HomeMy WebLinkAboutAVALANCHE Conceptural Design-2
SNETTISHAM
TRANSMISSION LINE
CONCEPTUAL AVALANCHE MITIGATION
REPORT FOR
TOWERS 3/4 TO 5/1
September 9, 2010
Prepared for:
ALASKA ELECTRIC LIGHT & POWER
JUNEAU, ALASKA
Prepared by:
TABLE OF CONTENTS
1. INTRODUCTION ....................................................................................................................... 1
2. BRIEF HISTORY ......................................................................................................................... 3
3. OUTAGE HISTORY DUE TO TOWER DAMAGE .......................................................................... 5
4. CONCEPTUAL DESIGN .............................................................................................................. 5
5. SUMMARY................................................................................................................................ 7
Appendix
A – Sample of Events
B – Original Tower Data
C – Estimated Avalanche Forces
D – Cost Estimates
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 1 Page 1
1. INTRODUCTION
The Snettisham transmission line extends from the powerhouse at the end of Speel Arm to the
Thane Substation south of Juneau. This single line is the only connection for electrical power to
Juneau from this hydroelectric facility. The line is 45 miles lo ng and is located along the very
rugged terrain adjacent to the water. The line was placed in service in 1973 (Salisbury Ridge
relocation was completed in 1975) by the Corp of Engineers and traverses several avalanche
paths. Avalanche and transmission line conflicts are not uncommon in several Alaskan
locations and the Snettisham line is further exacerbated by the Southeastern topography. Over
the last 34 years, the transmission line has experienced six major outages due to avalanches or
landslides that required tower repairs.
In winters 2007/2008 and 2008/2009 the line was damaged by avalanches and created
extended operation of the backup diesel facility in Juneau. Operation of the backup diesels
means a significant increase in the cost of electrical p roduction which is passed onto the
consumers. With a repeat of avalanche damage in successive years, AEL&P has explored
possible means to mitigate the avalanche damage. An analysis of the various avalanche paths
identified the line section between Towers 3/4 and 5/1 as the most vulnerable. The following is
a conceptual design for mitigating avalanche risk for this line section. The intent is to provide a
cost estimate and schedule of a reasonable mitigation measure at each site. This is not a design
document and once field parameters are known, the final design could be different th an what is
proposed here. However, it is expected that the final design will be similar to the conceptual
design and near the same cost.
Figure 1 on the following page is a map of the Snettisham Transmission Line from the
Powerhouse to the Thane Substation along with each tower location.
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 2
Figure 1 – Map of Snettisham Transmission Line
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 3
2. BRIEF HISTORY
The Snettisham 138kV transmission line provides the normal electrical supply to Juneau and
traverses a very difficult coastal terrain. The Corps of Engineers (Corps) designed the line as
part of the Snettisham hydroelectric facility with commercial operation beginning in 1973.
Several wind problems prompted the relocation off of Salisbury Ridge and sporadic avalanches
have caused some damage to the line. However considering the 45-mile line’s environment, it
has been reasonably reliable. Recent avalanche damage in April, 2008 and January, 2009
encouraged a look at possible mitigation of future avalanches.
The following report provides a brief history of the line problems and possible conceptual
designs of mitigation measures at each of the tower sites. Mitigating potential damage to
transmission lines is always a balancing of costs versus benefits and is far from being exact
when the forcing function is highly variable events like avalanches. The impact force from an
avalanche is almost always different because the variables are continually changing. In 2009, a
process of identifying the avalanche forces at two sites was completed. The process required
extensive modeling of the avalanches and their associated forces that could impact the towers.
This is further described in the Avalanche Mitigation Report of February, 20 10. Based on this
work, the design avalanches at Towers 4/3 and 4/6 were estimated to allow for specific
mitigation. Tower 4/3 was modified to absorb the avalanche impacts and Tower 4/6 required a
defense structure.
Tower 4/3 is in the path of a small avalanche with possible forces at the tower site of 600 psf.
Tower 4/6 is in the path of a very large avalanche with possible forces over 5,000 psf. With
these two limits we can look at other avalanche paths and very generally relate them to these
limits. This approach provides a ballpark estimate of avalanche forces at best. More accurate
estimates require sophisticated modeling of each specific site. This report is in tended to
provide the “first cut” to establish reasonable measures that are likely to be required at each
site.
The map on the following page shows the recognized avalanche paths along the Snettisham
transmission line.
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 4
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 5
3. OUTAGE HISTORY DUE TO TOWER DAMAGE
Since the 1976 relocation, the Snettisham project has had a respectable performance record,
considering the extremely difficult terrain and the many angles and deadends (41% versus a
more typical 20%). The records of major damage can be summarized as:
Several initial wind problems on Salisbury Ridge – The line section was relocated off of
the ridge in 1976 and there have been no recurring problems.
Tower 4/6 destroyed by avalanche in 1976 – The tower was relocated in 1983 and
functioned until it was destroyed again in 2008.
Landslide in 1994 at Tower 12/2 (no outage) – The tower was relocated with no
recurring problems.
Avalanches on April 16, 2008 destroyed Towers 3/4, 3/5 and 4/6. Towers 4/4 and 4/5
were damaged. The towers were replaced and repaired and the line was back in service
on June 1, 2008.
Avalanche on January 2009 destroyed Tower 3/5 – The tower was eliminated with a
long span across canyon.
From the summer of 1976 (after Salisbury Ridge relocation) to the present, there were no
major outages to the line until 2008 and then a repeat in 2009. Without counting minor
outages, this 34 year performance is very good considering the avalanche and landslide
potential along the line route (See Appendix A for record of events).
4. CONCEPTUAL DESIGN
The avalanche chutes between Towers 3/4 and 5/1 run the gamut from non-threatening (the
existing tower can likely handle the avalanche force) to very large. The first step in determining
the capabilities of the existing towers is to determine the original design criteria. There are two
types of towers in this section, SS and TH. Designs for both are noted in Appendix B. The
modeled avalanches at Towers 4/3 (600 psf) and 4/6 (5,000 psf) are used as the basis to
estimate the avalanche forces at the other sites (See Appendix C for estimated avalanche
forces). The original design criteria by the Corps were fairly conservative and this has
undoubtedly contributed to the reliability of the line. However based on the recent avalanches
in 2008 and 2009, it appears the historical performance may be changing. This may be a
recurring phenomena, such as the Pacific Decadal Oscillation (see the Avalanche Mitigation
Report of February 2010). If this is true, the Snettisham Line may be in for several years of
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 6
increased avalanches. In 2009 and 2010, an active avalanche control program was started to
manually create small avalanches when conditions were correct in order to reduce the chances
of a much larger and possibly damaging event. This program is expected to continue for some
time as a proactive approach.
TOWER 3/4
Tower 3/4 was originally designed as a small angle structure, less than 12 degree line angle.
The adjacent Tower 3/5 was destroyed in 2008 and replaced at the same site. It was again
destroyed in 2009 and it was decided to eliminate this tower location and span the conductors
directly from Towers 3/4 to 4/1. This reduced the time for line repairs in 2009. This path
change also changed the line angle on Tower 3/4 from a small angle to over 25 degrees. The
increased angle reduced the capacity of the tower to support wind and ice loadings on the
conductors. The benefit of eliminating Tower 3/5 outweighed the risk of reducing the
conservative strength capacity of Tower 3/4. Tower 3/4 has made it through the winter of
2009/2010, but it does not have the same degree of structure capacity as the other towers.
Tower 3/4 has only a minimal avalanche risk; approximately 5% of the estimated force at Tower
4/3. Tower 3/4 should be replaced with a tower designed for the new 25 degree line angle and
the appropriate avalanche forces. The terrain surrounding Tower 3/4 is reasonably level, but a
multi-legged tower would require precise leg differentials. Instead, three individual, guyed
masts (also used at other places on the line) will better adapt to the terrain. This will be a
deadend structure that, along with Tower 4/1, will create a deadended span across the largest
avalanche path on the line. The replacement of Tower 3/4 is estimated to cost $1.4 million (see
Cost Estimate Details in Appendix D).
TOWER 4/1
Tower 4/1 is a deadend tower vulnerable to an avalanche with about 15% of the impact force
estimated for Tower 4/3. This force can probably be handled by modifying some of t he
structural members of the tower. The estimated cost to modify Tower 4/1 is $60,000.
TOWER 4/2
Tower 4/2 is subject to an avalanche force of approximately 30% of the force established for
Tower 4/3. Like Tower 4/1, it can probably be modified to handle the loads. The estimated cost
to modify Tower 4/2 is $80,000.
TOWER 4/4
Tower 4/4 can potentially be hit with an avalanche twice as strong as the design avalanche
predicted for Tower 4/3. This is beyond the capabilities of tower modifications and requires the
tower be protected or replaced. The large defense structure installed in 2009 to protect Tower
4/6 cost $1.7 million to construct. Since the forces at Tower 4/4 are about 25% of the forces at
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 7
Tower 4/6, it is expected that a smaller structure can be constructed for protection. The exact
design of this smaller structure is unknown, but is estimated to cost $600,000.
TOWER 4/5
Tower 4/5 is on the edge of the large avalanche that destroyed Tower 4/6. Although not
directly in the path, it is still subject to major forces estimated at 50 to 70% of the predicted
forces at Tower 4/6. These forces are large enough that a similar defense structure to the one
constructed at Tower 4/6 is anticipated. The cost of this structure is estimated at $1.8 million.
TOWER 5/1
An evaluation of the terrain above Tower 5/1, along with the absence of damaging events,
leads to the conclusion an avalanche damaging Tower 5/1 is unlikely. No mitigation measures
are recommended for this site.
5. SUMMARY
The following table provides the estimated costs of avalanche mitigations in the line section
from Towers 3/4 to 5/1 of the Snettisham Transmission Line.
Tower No. Work Estimated Cost
3/4 Replace tower $1,400,000
4/1 Modify tower $60,000
4/2 Modify tower $80,000
4/4 Small deflector $600,000
4/5 Large deflector $1,800,000
5/1 None
Due to the limited resources in Southeast Alaska, the work described above should be
performed in at least two construction seasons. Tower 3/4 is the highest priority and could be
completed in one season along with the modifications at Towers 4/1 and 4/2. Defense
structures could be constructed at Towers 4/4 and 4/5 the following year since their
construction will be similar.
Conceptual Avalanche Mitigation Report for Towers 3/4 to 5/1 Page 8
Construction access is similar for all sites; it is helicopter only for crews, equipment and
materials. Large equipment cannot be transported and designs will allow for the use of smaller
equipment.
APPENDIX A
Sample of Events
Sample of Events
The following is a sample of events that impacted the Snettisham Line (based on known
records):
1973 to 1975 – Numerous failures of towers on Salisbury Ridge due to high winds.
1975 – Complete relocation of Salisbury Ridge transmission line. Relocated section has steel SS
towers, shorter spans, and a larger conductor.
April 1976 – Avalanche destroys Tower 4/6. Rebuilt in 30 days (?)
1979 – Avalanche control study by BPA? Barry Wright? (I’ve never seen this study. My guess it
was to evaluate the area around 4/6 and make recommendations. The next year APA started
designs for relocation of 4/6.)
1981 – Avalanche protection measures begun.
March 1982 – Failure of 32/1. Guy grips failed. Rebuilt in 30 days (?)
1982 – Inspect 91 towers, 225 preforms replaced, 4 towers found with same failure potential as
32/1.
1982 – Complete avalanche protection. Relocate 2400 feet of line, remove 4/7, relocate 4/6,
raise 4/5, and install snow legs on 4/5 and 4 /6.
1983 – Complete inspections started in 1981. All guyed structures inspected. Four steel and 14
aluminum SS towers inspected. Only problem noted was on rebuilt 4/6 where some bolts were
loose and members bent.
1984 – Annual tower inspection program started.
January 1989 – Small, freak avalanche knocks center pole of 3/5 off its base. Temporarily
righted six days later. Permanently repaired in two days in February.
1991 – Begin tree clearing on line. Start annual program of clearing.
1992 – AEL&P begins maintenance of Snettisham transmission line.
May 1993 – Tree slides down hill and knocks center pole of 12/2 off its base. Repaired and put
into service two days later.
June 1993 – Mystery outages start. Seven outages occur between June and September. All are
B-phase to ground faults. The line is successfully reenergized after each one. No external cause
can be found. Tremendous effort and money are expended to try to determine the cause.
May 1994 – A landslide at Tower 12-2 brings a large number of boulders and trees to within
feet of the tower, and a few smaller rocks and limbs cause minor damage. Investigations by
geologists report an unstable rock cliff that will likely shed more landslides. A decision is made
to relocate this tower out of the landslide chute.
July 1994 – Mystery outages return with outages on the 6th, 7th, 10th, and 12th. All are B-phase
to ground. All are able to immediately reenergize. Bird perching guards are placed on Towers
28/1 and 28/2. The faults stop.
January 2000 – A-phase conductor in span over Limestone Inlet “breaks” in two. Investigation
shows that the aircraft marker balls cut through the conductor. Damage is noted on other
phases in that span. Damage is suspected in other spans. New section of conductor is spliced
in on that phase.
Summer 2000 – All old aircraft marker balls are removed. All damaged conductor is replaced.
New marker balls with different attachment detail are installed. A detailed helicopter
inspection is also performed.
September 2002 – Landslide hits guy wires of Tower 18/3. It pulls so hard that the top of Tower
18/4 is damaged. Repair was completed in 5 days.
April 2008 – Avalanches on April 16 destroy Towers 3/4, 3/5 and 4/6. Towers 4/4 and 4/5
sustain slight damage. The towers were repaired and the line back in service on June 1, 2008 .
January 2009 – Just nine months after the catastrophic avalanche, another avalanche destroys
Tower 3/5.
APPENDIX B
Original Tower Data
Original Design Criteria
Twr
Type Qty Tower
numbers
Case
No. Case Description
Wind
Cond
(psf)
Radial
Ice
(in.)
Wind
1
tower
face
(psf)
Design
Angle
(deg.)
Design
Wind
Span
(ft.)
Design
Weight
Span
(ft.)
Design
Tension
(lbs)
Transv.
OLF
Vert.
OLF
Long.
OLF
Cond.
Wind
OLF
Tower
Wind
OLF
Tower
Dead
Load
OLF
SS 6 3/3, 4/1,
4/2, 4/4,
4/5, 5/1
I 113 mph wind, no ice 32.7 0 162 0 1400 1800 NA 1.15 1.15 NA 1.15 1.15 1.27
II 40 mph wind, 3/4" ice (2" rime) 4.1 0.75 24.4 0 1400 1800 NA 1.15 1.15 NA 1.15 1.15 1.27
III No wind, 1 1/2" ice 0 1.5 0 0 1400 1800 NA NA 1.15 NA NA NA 1.27
IV 40 mph wind, 1" ice, 24 deg angle 4.1 1 24.4 24 1400 1800 12000 1.15 1.15 NA 1.15 1.15 1.27
V
40 mph wind, 1" ice, 24 deg angle,
any one conductor broken 4.1 1 24.4 24 1400 1800 12000 1.15 1.15 1.1 1.15 1.15 1.27
TH 2 3/4, 5/2 I 113 mph wind, no ice 32.7 0 135 0 1800 2400 NA 1.2 1.27 NA 1.2 1.2 1.27
II 40 mph wind, 3/4" ice (2" rime) 4.1 0.75 9.6 0 1800 2400 NA 1.2 1.27 NA 1.2 2.54 1.27
III No wind, 1 1/2" ice 0 1.5 0 0 1800 2400 NA NA 1.27 NA NA NA 1.27
IV 40 mph wind, 1" ice, 12 deg angle 4.1 1 9.6 12 1800 2400 12000 1.65 1.5 NA 2.54 2.54 1.27
V
40 mph wind, 1" ice, 12 deg angle,
any one conductor broken 4.1 1 9.6 12 1800 2400 12000 1.65 1.5 1.1 2.54 2.54 1.27
TWR TYPE HEIGHT LEG ELEV
P&P
ANGLE
SPAN
OUT
LIDAR
SPAN
AHEAD DIFF. COMMENTS
808 813 5
03/3 SS 50:20,25,25,20 884 10°33" 1082 1058 -24
03/4 TH 70 900 1582 1590 8
03/5 3PDE 110 930 61°26" 1186 1175 -11 REMOVED
04/1 SS 70:5,5,10,15 987 9°31" 603 619 16
04/2 SS 50:5,10,15,10 946 497 523 26
04/3 SS 50:5,5,5,5, 906 19°28" 784 767 -17 REINFORCED
04/4 SS 50:15,15,20,20 752 22°23" 1086 1086 0
04/5 SS 90:5,10,15,20, 633 18°13" 740 1048 308
04/6 SS 110:5,5,30,30 676 9°21" 693 1339 646 WEDGE
05/1 SS 70:15,20,30,30 667 23°52" 817 800 -17
05/2 TH 100 700 715 741 26
APPENDIX C
Estimated Avalanche Forces
AEL&P Towers 3-4 to 5-1 Avalanche Size Estimation
Prepared by Mike Janes
September 4, 2010
Summary: The goal of this paper is to give an initial rough approximation of avalanche sizes we
might expect to see at Towers 3-4, 4-1, 4-2, 4-4, 4-5, and 5-1. Detailed studies have already
been conducted on Tower 4-6 (Alaska Avalanche Specialists and Mears/Wilbur) and 4-3 (Alaska
Avalanche Specialists/McClung). To come up with a general framework, the remaining towers
on this stretch are compared to either 4-3 or 4-6. Avalanche size estimation is based upon path
characteristics including: slope angle, path length and elevation of start zone, elevation of run
out, and ground cover/tree core data. In addition, a strong emphasis is placed on actual
avalanche events from 2008 up to the present.
To fit the remaining towers into the “high and low” spectrum created by studies on Towers 4-6
and 4-3, a percentage will be given relative to either Tower 4-6 or Tower 4-3. The tower that
best shares similar path characteristics and history will be used for comparison. This is to be
used only as a general “first cut” and not as a detailed study.
Tower 3-4
This tower falls on the low end of the spectrum. With minimal exposure it actually falls below 4 -
3 in slide size. The benchy, convex terrain above 3-4 does not make for a good start zone
because often the small slides are diverted off the broad ridge leading up to Bride Peak. There
is one rock slab directly above the tower that could send a small slide down onto the bench 3 -4
sits upon. It would have to be an exceptional event and even then, slide size would be small. It
is likely a slide would not be able to cross the bench to the tower foundation. In addition, the
bands of small trees would slow small slides and prevent them from gaining much speed. This
tower would seem to have avalanche events 5% the size of 4 -3 but likely much smaller if not at
all.
Tower 4-1
Tower 4-1 failed to receive debris from the large event originating in the alpine in April 2008
(above photo). That being said, small avalanche debris often comes just shy of the tower on the
East Crater Bowl side (looker’s right). The main threat to this tower is from the rock slab directly
above the tower. We used charges on this rock slab occasionally but did not se e debris hit the
tower base. The small stand of trees directly above the tower usually filters most of the snow
and slow it as well as redirecting it away from the tower. The vertical fall from the top of this
slab to the tower elevation is only 300 ft so slides here are small. Slide size is likely around 15%
of those seen at Tower 4-3.
Tower 4-2
In three seasons of blasting this section of the line, we have not seen debris from natural or
explosive triggered avalanches reach the base of this tower. T his includes the April 2008
avalanches as seen in the photo above. This is largely due to the fact that the tower lies out of
the fall line from the gully above it as well as the wide bench it sits out at the edge of. The
looker’s right portion of the start zone (north side) has a lot of trees as does the run out, so
most slides originating there get slowed significantly by the trees. The bulk of the path is a
confined gully that can produce fast slides, but it does not have a very large start zone area.
Avalanche frequency is not often, but size is probably 30% of those seen at Tower 4-3.
Tower 4-3
This tower is considered to be on the lower end of the spectrum for avalanche size in this
section of line. This tower likely has the highest frequency of avalanches of all the towers on
this stretch, but average slide size is on the smaller end.
Tower 4-4
Tower 4-4 lies just at the edge of a gully that transports most of the avalanche debris from
above away from the tower. When this gully fills in with snow as seen in the photo above, the
threat to the tower drastically increases. The above photo is two days after the April 2008 slide
and debris is visible at the base of the tower. It can be seen that if the tower was located
slightly to the right, it would be out of the debris flow from this particular slide. Tree core data
from the April 2008 slide show some smaller trees taken out by the slide to be in the 30 year
age class while a larger tree taken out by this event was over 300 years old. The start z one from
this slide was around 1620 feet and the tower elevation is 760 feet, giving us a total fall of 860
feet (versus a fall of 880 for 4-3). Above the 1620 foot start zone, Tower 4-4 has a bench visible
on LIDAR and photos that appeared to prevent sno w above from making a larger slide as seen
above 4-6 and 4-5. In addition, the terrain is convoluted and broken preventing a continuous
fracture line from propagating across the whole slope but rather smaller slabs. As a result, the
crown face of the slide in April 2008 was at a lower elevation, and the start zone area smaller,
yielding a lesser volume slide than seen at 4-5 and 4-6 (as seen on LIDAR/Aerial Photo overlay
below by Alaska Avalanche Specialists). While start zone area and elevation are similar to 4-3,
the gully adjacent to 4-4 has a tendency to funnel debris to the tower, whereas the 4 -3 run out
is more of an alluvial fan thinning debris depth at that site. The frequency of avalanches
reaching 4-4 is much smaller than at 4-3. Avalanche size is likely to be similar to those seen at 4-
3 in volume, but debris depth has the potential to be much greater when the gully is filled in.
Also, slides reaching 4-3 are decelerating, whereas slides large enough to reach 4 -4 will be
accelerating. As a result, avalanche size at 4-4 is expected to be double or 100% larger than
those at 4-3.
Tower 4-5
Tower 4-5 has the most similar path characteristics to Tower 4-6. However, it is not close to the
middle of the path but rather off to the side. All of the slides this path produced from the last
three years of blasting stayed in the main open path visible in the photo above (lookers left of
the tower). In fact, the small slides do not usually threaten this tower - it is the larger events
that pose a greater threat. Tree core data from the April 2008 slides indicate some trees taken
out 50 feet above this tower to be in the 200 year age class. This indicates that i t would likely
require a substantial event to reach this tower. The staring zone elevation and character of the
start zone is very similar to that of 4-6, however terrain features on both sides of the tower fan
debris away from the tower. The avalanche size at Tower 4-5 is likely to be 50-70% of the size
expected at 4-6.
Tower 4-6
Considered the high end of the spectrum for avalanche size in this section of line , frequency
was low but is now increased due to deforestation. Slide size is potentially lar ge. The above
photo was taken in the fall after the April 2008 slide and serves as a good comparison of 4 -5, 4-
6, and 5-1.
Tower 5-1
This tower falls on the extreme low end of the spectrum if not completely free from avalanche
danger. The upper elevation start zone above 5-1 is convex and does not lend to funneling of
debris towards the tower but rather away to the sides. In addition there is an alpine bench with
gullies leading away from the tower to both sides that would funnel (and did funnel in Apr il
2008 as seen in above photo) debris well away from the tower. In addition, the start zone
above the bench is the only place slides could originate and it has good tree cover. Based on the
April 2008 slide characteristics, most of the start zone fall line is directed towards the 4-6 path.
APPENDIX D
Cost Estimates
Cost Estimate 2010
Replace
Tower
Large
Deflector
Small
Deflector
Mod.
Tower
45 days 60 days 30 days 5 days
Helipad $25,000 $25,000 $25,000
Camp $90,000 $120,000 $60,000
Helicopter(Sm) $162,000 $216,000 $36,000 $12,000
Helicopter(Lg) $250,000 $250,000 $150,000
5-Man Crew $288,000 $384,000 $192,000 $32,000
Mob/Demob(sm) $150,000
Mob/Demob(lg)
$400,000
Eng/Survey/AELP(sm) $60,000
$40,000 $6,000
Eng/Survey/AELP(lg)
$100,000
Material(sm) $60,000 $40,000 $10,000
Material(lg)
Outage $100,000
SUBTOTAL $1,185,000 $1,535,000 $513,000 $50,000
Misc 20% $237,000 $307,000 $102,600 $10,000
TOTAL $1,422,000 $1,842,000 $615,600 $60,000