deter-
mined by routing actual stream flows which occurred for the 1969-79 period
through the system. Since this was by far the period of lowest stream flow
over 70 years of record, the Panel concurs that this is· a satisfactory
basis for establishing firm energy output.
DESIGN DEVELOPMENT
Acres described various alternative schemes for optimizing design of the
main dams, coffer dams, saddle dams, spillways, power facilities and
diversion tunnels for the two dams. The Panel was very impressed w_ith the
many specific alternatives which will be studied to arrive at the most
functionally satisfactory and economical plan. We desire to emphasize~ -
however, that full consideration should be given to the effects on ease of
construction and construction schedules, as \'Jell as costs, for the various
alternatives. Specific comments follow on some of the design features that
will be considered in the optimization studies.
Multiple Level Outlets
There is some question whether multiple level outlets will be required in
_ the power intakes, particularly at Watana Dam. Some experience in se'!eral
Alaska lakes indicates that a marked thermal stratification may not occur
in the two reservoirs and that the reservoir waters may never be free of
turbidity, in which case multiple level outlets would not effectively
enhance downstream water temperatures or quality. The Panel is of the
opinion that sufficient studies should be made of other lakes to make a
better assessment of what is most likely to occur in Wa~ana and Devil
Canyon reservoir. If the studies are inconclusive, then the Panel suggests
that multiple level outlets be provided at both dams, since their costs
would not be excessive and prototype experience may prove them to effec-
tively enhance water temperatures and qua1ity downstream of the dams. An
exception to this statement, however, is that in the event Devil Canyon
will be constructed earlier than anticipated due to greater power demand~
then multiple level outlets may not be required at Watana Dam.
Low Level Outlet
Acres has given preliminary consideration to providing low level outlets at
both darns for lowering the reservoirs in the event cif an emergency. Based
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on general guidance information used by the Corps of Engineers, a low level
outlet capacity of approximately 100,000 cfs would be required~ This would
require construction of an additional large gated tunnel at great cost. A
low level outlet was provided at Mica Creek Dam in British Columbia by
providing a tunnel plug and gates in the diversion tunnel which would allow
substantial lowering of the reservoir in a period of 8 months. The Panel
believes that this type of low level outlet should be installed in the
diversion tunnels at Watana and Devil Canyon. This low l~vel outlet would
provide for regulation of initial reservoir filling, minimum flow release
when the powerhouse is not in operation and emergency lowering of the
reservoir over a substantial period of time for repairs in the event that
seepage problems should develop.
Service Spillway
One ~.1 ternati ve scheme for Watana pro vi des for a service spillway v-Ji th a
stilling basin designed for a 1 in 10,000 year flood and a fuse plug
spillway to handle additional flows up to the PMF. While there may be some
reduction in cost by reducing the size of the service spillway and increasing
the size of the fuse plug spillway, the Panel is of the opinion that the
service spillway should not be made smaller than required for a 1 in 10"000
year flood. However, some reduction in cost can be made by designing the
stilling basin to function as a hydraulic jump basin fo~ a smaller discharge,
say 50 percent of the 1 in 10,000 year flow, and sweep out of the basin for
larger discharges, if this would not endanger the stilling basin structure.
Spillway Outlets in Arch Dam
Although technically feasible, the.Panel suggests that consideration be
given to eliminating the spillway outlets through the arch dam at Devil
Canyon and the concrete lined p 1 unge poo 1 r-•:=ar the toe of the dam by in-
creasing the size of the service spillway. If there is not a substantial
increase in cost, the Panel would prefer to eliminate the outlets through
the arch dam.
Watana Dam
An embankment structure has been selected for feasibility studies at the
Watana site. It appears that very little effort has been expended to study
other .. types of dams for this site. A preliminary design has been prepared
for an arch dam, but) to our knowledge, essentially no attempt has been
made to compare the cost of these tvJo structures, to evaluate construction
time or difficulties, or to otherwise evaluate potential alternatives.
As a basis for proceeding with feasibility studies, we consider 1t important
that economic comparisons be prepared for viable alternative dam types for
the ~Ja tan a site.
Devil Canyon Dam
An arch dam appears to be the most appropriate structure. for the Devi 1
Canyon site. This conclusion has been ;-eached by essentially all inves-
tigators, and, we assume, is based on comparisons 'lsi th other dam types for
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the site.
Acres has developed a satisfactory ar:h dam design for the Devil Canyon
site. Stress levels app~ar to be acceptable for all normal loading con-
ditions studies. A dynamic response spectrum analysis, assuming 0.5 gravity
ground acceleration and a 5 percent damping rate, was conducted. The re-
sulting stresses indicate that construction joints in the upper part of the
dam would open intermittently. Some horizontal surface cracking may also
occur on both faces.
We believe this loading to be extremely conservative. A damping rate of 10
percent is more appropriate for this situation, and a ground acceleration
no greater than 0.4 gravity appears to be more realistic.
ENVIRONMENTAL CONSIDERATIONS
Substantial progress is being made in the study of various environmental
considerations, such as the current status of fish and wildlife populationss
cultural resources (archaeologic remains), vegetation types, and alternative
location of access roads. Some crucial environmental issues, however, have
not been adequately addressed. These will require extra attention in the
1981 field season. In this category are downstream effects of the dams on
the river channel itself with potential secondary effects on fisheries and
wildlife, effects of the dams on water turbidity, and possible effects of
leaving standing timber in the impoundment areas.
Fisheries
Studies of fish population in the Susitna River Basin were late in starting
in 1980, but considerable data were accrued through the fall and winter
( 1980-81) . An acce 1 era ted program is underway in June 1981 , \'I hi ch by 1982
should yield a preliminary picture of the existing situation.
The Susitna River above Devil Canyon apparently supports a substantial
population of grayling, but few if any salmon are able to ascend the stream ..
Presumably, the grayling and probably lake trout will thrive in the iwpound-
ments. The question of whether they will constitute an important recrea-
tional fishery depends on the ultimate clarity or turbidity of the im-
pounded waters. Even if the water is turbid, there will be some sport
fishing at the mouths of clear streams entering the impoundments.
The lower Susitna River and its many tributaries and back waters carry
substantial populations of salmon that support an important commercial ·
fishery in Cook Inlet, as well as a sport fishery in the river channels and
at the river mouth. There are additional populations of grayling and
rainbow trout in many of the tributaries. On-going studies are intended to
shed light on the relative importance of the various tributaries, backwaters
and main channels in supporting fish life~ Of particular significance in
this regard is gaining an understanding of the possible effects of the
impoundments on downstream hydrology. This can best be prognosticated by ·
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measuring the bed load of sediment now carried by the Susitna and its
various tributaries. When the silt load from the upper Susitna is cut off
by the dams, what will be the changes in the conformation of the lower
river and the chemistry and turbidity of the water? Data on bed load must
be obtained before this important issue can be predicted.
\~ildlife
The Alaska Department of Fish and Game is making commendable progress in
studying populations of moose, caribou, black and grizzly bears, wolves and
dall sheep. The moose will be directly affected by loss of winter range in
the Watana impoundment. In time, there may be a compensatory development
of new willow stands bordering the impoundment. Black bears will be a11
but eliminated from the Watana impounded area by flooding of denning areas
and loss of protective timber. Caribou may be somewhat affected by disrup-
tion of seasonal migration to calving grounds. Dall sheep, grizzly bears,
and wolves will probably be only peripherally affected by disturbance of
their wilderness habitat. $
The University of Alaska and the Alaska Cooperative Wildlife Research Unit
are studying populations of furbear .. ers, non-game mammals, and birds. As
far as we know these studies ar~ pr _."'essi!lg satisfactorily.
Downstream Hydrology
Change in the amount of bed load carried by the Susitna River may affect
fisheries and wildlife in a number of ways. There is some indication that
the backwaters and billabongs of the lower Susitna~may be important rearing
areas for juvenile saimon. Sumner flooding of these: backwaters, sloughs,
and ponds creates extensive waterfowl habitat. Peak floods cut into
timber stands and deposit open bars which are colonized.by willows that
constitute winter forge for moose. Understanding the dynamics of the lo\\'er
river is essential in predicting long-term effects of the Susitna project
on wildlife .. ~
The need for additional hydrologic stud.ies -especially bed load studies -
was discussed ·;n the March meeting of the External Review Panel in San
Francisco.· But as of June 1981, no firm plan of action has been imple-
mented. The Pan~l urges immediate action to assure that some useful data
on bed load will be available for consideration in October, 1981. Without
it, there wi 11 be no way that do\'mstream effects can be eva 1 uated.
Water Chemistry and Turbidity
The water quality program is being prepared for Acres American by R & M
Consultants. No results have been made available to the Panel, nor even a
list of specific questions being investigated. From the standpoint of
fisheries it is important to know what may ·tJie the f~ture turbidity of the
reservoirs and the Susitna River below.
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In summer, a substantial flow of turbid water will enter Watana Reservoir
from the glacier above. Heavy materials will be deposited in the reservoir
head, and smaller particles will be carried on toward the dam. To what
extent will the water clear as it approaches Watana dam? Will the water in
Devil Canyon reservoir be clear or cloudy? And what of water passing Devil
Canyon dam into the mainstream of the river below through summer and
winter alike? Clouded water blocks the passage of light and reduces or
precludes the growth of phytoplankton which form the base of the aquatic
food chain. The productivity of these waters for fish will be an inverse
function of turbiditye Are adequate studies underway to prognosticate
post-project water conditions?
Timber in Impoundment Area
At the January, 1981 meeting of the !'anel, the suggestion was made that
consideration be given to stripping the timber from areas to be impounded,
for the purpose of reducing the load of floating tra$~ in the reservoirs.
Has this idea been considered? Has the cost been e~timated?
Nitrogen Supersaturation
To protect fish li-fe in the Devil Canyon reservoir and in the river below,
the design of both dams -including penstocks and overflow structures -
must minimize or preclude the incorporation of nitrogen into solution if
current studies by ~1r. r·1i1o Bell suggest this possibility.
Ac\:ess Roads
Selection of the route or routes for constructing access roads should
avoid, insofar as possible, disturbance. of caribou or Dall sheep. These
two species are expecially susceptible to environmental disturbance. The
area south .of the two reservoirs is of particular importance to sheep. The
calving ground of caribou adjoins the upper reaches of Watana impoundment
on the north.
ECONOMIC FEASIBILITY AND FINANCING
Battelle Pacific Northwest is respons~ble, under separate contract, to
review and analyze alternatives to the Susitna project. Dr. Rohan met on
June 2, 1981 at Battelle's office with Mr. Swift, the project manager and
several of his staff to review Battelle's progress and to gain·a better
understanding of their approach. Battelle has addressed its initial. effort
at understanding the gas supply situation, and in improving the demand
forecasting methode 1 ogy. Copies of \t~orki ng draft reports on these subjects
are being forwarded fo) .. review by the External Review Panel. Because th~ ;
results of the Battelle study wi11 be employed in Acre's final report due
in April 1982, it is recommended that the Alaska Power Authority monitor
the timeliness and work quality of Battelle$,
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From the initial Battelle meeting it was learned that Battelle's approach
to comparing alternatives is not totally consistant with the work of Acres.
In this respect, it clearly is advisable that Battelle and Acres meet in
the near future to arrive at a common basis to make economic comparisons of
the various alternatives.
Because of the high level of uncertainty in estimating a) the future
markets for electricity, b) the capital costs and construction time to
build power plants, c) the availability and prices for fossil fuels and,
d) future regulatory environments, it is recommended that all economic
analysis incorporate this uncertainty. Techniques for making economic
comparisons under uncertainty are well known and include sensitivity analysis,
probabilistic assessments and deci$ion analysis. Acres' current approach
needs some improvement as it is narrowly focused. The External Review
Panel would like to revie\'1 in October, progress in developing a consistant
approach to evaluating alternatives under uncertainty.
The issue of financing mechanisms for the Susitna project and the corre-
sponding electri~ rates to the customers needs further a~alysis. Because
of the financial risks, it is likely that the Susitna project cannot be
financed without support in the form of equity participation, guarantees
and the like by the State of Alaska. A determination of available and
likely financing mechanisms needs to be further developed by Acres and
available for review in October. ·
If the Susitna project is financed through direct state funding, and the
corresponding rates for electricity are set less than the cost of gas or
oil heating, there will be economic ·incentives to convert to electric heat.
This would greatly accelerate the demand for electricity and have a major
impact on Susitna and other power projects. The full impacts of this 6ase
need to be investigated.
From an economic viewpoint, it appears that gas is the competitive alter-
native to the Susitna project. Chugach Electt·ic Association, which repre-
sents about half the power requirements for the Railbelt region, is favorably
disposed to this gas alternative. The gas reserve situation and future
prices for gas needs further investigation. Particular emphasis should be
given to understanding potential long term contracting agreements for gas
from the oil and gas companies.
The Panel would like to examine the criteria that FERC will employ in the
market and economic area to be certain that Acre's report fully addresses
these issues.
AGENDA FOR NEXT MEETING
The next meeting of the Panel is tentatively sched~led for the week of
October 5, 1981 at the Acres Buffalo location. The Panel desires to make
the following recommendations regarding this meeting':
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1. A site visit should be made by Panel members who desire to do so
before the October 5th before the full meeting .
2. Geotechnical problems should be resolved and discussed in more
detail.
3. Results of design development studies for various alternatives
schemes should be discussed in more detail.
4. Environmental study results should be presented and.discussed
more fully. "
5. Battelle should present the results of their studies for Panel
consideration"
6. Consideration should be given to having a FERC representative
attend the meeting if this will be useful in speeding up their
review process and earlier license apprqva1.
CLOSING REMARKS
The Panel expresses its appreciation to the staff of the Alaska Power
Authority and the staff of Acres American Incorporated for the many cour-
tesies extended during the meeting.
Merlin D. Copen
A. Starker Leopold Andrew H. Merritt
Dennis fw\. Rohan H. Bolton S~ed
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INTRODUCTION
SUSITNA HYDROELECTRIC PROJECT
EXTERNAL REVIEW PANEL
. REPORT NO. 3
DRAFT
October 8, 1981
The third meeting of the External Review Pan~l for the Susitna Hydro-
electric Project was convened on October 6-8, 1981 at the Acres American
office in Buffalo. In addition to Panel Members, representatives of the
Alaska Pow.er Authority and .. ~cres American wt:re present. Various members
of the Acres American staff presented discussions regarding progress in
geotechnical areas, seismicity, hydraulics, hydrology, and design. The
discussions were we11 prepared and presented in such a manner as to give
a maximum amount of information in a reasonable timeo
Prior to the meeting Panel Members received a document entitled "Susitna
Hydroelectric Project, External Review Board, Meeting.;¥3, Information
Package, October 6-8, 1981 u. During the meeting other printed information
was presented to the Panel as required.
The Panet appreciates the efforts of the Acres American Staff in planning .
and preparing for this very informative and successful meetinge
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SEISMICITY AND SEISMIC GEOLOGY
Excellent progress has been made during the summer months in resolving
most of the uncertainties regarding the possible presenc~ of active
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faults in the vicinity of the dam sites, in developing an adequate model
of the seismic geoJogi of the region, and in assessing the maximum levels
of earthquake shaking which could result from events occurring along
the major seismic sources~ These studies have led to the following
preliminary conclusions:
WATANA DAM SITE
Four major lineaments were originally identified as being possible faults
in the vicinity of the dam:
(1) The Talkeetna Thrust Fault
(2) The Fins Feature
(3) The Susitna Feature
(4) The Watana River Feature .
Field geologic studies during the past several months have developed
evidence indicating that:
(1) The Talkeetna Thrust Fault is not an active fault.
(2) The Watana River Feature is not a fault.
(3) The Susitna Feature is not a fault.
and (4) The Fins Feature may well be a fault but it is relatively
. short in length and, since there are apparently no other active
fau1 ts in the area, it is very un1 i ke1 y that it caul d be active.
In any case its length would preclude the possibility of it ~
being the source of a. significant earthquake.
In consequence, there are apparently no active faults crossing the site
and the major sources of earthquake shaking at the site mr;~ ~ attributed
to earthquakes occurring on the Benioff Zone underlying the site at depth,
the Denali fault, the Castle Mountain Fault, and smaller local earthquakes
occurring with no apparent surface expression in the crust of the Talkeetna
terrain. Considerations of fau1t distances and possible earthquake mag-
nitudes le~ds to the coQclusion that the approximate maximum levels of
shaking will be due to the following sources:
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Source Closest Oi stance Magnitude (Ms) Peak Ace.
Benioff Zone = 63 krn = ~ ::: 0.35g
Benioff Zone = 48 km = 7~ ::::: 0.32g
Denali Fault = 70 km = 8+. = 0.22g
Local Event * ,* *
Seismic geology considerations have 1ed Woodward-Clyde consultants to
suggest that the maximum local earthquake which needs to be considered
(Mean)
is a Magnitude 5~ to 6 event occurring at a distance of about 10 km from
the site. Such an event would produce a peak acceleration (mean value)
of about 0.35g and would therefore not be a controlling event. However~
the Panel believes that in view of the past seismic history and other con-
siderations it would probably be prudent to consider the possibility of
a somewhat larger event at a slightly shorter distance. In which case
the local earthquake would be responsible for the maximum accelerations
likely to develop at the dam site. This does not mean 1lowever, that it
will necessarily control the design .
For the Benioff Zone event, which seems to be controlling at this stage,
the motions reco~mended by Woodward-Clyde Consultants for preliminary
design evaluations appear to be entirely appropriate.
DEVIL CANYON SITE
At the end of 1980, nine lineaments were ident·ified in the vicinity of
the Devil Canyon site which could possibly be active faults. Field
geologic studies during the past 6 months have led to the conclusion
that only 3 of these features are faults, that the three features recog-
nized as faults are inactive, and that in any case they are so short in
length that they could not generate earthquakes which would be centro 11 ing
events with regard to earthquake motions at the dam site. Thus since there
are no active f,Ju1ts in the vicinity of the dam site, the design earthquake
motions will be determined by similar considerations tp those applicable
for the Watana site. The Panel agrees with those conclusions.
* Information to be provided in Fin a 1 WCC Report
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Consideration of the most s~gnificant seisrnic"sources of ground shaking
leads to the following:
Source C1 osest Distance Maqnitude (Ms) Peak Ace . (Mean)
Benioff Zone :: 90 krn :: ~ ::: 0.3g
Benioff Zone ::::: 58 krn :: 7~ = 0.3g
Denali Fault :: 64 km :::: 8+ :: 0.24g
Local Event * * *
As for the Watana site., there is a need to establish very soon the signi-
ficant characteristics of the 1oca1 earthquake (in the crust of the
Talkeetna Terrain) in order to finalize the seismic criteria to be used
for project design.
In the light of the information presented at ~his meeting and on the basis
of past experience~ the Panel believes that through the use of appropriate
design and c·onstruction procedures, darns with ample margins of seismic
safety can be constructed at_both sites. The Panel be1ieves, however)
that the·question of seismic effects due to local crusted earthquakes
shou1 d be reso1 ved in the next few weeks so that more definitive_ design
siudies can be comp1~ted.
ROCK ENGINEERING CONSIDERATIONS
As a result of discussions during this meeting as well as observations
made in the field by Panel member Merritt during the period of 23-25
September, we have the following comments regaraing present designs •
WATANA
Every effort shou1 d be made to reduce the height of the cut s1 ope at the
inlet to the diversion tunnel. The structures can probably be moved
closer to the river and perhaps shifted sl ight1y in a downs~ream direction. ~
The surface excavation at the outlets of the tailrace tunnels and spillway
structures is 1 i kewi se very extensive. Further deta i1 ed examination is
warranted to minimize possible slope stabi1ity problems.
* To be provided in, final WCC Report
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Recent borings in the proposed underground powerhouse site encountered a
zone of soft hydrothermally altered diori.te. This is not acceptable
material to have in a major undergr:ound excavation. Some shifting of these
openings is required. Considering all borings made in the right abutment,
the general quality of the diorite ;is quite high and we foresee that
acceptable rock can be found for the proposed structures.
DEVIL CANYON
The graywacke and argillite at this site appear to be of acceptable quality
for the proposed underground structures. No major shear zones have been
recognized in these areas. The undergrourid openings have been oriented
with respect to the major known joint systems and bedding planes. The
present layout is acceptable and it is recognized that some slight shift
could result based upon the results of future exploration.
The axis of the proposed surface spi 11 way on the right abutment" will nearly
parallel the strike of the bedding of the rock. The required cuts wi~l .
daylight the bedding which dips at about 50 degrees into the excavation.
Potential major rock stability problems could result ~hich might not be
solved by simple rock bolting measures. This design likewise requires
your review.
D
BURIED CHANNEL
The results of al·i geophysical surveys completed tb date have defined a
major channel beneath the plateau on the right abutment at the Watana Site.
The channel is approximately 15,000 ft wide when measured with respect to
that portion of the bedrock channel below the proposed reservoir pool 1eve1.
The deepest portion of the channel lies about 450ft below pool level;
however, perhaps as mur:h as 60-70% of the channel 1 i es 100 ft or 1 ess
below maximum pool level.
The borings comp1 eted during the Corps of Engineers study indicated that
the channel is filled with glacial ti11, outwash, and perhaps lacustrine
deposits~ The boring logs show that bou1 ders (some as large as 12 ft) can
be expected in these heterogeneous deposits, either as individual units
or as thick layers. Contour maps made of the bedrock surface suggest a
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wide entrance channel or channnels upstream of the damsite and a relatively
narrow exit into Tsusena Creek downstream of the damsite.
The buri_ed channel on the north s1 ope C'Jf the reservoir at Watana Dain is
much greater~ in extent than was anticipated a year ag-o and represents one
of the greatest uncertainties associated with the Watana Dam project.
Major problems posed by the presence and extent of this channel are
(1) The magnitude of possible seepage losses through the channel.
(2) The possibi1 ity of piping within the channe1 resulting from
seepage from the reservoir towards Tsusena Creek.
(3} The possibility of seismic instability in the soils comprising
the buried channel under strong earthquake shaking.
It appears that problems {1) and (2) above could be eliminated by construc-
tion of a cut-off wall and grout curtain through the soils filling the channel.,
However, the provision of such a cut-off would not solve any problems of
seismic instabi1ity on the upstream side of the wall.
Since very little information is available concerning the nature cf the
soils forming the channel fi11 it is not possible to assess the magnitude
of the sei~smi c instabi1 ity prob1 em, if indeed it exists at all, or .the
need for an extensive cut-off wall, currently projected to be about 15,000
feet long and varying from a few feet to 450 feet in depth. However~ it
is clear that both th.a possibility of seismic instability and the cost
of a cut-off woulo be dramatically reduced if the reservoir 1eve1 were
about 100 feet lower than currently planned. Such a lowering could reduce
the length of the cut-off to about ·4,000 feet, facilitate its construction
and by lowering the_.;.water table in the soils, increase their seismic sta-
bi1ity. In view of these advantages, together with the. fact that economic
advantages associated with the top 50 to 80 feet of Watana Dam do not
appear to be very great, the Panel believe~ that careful consideration
should be Eiven to the potential benefits of· reducing the height of Watana
Dam by 50 to 1 DO f~et. Such a reduced height might a 1 so fa ci 1 J tate 1 a yo ut
problems for the dam.
The Panel cannot be sure that a reduction in dam hei'aht wou1 d be advanta--
geous but believes that a careful study of the questiqn is warranted in
the nex_t severa 1 months.
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WATANA DAM EMBANKMENT
The Panel bf.:l ieves that the preliminary design section se1 e.cted for Watana
Dam is satisfactory and will produce a stable and economical structure.
It is suggested however, that consideration be given to the following items:
(1) If the shells are constructed of densely compacted gravel
and/or
(2)
( 3)
or rockfi 11 and the core of a much more compressib1 e s_andy-
silky-clay~ there is a danger of deleterious stress redistribu-
tion due to differential settlements. Thus consideration should
h~ given to minimizing this possibility ~y:
(~) ~ inclining the core slightly upstream~ providing
this can be done without jeopardizing stability.
(b) 1t..cating a relatively incompressible core material
which is adequately impervious. Such a material appears
to be available as a GC material in one of the borrow
areas.
Deformations of the upstream shell of the dam due to strong
earthquake shaking can be minimized either by densifying the
shell material to such extent that high pore pressures cannot
develop or by using highly pervious rock-fi11 which will
dissipate any pore pressures resulting from earthquake shaking
almost as rapidly as they develop. Consideration should be given
to using grave1-fi11 and rock-fill in the upstream shell in such
a way as to optimize their use from a seismic design point of view.
There is apparently ice in the rock joints in the abutments at
Watana dam site and this wi11 have to be thawed before grouting.
It would bs desirable to determine whether construction costs
have allowed for this.
. (4) It appears that there may we11 be permafrost in the foundation
soils for the saddle-dam. When this melts it could leave the
soils in a very loose condition which may be adequate for static
stability but inadequate for seismic stability. It would be
desirable to explore this possibility further and examine the
need for exacavation of frozen foundations soils prior to saddle-
dam or dike construction.
DEVIL CANYON 0~~1
Sufficient study has been completed to adequately support the present arch
r~--, -----.
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dam design for feasibility purposes. However, the linear feature through
the pond areas where ~he wing dam will be located should be further explored
in the near future. Similar considerations to those discussed for the
Watana Site should be given to the foundation soils under the Devil Canyon
wing dam.
WATANA DAM DIVERSION TUNNELS
Two diversion tunnels are proposed for diverting up :to a 1 in 50-year
flood during construction of Watana Dam~ One tunnel would be ·located at a
low level so that it would flow full at a11 times. The second tunnel)
1 ocated at a higher 'i eve 1 _, would have free f1 ow. After diversion the 1 ower
tunnel would be plugged. Two plugs would be constructed in the upper
tunnel with gated out1 ets through them to permit re1 ease of 1 ow f1 ows until
Devil Canyon is completed and serve to lower the reservoir in case of an
emergency. The Panel concurs in the general concept of the diversion .
tunnels and modification of the hioh level tunnel for use as a low-f1ow w •
and emergency release outlet, subject to refinements discussed by Acr_e·s.
WATANA DAM SPILLWAY
Spillway flows at Watana Dam waul d be handled by three separate flow re1 ease
structures. Discharges corresponding up to a 1 in 1 00-year flood, waul d
be released through a 1ow-1eve1 tunnel controlled by three or more Hewell-
Bunger or similar valves located at the downstream end of the tunnel.
Discharges corres pon ding to f1 oods in excess of 1 in 1 00-years and up to 1
in 10,000-years would flow through an open chute spillway with a flip
bucket. Discharges in excess of the 1 in 10,000-year flood up to the PMF
would pass through a bypass channel controlled by a fuse plug • .
The Panel concurs in the proposed concept of handling spillway flows.
Release of floods up to 1 in 100-years by·low level valves would maintain
the nitrogen supersaturation level to an acceptable limit. The Panel
suggests that fixed cone valves, as installed by the Corps of Engineers at
New Melones Dam be used, since its greater rigidity makes it more suitable
for high-head operation. The smaller spi11way/chute flows reduce erosion
in the downstream river channel. Hydrau1 i c model tests wi1 1 be required
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to determine the extent of material that should be pre-excavated in the
plunge pool area. In view of the infrequency and short duration of spillway
ope~ation and the relatively high quality of rock in the steep river banks,
the Panel is of the opinion that excessive ero$ion would not occur due to
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service spillway operation. With respect to the emergency spillway bypass
r.hannel, the Panel is concerned over the 45-ft height of the fuse plug.
This high plug would nee~ to be designed as a small earth dam to retain the
power pool at maximum levels and also be capable of fai1ure as a fuse plug
when it is overtopped. It is suggested that the entrance to the bypass
channel be widened, thereby requiring a smaller height of fuse p1 ug •. This
(1 would also reduce the amount of reservoir lowering in the event of fuse plug
failure.
DEVIL CANYON DIVERSION TUNNEL
One diversion tunnel is proposed for Devil Canyon Dam to divert flows up to
a 1 in 50-year flood during dam construction. The tunnel would be plugged
after it is no longer needed for diversion. The Panel suggests that this
tunnel could be used for spillway flow releases in an a1ternative spi11way
design discussed hereinafter.
DEVIL CANYON SPILLWAYS
As for Watana Dam, spi 11 ;,•ay f1 ows at Devil Canyon would be hand1 ed by three
separate flow release structures. Flows up to the 1 in -100-year flood
would be released by four or five outlets through the base of the concrete
arch dam controlled by Hewell-Bunger or other type high pressure valves.
Discharges in excess of 1 in 100-years and up to 1 in 10,000-years would
f1ow through an open chute spillway with a high level flip bucket. Dis-
charges in excess of the 1 in 10,000-year f1ooJ U? to the PMF would pass
through a bypass channel control 1 ed by a fuse !J'! • ... g.
The Panel concurs in the· concept of handling the spi11way flows subject to
one question discussed below. Release of sma11 flows through valves at
the base of the darn wi11 prevent excessive nitrogen supersaturation in
the downstream river channel, as well as reduce discharges and f1 ow fre._
quency and duration in the chute/flip bucket spillway, thereby reducing
p1 unge pool erosion. Based on a ground and. air inspection of the river
channel at the Devil Canyon Site by Panel member Douma and A.cres repre-
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sentatives on September 17, 1981, the Panel is of the opinion that the ~/ery
high quality rock in the canyon walls should not experience excessive
erosion due to spillway operation. In this case, pre-excavation of streamed
material and weathered rock is probably not required. The Panel is con-
cerned, however, over the deep sidehi11 rock cut required for construction
of the spillway chute. It suggests that consideration be given to an a1ternate
plan of providing spillway tunnels, as required, instead. of the chute spillway.
In this alternate plan, the diversion tunnel and probably only one addi-
tional tunnel would be required. With respect to the emergency bypass channel
spill way, the Panel is concerned over the 57-foot high fuse. p1 ug for the
reasons stated for the Watana fuse plug. Consideration should be given to
increasing the length and reducing the height of this fuse plug as desc~it~ci
for Watana.
DEVIL CANYON POWERHOUSE TAILRACE
The Pane1 concurs in extending the tail race for the Devil Canyon powerhouse.
about 1 1/4 mile to take advantage of the additional approximately 30 feet
of head •
CLOSING REMARKS
The Panel requests that the topics raised in this report be thoroughly .
discussed in the next Externa1 Review Board Meeting tentatively scheduled
for the week of January 11, 1982 in Anchorage.
~
The Panel greatly appreciates the many courtesies extended to it by the
staff of the Alaska Power Authority and the staff .of Acres American, Inc.
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Merlin D .. Copen Andrew H. Merritt
-------------------------~--Jacob H ~ Douma H. Bolton Seed
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January 13, 1982
i SUSITNA HYDRORT~CTRJC PROJECT
I' EXTERNAT, JH::VJF.t-J PANEL
J?RPORT No. 4
' J
-INTRODUCTION 0
•.
The fourth meetinq of the External Review Pa.neJ for the
Susitna Hydroelectric Project was convened on ~anuary 12 -
13, 1982 at the Al,ska Power Authority office in Anchorage.
Jn addition to Ponel Members, rcpresentntives of the Alaska
t • • • /" Power Authority and ~cres Americ~~,staff presented
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di.scussions regarding progress in qentechnical areas,
seismicity, hydraulics, desiqn and e~onomics.
Prior to the mcetinq Pane] Members received documents
entitled "Susi tna Hydro~l ectric Proi ect, External Rcvi ~\-:
noard, Meeting #4, Information Pack?q~, 1Januarv 129'13,
,;
1982''; Susitna Hydroelectric PrniPrt, Arres Speciali~t
Consultants Panel, Report, November 18, l9Rl"; "Final Report
on Seismic Studies for Susitna TTydrnelectrir. Project,
Fcbruarv 1 9R?., prepnred by Woor1\·lnrci-C'1 ydc rnnBuJ tant$" .. . ,/
~he Pane] appreciates th0 pffortR of the AcrcB American
staff in planninq the meeting a~d prcpArinq the discu~nions
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prcsrnted th0r~in.
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SEISMICITY AND SEISMIC GEOJ,OGY -"
The seismic geology and seismicity studies have
progressed satisfactorily since the last meeting of the
Panel. At that time, the major sources of earthquake g~und
motion had been determined and the on 1 y · 1n-.eaffi'ly· uncertainty y .r ,.;," ·' ' , ' r •
was the establishment of the significant characteristics of
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the 1aw earthquake (occurring in the Clil!Ye of the Talkeetna , -~
Terrain} which could affect the d~sign of the dam.
Woodward-Clyde Consultants have add~ssed this issue
in their draft of the "Final Report on Seismic Studies for
Susitna Hyd~oelectric Project• and in their presentation at
this meeting. They te;:-m this sourcP "the detection leve]
earthquake" and conclude thqt sue!"> earthquakes would have a
r t.?!. •·• '~ ', ' .
magnitude of 6 3nd 'dtuldjoccur· v0ry c]ose to eit:h~r dam -site.
1
Based on this cone] us ion and other kno~1n source$' o:f
earthquake ground motions, recornm0n~atinns ha~e been
presented concerning the lcv~J of qround motions Which
project Ptructures shnuJd he desiqned ~o withstand. These
are resented in terms of mean response spectra and the
Panel considers the recommendations from mean ground motion~
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to ba entirely appropriate. We wnuln H:ke, however, t.hat
"crj tical" structures such as major clnmR are normnlly
.• t ~ t t designed to wi~hstand carthquRkn ~ntinns at~~he 80
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percentile level and the characteristicR of such motions
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should be developed and considered in evaluations of the
seismic stability of the project structures.
The Panel has nonsidered the characteristics of
possible motions resultjng from earthquakes on the various
sources (Benioff Zone, Denali FauJ.t, Castle Mountain Fault,
and Talkeetna·Terrain) and concludes that it is feasible to .
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design both the gra\rel .... f)All dam at Watana ana the concrete
arch dam at Devil C~nyon, as welJ RR the appurtenant
<'" "-f.-. I:;
structures, to ;~le±y withstand the effects of such
earthquakej shakinq.
-----...... ~-----··· ___ & ___ _
WATANA DAM EMBANKMENT --
THe Panel believes that the d0Rign saction for Watana
Dam, presented at this meeting, j:;; sntisfactor~' and will
produce a stable and economical structure .. with regard to
the questions raised in our previous report, we note that:
(1) It is proposed to construct the core with the
(2)
( 3)
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well-graded glacial moraine material from Borrow Area
"D". This material is satisfactor~ for construction
of an impervious core and furi:her studies of its
properties can be made in th~ design stage.
It has been decided to use an Assentiallv vertical core
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vli th a viebz!e RufficientJ.y l~rqe to prevent arching of
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the core caused hy diff0rcntiaJ scd:intt_fl~ betv?cen the
core and the shell materials.
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It is proposed t;.he/\ the upstrc~am she1J he constructed~
compacted clean river alluvium qravelfir this materia]
e "'· • 11 • , 1 ( , ' , d • . ' . ! :".. .. \ o ~I. w J ~J.. .t ..,.... "~ t, L \, ~.. (J w":)
being processed to r-emove a~].]-f-4-ne.s-J.e-ss-than ~'-si·2-e ( , I •
1.'-{ ....... \ ~.~. -· ~1 '.( 1
in order to provide a hiqh coffficient of permeability
and thereby facilitate rapid dissipation of any pore
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water pressures generated by Lseismic event. This
treatment, together wjth placement in 2 ft. lifts,
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should ensure adequntf' !'<tnhi J i t:y for stat~L and seismic
""'""-'*"'--'-r ... .Jl' it\ ....... loading aii)OehetL.c;;.
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(4) The crest level of the dam and associated reservoir
level~ have heen lowered by 30 feet so that the saddle
}(.!,. ... dike has no water-retaining function except in teh case
of the probabl~ maximum prninct flood. This chanqe
greatly reduces the siqnific«nce of foundation
stability associated with the thawing of permafrost
zones in the foundation of the saddle dike after
reserv<l?gr filling.
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BURIED CHANNEL
In our Report No. 3, we noted potential problems posed
by the buried channel to be as fallows:
1. rnagnituoe se~page losses throuqh.the mixed glacial
and alluvial deposits.
2. pipinq of these materials tO\'lards Tsusena Creek.
3. J ~eismic instability of th€' ~'aiJ s under stronq
·2arthquake shakinq.
Acres has addressed these concerns and has concluned that
seepage losses are not significant, piping can be controlled
if necessary by filt0.r blankets p1c:tced on the sJopes
-~ ....... · . ,.t. J{... • ,.: -'~-· ,J. 11.
adjacent tn)liquefnction is not n problem,especially since
the saddJ e darn ha!~ been reduced in hej ght and the reservoir
level drbpped about 30 feet.
I At this stage in the proiect, only limited information
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iF availBhle on the en9ineerinq ~n~ qcoloaical properties of
the materials within the -channeJ. Thu~ ~ny presept
assessment of seepage, piping, and liquefaction potent~al
is haRed upon the brnc:td~~t assumptionso
The R~ternal Review Panel ~ontinues to believe thRt the
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behavior of the buried channel under full reservoir as well
as seismic events is important to the performanr:e of the
project. However, the lack of ~pecifc knowledge ·of material
properties at this time does not compromise project
feasibility. In our opinion, technical solutions are
available to handle the concerns mentioned above at a
reasonable cost. These solutions might include a filter
blanket, partial or compJ ete cut-off, pumping tcfeduce
porewater pressures, or possihle clensificatjon of loose
soils. The potential for liquefaction incree~es with the
h,..e.v. {v{;-
• .J ~aahest of the reservoir or incr~ase in water level in the
channel s~ils. Thus any further economically justifiable
reduction ib dam height has positive geotechnical benefits.
·The External Review Panel gives i tR Lqualified support
to on-going exploration within the ~hannel area. We agree
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with Acres· that bearings are required to c1efine the extent
\JJ ta :~{!..,~ ht.-{\ ev~
and properties of the various anticipated deposits.~
f~t.{: ~ .
I on~e de~ined, large-scale pumping test~ will be required to
......
. determine general values of permeability. Acres has noted
that buried channels have been founa on oth~r projects which
l ·1-!;r,. ..........
have not permjttod large Wflt0r 1of;Bns n:r cetusnd pipina w.tm~
' the reservoj r \,ras f~lled. They have e~qreed to document
these cas~:::> and p~sent this information prior to our
receipt of the feasibilitv report.
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DEVIL CANYON D~~
The Panel requested in jts Third Report that the linear
feature through the pond areas adjacent to the Devil Canyon
damsite where the wing dam will he located,.~ . I ,
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be fLrther explored in the near future.· Acres agrees that
the investigation iB necessary but, because· of time
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limitations,Lar~ unable to conduGt this work prior to
submission of their Feasibili~y Peport. They do no~ believe
that delaying this inve.stigatinn \\'ill afftect th~ r
feasibility of the project .
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WATANA DAM SPILLWAYS _____ ,_, ________ _
In its Report No. 3, dated October 8, 1981, the Panel
concurred in the concept of hannling spillway flows at
Watana dam by three separate flow release structures, as
follows: Discharges corresponding to floods up to the 1 in
100-year flood through a tunnel controlled by downstream
valves~ discharges correspondinq to floods in excess of 1 in
100 years and up to 1 in 10,000-venrn through a gated chute
spillway with a flip bucket: and c1.ischarges in excess of the
1 in 10,000-year flood up to the probahle maximum flood
through an emergency fuse pluq spillway.
The Panel suggested that fixed cone valves be used
instead of Howell Bunqcr -valve?~ for the tunnf:] spillway
since fixed cone valves give better service for hiqh . ~
operation. The Panel also sugqested that consideration be
given 1:o adopting a wider entranrP and lnHer fuse plug for
,.
the emergertcy spillway. These two suggestions have been
-/!](_•
adopted. The Panel concurs in teh general layout -of a
manifold at th~ down~tream end of the 28-font. rliamcter
spillway tunnel with six 8-foot diameter condujts each
ter~inating with a 96-inch fixed rone VRlve. We also concur
in thE! rroposed wider entranc~ to the emer9ency spillway
with the lower 31-foot high fuse plua.
The service spjllway is rlcsiqnrd so that in combinAtion
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with the tunnel spillway the 1 in 10,000-year flood will
have a maximum reservoir elevation of 2193.-The
corresponding service spillway discharge is 114,000 cfs.
Since the fuse plug crest would bP a"..: elevation 2200, flotvs
through the service spillway would become larger than
114,000 cfs until the reservoir level increases to about
It'/,~,./·,~
2202 when the fuse plug .ahoald have failed':" The
corresponding service spillway discharge at reservoir
elevation 2202 would be 147,000 cf~"t.,;hich is being used for
design of the service spillway. Thusr the tunnel and
service spillt-.ray would handle a flood somewhat larger than a
1 in 10,000-year floodc The PanpJ suqqests that
consideration be given to reducing the size of the service
spill\-lay so that in combination with the tunnP-1 ::;~pillway the
1 in 10,000-year f]ood would have a maximum reservoir
elevation of 2202.
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WATANA SERVICE SPILLWAY CHUTE
Consideration should be ·given to providing concrete
paving for a short distance on the invert of the approach
e . . channel upstream of the ogre crest. Four a-erat~on slots ~n
the chute invert should b~ located at approximate] y stations.
5+~00, 10+00, 14+00 and 17+00. A small ramp should be
located just upstream of each slot. -rhe sJots should be open
on top and a bevelled curved surface should be provided from
the downstream edge of the sJ.ob; to the main invert slope.
The slot oesign should be similar to that developed for
Tarbela dam at Colorado State Unjversity in a 1:12 scale
model and f6und to function satisfactorily in the prototype.
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., DEVIL CANYON SPILLWAYS
·-In its last report, the Panel suggested that fixed cone
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valves be used instead of HowelJ. Bunger vn.lves fo.r the low
level spillway outlets. The Panel also suggested that its 1 •• 1 .r:
entrance to the emergency spill~av channel be widened and
I the fuse plug height be reduced. These suggestions have
been adopted.
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The Panel suggested that consideration be given to
using one diversion tunnel and an additional tunnel instead
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of the gated chute service spillway. Acres has studied this
alternative and found it to be significantly more costly.
The Panel is-satisfied that a tunnel spillway is not an
economic alternative~
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SEDIMENTATION AND RIVER MORPHOLOGY STUDIES
n :,_:{:
Panel member Douma ~ with R & M Constultants, Inc
and AcreG representatives in Anchorage on December 9 and 10,
1981 to review reservoir sedimentation; sediment yield and
river morphology studies for the Susitna. Hydroelectric
Project.. A report dated December 10, 1981 was prepared and
submitted to the Alaska Power Authority.
~he report generally concurs with the study's main
conclusions, as follows: 1) Reservoir sedimentation wouJd
be of no concern to the project as less than 5 percent of
the reservoir storage would be depleted in 100 years; 2) it
will be important to identify locations in the Susitna River
main channel bet\o~een Devil Canyon anrl the Chul tina River
conf "..lence where po~t-prnject channel conditions · . be ;;_j?
detrimental to the fishery a.nd whether or not remedial work
can be accomplished at reasonable cost to minimize oamnge to
fish spawning. a+eas; 3) sta.ges of 1. 5 to 3. 5 feet lot-1er,
~
depending on the reach in the ]ower Snsitna Fiver, will
occur after flow regulation which shouJd not cause excessive
flooding and navigation impacts; and 4) under post-pt"i'ject
conditions, the frequency of occurrence_of dramatic changes
in river morphology \·Jj 11 decrease, resul tj ng in a more
0..
sta~ilizea flood plain ,L_decrease in. number of sub<"::hartne1 s ..., . ..,. ....
and)increas~ in vegetative cover.
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CLOSING REt-1ARKS
L d :.-.!.·::., ,dn<;
The Panel is of the opinion that theLtopic~ discussed
~n -t*& this report must be resolved in the very near future.
It is therefore suggested that Pane]· Member-s ;neet with the
Acres Specialists Consultants Pane] on February 17-18, 1982
in Buffalo to reach agreement tvi th Acres American on the
unresolved issues.
The Panel appreciates the courtesies extended to it by
the Alaska Power Authority and Acres American, Inc.
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ALASKA POWER
A.UiHORITY
SUSiTNA
FILE PS700,
"' ..,. , >A
• ""._'?.~t
SEQUENCE NO~
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ENS
SNT
nWL
April 15, 1982
RECE.-IVEQ __
!~PR l9 1982
. Pt:PCRT TO ACRES hmt.kif;AH Ul&QB.PDRATED
BOARD OF 0 !RECTORS, ALASKA PCv/ER AUTHORITY
From
EXTERNAL REVIEW PANEL, SUSITNA HYDROELECTRIC PP0"1ECT .
After reviewing the comcrehensive Feasibility Report prepared by
Acres American Inc.., the External Review Panel offers to·the Alaska
Power Authority the following unanimous comments on the proposed Susitna
Hydroelectric Project:
1. It is recognized that the project wi11 have environmental
impacts on wildlife, fisheries, and botanical resources.
However, the extent and severity of thes~ impacts appear to be
relativeiy small and furtherrrtore manv of these environmental
losses can be mitigated in full or in part.
2. The high dams p~nposed for Watana and Devil Canyon can be
designed t~ safely withstand the maximum anticipated earth-
quake forces~ ·
3. The proposed design adequately responds to the ~~drologic
environment in terms of spi 11 way capacity and deptnd,~bi 1 i ty.
4. If the project is financed at an oopoy-tunE time when bond
interest rates and oil r~venues are favorable, the pote~tia1
long term benefits of the Susitna project will be
considerable •
Accordingly we consider that thP overall impact of the project
on the State of Alaska could be attractive.
6. To this end we endorse the plan to apply in September 1982 for
a permit from the Federal Energy Regulatory Commission.
7.
8.
\
Mnr~ovPr, WP ~ndnrse the prnpn~~l tn prncPed with site invRs-
tigations and design of the pr~ject, with concurrent work on
some of the critical environmental studies, particularly those
concernino downstream pffects of the dams on the stream and
its fish 1 i fe.
........::t..Li:.--:ut .... ' c)Jt.t., )
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The arrival of any opportune time to proceed with construction
will depend on critical issues of finance aPd marketing of
power which cannot now be accurately forecast. Our
recommendation is that tendP.r documents with all supportir!c
aeotechnical investiaations and desian stu~ies be developea.
Qe estimate that a t~ta1 period of t~ree to four years will be
required for this phase of work. The prnject will thefl be
ready to be implemented whenever thP. financial climate for
contracting becomes favor-able. The advantages ·of proceedin~
in this manner are:
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il!iz T . iii¥~-
9~
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10.
(1)
(2)
(3)
The economic benefits of being ready for financin~;
the momentum of the onaoing studv and an informed
~ . -staff; and .
the ability to avoid a crash design progr?.m.
The disadvantaoe is the small risk of loss of the desiqn costs
in the event that, for some reason, the project is nev~r
bui 1 t ..
We recormnend that the Alaska Power Authority develop a de-
tailed business plan which incorporates a financing and
marketing plan into an overall business strategy. The plan
would describe the critical events that need to be accom-
plished, the interrelationship of these events, the approach
to accomplishing these gonls, the management and control
practice that are appropriate, the most ecnnomic financing
strategy, and power alternatives if the Susitna prci.iect is
delayed or the demand forecast changes.~
This Panel is of the opinion that the economic climate wi11
eventually indicate that it is advisable to proceed with the
construction of the Susitna project and at that time it wiil
.be in the best interests of the State ~f Alaska to develop
this important natural resourceQ
a~., .LO lL0.,7:4
Andrew H. Merr1'1tt
;:.,. Starker Leopof d H. Bolton Seed~ ·
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AiLASii.A. POWER AlJTHOBLITY
334 WEST 5th AVENUE .. ANCHORAGe, ALASKA 9950i
Mr. Charles Conway, Chairman
Alaska Power Authority
334 West Fif·th Avenue, 2nd Floor
Anchorage, ZJ,~.l?iska 99501
Dear Mr. Conway:
April 14, 1982
Phone: {907) 277· 7541
(fJ07) 276-0001
In re!;ponse to your letter of J!'ebruary 3 to members of the
Alaska POWfer Authority External Review Panel for the Susitna
Project an.d your request for a cri t.ical evaluation of the Acres
American Inc. Feasiblity Report and findings and tne responses of
individual Panel members to specific questions, we offer the
following attached coillhlents on the various aspects of the .-)tudy.
It has been a pleasure working with members of the Ala~ska
Power Authority staff and Acres American, Inc. on this important
study and we would like to expres.s our appreciation to you and
all concerned for the help and support we have recei11ed in
preparing our reports and recommendations over the past two
years.
'17~7<~· ~is M. Rohan
Attachment~ as stated
Sincerely,
EXTER...~AL REVIEl-1 PANEL
-MEMBERS
Douma
H. Bolton S'eed ·
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ENVIRONMENTAL CONSIDERATIONS
Development of the Susi tna Hydroelect·ric Project will impact the
environment of the Susitna basin in a number_ of ways. The two reser-
voirs will inundate substantial areas which now support forests and
some kinds of wildlife; the constructio~1. camps, roads, and transmis-
s.ion ·lines will disturb various upland eoosyst,ems; and the flow of the
Susitna River below the. oams will be modified as salmon spawning and
rearing habitat. A number of on-going studies have shed considerable
light o~ existing animal populations and vegetational types. Although
some information is still far from complete, it is possible now to
anticipate some of the impacts t.hat the project will impose on these
communities. In the aggregate~ the total impact will be relatively
small. Moreover, by judicious management, it will be possible to mit-
igate some of the habitat losses by-improving habitats eJ.sewhere. The
discussions which follow summarize the environmental problems as they
are now understood.
Reservoir Areas
The two impoundments, with an aggregate area of about 71 square
miles, will obv~ously be converted from terrestrial to lacustrine hab-
itat with a loss of all the plants and wildlife that use these areas
now. Among the larger animals whose numbers will be reduced are
moose, black bear, and several species of mustelid fur-beaJ:ers. A
wide variety of small birds and mammals will be evicted. Yet most of
these species are connno.n in this part o£ Alaska; there are no known
endangered species of either plants or animals. . In the case o;t; the
moose, it is proposed to manipulate vegetation along tl~e lower
Susitna, by burning or mechanical means, to create more winter range
and hence to increase moose populations there to compensate for losses
of moose in the impou.ndment areas. A somewhat reduced moose popula-
tion in the upper Susi tna basin might mean some reduction in the
dependent wolf population. The Watana impoundment intersects a migra-
tion route used by the Nelchina caribou herd. Although caribou swim
well, and eas1ly cross natural water barriers, there is a possibility
that ice shelving along the shore of the Watana reservoir might inter-
fere with caribou movements. If such a problem is detected, the ice
shelf could presumably be blasted. Of greater importance, perhaps, is
the ne.cessi ty to clear and remove all tpe timber from the impoundment
areas to preclude the formation of floating log jams that could create
a truly dan9erous barrier to migrating caribou.
The upper Susitna Ri~er supports several native fish,. of which
the grayling is the primary game species. Although the river habitats
that are inundated will be lost to grayling production, it is possible
that the reservoirs themselves may support modest populations of gray-
ling and perhaps lake trout.
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Downstream Effects
Below the Devil Canyon dam the flow of the river w.ill be substan-
tially altered from its natural cycle. High summer flows will be
captured in tbe reservoirs to supply winter discharge. The red.uced ·
summer flows in the ri ve1: might adversely affect salmon spawning and
rearing habitat as far downstream on the confluence with the Chulitna
River, naar Talkeetna a Side sloughs that are used as spawning areas
by chum and sockeye and as rearing areas by juvenile coho and chinook
will be cut off from flushing flows which normally occur at high
·levels of di§icharg.e.. Considering the total runs of salmon that spawn
in the Susi tna drainage and its tributaries, the proportions that uti-
lize the reach between Talkeetna and Devil Canyon are as follows
(figures from Schmidt and Trihey):
SPecies
Coho
Chinook
Sockeye
Pink (odd
years)
Chum
Total Susitna
runs ( approx. ) .
33,000
76,000
340,000
113,000
286,000
Percentage spawning
above Talkeetna
. 8%
2%
1%
3%
15% •
Chum and coho salmon are the two species that might be adversely
affected by construction of the dams. There are good prospects for
mitigation of those potential losses. Thirty-two sloughs have been
identified along this stretch of the river. Mechanical opening of in-
take channels might permit flushing flows at discharge levels planned
for normal power production. Occasional higher flows might,. be re-
leased, if needed. Additionally, artificial spawning channels might
be constructed. If proper multiple outlet structures are instal.led in
the dams, wate~ temperature can be re~ulated as well ·as flows.. Much
of the silt in the upper river will settle in the reservoirsr result-
ing in clearer water flowing from Devil Canyon dam, which may be
highly advantageous for rearing of young salmon. All of these mitiga-
tion measures could preserve the salmon runs d.t nearly pre-project
levels, or potentially at even higher levels. Below Talkeetna, no
significant changes in the salmon habitat are anticipated.
Elimination of peak floods may result in. stabilization of bars,
islands, and river banks in the river bottoms below Devil Canyon Dam,
with the result that riparian forest may de·velop in areas now in wil-
low brush. Such advance in plant succession will be unfa'-"Orable to
moose, since willow is a prime \<linter food. This trend can be
reversed by a program of logging of the bottomland forest or by judi-·
cious controlled burninge
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Summary
Considering the environmental impacts as a whole,· and the possi-
bilities for partial mitigation, it does not appear that environmental
considerations should preclude the development of the Susitna
Project.
GEOTECSNICAL CONSIDERATIONS
General
The External Review Panel, as a group and individually, has
visited the proposed dam sites, inspected. the rock formations,
reviewed the results of the exploration program, and read the
interpretations and conclusions presented by Acres in their
Feasibility Report. We recognize that the site exploration has been
done in variqus stages over the past years and note that the
Feasibility Report has included the pertinent portions of these
earlier studies.
~ie · conclude that the amount of site geologic investigations
completed for the Feasibility Report is adequate to effectively
preclude unknown geotechnical conditions which would have a major
adverse impact on project design and costs.
Geology and Project Layout
The geologic . conditions revealed in outcrops and borings are
generally very favorable for the structures required for the project.
Where local shear zones or other areas of poorer quality rock have
been ideptified, the proposed project features have been positioned to
avoid them to the degree possible.. For example, the diversion tunnel
inlet structure at Wata.na has been moved downstream to avoid the
"Fins" feature, the major underground chambers at Watana have been
moved· to the right abutment to avoid the "Fingerbuster• she,ar zone,
and the orientation of· the open cuts and underground chambers have
been located where possible to obtain the ~ost favorable orientation
with respect to the joints and shear zones and thereby avoid major
rock stability problems. ·
The very good rock conditions revealed in the borings are
favorable for the major underground ope.nings proposed and we foresee
that the excavation and support of the chambers will proceed using
well established construction methods. We expect that sub$equent
exploration will provide the information required to establish the
most favorable final position for the chambers as well as providing
more detailed information on the most appropriate excavation and
support methods for the large diameter tunnels and high slopes. ·
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§pecial Geologic Conditions
The result$ of the exploration program at both sites have
revealed no geologic· structures that can not be hand~ed by
conventional methods. Moreover, the field work has been sufficiently
widespread to embrace che general geologic conditions so that no majot'
adverse feature is likely to have been overlooked.
One of the most important geologic aspects that will receive
careful attention during future field work is the buried or relict
channels on both abutments at Watana. To date the studies have
identified a deep channel on the right side that passes . between
Deadman's and Tsusena Creeks that has been filled wfth varied glacial
deposits. The geometry of the channel and general nature of the
deposits have been defined by geophysical surveys and borings. More
recent studies on the left side in the Fog Lakes areas indicate that a
similar ehanne1 exists here also.
The importance of this channel and its deposits for the Watqna
site are threefold: 1) magnitude of seepage, 2) piping of materials
towards Tsusena Creek, and 3) seismic instability of the soi.ls under
strong earthquake shaking. These items have been fully addressed in
our meetings with Alaska Power Authority and Acres and among other
items, modifications have been made in the level of the reservoir to
decrease the height of water against the saddle dike on the right
side. It is clear that further field studies are required (and are
planned) to assess the importance of the above mentioned three
factorso However, as has been clearly pointed-out in previous
reports., we believe that there are technically and econotnicall.y viable
solutions to these potential problems... Acres and their External
Review Panel hold· the s.ame opinion. For the various possible
solutions, estimates have been developed and are reflected in the
project costs. We believe tha·c the estimate is reasonable and should
cover possible contingencies that may develop as more informatic.)n
becomes available.
SEISMIC DESIGN CONSIDERATIONS
The Susitna Project .is clearly located in an area of potentially
strong seismic activity and must be designed to safely withstand the
effects of earthquakes. For this reason, a greater than normal effort
has been devoted during the feasiblity studies to determining the pos-
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sible sources and magnitudes of seismic events which could affect the
project and .the intensity of shaking which these events {:ould produce
at the proposed sites for Watana Dam and Devil Canyon Dam.
The extremely comprehensive studies of the seismicity of the pro-
ject area are probably more extensive than those conducted for any
other hydropower project in the world. They have been conducted by a
high~y competent group of earth scientists and engineers and they have
identified the major potential sources of seismic activity, the
potential magnitudes of earthquakes which oculd occur on tht1se souroes
and the levels of ground shaking which could occur at the project
sites as a result of the largest earthquakes likely to occur on these
sources.
Design ground motions for the requi~' d studies have been selected
with a degree of conservatism appropriate for critical structu.res,
taking into account the possibility of-a great earthquake (Magnitude
8 e 5) occurring on the Benioff Zone underlying the dam-sites as well as
the possibility of local earthquakes (Magnitude about 6 1/4) occurring
within a few kilometers of either of the sites.
Watana Dam
The preliminary design of the Watana Dam is a high embankment dam
with gravel shells and an impervious central core. The design is sim-
ilar to that successfully us~a for other very high dams (Oroville Dam
in California and Mica Creek Dam in British Columbia, for example) and
generally considered to be the most desirable for embankment dam con-
struction. Sources of the required types of soils have been located
and investigations have shown that ample quantities are available.
The proposed section · of the dam is appropriately conservative
with a proven capability to withstand normal loadings and excellent
characteristics to enable it to w.ithstand any anticipated earthquake
loadin·g. The proposed design is in fact very similar to that of Oro-
ville Dam in California which has probably been subjected to ·more de-
tailed analysis of seismic stability than any embankment dam in the
world. These studies have shown that the Oroville Dam would be stable
even if a Magnitude 8 1/4 earthquake should occur within a few
kilometers of the dam-site. The controlling design earthquake for
Watana Dam is comparable in magnitude but is source is located about
65 kms from the Watana site so that the shaking intensity is less than
that used in the Oroville Dam investigation. Furthermore, the
proposed materials for construction of the upstream shell of Watana
have equally desirable characteristics as the Oroville Dam shell
materials. Consequently, there is no reason to doubt, and preliminary
analysis by Acres American, Inc., confirm that, with appropriate
attention to engineering details, the proposed Watana Dam section will
be able to withstand the effects of the conservatively evaluated
earthquake shaking with no detri1nental effects.
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Devil Canyon Dam
. .
The proposed design of Dev-il Canyon Dam is a concrete arch and an
evaluation of the design is presented in the following section. With
regard to earthquake-resistant design, dynamic analyses have been made
to determine the stresses developed by conservatively-selected design
earthquakes: a magnitude a l/2 event occurring at a distance of 90
kms and a local earthquake of magnitude 6 l/ 4 occurring very near the
dam-site. The computed stresses are with the acceptable limits for
concrete arch dams.
Furthermore, the ability of such dams to safely withstand
extremely strong earthquake shaking has been demonstrated by th~
excellent performance of the Pacoima Dam in California in the San
Fernando earthquake of 1971. This 350 ft. high dam safely withstood
the effects of a _Magnitude 6 1/2 earthquake occurring directly below
the dam and producing some of the strongest earthquake motions ever
recorded. This full scale test of a prototype structure provides
convincing evidence that such dams can be designed to safely withstand
the effects of strong earthquake shaking.
Other structures
In final de·sign careful attention will have to be given to the
earthquake-resistant design of other features of the project including
spillways, powerhouses, intake structures, etc.. The safe design or
these structures is well within the state-of-the-art of engineering
design for the anticipated levels of earthquake shaking and should
present no major problems with regard to unacceptable levels of damage
or public safety.
Un~ertainties in Design
Probably the greatest uncertainty with regard to seismic design
is in the required treatment of the buried channel on the right bank
of the Watana reservoir. This uncertainty stems mainly from the fact
that it has not been possible at this stage of project development to
ascertain by bo~· · ngs the types of soils filling the buried channel and
their engineering characteristics.
However, this is not_ a major problem since even if very
unfavorable characteristics are assumed for these soils (and this ·wi.ll
not necessarily be ·the case) , re·medial design measures have been
explored and developed to eliminate any problems which could arise.
Provisions for the costs of these measures are included in the
cos.t-estimate even though the mitigation measures themselves, which
may not be required, are not presented in the feasibility design
reports.
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Conclusion
In summary, it may be stated that the feasibility studies for the
S.usitna Project included an extremely comprehensive investigation of
the seismicity of the project area and the development of design
concepts for the major critical s,tructures ·which, with appropriate
attention to details in the final design and construction, should
certainly eliminate any concerns regard.ing the provision of an
adequate level of public safety and the prevention of any significant
damage to the project as a t"esult of earthquake effects.
DEVIL CANYON DAM
The Devil Canyon Darnsite is ideally suited for an arch dam. The
canyon is narrow and v-shaped. The abutment rock is sound and compe-
tent.
Devil Canyon arch da.rn has been-designee and analyzed by use of
the Arch Dam Stress Analysis System (ADSAS) computer program, whit.:h is
the computerized version of the Trial Load Method of Analysis. This
method was developed by the U. s. Bureau of Reclamation and has been
thorough~:;{ examined by rigorous mathematical analyses. In addition,
results from this method have been successfully compar:ed with
structural models and prototypes in service.
Tha design selected for Devil Canyon is a thin double curvature
arch. It is curved in both horizontal and vertical planes to produce
the most efficient distribution of stresses possible under the site
and loading conditions to which it may be exposed at this site.
The static loading conditions examined are the most severe combi-
nations of gravity, reservoir and temperature loads anticipated at the
site. The resulting stresses indicate a factor of safety greater than
four, based on the anticipated compressive strength of concrete in the
structure. The maximum tensile stresses occur on the downstream face
of the arch, where, if cracking were to o~cur, no damage would
resu~t. The magnitudes of tensil.e stresses indicated will not occur
since a redistribution of load in the dam will result as such stresses
develop.
The dynamic loads applied to the dam are considered to be very
conservative. Even so the resulting stresses will not cause serious
damage to the structure. The analytical method used for stress stud-
ies is based on elastic theory. If the stresses indicated should
occur, contraction joints in the upper part of the dam may open momen-
tarily but would n!:>t result in major release of water or permanent
damage to the structure.
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The preliminary design for Devil Canyon Dam does, in every re-
spect, respond to the seismic environment of the site •
With proper construction control, the dam will provide adequate
safety under all loading conditions. It is extremely important that
the very best construction techniques be employed in this dam.. .Proper
concrete mix designs, ccns.istent consolidation of the concrete and
careful treatment of the rock contact and construction joints are of
the utmost importance. The resulting concrete must be a homogeneous
and· isotropic product.
There are always risks of inade~~ate or inconsistent construction
practices which would present problems in the behavior of a dam. For-
tunat;ly an arch dam has the capability of distrib•tting l9ad from
weak areas to stronger, more capable concrete. This is not mean.t to
excuse any but the best concrete control possible, because any weak-
nesses are not accepta:,le in this important structure.
Additional foundation investigations and insitu measurements will
be required before a final design for Devil Canyon Dam is completed.
Deformation moduli, joint orientation and continuity, and shearing re-
sistance along joints will be required. Because of the preliminary
nature of the present studies, such investigations are not considered
necessary at this time. Instead, conservative assumptions have been
made to assure a safe and satisfactory structure ..
The proposed foundation treatment, consisting of consol.idation
and curtain grouting and adequate drainage, is satisfactory.
The engineering consultant has used adequate conservatism
throughout th.e design for Devil Canyon Dam. Very little change from
the preliminary design is anticipated for a safe and ·efficient fil}.al
design for Devil Canyon Dam.
HYDROLOGY AND HYDRAULIC DESIGN CONSIDERATIONS
Flood Potential
The engineering consultant's assessment of the flood potential in
the project area has properly identified the potential magnitudes and
frequencies of flood flows.
The assessment utilized all available-precipitation, snow survey
and stream gaging data for stations within and adjacent to the Susitna
River Basin. The probable maximum flood is based on the mos.t. critical
combination of precipitation, snow melt, infiltration losses and flow
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concentrations that is reasonably possible. The hydrologic analyses.
are in accordance with accepted engineering practice which has been
developed in the United States and is being used in many parts of the
world.
Spillway Capacity and Dependabilit;(
The proposed design adequately responds to the hydrologic envi-
ronment in terms of spillway capac.-ity and dependability.
Both Watana and Devil Canyon dams will have low-level valve-
controlled ou~lets to pass the once in SO-year flood, a gate control~
led chute spillway in combination with the valve outlets would pass
the once in 10, 000-year flood and a fuse plug emergency spillway
in combination with the valve outlets and chute spillway would pass
the probable maximum flood without overtopping the dams. Similar
valve outlets and emergency spillways have been constructed and
operated elsewhere with successful service. There is no reason to
believe that they would not be successful at the Susitna project.
Public Flood Safety
The proposed project adequately protects public safety . in terms
of the flood danger and there are no increased flood risks inherent in
building the project. 0
The rese.rvoirs will be drawn down in winters providing signifi-
cant amounts of reservoir capacity for storage of summer floods.
Virtually all normal river flows would pass through the powerhouses
with very little spillway operation. Peak discharges for major floods
would be reduced substantially. Consequently, project operation would
enhance the public safety by ;-educing the magnitude and danger of
floods in the lower Susitna River.
Spillway capacities and heights of dams are designed with conser-
vative safety factors. The dams and water conveyance structures are
designed and would be constructed with high safety factors in accord-
ance with best engineering practice. For these reasons, there would
be no increased flood risk inherent in building the project.
Project Damage or Shutdown
There is no reason to expect t~at the project would experience
damage and/or require shutdown as a result of floods.
Major floods may cause som~ cavitation erosion in spillway
chutes, river bank and bed erosion downstream of flip buckets and
valve outlets, and eros ion in the unlined emergency spillway channel.
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Because of the infrequent occurrence and relatively short duration of
major floods, none of these t~.~s of damage would become so extensive
during any single flood to require project shutdown •
One or more of the valve controlled low-level outlets may sustain
daro.age during a major flood requiring temporary shutdown for repairs •
This shutdown would not significantly affect flood regulation since
each outlet discharges a small percentage of the total flood flowo
As the powerhouses will be undergroundv floods would not cause
them to be damaged or shutdown •
Design and 0Eeration Assumptions
The engineering consultant has not made any major assumptions re-
garding design, operational modev etc() of water conveyance structures
that lack a satisfactory level of conservatism.
The low-level outlets, main spilll\·ays, and fuse plug emergency
. spillways have all been designed in accordance with current engineer-
ing practice which is based on conservative assumptions. Fixed cone
valves are superior to any other type of valve for high-head opera-
tion. Air slots will be provided in spillway 4 chutes to prevent
cavitation erosion by high velocity flow. Pre-excavated plunge pools
and/or bank protection will be pro,rided downstream of flip buckets and
fixed cone valves to prevent excessive streambed and bank erosion.
The fuse plugs are designed conservatively. to withstand. reservoir
pressures until they are overtopped and then wash out rapidly to
activate emergency spillway operation. The assumption that excess.ive
erosion would not occur in the unlined emergency spillway channel is
conservative in view of the mild channel slope and favorable rock
quality.
The proposed operation of the water con~eyance structures is be-
lieved to be the most reasonable and practical operational mode which
provides a satisfactory level of conservatism with respect to down-
stream effects and project safety.
Reservoir Sedimentation
The effects of reservoir sedimentation have been properly assess-
ed in design of the project.
Based on conservative values ~f the sediment inflow and reservoir
trap efficiency, less than 5 percent of Watana reservoir would be
filled in 100 years, and deposits in Devil Canyon would be less than
25 percent of that deposited in Watana reservoir. A large percentage
of the sediment would be depo-sited in the dead storage portion of the
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reservoirs. Reservoir sedimentation is not a controlling factor in
·project design as larger reservoirs or higher dams are not required
and power production due to reservoir sedimentation would not be
affected for w~ll over 500 years.
Potential Downstream Effects
The proposed design and operation of the water conveyance
structures adequately addresses potential downstream effects on river
morphology, fisheries and wildlife.
Multi-level intakes will be provided for the power intakes and/or
low-level outlets, as necessary, to permit release of reservoir water
in the temperature range sui table for the downstream fishery. The
valved outlets will discharge into relatively ·shallow basins, thereby
preventing nitrogen supersaturation conditions harmful to fish.
Spillway flip buckets and plunge pools will be designed to minimize
nitrogen supersaturation. Their infrequent operation of once in SO
years ~..rould also greatly reduce any potential for serious effects on
fish by nitrogen supersaturation. Planned increased reservoir
releases during critica,l spawning periods together with remedial river
channel work in spawning areas would minimize detrimental effects.
caused by lower river water levels due to project operation. While
turbidity levels of reservoir releases would be sharply reduced in the
summer, winter turbidity levels may be above., natural levels due to
suspension of fine sediments in the reservoirs; but this is not
believed ~o be significant. Project operation will cause the
following addtional 'effects in the Susitna River downstream of Devil
Canyon Dam:
1) Eliminate and/or reduce thickness of ice cover for 2.0 to 30 miles
downstream of Devil Canyon Dam in the winter due to 't'elease of
reservoir flows above freezing temperatur,es which would prevent
river.crossings over ice by someawildlife and humans.
2) Sediment leads would be reduced in the Susitna River upstream of
the confluence with Talkeetna causing some degradation of river
channels. ~
3) Sediment loads would be essentially unchanged below the ·
confluence because of the extremely large volume of sediment in
the flood plain and contributed by tributary streams below ·the
Talkeetna confluence.
4) · Summer water stages in the lower Susi tna River will be reduced by
1. 5 to 3. 5 feet which would reduce flooding in some areas and
should. not cause major impacts on navigation and other river
operations.
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5) The lower river will become more stabilized, resulting in a
decrease in the number of small subchanr..~ls and an increase in
vegetative cover4
6) The absence of annual floods may re:?ul t in some loss of new lands
for moose browse.
In summary, the potential downstream effects do not appear to be
of such significance as to seriously jeopardize project construction~;
Mitigation Measures in Water Conveyance Structures
Based on successful experience at other projects,
measures that will be incorporated in the design of
conveyance structures should be reliable and effective.
mitigation
the water
Multi-level intakes would have ports at several reser-voir levels
and a gate control system which would permit reservoir \~Tater to be
released at the best possible temperatures sui table to the dow.nstream
fishery. The fixed cone valve sizes and operating heads for the
.Susitna project are well within their acceptable limits. Additional
reliability of operation is provided by the use of 5 and 6 valved
outlets at Devil Canyon and Watana, respectively.. This enables
continued operation at a high level of reservoir release in the event
that one or two outlets would need to be closed. Operation of the
valved outlets, as proposed, will reduce operation of the main spill-
way to once in 50 years, thereby reliably and effectively minimizing
nitrogen supersaturation effects on the downstream river fi.Jhery.,.
Conclusions
In summary, it may be stated t.lotat the feasibility studies for the
Susitna Project includes a thorough development of hydrologic aspects
of the Susitna River and the development of design concepts for the
major water conveyance structures which, with appropriate attention to
details in the final hydraulic design, would assure an adequate level
of public safety against flooding and the prevention of· excessive
detrimental downstream effects on river morphology, fisheries and
wildlife •
MARKETS, ECONOl-iiCS AND FINANCE FOR THE PROJECT
This section responds to the basic issues of the macroeconomic
forces impacting the economic viability of the project, the future de-
mand for power, economic measures and risks for the project, financial
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opportunities and problems, marketability of power and suggestions for
an overall strategy.
Macroeconomics
Two factors, future world oil prices and market rate of interest
strongly impact (if not dominate) the economic and financial viability
of the project. Bot~ of these factors are in a large measure outside
the control of the Alaska Power Authority.
Oil prices strongly affect the State • s revenues, which in turn
influence the State's economy, the rate of economic development in
Alaska and correspondingly the future demand for power.. These prices,
through competitive market forces, establish the long run competitive
price of natural gas and influence the price of coal and thus strongly
influence the cos.ts of thermal alternat.ives to the Susitna Project.
Thes~ same prices af£ect State revenues and available funding from the
State for the project, and the marketability of power.
More than 90% of the direct costs of operating a hydro facility
are interest charges. The market· rates of interest, thus strongly de-
termines the cost of the Susitna Project and its relative
economics a
The Susitna project is economically attractive in an environment
of ri.sing oil prices and low interest rates. Interest rates for State
Government bonds are the highe-st they have been in fifty years. With
a growing surplus of crude on world oil markets 1 the spot prices of
crude have declined and future price trends are uncertain.
Demand For Power
We have reviewed the range of demand forecasts developed by ISBR
and Battelle and employed by Acres in their report and it is our
opinion that these. forecasts appear reasonable. Actual growth rates
will probably lie between the expected and low cases. This is true
because essentially all of the p~wer will serve the residential and
connnercial market, which tracks populati<?n and employment trends.
Economics of the Susitna Project
The present value of the cost of the Susitna Project versus
another source of power is related to the time horizon of the
evaluation and the discount ·rate. The time horizon is important
because the economics may b~ different depending on the period of
evaluation.
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Work done by Acres and Battelle, and supported by our independent
evaluation show that over a 30 year period through the year 2010, the
Susitna project would probably yield no net benefits. With current
interest rates and oil prices, over a thirty year period, power from
the Susitna could very likely be more costly than a thermal
alternative.
However, hydro projects usually have long useful lives of many
decades, and over a 60 year period, the Susitna project appears to be
economically attractive.
With this framework, there is a value trade-off for Alaskans to
choose between
* Receiving the current benefits from funds that would be
invested in the Susitna Project
or
* Investing and receiving the potential long term benefits of
hydro power in the next century ..
Sensitivity and Risk Analysis
The net economic
alternatives are highly
rates, fuel escalation
financing strategies.
benefits for the Susitna project versus
sensitive to load forecasts, real discount
costs, capital costs of the project, and
For the Acres' base case ·analysis, which has escalating energ·y
prices of 9-10% per year based on inflation of 7% per year and an
implied interest rate of 10%, the net gain over a 60 year period is
about $1.3 billion (1982)... The investment in the Susitna Project
corresponding to this gain is $5.1 billion (1982). If the load
forecast follows a low growth scenario, the net gain is reduced to
nearly zero, or if the discount rate is reduced to 12% (5% real) the
project would yield a .l-oss of $500 million or more.
If the fuel costs escalated at an inflation rate of 7% per annum,
the impact would also be a loss of $1.1 billion dollars. Conversely,
if the escalation rate for fuel is 10%, the impact would be a net sum
of about $1.5 billion. If the capital costs of the project were 20%
more than e.stimated, the cost of the Susitna Project and a thermal
alternative would be essentially the same.
There is a wide range of possibilities for forecasts of these
variables and corresponding values for the net benefits or losses.
Through .a probabilistic assessment of each of these variables, Acres
estimated that there is about 25 -30% charice for a net loss and a 70
-75% chance for a net gain. Thes.e assessments were made in an
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environment of increasing oil prices and medium increases in load; and
did not directly account for the financing and marketing risks in
these economic analysis. If we include these factors in todays _
environment, the risks increase although the weight of the economics
still slightly favor~ the Susitna Project.
The major economic risks for the project are:
( 1} Inability to obtain favorable bond rates and corresponding high
financing charges for the project.
(2) Lower than expected energy price increases could make the project
economically nonviableo
( 3) Capital cost estimates may be too low, placing severe financial
strain on the project.
( 4) Possible opportunity losses, that is, foregoing the benefits of
other investments in Alaska, for example, industrial development
in enterprises which might generate net revenues or a stable long
term employment base. The Susitna project would generate jobs
during construction. However, in the long term during operation,
the number of jobs added to Alaska's economy is minimalo
( 5·) Difficulty in entering into long term contracts for the power.
(6) A possible combination of the above.
Management of ·Economic Risks
Many of these risks can be managed, thereby substantially
increasing the possibility of favorable economics for the project.
The essence of this management is ( l) timing and ( 2) additional
low-cost studies.
A strategy of waiting patiently for favorable bond interest rates
and an increase of oil prices would substantially reduce the risks.
Taking a long term view, over say ten years, there is a strong
possibility that interest rates will decline giving the Power
Authority a window to obtain inexpensive financing. Correspondingly
in the same time frame, it is li~ely that oil prices may st.art to rise
again. In order to finance and start construction when these
favorable events occur requires positioning now. This includes
obtaining in advance all permits and licenses, and completing the
engineering design and environmental studies.
To further reduce the risks,
Authority develop a business plan
identify viable power alternatives
or the demand forecast changes ..
it is recommended that the Power
which would, among other things,
if the Susi tna project is delayed
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Financing
In the current inflationary environment, the Susitna Project
would probably need· state goverment participation of about 50% of the
project's value --$2,500,000,000 in 1982 dollars and more than
$3,500,.000,000 in actual costs. Because of the high level of risks,
the debt portion of the project would probably require implicit or
explicit state guarantees, or possible general obligation bonding.
The State of Alaska effectively takes all the risk on the E~ntire cost
of the project including potential bonding of $2,800,000,000 in 1982
dollars and a correspondingly greater numbers of actual dollars ..
A combination of escalating construction costs, high interest
rates, and declining state revenues could put a revenue cash flow
squeeze on the project. Posi t.ioning, patience and timing are critical
to minimizing this risk.
These are some major opportunities .in the financi.ng area
including the arbitraging of funds during the construction period or
obtaining low cost debt financing. For example,. if the project could
be financed today at the lower rates that prevailed in 1977 and 1978
( 7 to 8%), the present value of the costs could be reduced by about
$1,500,000,000 (1982 dollars). A recurrence of low rates would
markedly affect the financing of the project.
The tactics and strategy· for financing needs further study and
should be developed in the business plan.
Marketabilitv
The power from the Sus_itna Project probably could not be sold
unless it were less costly than al terna ti ves. Anchorage, Fairbanks,
and other reg ions within the Railbel t Area have different power
sources and, correspondingly, different cost. bases for power. This
means that if uniform electric rates were used for Susitna power, the
cost of power may be pegged to the least costly alternative. This
would further exacerbate the financing and contracting problems.
A solution lies in organizational changes and a possible state
referendum to gain support from the interested parties. Thi$ problem
of marketing needs further study~in the suggested business plan.
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SECTION B
REPORTS BY .. ACRES EXTERNAL=atVlEW PANEL
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25 October 1980
Mr.· John Lawrance
Project M~nag;.r
Acres American Inc.
900 Liberty Bank Building
Buffalo NY 94202
Subject:. Susitna Project
FirstSpecialist Consultants Panel Meeting
October 20 through 24, 1980
Dear Mro Lawr=nce:
.... Introduction
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The undersigned members of thz Panel visited the .site on
October 22, wera briefed in the office of Acres American In-
corporated on October 21 and 23, and had previously reviawed
a package of information dated October 1980. This report pre.,
sents our consensus of the informa·tion obtained and suggestions·
regarding future investigations on the project.
'Nc. consider the Su$itna Project, as now conceived, to be
viable and worthy of continued investigation.
General Geology and Seismology
The WCC pres€ntation dealt with the wall knotm fe.atures
such as th:: D~nali fault, the Castle r~ountain fault, the Bor-
der fault and the Talkeetna fault; as well as th~ hypothesizeo
"Susitna .fault" and other linears defined in the WCC study tn
date. The Denali fault, Castla: f.iountain fault, and the· Border~
efault are all wall known, recent, activs features that show
evidenc~ of displac~ng or offsetting Plsistoczne features.
The maanituds and minimum distances to the site of credible _,
events on thes~ structures are. not controversial and dasign
motions predicted from svant.s on these structures are r;lativ"';..:
ly straightforward. The possible influEnce of the Talkeetna
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John Lawrence -2-25 October 1980
fault and the Susitna linear on the design motions needs more
study. The Talkeetna rault is a relatively old thrust fault
which brings Triassic volcanics and Permian strata from the. south-
east over Cretaceous argillites on the northwest side of the
fault, Although this feature does not appear to cut Pleistocene
deposits, wee has tentatively assigned to the feature a magnitude
7.5 to 7.9.event at a distance of 4 mi from Watana Dam. There is
a aood oossibilitv that this is an old feature that mav not be -. ~ ~
a "capable" structure. Thus it is of very high priority to per-
form detailed field work along this structure to investigate the
· age of overlying materials not displaced by this fault or to de-
fine the observed offsets of formations of known age that cross
the fault. Observations in the Watana creek area may prove to
be of great valu~ since Tertiary deposits appear to cover both
the Cretaceous argillites and the Triassic volcanics in this
area.
Field studies also need to be condt.' .::ted along the Susi tna
linear to establ~sh if it is a real feature which has experienc-
ed offset and~ if so, what is the evidence of the time of last
movement an.d of the magnitude of the offset.
Other linears or possible faults close to Watana Dam should
be investigated to such an extent that a statement can be m~de as
to whethe-tr the feature is truncated by Pleistocene or older geo-
logic formations.
If possible, a statement should.be made regarding any pos-
sible structural explanation for the :wo clusters defined from
the micro-earthquake observations.
Engineering Geology and Rock Enginee_ring
The "fins" and u finger busters •• in the 'rerti ary diorites as
well as other rock ribs exposed in the canyon indicate that
there are wide shear zones in the diorite intrusion. !-iore ex-
ploration in the form of borings and possibly adits al!e necessary
in the right abutment area to confirm that. the rock quality is
good enough to permit a reasonably accurate estimate of the cost
of an underground powerhouse. Preliminary observations indicate
that the construction of an underground powerhouse at Watana may
be difficult or infeasible due to the wide .shear zones. Reorien-
tation of the pov1erhouse to minimize wall and roof instability
may lead to unfavorable orientations for the penstocks.
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John Lawr::nce -3-25 Oc~tobe.r 19 80
Additional exploration of the relationship of ~~e Ter~iary
clastic volcanics and andesites to the underlying diorite down-
stream of the dam on the right abutment is also necessarj' to
evaluate the possible effects on the. tailrace tunnels and on the
possible long power tunnal. The andesites may fill em old buried
valley in the diorite.
1\n estimate o£ the tunneling difficulty for the long power
tunnel alternativa can only be made after the various formations
and the nature of the contacts between formations are mapped from
Watana Dam to the downstream end of the tunnel. First priority
should be assigned to this mapping for Scheme 3.
The argillite formation of Devil Canyon appears stti.table for
an underground powerhouse. 1-iore exploration is needed to delin-
eate the rock quality and orientation of fractures and shaars to
permit an optimization of the orientation and to aid detailed
roof and sidewall design.
The nature o£ the sheared and weathered zone of ths band in
the river just upstream of ·the Devil Canyon site needs to be stu-
died to determine the nature and possible origin of the fsature.
v7atana Site
General. Although an embankment dam w-ith a height of about
800 ft 'trTould be comparable .to the highest in North Amarica and
among the highest in the ,.,orld, we consider the topography and
a:vailable materials favorable to the construction of Watana Dam.
The foundation and abutment conditions, although not yet ful.l:y
explored, present no kno~m unusual difficu~ties. We believe that
further investigations of seismicity are most unlikely to indi-
cate unfavorable featuras for which adequate provisions cannot ba
made in design~ We believe that emphasis in the next exploratory
phase: should·be placed on dsfining the boundaries of the pluton
and the natuxe and effects of its contacts wi1:.h the ·adjacent 4 ...,
rocks in the gsnaral vicinity of the damsite.
Spillt.;ay. We concur that ~~e spillway should not discharge
into or through the buried valley to ths right of the dam, and
believe that a layout entirely in rock, closer to ths dam,
should be adopted. As the geologic situation becomes better de-
fine.d, an upstream shift in t.he axis of the dam may prove advi-
sable~
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John Lawrence -4-25 October 1980
Reservoir Slidese Our overflicrht of the reservoir area for ~ '
several miles upstream of the dam indicated to us that the topo-
graphy and the nature of the materials near the reservoir rim are
such that major landslidas into ·the reservoir, such as to endan-
ger the dam o:c control works, is remote eve:n under seismic condi-
tions. Therefore, we consider that special investigations of
this ·possibility are r~ot naeded to establish the feasibility of
the project. ··
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Cross Section and Materials~~~ ~~1e concur that a conventional
embankment dam section with near-c=ntral core is· appropriate.
For estimates, the upstream and downstream slopes of 2.25;1 and
2:1 are reasonable. 'tle would prefer that tha do'Wilstrearn slope of
~he core be at least slightly positive to assure that sattlament
of tha sh~lls \-lould induce compression in the core.
Wa consider that the riverbed alluvium should be removed be-
neath the cor~, filters, and transitions, and within a zone da-
fined by linss e:xtending from the outer edges of the crest down-
ward at slopes of 1.5:1. For ths fsasibility studies we consider
it advisable to assume that the material will be removed beneath
the remainder of the embankm~nt axcept whare neadad to support
the cofferdams. ta1hsther soma of this material can remain can
best be decided during ths required excavation of the central
portion.
We consider rounded gravsls, cobbles, and bouldeJ:s to be
superior to rockfill for the shells of such a high dam and sug-
gest that the upstream shell, in particular, should consist pri-
marily of rounded material b;nsath a near-surfac:: zone of rock-
fill that may serve as riprap. Such material, which does not
s;uffer corner-breakage on saturation, reduces ths likelihood of
longitudinal cracking near the crest and tends to dilate under
small strains. The latte~ property substantially increases tb.e
resistance during seismic shakiRg. Do\·mstream of. the core, usa
of r<:'~ed materials near· the transitions is also advantageous,
but compacted rockfill in a substantial portion further down-
stream \-vill be satisfactory to accommodate suitable material £rom
structural or other required excavation.
In our judgment, static and dynamic analyses can be deferred
until t-1-}e general quality and availability of borro'\•1 materials
has been established. To this end the emphasis in the next ex-
ploratory phases should be placed on de.termining the character of
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John Lawre!nca -s-25 October 1980
t.lle riverbed materials, particularly their grain-size, and on
the extent and thickness of lodgment till dep~sits that might be
suitable for cora. Atten-.':i,on should be given to locating deposits
of sufficient thickness to permit exploitation in near-vertical
£aces so that the moisture content .will be increased as little as
possible before and during excavation and transportation. The
possibility of routinely processing all or most of the alluvium
for optimum use in the dam should be considered.
Continuing investigations of the permafrost conditions in
the south abutment are considered of high priority.
Devil Canyon Site
We have visited Devil Canyon Site and have examined the en-
gineering and geologic data pertinent to it. l-7e consider the
site to be well suited for the construction of an arch dam.
Adits are not considered to be essential for furth~r defini-
tion of foundation characteristics prior to a fs.asibility datermi..;
nation. Additional boring and laboratory invastigat:ions will be
necessary to define the locations 1 . directions and characteristics
of joints and shears.
The possibility of surface rupture at the Devil Canycn Site
must be resolved.
A more sophisticatad arch dam design bassd on well formulat-
ed critari.a should be prepared. Such a d~sign should be supple-
mented by well docurnsnted and generally accepted analytical meth-
ods. This is considered to be necessary to establish th= ~conomic
feasibility of the project.
Yours vary sincerely,
A. J. Handron, Jr.
~ Bf!el
Ralph B. Peck _
RBP /ajj
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RECEIVED FEB 2 G 1981
r-1r. John Lat-tren ce
Project Hanagar
21 Fs.brua:ry 1981
Acres America.~ Incorporated
900 Libarty Bank Building
Buffalo ~Y 94202
Susitna Project
Second Spacialist Consultants Panel Meeting
February 17 and 18, 1981
Lawrence-:
Introdu'etion
The members o£ the Panel visited.the office of Acres A:meri-
ca..-x in Buffalo on February 17 an.d 18, 1981. Information r:gard-
ing prograss on the susitna Project since the First Panel Ms.et-
ing was provided to each Panel Member prior to this visit.
Dr. L. R. Sykes participat~ed in the discussions on February
17, but because of otner commitm~nts was unable to remain ~n
Buffalo on February··lB and therefore did not assist in preparing
t..~is report.
This report presents our consensus of the information ob-
tained and suggestions regarding future invsstigations on t..~e
projact ..
Ganeral Geology anc Seismology
The cornmsnts on geology and saismology in the Panel 1-.:tter
of 25 October 1980 ramain the view of the ~an~l and are naither
rapaated nor changed as of the date of this re.port. Since Oeto-
b-tar of 1980 wee have indicated that t..~e maximum ground acce1era-
ti.on at both Devil Canyon and Watana Dam si t.as from a magnitude
8. 5 . earthquake on the Benioff zone would be: on t..l}e order of 0. 40
g. In addition, an earthquake of magnitude 8.5 on the D~nali
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John Lal-lrsnce · -2-21 February l9Sl
Fault would produce about 0.20 g at each site. These values seem
reasonable to the Pa.."lel. Comments on this subjaet are also givc;n
in t~e report by Dr. Sykes dated 4 February 1981e
Tha main items of work that ramain to be resolved are the
invastigation of features and hypothesized faaturas ~~at pass near
or through the dam sites. Such features for Watana Dam are:
(1) The Talkaetna Overthrust (sea letter o£ ·october 25).
·{2). KD3-7, a linear drawn on the basis of air and sata1lite
photography through the r7atana dam site parallel to i;.'le
Susitna ·River.
{3} The Susitna feature, another linear which has been
drawn to t.;,e northwest of \.Yatana dam site.
As stated praviously, geologic field work needs to be done
to substantiate if there is a feature~ if there is, how continu-
ous it is and t'lhat is its dat.s of last movement. Of all the.
items listed above, the Talkeetna Ovsrthrust is the only ~~ll de-
fined tectonic feature, and the affort is definitely justified to
gathar evidence on ~~e data of last significant movement. Up to
b~e present, no other featurs mentioned above has been substan-
tiated by direct fisld evidence.
Sykes (February 1981) states that the 1912 and 1943 earth-
quakes ~ndicate that a floatin~ earthquake of magnituda 6.5
should be considered in the Talkeetna terrain. On this pre~~se,
tha need to investigate some of ths shortsr linears dissappsars
unless they are in the immediate arsa of the dam sites and could
rasult in the offsetting of the p-roposed structur~s. In this
connaction, it is suggested that the recurrance interval be com•
puted for a floating ear~~quake of magnitude 6.5 occurring w£thin
a distance of 10 km ;:>f the Wat~"'la Site~ taking into account t:.'le
area of the Talkeetna terrain, the p~riod of observation, and. the
1912 and 1943 observations. It is suggasted that both ~7CC and
Dr. Sykas indapendsntly assess th~ probability of occurrence of
this floating earthquake closer than 10 km to the siteo
The forsgoing con-:.rnents lead us to the follo~Ting suggestions
and conclusions concerning the pr~liminary budt.ret layout for
1981 activities •,.;eing conside.rad for the NCC aft~ort..
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-3-21 February 1981
We believe that t.~a main effort should be devoted to Activi-·
ty (1) , field mapping and Quatern,ary geology, particularly to
dev=lop the geological structure.naar· the Hatana site, to raach
conclusions raqarding the nature of the linaars or other faatures
close to the site (KD3-3, KD3-7), and to obtain whatsvar perti-
nent data can be assembled regarding the time since any nearby
proven faults 'v-ere active (whether post-Pleistocene 1 ·Tertiary,
etc., without reference to an arbitrary age such as 100,000 years).
To, the :xtant t.~at trsnching at critical points may aid the mapping
and dating, tY"e consider it to be a desirable adjunct, not a pri-
mary effort directad toward determining an age older or younger
than 100,000 years.
In !7-iew of the possibility that the application for license
may include the two-dam project, wg favor a similar effort for the
Devil Canyon site (Activity 2).
!'la do not endorse t;.;.g j?roposed calibration sffort to test
out the e_f£i.cacy of dating procsC.ures. "t-le question whether remote
sensing will provide further useful information, CL"'ld we consider
that geophys-ical and seismic refraction survays should be utiJ.-
ized primarily to extend the limits of knowledge of buried chan-
nals or ot.~.ar low-ve-locity zones already discovered, and aspS!cially
to explor; the depths and areal extant of the buriad channels close
to the dam sites, wher~ thsy may influence the layouts of t.'"l.a pro-
jects with rsspect to diversion 1 SP.illways, power plants a.~d watar
?as sagas, and fou.'rldation conditions. In short, we beliave t...~a ax-
panditur·es for these vital purposes should be primary obje=cti:yes,
and that inferences dratm therefrom regarding saismicity should be
consid~red as us~ful by-products •
't-ie bslieva the funds proposed for Acti vi tias ( 3) , { 8) , (9) ,
and (15) could better be spant as an incr::ased affort under Acti-
vities (1) and (2). Hodest expandituras U..'1.der 2\ctivities (4},
(5), possibly (6), (ll) 1 and (17) appear appropriate. Activity
{ 10) , installation and operation of a seismic nettqork in 19 81..#
would ba desirable for developing background information prior to
resarvoir filling, but in view of the likelihood o·f a period of
nearly 10 years before filling, the itsm could be dafarred. ~e
network could possibly then ba established under the aegis of a
permanent agancy. ·
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Jol"t.n Lawrance
Engineering Geology and Rock Enqineerins.
21 February 1981
Revised April 7, 1981
All co~ents from the October 25, ~980, report apply unchang-
ed. It is felt that at least two borings are necessary in the
area of the underground potv-erhouse at \iatana Dam. Although explo-
ratory adits will yield the. best information on t.~e feasibility
of the underground powerhouse, it would be preferable not to spsnd
the money on the adits at this tim~. It is suggested that a lay-
out be cqnsidered for evaluation '"'hich inclu<?-es a surface po\-rer-
house, in order that the relative economics of t.r,e surface and
underground layouts ean be compared before large sums ara expended
to invest.l:gate the undergroWld po"!erhouse further.
Since it has been found that the andesites i~~diately down-
straam of tiatana Dam ara extrusive, it is again emphasized that
t..~e base of ·the andesit~s and the underlying weathc:rad surface on
the diorit<$ -should be more extensively investigated to evaluat.s
ths possibility that tunnels. may inte.rsect this unconforrr.ity.
This -fsature could affect tailraca tu."'lnels from an underground
po'tt~erhouse or power tunnels to a surface powerhouse. Borings to
investigate the nature of this contact should be given a higher
priority than Boring B-ll presently proposed for the "fins" area
of Watana.
Additional borings supplemented by seismic exploration would be desirable
to delineate an approximate width of the buried channel just upstream on
the right bank of WATAN..~ Dam site. Eventually percolation tests and pumping
tests to determine the permeability of the channel should be conducted.
Piezometers should also be placed at several locations in the buried channel
between the Susitna River and Tsusena Creek to learn about any possible
existing hydraulic gradients in the present condition of the channel.
_ At the Devil Canyon site, two angle borings have bsan pro-
posed on the left river bank, one dipping·banaath the river and
the other into the canyon T.otall. Tha boring dipping baneath the
river is intend~d to chsck the possibility of a shear zona be-
neat;, tha rivar. Both borings are intended to explore the geolo-
gic structure as well. Inasmuch as the J'llOSt prominent jointing,
and some observable shears seem to be ori~"'lted parpandicular to
the axis of t.l-ta river, such borings may not disclose t.l-tem. On
the othsr hand, one or both borings might fortuitously be located
entirely in one of the shears characteristic of metamorphic rocks
and might give an $rrone.ous conception of the rock mass. tie sug-
qest that the need for these borings be rsviewed and that, if
they are deemed necessar.1, they be oriented to ~ross the geologic
~tructure.
Watana Dam Laxout
Sevaral preliminary layouts wsre presented and discussed for
an e-mbankment dam at ~\Tatana. \·~e concur that an external cross
~~-, .,;.,.~-::;-. --.,---.-.----. ,,
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.John La~·;ranc:: -s-21 Fa~ruary 1981
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section similar to that at Oroville rapresents a satisfactory ~.,d
conservative starting point·. A dam with these slopes requires a
lonq diversion tunnel, whsraas the length of the turil"lel is con-
strained by the configuration of the river and the quality of the
rock near the portals. The position of the cofferdams, similar-
ly constrained, may in part determine the amount of riverbed al-
luvium that ca.."l. be excavated baneath the da.T!t, The depth of all.u-
viumwill also be a significant factor in this determination.
Henca, determination of the configuration of the riv:r bottom and
of the depth and character of the alluvium are consider~d matters
of high priority for 1981~ A Becker drill, perhaps of large dia-
meter, may prove useful in rivsrbsd exploration.
We concur that an ample allowance for blanket and curtain
grouting, for fou.Yldation traatm:nt, and for drainage of founda-
tion and abutments should be made in the preliminary estimates.
A decision regarding the adoption of drainage or grouting gal-
le.ri-as c~"'l and should be d.aferrad until mor2 is known regarding the
character of the rock~
An. embankment dam has been investigated in scm::. detail for
the Watana site. To provide a reasonable evaluation of alterna-
tive design possibilitiss, a ~~in doubls curvatura arch dam d~
sign should be prspared and studied. The geological and topogra-
phical conditions at t.~e Watana site app~ar to ba. satisfactory
for a structure of this type.
Devil Canvon Dam Design
An accspt,able arch dam design has been pre:pared for the Devil
Canyon site. Stress analyses '<rlere made for normal full resarvoir
and maximum dra-c.-ldo"\-rn with appropriate concrete temperatures.. Ths
stresses computad for th::se conditions are satisfactory.
Some minor changes in the design can be made with minimal ef-
fort and should improve the structural behavior while reducing tha
concrate volume required for the darn.
An analysis indicating the affects on t.he das.ign of earth-
quake should be. made \'lhen app.ropriate ground accelerations are
datarmined. Response spectra analyses ara satisfactory for this
stage of developmant.
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John !.,awren ce ·
RBP/ajj
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Yours sincerely,
Hendron Jr.
21 February 1981
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Mr. John Lawrence
Project Manaqer
Acres American Inc&
900 Liberty Bank Buildinq
Buffalo NY 94202
18 November 1981
Subject: Susitna Project .
Specialist Consultants Panel Meeting No. 4
November 18, 1981
Dear Mr o Lawrence:
·INTRODUCTION
On this date. Profs. Hendron and Peck met in Buffalo to
discuss certain qeotechnical· features of the project. Brief-
inq and discussions followed the attached agenda.
This letter was drafted in the Acres Anreri;;.:an office at
the end of the meeting and was finalized by the undersigned
shortly thereafter.
WATANA CORE MATERIALS
The well graded materials from borrow area D are suitable
• for use in the core of Watana Dam; current thought regardinq
filter requirements for well graded materials should be taken
into account in the design of the filters {John Lowe III, 4th
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Naber Carr~llo Lecturet 1979). The well graded materials from
borrow area H are also suitable and have some plasticity whi~
poss~ly makes them slightly more desirable when considering
design against pipinq. However, the clayey materials may be
more compressible than the materials from area D; also, they
may exi~t at water contents too hiqh 'to be pla~ed at the de-
sired densJ.t:i.es and there wilJ. .oe little possibility of dryi.nq
them during the construction season. In summary, both mater-
ials are acceptable on the basis of present information.
l-iore information is necessary on insitu water contents and de-
sired densities in t.~e dam before the final selection can be
made properly.
WATJU~A DAM SHELL MATERIALS
We feel that the dam would perform better statically ~f
river gravel and cobbles were used for the upstream shell,
because rock fi.ll dams over about 500 ft high usually develop
longitudinal crapks upon first filling due to additional break-••
age at sharp contacts on saturation. Zones of processed gra-
vel could be provided to eliminate the fines and assure higher
permeabilities if excess pore pressures are thought to be a
problem during earthquakes.. It is possible that too low an
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cause for the hiqh pore pressures computed in dynamic analyses.
I Stiffness values for these materials could ~e approximated by
I back calculation from the observed settlement of Portage
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Mountain Dam in which both processed and pit-run compacted
qravels were usede
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WATANA CORE GEO~mTRY
~though static analyses may indicate that a more· favor-
I able stress distribution is achieved if the core is sloped
I upstream {on the assumption that the core is more compressi-
ble than the shells), we feel that a central core is prefer-
I al::Sle Ullder e~.'t'thqua."ce conditions because the shells will
I probably shake down more than the core. Thus the downdrag on·
ti1e core will tend to produce hiqher vertical stresses in the
I core and so reduce the probability of crackinq.
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WATANA RELICT VALLEY
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Control of seepage through this buried valley is required
I for safety; the cost of the lost water is of little import
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because the seepage loss merely offsets the requirement for a.
minimum downstream flow. Three alternatives have been consid-
I ered:
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.l) An upstream blanket over the entire inflow area.
This would be costly and, in _fact, impractical because of the
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limitation on its extent.imposed by the entrance to the di-
version works.
2) A cutoff across the pervious channel. This would be
extremely costly and probably ineffective. For nractical rea-..
sons it would hardly be p9ss:ible to construct a slurry wall
deeper than 200 ft. Attempts to create a grouted alluvial cut-
off between the bottom of the. wall and bedrock would have small
chance for success in view of the likelihood of encountering
permafrost and in view of the great variation of permeability
likely to exist. If such a cutoff were to be provided, it
would be necessary to monitor points of po.ssible emergence of
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seepage downstream in the Talkeei;_I}_a valley and, in all proba-
biltty, to protect part of the area by filter blankets. In
our judgment no further consideration should be given to the
cutoff alternative.
3) Prevention of piping or backward erosion by providing
suitable filters in the zone of seepage emergence in the Tal---
Js~~tna valley. This can be done, as the need is demonstrated,
in the following steps:
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a) Establish the location and reqLTL\e of spr·ings that ·
presently exist in the area of possible emergence, and
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install and observe piezometers at suitable locations
prior to reservoir filling.
b) . If discharges appear or increase during reservoir
filling (or thereafter as perma~rost zones melt), or if
piezometric levels so indicate; cover the emergsnea areas
with filter drains. If seepage emerqes high above the
.-'fl.
Talkeetna valley bottom, consideration can be given to
directing the seepage into lower strata by means of fil-
ter wells and providing filter protection £or the lower
strata.
We consider this alternative to be the most oositive ..
control measure. It wi.ll, in addition, be the least costly.
Similar treatment would be necessary to a lesser extent even
if one of the other alternatives were adopted. The procedure
requires a period of surveillance, adequately funded, for sev-
eral years until conditions stabilize, including the melting
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of permafrost until thermal equilibriwn develops. It ~lso
requires maintaining the ability at site to execute the mea-
sures that may be found necessary. It should be noted, h~w-
ever, that the requirements of surveillance and capability of
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remedial work would exist in any event, in view of the remote-
ness and rigorous climatic conditions at the site.
SADDLE 'DIKE AT WATANA. RELICT VAI·T.EY
In view of our preference to eliminate the cutoff in the
valley, the design of the saddle dik~ would not be premised on
the incorpc,ration of the cutoff in i:ts foun~dation. -The rela-
tively low head across the dike would permit conventional
seepage con~rol. However, consideration must be given to the
possible existence and thawing of J?ermafrost zones in the
foundation after the reservoir has risen and to the influence
of liquefiable zones. Exploration is presently inadequate to
determine if such zones exist. If the maximum reservoir level
would be no higher than the natural saddle, these considera-
tions would become insignificanta We believe the proposed
studies of reservoir elevation will be useful to determine . .if
there is an optimum level at which most of the project bene-
fits may be retained while the problems of the dike can be·
substantially reduced •
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WATANA UPSTREAM COFFERDAM
We are concerned about the space limitation that may re-
quire steepening the downstream slope of this cofferdam if the
bedrock in the ri ~er should be lower than anticipated where
the main-dam excavation would occur adjacent to the cofferdam.
We also have concern that constructing the proposed cutoff to
rock beneath the cofferdam may involve delays due to its depth
and to obstructions in the alluvium. We suggest that the
cofferdam desiqn be studied further •
PERFORMANCE OF CONCRETE DAMS
We be~ieve it would be pertinent to review the experience
in arctic climates of concrete dams, including the long-time
history of several dams in Norway. (For example, Hegqstad and
Myran, Investigations on 132 Norwegian Concrete Dams, 9th Con-
qress Large Dams, Q34, R28, Istanbul 1967; Berdal and Kiel ..
Skogfoss Hydroelectric Power Station, Norway/USSR; Civil Enqi-
neering Works, Proc. Insto CE, Vol. 30, pp. 271-290, Feb.
1965, discussion Vole 33, pp. 481-491, March 1966.) This in-
formation would be pertinent to several features of the pro-
ject, including possible consideration of a concrete-faced
rockfill dike at the side channel to the left of the Devil Can-
yon site.
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A. J. Hendron, Jr.
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INTRODUCTION
February 18, 1982
SUSITNA HYDROELECTRIC PROJECT
ACRES AMERICAN EXTERNAL REVIEW PANEL
REPORT NO. 4 ·
The Acres American External Review Panel for the Susitna Hydroelectric
Project met with the Alaska Power Authority Review Panel on February 18,
1982. The Acres External Review Panel had convened independently on
February 17. Both meetings were conducted at the Acres American offices
in Buffalo.
In addition to Panel Members~ Robert Mohn of the Alaska Power Authority
and representatives of Acres American were present.
The objective of these meetings was to discuss the few remaining topics
regarding the project which require resolution. Various members of Acres
American staff presented discussions regarding geotechnical questions,
seismicitY. hydraulics and design.
}
The Panel appreciates the courtesies extended to it by Acres American and
the planning and preparation of discussions presented in the meetings.
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Buried Channel
Regarding the feasibility of Watana Dam, it is our op1n1on that the possib'le
seepage losses through the buried channel are not large enough to impact
the feasibility of the project. Moreover, possible piping of alluvial
materials can be controlled-, if necessary, by weighted filter blankets
placed on t'·;e slopes between the reservoir and Tsusena Creek. The cost
of providing the downstream filter should be considered in the feasibility
reporto
The present reservoir elevation of 2185 is low enough such that the water
is not required to be permanently supported by the freeboard dike. In .
fact, the free-board dike will not be required to resist differential water
levels for the PMF (el. 2202) because the lowest point above the relict
channel is elev. 2202.
Recently, the possibility of liquefaction of the uppermost layers of the
buried channel fillings has been raised. If these materials liquified,
and if large volumes of these materials could move under the gentle slopes
' shown in attached section W-16 of Figure 6.34, Task 5 Report; then it would
be hypothetically possible to breach the reservoir. Recent stratigraphy
has been developed for the buried channel which is shown in attached
Figure 1. As shown in Fig. i, the lower unit K is the buried alluvium~
unit J is a preloaded till, unit Jl is an interglacial alluvium, unit I
is a preloaded till, unit His an alluvium, unit G is a waterlain till
or lucustrine deposit, and units A,B,C,D,E and F are more recent outwash
deposits. It is highly unlikely that liquefaction could be a problem from
the top of unit I downward as shown in the cross section given in Figure 2.
The alluvium in stratum H will be saturated by the reservoir, however, and
more information is needed to conclude whether liquefaction is or is not a
problem in stratum H. Stratum H is buried beneath the water laid till unit
G, which indicates it was saturated under the water levels which produced
unit G and was probably subjected to earthquakes during that time period.
Further development of the pleistoc~ne geology may clarify this point~ The
strata above unit G are outwash materials and more information is required
on density, gradations, and blow counts in order to make definite comments
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rage l:
on liquefaction susceptability. Because of the above, it is advisable to
consider the possible remedial action shown in Fig. 3 where in the worst
case, a compacted dike would be placed in a trench excavated down to the
top of the overconsolidated till, {Unit I). The costs. of this remedial
action should be included in the feasibility report, but the decision to
employ or omit this possible remediai action must be delayed until after
more investigations are conducted in the area of the buried channel.
At its meeting held on February 18, 1982 the APA review panel made recom-
mendations concerning the design earthquake motions for Devil Canyon con-
crete arch dam. We concur with these recorranendations.
Alfred Je Hendron
/ •
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SYMBOL
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B
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F . -
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-
-
QUATERNARY STRATIGRAPHY OF THE
BURIED CHANNEL AREA
..
UNIT MAXIMUM MATERIAL TYPE THICKNESS .
~· ~ . 5'-:-:. ~ • ..
SURFICIA~.: ORGANIC SILTS AND SANDS . . .
DEPOSITS -WITH COBBLES AND BOULDERS. . · .. . -. . . ..
i -·. --!> ~ . -. . .
OUTWASH :18' SILTY SAND WITH GRAVEL AND
COBBLES.
-
ALLUVIUM & . .... 15' SAND, SILT WITH OCCASIONAL
FLUVIAL GRAVEL ,
.
DEPOSITS
-
OUTWASH 55' SILT, SAND, GRAVEL, COBBLES,
PARTLY SORTED .. --
TILL/WATERLAIN 65' CLAYEY SILTY SAND, WITH GRAVEL
TILL AND COBBLES, OFTEN PLASTIC.
ALLUVIUM 40' SILT, SAND AND GRAVEL, SORTED~
·e
TILL i, 60' SILT, SAND, GRAVEL, COBBLES,
POORLY SORTED. '
INTERGLACIAL 45' SAND, GRAVEL V{ITH OCCASIONAL
ALLUVIUM . SILT, SORTED.
. . . . . . -.
,") TILL 60' _SILT, SAND, GRAVEL ,COBBLES, . . . . -.. .. .... ~ . . . . .. ~C?ORLY SORTED., .. . . .. .. . . ··-. . ' . . . .
' -·· _ ...... . •. . .., . -~ ~
i ALLUVIUM 160' GRAVEL, COBBLES, BOULDERS,
FE'ii FINES •
FIGURE I
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•• 0
N
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E.
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T
2300
.2100
2000
~
1900 ~ SUSITNA RIVER
0 200 400 600 800
CREST El. 2210 FT ..
PMF EL 2202
1000 1200 1400 1600 1800
HORIZONTAL DISTANCE IN FEET
WATANA RESERVOIR
FREEBOARD DIKE
.CROSS SECTION
2000 2200
FREEBOARD DIKE
2400
DR-2CI
TSUSENA CREEK ~ , I
2600 2800 3000
VERTICAL EXAGGERATION = 2
FIGURE 2
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2300
2200
2100
2000
1900
1800
®
RESERVOIR LEVEL
2185 FT.
CREST EL 2210 FT.
SURPLUS AND
UNSUITABLE
MATERIAL
TOP OF
GROUND
-----·.0 .. U,... .• , '"S.H ._ . . ·I. U M •·-·--
OVERCONSOLIDATED
DEPOSITS --------X X X X )( x-)c'V::--::-~---XXX)("'~
ALLUVIUM
I
600 soo 400 300
~ ~ x~-;-~~----~. JC X JC
BEDROCK X ----X X .
X X X X X :c l( X X X X X X )( X X X X I I I I 1 a
0 100 200 3Qtl 4~10 500 600
200 100
HORIZONTAl DISTANCE IN FEET
TOTAL CUT = 13 MILLION CU. YD.
DAM VOLUME = 4.5 MILLION CU. YD.
BURIED CHANNEL
ALTERNATIVE SOLUTION
•
X
700
fiGURE 3
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--
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~~-
,____, ---------
SL 80-1
t r---TSUSENA CREEK
EL. 1680
•L-~----I · - ----------jl~'fBi'll§lli-----~~--=-~-----
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u THE FINS u TO TSUSENA CREEK
SHORTEST FLO\V PAT,H
SECTION W-16 ---------------
• • • t!lo • • • • • • .... -. • • .. ~ • ' .... • • ·~ • • ••
PAt~ et= Fi<dJ/2£: 6 ·34.
/A-SK 5 · t2£fo ~1
2400
2000
1600
1·2o o
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2400 -
2000
l600
1200
~-------: ..... -315 °
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[}
NORMAL MAXiMUM OPERATING
LEVEL EL. 2185
-
SUSITNA RIVER
(LOOKING DOWNSTREAM)
F.L. 1470±
·-___ , _______ ._/"
-.
THIS AREA APPROXIMATELY
30001 UPSTREAM OF WATANA
DAM CENTERLINE
u THE FINS u
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OR-19
NORMAL MAXIMUM I OPERATING POOL
' EDGE ·
OM-A
OR-20
~ , ---., ---+---1--rtc--,
SL ao-2
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