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SUSITNA HYDROELECTRIC PROJECT
WATANA -ROCK MECHANICS ASPECTS OF DIVERSION TUNNELS
1 -GENERAL
The layout assumed for these tunnels was
SK 5700 C6 218 A
SK 5700 C6 226
SK 5700 C6 227
determined from dravli ngs
October 14, 1981
October 13, 1981 / ,., .. · October 13, 1981 ./
No:_
The plan position shown on drawing SK 5700 C6 218 A is not consistent
with the upstt"'eam portal location. It is understood that the drawing
SK 5700 C6 226 is the preferred portal location and the plan location of
the tunnel at the upstream end will be.mov.ed approximately 140 feet south.
The tunnel is assumed to be "Dee" shaped with a span of 35 feet and height
of 35 feet (see attached sketch). The tunnels are 4.200 feet long. It is .
understood that the tunnels ~re required to be lined with insitu concrete
' for hydraulic consideration. It is assumed that th~ lining would be at
least 1.5 feet thick,.thereby increasing the excavated span to 38 feet.
The left tunnel falls at 0.95% downstream from~~ ele-.·ation .1,490 feet
,·
to 1,450 feet. The right tunnel falls at 0.24% downstream~~ elevation
from 1,430 feet to 1,420 feet. The tunnels are spaced 2.5 D when D is the
spa~~of the tunnel center to center, i.e. 57 feet of rock pillar horizontally
/
between tunne 1 s. /.
The tunnel alignment is 255° (W.C.B.) which is approx.ima.tely parallel to
"'-·\'I •'· '-\ the river for about 80% of its length. The maximumrcover'to the tunnel
crown is 550 feet. tM. t~Jc .... ,......... ~'"' A:L 4~ 7: fi. 7~-w ~'"?:~ }17·n ..
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The tunnel alignment passes through 6 surface mapped fracture zones. The
fracture zones intersect the tunnel alignment at 50° to 60°. The width of
fracture zones, as mapped on the surface, total 220 feet, i.e. 5% of the
tunnel length but it is expected that there will be some shears not mapped
which will occur at depth and also there will be some improvements of the
.shea·r zones wi -ch depth. It will be assumed tr1at these two factors wi 11 tend
to cancel each other out.
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This is reflected in the ~esults from the exploratory drillings.
BOREHOLE NO.
2
6
8
% Borehole Length
BOREHOLE ROCK QUALITY DISTRIBUTION
RQD
0-25% 25-50% 50-75% 75-90%
19 15 22 17
5 2 13 22
4 7 18 16
28 24 53 55
9% Bet
'" 18% 18%
90-95% 95-100% --
13 14
15 43
18 38
46 95
15% 32%
Although the boreholes dril1ed from the surface vertically or near vertically
is not directly analogous to a horizontal tunnel, there~ sufficient
similarit~ to give a good indication of expected rock quality.
2 -ORIENTATION OF JOINTING
Tne tunnel alignment parallels the major joint set II and intersects major
joint set I at an angle of 35° to 75°. The major joint sets are steeply
dipping. See "b2ology of Diversion Tunnels 11
•
3 -UPSTREAM PORTAL
The portal is located just to the south of the 11 Fins". Minor shearing
associated with the "Fins 11 intersects the portal area at approximately 30°
to 40° with the line of the tunnel. The extent of this shearing is not yet
·defined. It should be assumed that the cut slopes will be at 1H:4V with
10 feet wide berms at 40 foot vertical intervals.
The portal arrangement, as-sh · ~700 GS-226; i~.gp~~~9ed
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mainly by the access required along the river bank. It is suggested that the
access ramp start within the tunnel at 1,430 feet elevation. This will shorten
the ramp and allow some saving in excavation of the cut for the access road.
The ramp and remaining excavation down to 1,430 feet.elevation within the portal
area could be excavated after the tunnel breakthrough and mucked out through
the tunne 1 . Us:i.-Rg..tf:l;j,s-met1lorl~-ef---wc:rrl<4·ng;-the-per-ta·1-cal 1 be lllovect-s~·ds
. '
t~4~r. If the rock surface adjacent to the river is lower than at presently
indicated, the level of the river retaining rock dyke will have to be raised by
a small embankment. B~cause of the strong river current at the outside of the
bend, a concrete ~r wall may be preferable.
It should also be considered that the minor shearing associated with the 11 Fins 11
will pass through the rock dyke and considerable rock support may be required.
The permeability of the clyke may need to be reduced by grouting or dumping
impervious material on the upstream face. In view of the fast flow of the
river at tr.is point, grouting should be allowed for at this stage.
Considering the probable shearing parallel to the 11 Fins 11
, extensive bolting
of the whole portal face should be allowed for a 25 foot long tensioned rock
bolt at 5 foot centers and two closely spaced rows of rock bolts around the
periphery of th2 tunnel. 50% of the area of the cut slopes will require to
be shotcreted to a thickness of 3 inches, 25% of the area reinforced with mesh.
The sides of the portal excavation will require 50% of the area rock bolted
with 25 foot long bolts.
4 -TUNNELS
For hydraulic consideratio~ the diversion tunnels will be lined with insitu
concrete. S~ctio~of the tunnel will require temporary rock support for the time
the rock is exposed between excavation and concrete lining. 17~ of the tunnel
....._ ,.\!;
length will require concrete lining from support considerations~ .See at~-Red
~pport-c~i~ri~.
Temporary support should be installed soon after excavation and light support
may be used to prevent excessive overbreak thereby saving on concrete to fill
the overbreak.
The orientation of the tunnel alignment relat :ve to joint set II is unfavorable
but due to the restriction on the location of the portal, the alignment of the
tunne 1 cannot be changed to h.Mi any s i gni fi cant effect on the .support required.
It is expected that the excavation will be in two stages. The semi-circular
top section followed by the lower bench to final invert level.
~ \
In good rock with minimum support1 advance rates vf 160 feet per week for the
top ~eading could be achieved. An average of 100 feet .per week overall is
expected. The bench excavation should average 300 feet per week. Enlargement
of the upper tunnel in two locations is required for energy dissipation
structures and gates. This will be mainly an enlargement vertically and,
therefore, will not increase the tunnel span. Extra supports in the form of
rock bolts .and shotcrete will probably be required. Slight adjustment of the
gate structures may be made to locate the enlargements in good rofok away from
shear zort_~ ·At the location of th~ctures and the plu. g iritne lower -~---~
tunnel, the concrete should be well into the rock. The normal irregular
overbreak associated with drill and blast .e-~cavation shoulrl be suffic-ient; but
if the quality of rock and bl_ast-i-ng··is-·such that a very smooth profile is being
obtained, tQen .some -trimmi~; or adjustment of peripheral holes will be required.
" _,. '"'
At the downstream end of the lower diversion tunnel, a junction will be formed
with the tailrace tunnel. This junction should be formed at the same time as
the diversion tunnel is constructed to avoid blasting close to the concrete
lining of the diversion tu~nel. A stub heading about 40 feet long on the line
of the tailrace tunnel should be excavated and left unlined until the tailrace
tunnel is completed.
The junction of the access tunnels with the energy dissipating gate structures
should also be formed prior to concreting of the upper dtversion tunnel.
5 -DOWNSTREAM PORTAL
The location of this portal is severely restricted due to the close proximity
of the downstream cofferdam, the chute spillway and service spillwt~· discharge
valves and tailrace tunnel. The geology of the portal area is controlled by
the major shears trending at 45° to the tunnel axis. A zone of alteration
with minor shears and fractures has been mapped on the surface crossing the
tunnel alignment 100 feet to 200 feet from the portal.
As at the upstream portal, a rock dyke will give protection to the portal area
during construction. It is anticipated that the excavation slopes will be
at 1H:4V. The portal face will be extensively bolted in the same manner as the
upstream portal.
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t;'' Drawing SK 5700 G6 218 shows an ups-tand between diversion'tunne 1 porta_ls_;___but I ' -_.. -"--·----_,. --...,,
( · with the tunnels ~t 30 feet differen-~--e1-evation with 57 feet spacing, this
.· upstat)d would probably...-require extensive SUJl.Uort. It may be m.o.re_economical
' /
to-· remove this rock upstand. Generally, a rock cover of 1.5 D (where ~ ~
the tunnel span is required . However 1 since the tunnels will be concrete
lined, it may be possible to reduce this requirement to 1.0.0 cover. Since
the portal is in a zone of·major shearing a'd alteration, a cover of 1.5 D should
be assumed until more detailed information is available on the portal geology.
6 -EXCAVAT10N SLOPE DRAiNAGE
Drainage channels should be provided at the top of all slopes to channel surface
run off away from the cut slopes. Pressure relief holes may be required. For
estimating, assu~e 25% of the cut slope ar~ea will require drain holes 10 feet
X 10 feet grid~ feet in depth for upstream and downstream portals.
l;
1 -INTRODUCTIO~
2 -SCOPE
$ubtask 6.21 -Watana Diversion Scheme
Closeout Report
3 -DESIGN CONSIDERATIONS
3.1 -General
3.2 -Reservoirs
3.3 ~ Cofferdams
3.4 -Inlet and Outl~t Structures
3.5 -Tunnels
4 -TUNNEL SCHEMES
4.1-General
4.2 -Hydraulics
4.3 -Capit · Costs
4.4 -Optimization
4.5 -Cofferdam Closure
4.6 -Low Level Outlets
5 -SELECTION OF DIVERSION SCHEME
5.1 -Scheme Selection
5.2 -Opei"ation
5.3 -Final Closure ·and Reservoir Fi11ing
5.4 -Continuing Studies
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1 -INTRODUCTION
The objec:tive of this closeout report is to present the results of the Diversion
Scheme studies and present information on the selected scheme.
Bastca11y a diversion scheme study is a simple economic optimization of the
tunnel diameter vs. cofferdam height. In optimizing the tunnel diameter
certain limiting crit';ria have to be adhered to. These would include but are
not limited to geologic conditions which would determine structural support
conditions for· the tunneling, foundation conditions which influence the type
of cofferdam and treatment, physical geometry of the site which would limit the
size or type of cofferdam, flows to be handled by the scheme, and construction .
s~quence and scheduling.
An addftional consideration that ha~ to be investigated and evaluated for the
Watana diversion scheme. is tbe tr;corporation of the low. level outlet into the
diversion tunnel. The requirement and parameters for the low level outlet
are presented in Closeout Report 6.17, Watana Spillways, Preliminary Design.
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2 -SCOPE
The scope of the study was to conceptua1iz~ a diversion scheme or schemes,
detenni·ne and evaluate the flows and water levels the scheme would be required
tc handle, determi·ne the various heights of cofferdams required for the various
tunnel sizes, estimate capital costs for the various schemes, select the tunnel
size and corr~sponding cofferdam height. Upon completion of the selection of
the optimum tunnel diameter, the preliminary design of the tunnel and cofferdam
used for the capital costing aspect are refined and c~nfirmed.
Operation~ during construction were cons.idered and incorporated into the pre-
liminary design of the tunnels. The operation of tunnels during final closure
and the finu1 closure scheme or sequence itself was detennined ..
As part of the hydraulic studies incorporated in the diversion studies was the
deve1opment of downstream tailwater elevations considering ice buildup in the
downstream reach of the river. This directly affected the water surface
e!avations which directly determined the upstream cofferdam height.
The incorporation of a permanent low level outlet into one of the diversion
tunnels was cons·idered. This involved preliminary design comparison of a
sepa?"ate low level outlet versus incorporation of a low level outlet ..
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1,_ -DESIGN CONSIDERATIONS
3.:_1:_ _-: __ Genera 1
The first -parameter to be established in any diversion study is the flow
or recurrance peri· ad flood that has to be handled by the diversion scheme.
This is detennined by an economic risk analysis in which the cost of the
diversion scheme is compa:"ed against the damage that would result from
a flood beyond the capabilities of the diversion scheme, and the risks
involved in exceeding the capabilities of the scheme.
Wt I{
This ~ be carried out during the detai'led design phase of the project
however· general criteria by others such as the U.S.S.R. and the U.S. Army
Corps of Engineers have established criteria and experience which is
acceptable for' feasibi1 ity studies ..
The general criteria adopted by Jnd recognized by ---.-------
international insurance companies is a 10% risk per annum is acceptable.
This translates to a diversion design for a 1 in 10 year exceedance flow
or flood per year of construction. Preliminary estimates show that a c~,f\t~\
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construction period of 5 years is required to-construct tt:le mair; aam·te
aR-se:1~ati'oe "'~ a f1ood overtopping the cofferdam would Jet be U'f\C~..tte~l~lv._
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detrimental to the main dam and Ot'~9:t~~~ Thus a 10 year exceedance
· risk per year on a cumulative basis for a 5 year construction period equates ~
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to a 50 year recurrance period flow or a 10% risk of exceedance. This ~
f1ow has been established as 83,000 cfs. (See Subtask 6.09, Design Cri'teria).
... 5
Once tfu: flow· or flood bydrograph ts selected several diversion schemes were
devi'sed and layed out to conform fo the site geometry and characteristics.
A.JJ.,l •o""-.. ( J.,~'""'-t"~~,JI?.N"~-~-~ OJ't""f..-
.!.)ii\er·iddftJOh¥' •aa;tz ·,is outlined in the following subsections.
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3.2 -Reservoirs
In the tunnel/cofferdam optimization studies the height of the coffer-·
dam, and thus the size of the resr~oir, is optimized by economic comparison
against the tunnel diameter.
The Watana Reservoir resulting from the cofferdam was anal tzed for
possible ·flood storage, and routing of the design flood. The reservoir
was too small to provide any storage and.the routing effects were very
sma11.
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3.3 -Cofferdams
The cofferdams will be earth and rockfi11 structure!i with the height
to be determined from the optimization studies. Ccmsideration wi11 be
gi·ven to founcdation treatment due to the alluvium present in the Susitna
riverbed.
(a) Foundation Treatment
Foundation treatment will consist of a s\~urry W9-11 throu~~h the
alluvium miiteria1 to bedrock excavation tl') sound rock in the abutment
areas ..
The depth of alluvium materi.a'J, in the river bed area ranges from non
e~i stant to a maximum of 100 feet. The a 11 uvi urn materia 1 is a si.l ty
sandy gr·avel w-ith numerous cobbles. The soil/bentonite slurry wall
wi11 be constructed through the closure dam and alluvium material
to ~~drock and will minimize the amount of seepage into the maindam
excavat.ion. The abutment areas will be cleared and grubbed with
excavation of all material to sound rock prior to placement of any
cofferdam material.
(b) Upstream Cofferdam
The upstr~am cofferdam wi 11 be a zoned embankment founded on the
closure dam. See Figure/Drawing --· The closure dam wi11 be
constructed to elevation 1475 based on a low water level of elevation
- 8
1470. It',wi11 consist of coarse material on the upstream side
grading to finer material on the downstream side. When the closure
dam is completed the soil/bentonite slurry wall can be eonstructed
to minimize seepage into the main i:iam foundation excavation. It the
slurry wall is not effective in preventing seepage into the excavation a
dewatering system can be established in the main dam excavation.
The cofferdam from elevation 1475 to 1545 will be a zoned embankment
consi·sting of a central rove, fine and coa.rse filters, and rock
and/or gravel shells with rip rap on the upstream face. The core
material will come from BoT"row Area uou and wi11 be the silme material
to be used in the main dam. The 'filter materials wi11 be obtained
from Borrow Area "E 11 and will also be the same as the materials to
be used in the. main dam. The rock for the cofferdam she11s will
come from either Quarry L or A. Gravel material wi11 come from
Borrow Area "E". The choice of rock or gravel for the shells
(c) Downstream Cofferdam
The downstream cofferdam will be a closure dam constructed frpm
elevation 1440 to 1472. See F.igure/Drawing • --It wi11 consist
of coarse material on the downstream side grading to finer material
on t~e upstream side. When the closure dam is completed the soil/
bentonite slurry wall can be' constructed in the finer material to
minimize seepage i'nto the ttain dam foundation excavation. A
dewater1ng system can be installed in the main dam excava~ion if
required. The downstream slope will be protected by a rip rap layer.
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The wi.dth of the crest of the cofferdam can be varied if. clearance
of the outlet structures are required.
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3.4 -In-let and Outlet Structuras
The inlet and outlet structures ar.e reinforced concrete structures. The
intake structure will support and house vertical lift fixed wheel gates
for contro·l and fina 1 c1 osure.
The intake and gates will be designed to operate under the following
condi'tions..
(a) Gates open, reservoir partially full
(b) Gates open, reservoir full
(c) Gates partially open, reservoir partially full
(d) Gates partially open, reservoir fu11
(e) Gates closed, reservoir partially full
(f) Gate closed, reservoir full.
The intake will have a rounded corner (bell mouth) entrance to reduce
energy losses and prevent cavita:tion, The outlet structures will have
slots for provisions of stoplog installation.
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3.5 -Tunnels
The*tunnels were designed to accomodate the structural geology of the
site. The major joints sets to be avoided ran -----------
Thus the range of unacceptable orientation i.s -----------·
The range of pr:eferred orientation . lS '-.. ~-------------------------_________ _, ________________________ ..
The 1ocation of the dam and the layouts indicated the tunnel alignment
of is acceptable. ----------------------------------
The tunnels are designed to handle the flow stated in Section~thu~ the
resulti.ng velocities at•e approximately 50 ft/sec and are presented in
Table • --This nece~ssitated the concrete lining of the tunnels be of
a thi"ckness sufficient to prevent scour.
Geotechnical data and evaluations indicated structural support of the
tunnels wtll be required. For the study purposes a percentage of the
.tunnel length i's estimated to require rock bolting and steel sets for
T ~ . ' support, This is presented i"n able .~e cost est1mates 1nclude
these porttons of support.
Geotechnical considerations for the sizes of tunnels investigated favored
a smaller diameter two tunnel sch~ over one large diameter single tunneL
It was also decided for security and risk considerations, two tunnels were
superior to a single tunnel.
_tt,l\.· ,.I,
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4 -TUNNEL SCHEMES
4.1 -~nera1 Description
The general schemes considered during the study consisted of a pressure
tunnel scheme and a free flow tunnel scheme. These pressure tunnel
schemes were further subdivided into a pressure tunnel with a free outlet
and a pressure tunnel with a submerged outlet. The pressure tunnels .flow
full and are designed for an internal pressure. The pressure tunnel with
a submerged outlet has the crown of the outlet portal a·lways submerged
under all flow conditions. This is the most common and widely used type
of diversion scheme. The other pressure tunnel with a free outlet has
the crown of the outlet portal never submerged under all flow conditions.
The free flow tunnel scheme is a scheme-where the tunnel flows free and
is not designed for internal pressure.
The various tunnel scheme? were optimized for optimum tunnel diameter.
This necessitated several diameter tunnel sizes be evaluated for costs,
assoctated cofferdam height, and physical layout arrangement.
Layout studi·es of the project site 1 oca_ted the diversion tunne 1 s on the r· cC!" t<;. ~
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Nortfl. bank of the Susitna River. The North bank was selected due to41ess
length of tunnel required.than the South bank. The geotechnical consider-
attons favored the North banko The South bank would have required the If
tl ""'1h e.. p, Itt s
tunnel to pass through a major shear 7.one. The North bank has tbe "fins 11
located near the upstream portal of the tunnel and a shear zone located
~ near the do\'lnstream porta 1 of the tunne \ the diversion tunne 1 can be
v ,,
-13
located between these two features however there is virtually no tolerance
or room for adjustment.
-14
4.2 -Hydraulics
Hydr.aulic studies were carried out to detenr;ine the 50 year· recurrence
period flow. This was established as 83,000 cfs. The initial step
estimated the routing effects to reduce the outf1 ow of the di \.•ersi on
tunnels to 76,000 cfs. Hydraulic calculations were then carried out to
determine the Headwater/Discharge relationship for the various types
and diameters of the tunnels. This is presented as Figure 1; 2, 3, and 4.
The tunnel diameters presented in the graph are for a modified horshoe
shaped tunnel. The area for this shape of tunnel is 13.7% greater than·
for a circular tunnel with the same diam~ter. The tunnel sizes investigated
establi'shed the maximum ve1oci·ty under the f1ooci flow in the 50 ft/s to
A \\ , ' 60 ft/s range. This dictated the requirement of concrete linir.g. ~ rt
vaiue of 0.014 was selected for the concrete lined tunnel.
.I, .. -'( I, : -J·' '
-15
4.3 ~ Captta1 Cost~
Capital costs were developed for the optimization studies.
(a) Cofferdams
An earth and rockfi'll·. cofferdam with a 30 ft crest width, 2H: 1 V
side slopes, and a cross sect~on that has 70% rock or gravel, lOS
filter, and 20% impervious materi.al was used for quantity take offs.
Several dam heights were selected and layed out. Quantity take offs
were carried out and unit costs applied. A g~aph showing the Dam
Elevation/Capital Cost rea1iion~hip was prepared and is shown as
Figure 5.
(.b) Tunnels
Major quantities for the tunnels were calculated for the various
diameter tunnels. These included rock excavation, concrete lin2r,
rock bolts, and support· steel sets. Unit costs were applied and
the capital cost of·tbe tunnels were developed. Portal quantities
for the upstream and downstream portals were calculated and con-
sisted of rock excavation and rock bolts for support, These were
included in the capital costs for·the various tunnel diameters.
The capital costs for the various tunnel diameters are presented
in Figure 6. The difference in costs for the same diameter tunnel
for the pressure vs. free flow tunnel are due to the portal costs.
The submerged tunne 1 s are at a 1 ower elevation than the free f1 ow
-16
tunnels and require a larger portal, thus a larger cost. The tota.1
costs do not include the costs ·for the intake structures or gates.
It i's esti rna ted these costs wi11 vary direct 1 y with the tunne 1
diameter and therefore wi11 not effe~the optjmization analysis.
-17
4.4 -Oettm1zation
The ~ptimization consisted.of developing the capita.1 cost of the dam
required for the vartous tunnel diameters. This.was.accomp1ished by
first sel e.cttng a 15 ft freeboar.d requirement.. 5 ft is for settlement
and wave run up. 10 ft is ·for ice. The .10 ft for ice is a provision for
' possible ice jamming downstream:. thus raising the tai1water elevation
and subsequent headwater elevatiun. Using Figure 4, the 15 ft freeboard)
requirement and Figure 5, the capital cos.t of.. the dam/tunnel diameter
relationship was produced and is presented in Figure 6. The total capital
costs of the various tunnel diameters were then produced and are presented
in Figure 7. Thi.s is a composit.of the indivi·dual costs presented in
Figure 6. From ·Figure 7 it can be seen that 30 ft is the optimum diameter
'"'Pre. Sa.S. u or ._ · 1
for the f,.-ee fiseH tunnel. w~ 't e... ~ S 4--\-l ~ ~'e.. op+~-""-J..'-,_ e.--+'-~
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-18
f.; 4.5 -Cofferdam Closure
An important constderati on .at this point· is the cofferdam closure. For.
the pressure tunnel scherne thi-s. is no problem for the invert of the
tunnel ;·s ·below the riverbed· elevation·. Once the tunr.e1· is complete
the riverf1ow will divert to the tunnel with only a 10 ft high closure
daiii. The free· flow tunnel scheme however has the tunnel invert some 30 ft
above the riv~,..bed. This will necessitate an end dumped closure dam
some 50 ft high. There are design problems invo1ved in dumped fill of
·these heights however it· can be done. A more serious prob·1em is the size
of particle (boulder) of the dumped fi11. As the closure dam crosses
the river.the water surface increases along with the vel:Jcity of the river
to be closed. This necessitates large boulders of sufficient weight be.
and dumped to resist the velocity of the river water. The size of boulders
~~pected to resist these velocities are large (greater than 3 ft diameter)
and availability of them is small. Therefore it is impractical to assume
the free flow scheme.
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19
~·~:) 4.6··-Low Leve 1 ·Out1 ets
(; ' X./
In keeping with current practice it has been decided to provide for
drai.ning of the reservoir.· The provision of draining of the reservoir
is for an extreme emergency case.
Preliminary studies showed the low. level outlets with capacity to draw
the reservoir down in 4 months (in cor~1pliance with Corps of Engineers
Regulation No. 1110-2-50) .was prahibitavely high.
fe\e(
A capacity of 30,000 cfs at full reservoir ~tor the low level outlets
was arrived at afte.r evaluating the conversion of one diversirm tunnel
into a pennanent low level outlet and if'\ keeping consistent with the
project spi11way.s discharge capacities.
Environmental considerations required the low 1eve1 outlel prevent nitrogen
supersaturation of the discharged waters. This requirement necessitated
the use of an energy di.ssipation device. Two alternatives for energy
di.ssi.pation were investigated. One ·alternative was Howell Bunger valves
and the other alternative was an ex'pansion chamber. The expansion chamber
was· selected as the preferred alternative. The valve alternative would
have rquired a large number and size of valves that were not compatible
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wi th thefl diversion tunne 1.
The "sudden expansion'' which makes the expansion chamber dissipate energy
is a proven scheme that has baen used on qther projects. The ~1ica project
a,\rJ -H-t_ ~1.~ Cc--'1•-~·~ "?u-~t~et.4·_, a.,.,.e_-fuo
i• iAE of the p~evious projects where this type of dissipation device has
. -20
worked sati·sfactori·1y. One constratnt of the dissipation chamber is the
requirement that the chamber be located above tailwater· in elevation..
If the chamber is submerged, cavitation wculd result in the area of the
discharge. jet enter~ng the expansi.on chamber. This constraint necessitated i\.c....+
the diversion· tunnel to be converted to a low level outlet be a 11 free
flow'' tunne 1 or a "pressure tunne 1 with a ·Free out 1 et 11 wh i 1 e it is being
.used as a diversion tunnel.
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-21
5 -SELECTION OF THE DIVERSION SCHEME
5.1 -Tunnel Scheme Selection·
From Figure. 7 the selection of·the tunnel scheme \-Jou1d appear to be two -
30 ft di'ameter pressure tunnels. However, if the pressurf~ tunnels are
selected a separate low level release wi11 have to be constructed to
avoid the cavitation problems that would occur if one of the pressure
tunnels was converted to a low level release. Taking the incorporation
of a low level release into·consideration, and referring to Figure i :it
wouild appe::ar the selected scheme would be two -35 ft diameter free flow
tunnels. However with the free flow (or pressure tunnel w·ith a free outlet)
tunnels there are major problems with cofferdam· closure explained previously
in Section 4.5.
The solution, and subsequent selected scheme is a combination one pressure
tunne~l and one free flow (or pressure tunne1 with a free outlet) tunnel.
Two 30 ft diameter tunnels were selected and investigated. The cofferdam
required will have a crest elevation .of 1595. This is about a 150 ft
hi'gh cofferdam. Layout drawings showed the 150 ft high dam would push
the inlet portal upstream and into the mi:idle at "The Fins". This is
very undesirable. Subsequent layout drawings and investigations showed
two -35 ft diarreter tunnels require a cofferdam with a crest elevation
.
of 15.40. This is about a 90 ft high cofferdam. The reduced size coffer-
~ .
dam is preferable over the higher cofferdam and the cost increase is
considered insignificant. The reduced size cofferdam also a11ows the
-22.
entrance portal of the diversion tunnels to ba moved downst-ream to the
edge of 11 The Fins 11
• For the reasons presented two 35 ft diameter modified
horshoe shaped tunnels wo~ selected as the diversion scheme. Dna tunnel
will be submerged and act as a pressure tunnel. The other tunnel will
be 1 ocated at a higher e 'J e.vati on and act as a free flow tunne 1. An upper
and 1 ower tunne 1 scheme was used successfully previous 1y in the 0 V"O v \ l \ e_
\) ~ project.
~-~
-23
~ 5.2 -Operation
. '
., ... t
As stated previously during diversionltunne1 will-operate as a pressure
tunnel at a11 times. This is the lower tunne1. The. other tunnel wi11
operate as a free flow tunneL This is the upper tunnel. Both tunnels
will have operating gates at the entrances to control flow during
operation and to enable final closure to take place.
To develop the operat·~ons more: accurately the discharge rating curve for
the selected scheme was developed. This· is presented as Figure 8. The
routing of the 50 recurrance pe.riod flow is presented as a hydrograph in
Fi gur·e 9.
An iwportant consideration in the operation of the diversion is ice. The
ice considerat.ions can be· broken down into two major headings. One
heading is ice janming downstr.e·am of the diversion .tunnels in the open
river channel. If this were to occur the tailwater elevation could rise,
and thus overtop the downstream cofferdam. By raising the tailwater
elevation the headwater elevation .would also rise, possibly overtopping
the upstream cofferdam. The other heading is ice jamming inside the
tunnel, or tunnel entrance, thus blocking the outflow and possibly over-
topping ~he upstream cofferdam.
This potential problem of ice jamming inside.the tunnel is eliminated by
using the upstream control gates to keep the tunnel entrance submerged
during operation., th:1s ·eliminating ,the possibility ·of ice entering the
tunne 1. The 1 ower tunne 1 (.pressurt' tunne 1 ) is a 1 ways subme-rged c:o the
..
•
•
,,
-24
gates do not need tr> be lower.ed. The·upper tunnel (.free flowl wi11 not
be in use except during the higher flows. It is ?lanned to operate the
diversion with titt; up-pc;r tunnel' closed at a11 times. Using only the
lower t.unnal th~ reservoir upstream of the cofferdam will be allowed to
rise to E1 1525.. This will discharge approximately 50,000 cfs. This is
the yea:r recurrence period flow.. For any 'flow above 50,000 cfs .up
to i6,000 cfs the upper tunnel gatss will be opened, eith~r partially or
fully, to pass the required flow. At a11 times while the upper tunnel is
in operation the entrance to· the tunnel .will be ~ubmerged thus preventing
ice from p.ossi'bly entering the tunnels This may require operating the
upstream gates only partially open.
The first heading of potential ice considerations, ice jamming downstream
of the tunnels in the open river, was studied and it was found this is
not a problem. The stretch of river from the downstream diversion portal
to the Devil Canyon site, an approximate 500 ft drop in riverbed elevation
wa·s:examined for possible ice jarrming. It was found during the investigations
that stretch of river had no location large enough in su"'face area where
a large ice mass could form, thus fostering the possibility of an ice jam
fanning.
Another consideration regarding ice during diversion operation that was
investigated was the possibility of i1!e in the reservo.ir being pushed
up against, and possible overtopping the cofferdam. This ~~ c~~s~
erosion and structural damage to the cofferdam. Studies determined that
this problem could be eliminated by constrttcting the dam 10 ft higher or
allowing 10 ft additional freeboard for ice buildup or installing an .
..
•
-25
ice boom. From the cost curve presented in Figure 5 an additional 10 ft
of freeboard ~n the dam costs app~oximately $700,000. It was decided an
ice boom can be installed more econonrlcai1y~
The operation of the tunnels .during· diversion is as stated previously.
The lower tunnel wi11 handle the majority of the flows alene. The entrance
will be submerged a11 the time therefore the gates will always be open.
Tha upper tunnel will alw~s be closed until the reservoir teaches elevation
1520. Then the gates should be partially opened half way. From the
rat1ng curve presented as. Figure 10 it can be seen that this can handl~
flows up to ----cfs. If the reservoir cont1ftues to rise open the
gates fully. If the reservoir elevation falls, close the gates accordingly.
The operation.of the upper tunnel will operate as a free flow tunnel. The
slope is ~reat eno~gh that f1ow in the tunne1 is supercritical and a
hydraulic jump wi11 not take place.
After final closure the 1owe.1eve1 outlets will have the capacity to
discharge 30,000.cfs at full reservoir elevation of 2020~ They will be
l
capable of operating at heads up ·to 550 ft and capable of withstanding
static heads up to 750 ft. ihe rating eurve for the low level outlet
discharge is presented in Figure 11.
~~._v) ~,~t.
The reservoir~down A incorporating the low 1ev~1 outlets is estimated
at 14 months and is presented graphically in Figure 12 for various
"start" times of the year.
0
.)
-26
Conv~rting one of the diversion tunneis to a low level outlet after
using it as a diversion tunnel necessitsted a scheme or seque:oce for final
ci osure.
It was estimated a one year duration is required to construct and install
the penncnent low levei outlets in the existing diversion tunnel. This
re.qui red the 1 ower or su;.;merged tunne 1 pass a 11 f1 OlliS. The 1 ower tunne 1
can pass 50,000 cfs alone without overtopping the cofferdam. This is a
----recurrance period flow. Considering the construction duration
is oniy one year, a· 10 year recurrence_ period flow to be handled is
. -""\~ \t ~
consistent with the entire project de.sign flow.~ Therefore the lower
tunnel alone is considered to have sufficient capacity to ac-e:. as the
diversion for one year.. During the construction of the low level outle.ts
hA -1k ~~'PW -t.J-~t..\
~he intake operating gatefwi11 be closed. The gate and intake structure
will be de~igned for reservoir elevation of 1540 ft which is an externa1
pres~ure head of 120 ft. Prior to commencing operation of the low level
outlets coarse racks wi11 be installed in the upstream intake structure
in slots provided~
Upon completion of the iow ... leve1 outlet in thf; upper tunnel the intake
gate will be opened and the 1tM 1eve1 outlets wi11 commf:mce operation.
Upon conrnencing operation of the lO.IfJ level outlets the 1ower tunnel wi11
.be temporarily closed with the intake gates and construction of the penn-e\" ~ . v~~(\
anentf·wi11 a1so commence~ filling of tha reservoir. It is estimated
it will take a. years duration to complet~ly place and cure the plug.
•
•\
. /
•
•
-2.7
During th.is time the upstream gate and intake structure will be designed 1 f\.~ll.CI
to for a reservoir elevation of 1800 ft. whi·ch is an external pressure/ .. ,f
400 ft.
The fi 11 i ng· of the rese·rvoi r wi 1 i take 4 years to comp 1 ete to fu 1 1
reservoir operating e:evation of·2185~ After 3 years of filling the
reservoir will be at elevation 2150 and will allow operation of power
plant to commence.
The filling sequence was determined· from the main dam· elevation at that
time during construction, th~ starting reservoir pool elevation at that
time during cunstruction, and the capability of the reservoir storage
to absorb the inf1o~vo1ume from a ~OC)year recurrence period i~fiow
~-""' without overtopping the rna in da.m. The 5b e; ... recm"rer.:e period fl cod
volume was se·lected to ~ consistant with the recurrance period flows
and risks used for the design of the diversion a~d entire project.
This information is presented graphica11y as Figure 13 .
••
0
<:: 28
5.4-Continuing Studies and Reco111ne.ndations
Subsequent studies have shown the structural design of the tunnels to
be better accomodated if the tunnels are circular in diameter. Due to
the tunnel size optimization determining a 35 ft diameter modified
horshoe shaped tunnel to be the optimum size, this equates to a 38 ft
diameter circular shaped tunnel section.
Layout studies of the energy dissipation chamber in the ~ow level ou.tlet
necessitate the chamb~r be 45 ft diameter to physically acccmodate the
gates· and passages required for the chamber. It is necessary to construct
the larger diameter tunnel section in the initial construction of the
diversion tunnel rather than afterwards which would require removal of
the tunnel lining and enlarging.
St'Jdies conducted or constructing th;; lower tunnel showed that leaving
an 11 origina1 rock cofferdam 1
L in place and removing it prior to corrmence-
ment of operation of the diversion tunnei, is economically advantageous
over constructing a sheet pile cofferdam for ·construction of the portal
and the ·tunnels .
.
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