HomeMy WebLinkAboutH-HBC~19R&M CONSULTANTS, INC.
9101 Vanguard Drive, Anchorage, Alaska 99507
February 15, 2008
Mr. Clay Koplin, P.E.
CEO
Cordova Electric Cooperative, Inc.
PO Box 20
Cordova, AK 99574
RE: Humpback Creek Project FERC No. 8889
(907) 522-1707, FAX 1907) 522-3403, www.rmconsult.com
Subject: Rehabilitation of the Intake/Diversion and Conveyance
Dear Mr. Koplin:
R&M No. 1476.01
The project team comprising R&M Consultants, Inc. and Hatch Acres Corporation personnel have
performed feasibility studies for rehabilitation of the Humpback Creek Project Intake / Diversion
Dam and appurtenances damaged by the flood of October 2006. Our conclusion based on the
feasibility studies is that the replacement of the Original timber frame intake/diversion dam and
appurtenances, if the timber structure were to be approved by FERC, would be more expensive
than constructing a new intake / diversion dam and appurtenances using concrete as the primary
construction material at a location 120 to 150 feet upstream from the original location. Study
considerations and conclusions/recommendations regarding the Original project features using
wood, timber and steel (Option 1) and the Proposed project features using concrete and steel
(Option 2) are presented in detail below.
BACKGROUND — Humpback Creek Proiect
The original Humpback Creek intake/diversion and conveyance comprises a run of the river
timber frame dam clad with planks and boards and surmounted by a steel frame and deck with
swing gate for sluicing bed load accretion over the spillway. Water is diverted to a side intake
which directs the water to a flume that conveys the water to the upper penstock.
Gabions have been installed for tailwater erosion control and to prevent erosion of the right bank
of the stream where the upper penstock and conveyance flume are located. Gabions are also used
for streambed erosion control for some distance downstream from the diversion and behind the
diversion spillway training walls.
In October 2006 extreme weather event(s) resulted in very high stream flows, estimated to exceed
1,800 to 2,000 cfs at peak conditions. The magnitude of the event can be appreciated when
Mr. Clay Koplin, P.E.
Cordova Electric Cooperative, Inc.
RE: Rehabilitation of the Intake/Diversion and Conveyance
February 15, 2008
Page 2
considering the average annual stream flow is around 48 cfs and the normal annual peak stream
flow has a recurrence interval estimated to be 300 cfs based on preliminary hydrologic studies that
are part of the feasibility studies. The result of the extreme event was extensive damage to the
intake/diversion dam and conveyance as well as the gabion streambed erosion control and stream
bank erosion control structures along the stream channel from the intake/diversion for some 600
feet downstream to the existing old log crib dam. Other damage resulting from the extreme event
includes rockslides from the canyon walls along the right bank of the creek that have surcharged
the soils along the right side of the conveyance flume for 200 feet of the flume downstream from
the intake/diversion.
Because of the damage described above and in reports previously provided CEC by others, it was
not possible to return the intake/diversion to function after the October 2006 flood without total
replacement of the intake/diversion dam and conveyance flume including all gabion erosion
control structures to protect the conveyance and upper penstock from loss of structural integrity.
Feasibility studies for rehabilitation of the Humpback Creek Project were implemented to
determine the feasibility and cost of replacement of the existing timber frame intake/diversion and
conveyance in -kind and to determine if an alternative approach to rehabilitation using concrete
materials was feasible. The results of the feasibility studies resolved to two options 1) Replace -in -
kind with timber and wood and 2) Replace With New concrete structure and pipe conveyance in
tunnel. Details of the feasibility studies are as follows.
OPTION 1 - Replace -In -Kind
For this option the existing timber frame, wood and steel intake structure and flume conveyance
and all erosion protection gabion mats and structures would be totally removed and reconstructed
to the original designs using all new materials. The estimated life of the replace -in -kind structure
is 12 to 15 years for the project setting, that is to say the intake/diversion dam structure will likely
require total replacement in 12 to 15 years because of hydraulic loading and bed load impact and
abrasion effects on the dam structure.
The gabion erosion control structures (retaining walls) along the stream from the intake/diversion
location to the old log crib dam will require heavy maintenance during years with extreme
hydrologic events in order that the flume conveyance and upper penstock will be protected from
loss of function and capacity. Based on feasibility studies, the 10-year recurrence interval stream
flow is approximately 650 cfs which is a flow high enough to easily mobilize bed load particles
over 6 inches in size and this would indicate very large amounts of gravel of 4-inch minus size is
being transported and impacting the structure; the recurrence interval for movement of 4-inch
sized material is estimated as 2 years based on past stream flow history of Humpback Creek and
Olsen Bay Creek nearby.
There is a great concern that Option 1 is not viable as a timber structure will not meet FERC dam
safety and operational technical requirements due particularly to the performance of timber
structures in settings such as Humpback Creek; FERC will not approve a structure for installation
Mr. Clay Koplin, P.E.
Cordova Electric Cooperative, Inc.
RE: Rehabilitation of the Intake/Diversion and Conveyance
February 15, 2008
Page 3
that is destined to fail in a few years. There is also concern with the stability of the old existing
log crib dam and the associated buttressing log crib installed in 1998 (old log crib dam) located
some 600 feet downstream from the intake/diversion. If the old log crib dam were to fail, the
streambed between the intake/diversion and the old log crib dam would cut down several feet,
compromising the foundations under the gabion retaining wall/erosion control structure along the
right bank. An allowance for initial stabilization of the old log crib dam is included in the
estimated cost for the Replacement -In -Kind option but further investigation is needed to determine
the feasibility of the immediate and future remedial measures and consequences of failure of the
old log crib dam.
Construction access will be by pioneer road and much of the material handling would be done by
helicopter. A skyline would be rigged to allow a certain amount of heavy lifting from the canyon
site to the rim for transport by all terrain vehicle.
The estimated cost to rehabilitate the intake/diversion by Replacement -In -Kind is $6.7 million.
OPTION 2 - Replace -With -Concrete
For this option the existing timber frame, wood and steel intake structure and flume conveyance
and all erosion protection gabion mats and structures would be totally removed and the
intake/diversion would be reconstructed using all new concrete and steel materials to modern
standards that are compliant with FERC engineering guidelines. The conveyance would be HDPE
pipe passing through a tunnel to a point of connection to the penstock near the old log crib dam.
The estimated life of the new concrete structure option is 50 years for the project setting, that is to
say the intake/diversion dam structure may require total replacement in 50 years but could have its
life extended beyond 50 years by performing maintenance in accordance with practices consistent
with the FERC guidelines and license provisions. Hydraulic loading and bed load impact and
abrasion effects on the dam structure will be considered in the design development to ensure long
term performance.
The gabion erosion control structures (retaining walls) along the stream from the intake/diversion
location to the old log crib dam would not be replaced with new structures under this option as the
pipe conveyance will be protected from loss of function and capacity as it will not be exposed to
stream bank loss or canyon wall slides even during years with extreme hydrologic events due to
being contained within a tunnel.
The proposed Replacement -With -Concrete, Option 2, will meet FERC dam safety and operational
technical requirements in settings such as Humpback Creek.
The estimated cost to rehabilitate the intake/diversion by Replacement -With -Concrete is $7.7
million.
5 3 MD�A �-
Mr. Clay Koplin, P.E.
Cordova Electric Cooperative, Inc.
RE: Rehabilitation of the Intake/Diversion and Conveyance
February 15, 2008
Page 4
COMPARISON OF OPTIONS
Comparison of costs of Options 1 and 2 is based on structure construction/replacement only, no
annual O&M is included in the costs presented.
As mentioned above the estimated one-time replacement cost of Option 1 - Replace -In -Kind is
$6.7 million. However, when a replacement cycle of full replacement every 15 years up to a 50
year project life is taken into account, the present worth values of future replacements would be
approximately $8.9 million. Thus to provide 50 years of useful function in 2008 dollars would
cost approximately $15.6 million. Option 1 is unlikely to be approved by FERC.
The estimated cost of Option 2 - Replacement -With -Concrete is $7.7 million with no replacement
costs for new structures in the 50-year life.
The indicated saving in cost for a 50-year project of Option 2 over Option 1 is $ 7.9 million.
Option 2 at $7.7 million estimated initial replacement cost is the preferred option based on first
cost and the fact the structure has a 50-year life with out interim replacement, and will meet
acceptance criteria of FERC for construction and operation.
RECOMMENDATION
Based on the results of our feasibility studies and construction cost estimates for Options 1 and 2,
we recommend selection of Option 2 at an estimated cost of $7.7 million and that the design be
developed based on constructing a new concrete structure approximately 120 feet upstream from
the existing original intake/diversion and that the conveyance from the diversion to the penstock
connection point be a pipe installed inside a tunnel.
Please contact the undersigned at your convenience for any clarifications you may require on the
information presented above.
Thank you for this opportunity to support Cordova Electric Cooperative Inc. in rehabilitating the
Humpback Creek Project.
Sincerely,
R&M CONSULTANTS, INC.
John K. Magee, P.E.
Project Manager
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