Water Power and Dam Construction. Issue March 2010
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WWW.WATERPOWERMAGAZINE.COM MARCH 2010 21
NEW TECHNOLOGY
are led into a closed system as illustrated in Figure 2. Before entering
the membrane modules, the seawater is pressurised to about half the
osmotic pressure, approximately 12-14 bars. In the module freshwa-
ter migrates through the membrane into the pressurised seawater.
This results in an excess of diluted and pressurised seawater which is
then split into two streams. One third of this pressurised seawater is
used for power generation in a hydro power turbine, and the remain-
ing part passes through a pressure exchanger in order to pressurise
the incoming seawater. The outlet from such a plant will mainly be
diluted seawater (brackish water) that will be led either back to the
river mouth or into the sea.
Consequently, the higher the salinity gradient between fresh and
saltwater, the more pressure will build up in the system. Similarly,
the more water that enters the system, the more power can be
produced. At the same time, it is important that the freshwater
and seawater is as clean as possible. Substances in the water may
get captured within the membrane’s support structure or on the
membrane surfaces, reducing the flow through the membrane and
causing a reduction in power output and overall system efficiency.
This phenomenon, commonly known as fouling, is linked to the
design of the system, to the characteristics of the membrane, to
the membrane module, and to the pre-treatment of the freshwater
and the seawater.
The majority of an osmotic power plant will be designed of existing
‘off-the-shelf’ technology. The key components are the membranes,
the membrane modules, and the pressure exchangers. The lion’s share
of efforts to commercialise osmotic power is dedicated to improving
and scaling up these components.
OSMOTIC FO CUS
Statkraft’s focus on osmotic power began in 1997 when the
Norwegian research organisation SINTEF was engaged to perform
feasibility studies on behalf of the company. The result of the study
was that osmotic power could have significant global potential.
However, it was also revealed that one component would require
improvement, and this was the semi-permeable membrane.
Based on the early findings, the work since then has mainly been
focused on the design and production of a semi-permeable mem-
brane optimised for osmotic power. From economical calculations
and estimations of the development in the energy market, a target
for the efficiency of the membranes has been set at 5W/m
2
for
producing osmotic power on a commercial basis. During these
years the power density of the membrane developed by Statkraft
has increased from less than 0.1W/m
2
up to today’s membranes
producing close to 3W/m
2
. The development has been aimed at
testing commercial membranes, as well as developing new mem-
branes designed for osmotic power.
In addition to the development of the membrane, there are also
significant activities on the design and development of the membrane
modules. The standard spiral wound module design has limitations
both in the internal flow pattern and pressure losses, and there are
also limitations with regards to the common design for scaling up to
larger units. Since an osmotic power plant will require several mil-
lion square metres of membrane, the modules should contain several
hundreds or even thousands of square metres. In this respect, the
following design criteria have been suggested:
U The elements must be able to have flow on both the freshwater and
the seawater side of the membrane.
U The elements must contain a large membrane area.
U Fouling must be minimised.
U The design must be cost-effective.
Top left: Official opening of the osmotic power prototype in November 2009;
Top right: The turbine generating electricity in the osmotic power prototype;
Bottom left: The PRO membranes with pressure vessels and PE piping;
Bottom right: Two energy recovery devices recycling the PRO pressure
22 MARCH 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
NEW TECHNOLOGY
The development of osmotic power is managed by Statkraft, and is execut-
ed mainly by research groups in Germany, Norway and the Netherlands,
as well as in the US. There are, however, several other groups working on
elaborative topics both in North America and in Asia.
Based on the development of the membranes, along with an
improved understanding of the system technology and potential,
Statkraft decided to build a complete system for testing the osmotic
power concept in 2007. It has designed a prototype plant where pres-
sure retarded osmosis is used to drive a turbine, based on feed of
seawater and freshwater from natural sources.
THE OSMOTIC POWER PROTOTYPE
The world’s first osmosis driven prototype for power generation was
put into operation on 24 November 2009 during a grand ceremony.
The first electricity produced was used for preparing a cup of tea for
the Crown Princess of Norway Mette-Marit.
In the southern part of Norway a complete prototype of an osmotic
power plant has been built. Several considerations had to be made related
to the choice of location. Firstly, fresh water and seawater with satisfactory
quality must be available. Secondly, the location had to be readily available
for researchers, suppliers and governmental representatives, hence it could
not be too far from a large city. Based on these requirements, the small
community of Tofte, one hour drive from Oslo, was chosen.
The prototype represents a major milestone towards the com-
mercialisation and creates a unique test site for future technology
development. The plant did generate the first small kWh of electricity
from osmosis in November 2009, and the first proof of the concept
of producing power by osmosis has been recorded.
The prototype is designed to be used as a laboratory for the ongo-
ing development of the technology. In this respect, it will contribute
to technology enhancements in order to reach the objective of pro-
ducing power at a competitive cost, and creating the basis for upsiz-
ing the various components to commercial scale.
In addition to the on-going research, with the main focus being on
the membranes and the membrane modules, the prototype will serve
as a catalyst for developing partnerships and building relationships
with interested parties. The prototype facilitates the creation of part-
nership for development of osmotic power outside Statkraft’s core
geographic area, and it increases awareness among governments and
manufacturers. Furthermore, the prototype will be a starting point to
test and measure environmental challenges such as measuring poten-
tial algae bloom related to the discharge of brackish water.
Advantages of osmotic power
Competitiveness
The estimated energy cost of osmotic power is comparable and competitive
with the other new renewable energy sources, such as wave, tidal and offshore
wind being in the range of EUR 50-100/MWh. This cost analysis is based to
a major extent on the existing market pricing for the individual components in
large scale projects. For the membranes, and other units that still require more
extensive development, one has used the assumption that the future pricing
will not be very different from similar technology today due to the enormous
potential in the economy of scale for this global market.
The market potential
To establish an understanding of the potential addition of power generation
capacity osmotic power might represent, surveys of the sites where freshwater
meets seawater have been made. To evaluate the potential power production
from a river, detailed information about water quality, seasonal variations,
seawater salinity and quality, and the amount of freshwater available is
required. Based on this information there are several regions in both the
northern and southern hemisphere that have a significant potential.
The worldwide potential is more than 1600TWh/yr, whereas 170TWh/yr could
be generated in Europe. It is likely that osmotic power can make a sizeable
contribution to the growth of renewable energy in the future, and help regions
such as Europe to reach their targets for environmentally friendly solutions.
Environmental issues
The mixing of seawater and freshwater is a process that occurs naturally all
over the world. Osmotic power plants will extract the energy from this process
without polluting discharges to the atmosphere or water. Moreover, this
process produces no other emissions that could have an impact on the global
climate. Osmotic power’s excellent environmental performance and CO2-
free power production will most likely qualify for green certificates and other
supportive policy measures to increase the share of renewable energy.
One area where there has been some discussion is whether there will be a
negative effect on the marine environment due to the discharge of brackish
water by the osmotic power plant. This may alter the local marine environment
and result in changes for animals and plants living in the discharge area.
However, the osmotic plant will only displace the formation of brackish water
in space without modifying the water quality so this will not be a significant
environmental impact.
Since most rivers run into the ocean at a place where people have already
built cities or industrial areas such as harbours, most of the potential sites
for osmotic power generation can be utilised without affecting pristine areas.
Moreover, the plants can be constructed partly or completely underground (e.g.
in the basement of an industrial building or under a park) which will make them
very discreet. In these areas the environmental impacts onshore are often
considered to be of minor importance. These impacts will mainly be related
to the building of access roads, channels and connections to the electricity
grid. A power plant the size of a football stadium could supply around 30,000
European households with electricity.
Distributed power production
The most likely location for the construction of osmotic power plants would be
in areas that are already populated and have both urban areas and production
facilities. One additional angle to consider is the possibility to exploit osmotic
power as a solution for distributed production of electricity in more remote or
less developed regions.
100
150
100
50
0
Nuclear
Closing on non-renewables by 2030
Gas
CCGT
Oil
CC
Coal
PCC
Hydro
dam
Hydro
running
Biomass Wind
offshore
Wind
onshore
Solar
CSP
Osmotic
power
50-100
/MWh
LCOE /MWh
1
Forecast
2020
45
12 5
115
16 0
65
85 85
88
90
110
48
80
65
Potential of 16-1700 TWh, of which 180 in Europe
Seawater
Brackish Water
Freshwater
Water filter
Water filter
Pressure
exchanger
Freshwater bleed
Membrane modules
Membrane
Turbine
Power
Power
Below: Figure 2 – The principle of osmotic power utilises the energy outcome of
mixing water with different salt gradients. In the process the water with low salt
content moves through the membrane to the side with the higher salt concentra-
tion and creates increased pressure due to osmotic forces. Given the sufficient con-
trol of the pressure on the saltwater side, approximately half the theoretical energy
can be transformed to electrical power, meaning that the operating pressure is in
the range of 11-14 bars enabling the generation of 1MW per m
3
/sec of freshwater
WWW.WATERPOWERMAGAZINE.COM MARCH 2010 23
NEW TECHNOLOGY
DESIGN OF THE PROTOTYPE
The prototype is designed with all necessary systems and compo-
nents for continuous PRO operation. Based on the assumption that a
membrane with an efficiency of 5 W/m
2
will be developed during the
lifetime of the plant, 10kW installed power capacity was set as the
overall design criteria. This gave the lead for water supply for both
water qualities, as well as sizing of the individual components.
The seawater feed to the plant is supplied through water pipes from
approximately 30m below sea level, just outside the harbour. The
water is filtered through a mesh before it enters the plant.
The freshwater at the Tofte location flows from a small lake up
in the hillside. A major focus activity will be to identify the minimal
pre-treatment necessary to operate the plant, and to design the appro-
priate system optimised to fulfil the requirements for operating the
membrane system in a continuous mode through the entire lifetime
of the membranes.
For this plant 2000m
2
of membrane has been installed based on
a modified spiral wound 8” module. This is a convenient and stand-
ardised design where membranes can be replaced easily and also is a
standard that other suppliers can relate to. After some time in opera-
tion we also expect to test alternative module design optimised for
PRO operation. The first membranes installed are based on conven-
tional cellulose acetate membranes, redesigned for PRO operation.
The membranes will be replaced when new and improved membranes
are designed and produced in sufficient amounts.
Besides the membrane system, the plant is equipped with two spe-
cially designed energy recovery devices. Although this technology is
well proven in desalination systems, the installation in this plant is
unique due to the low operating pressure. It will be very important
to learn the operations of these units in PRO, and also to test the
efficiency and leakages experienced in a low pressure system.
A turbine with a generator is installed to generate electricity from
the pressurised water. With continuous flow of water at approxi-
mately 12 bar, a Pelton turbine was chosen. To be able to generate as
much electricity as possible from the membranes installed, the turbine
must be optimised for the correct flow of water at the given pressure.
In a full-scale installation, the combined efficiency of the turbine and
generator is expected to exceed 85%.
The overall objectives of the prototype are two-fold. Firstly, confirm-
ing that the designed system can produce power on a reliable 24-hour/
day production. Secondly, the plant will be used for further testing of
technology achieved from parallel research activities to substantially
increase the efficiency. The performance and efficiency of the individual
component, as well as the system efficiency as a whole, will be directed
towards the targets for commercial production of osmotic power.
These activities will mainly be focused on membranes, membrane mod-
ules, pre-treatment of water, pressure exchanger equipment, and power
generation (turbine and generator).
THE ROAD TOWARDS COMMERCIALISATION
What is necessary for establishing osmotic power as a major con-
tributor to renewable energy generation in the future?
Statkraft has spent significant time and effort on the development
of osmotic power, and will continue to do so. The solution is very
attractive due to the environmentally friendly solution it offers, but to
really make this a new and attractive solution in the renewable energy
market it will depend on three major factors.
U 1) Supplier industry: It is well known that osmotic power was
founded in the field for desalination. It is crucial that the future
suppliers for osmotic power, such as membrane manufacturers, are
willing to spend time and resources on bringing the technology from
where it is today and improve and scale it up to an industrial size.
U 2) Energy utilities: Statkraft’s strategic long-term interest in osmotic
power is to include it in its renewable energy portfolio. With the
increasing focus on the environment, this will also be the case for sev-
eral other utilities around the world. Other utilities must show their
belief in osmotic power – Statkraft alone will not be able to establish
a global osmotic power market necessary to realise its potential.
U 3) Governments and framework conditions: During the last few dec-
ades one has seen the growth of new solutions for harvesting renew-
able energy in Europe. These days, several European countries give
substantial economic support to the establishment and growth of
new, renewable solutions. Such framework conditions will also be
critical for osmotic power. With a predictable support scheme and
incentives for competitiveness also in the early maturing phase, both
the supplier industry and the utilities will be ready to participate.
Statkraft has already encouraged the European Union to include
osmotic power as a recognised part of the marine energy sector, and
will continue to do so also for individual countries.
Stein Erik Skilhagen, head of osmotic power, Statkraft
AS, Lilleakerveien 6, 0216 OSLO, Norway. Email: Stein.
erik.skilhagen@statkraft.com
IWP& DC
References
[1] Stein Erik Skilhagen, Jon E. Dugstad, Rolf Jarle Aaberg, “Osmotic power
— power production based on the osmotic pressure difference between
waters with varying salt gradients” Desalination 220 (2008) 476–482
[2] Thor Thorsen, Torleif Holt, “The potential for power production from
salinity gradients by pressure retarded osmosis” Journal of Membrane
Science 335 (2009) 103–110
[3] S. Loeb, “Osmotic Power Plants,” Science, 1975, vol.189, pp. 654-655.
[4] Lee, K.L. Baker, R.W. and Londsdale, H.K., “Membranes for power generation
by pressure- retarded osmosis”, J. Membrane Science, 1981, vol. 8, 141.
[5] Loeb, S. “Method and apparatus for generating power utilizing pressure-
retarded osmosis” Patent US 4,193,267 Assigned Ben-Gurion University of
the Negev, Research and Development Authority, Beersheba, Israel
[6] Loeb, S., T. Honda, and M. Reali, “Comparative mechanical efficiency
of several plant configurations using a pressure-retarded osmosis energy
converter”. Journal of Membrane Science, 1990. 51(3): p. 323-335.
About Statkraft
The Norwegian utility Statkraft develops and generates electricity from water, wind,
biomass, sun and natural gas, while also being a major player on the European
energy exchanges. The company is owned 100% by the Norwegian government
and has more than a century of experience with hydropower. Statkraft is today the
largest producer of electricity from renewable energy in Europe, with the majority
of power generation coming from hydro power. The company’s portfolio contains
225 hydro power projects – 149 in Norway, 58 in Sweden, 11 in Germany, four in
Finland and three in the UK. In total, the company has an installed hydro power
capacity of more than 12,000MW, with an average annual production of 50TWh.
Statkraft has ambitions for further European growth in France and Southeast
Europe. It is also developing and operating hydro power capacity in emerging
markets outside Europe through its subsidiary SN Power. This company is active in
countries such as Peru, Chile, India, Nepal, Sri Lanka and the Philippines.
Decreasing
cost
Pilot
Demo
Prototype
D
ecreasin
g
cost
Membrane effect
1 W/m
2
3 W/m
2
5 W/m
2
System scale-up
Tofte pilot Demo
System efficiency
40% 60% 80%
Increase membrane element size
216m
2
1000m
2
5000m
2
The four major technological drivers toward commercialisation of osmotic power
24 MARCH 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
FISH PASSAGE
I
N Thompson Falls, Montana, US, a fish passage ladder is
being constructed at the Thompson Falls hydroelectric plant
on the Clark Fork river. This precedent-setting project is the
first full height fish passage ladder in the US built specifically
for the threatened bull trout, and is being constructed specifically
to provide over-dam passage for this species.
The project is being developed as a collaborative effort through
an inter-agency, multi-disciplinary technical advisory team (TAC)
composed of:
UÊ PPL Montana.
UÊ US Fish and Wildlife Service (USFWS).
UÊ Montana Fish.
UÊ Wildlife and Parks (MFWP).
UÊ Montana Department of Environmental Quality (MDEQ).
UÊ Federal Energy Regulatory Commission (FERC).
UÊ Confederated Salish and Kootenai Tribes.
UÊ GEI Consultants GEI and others.
With an estimated construction cost of just over US$6M, the project
is expected to be completed by the autumn of 2010.
PROJECT OVERVIEW
The Thompson Falls plant, which began operation in 1915, consists
of a main dam and a dry channel dam with an island in between. Both
dams are used to regulate the reservoir, which has a storage capacity
of 10Mm
3
, and control flow during high spring runoff. The main
dam is a 278m long and 9.8m high concrete arch gravity structure
with 6m of flashboards and stoplogs above the concrete spillway. The
Making way for bull trout
A full height fish ladder is being constructed to enable the safe passage of fish over the
Thompson falls hydro project in Montana. L Brent Mabbott, Ginger Gillin and Chad
Masching explain why this is the first ladder in the US to be built specifically for bull trout
Work bridge and fish ladder in winter
End of work bridge with fish ladder and
log sluice looking from top of dam
WWW.WATERPOWERMAGAZINE.COM MARCH 2010 25
FISH PASSAGE
dry channel dam is a concrete gravity structure with an overflow spill-
way and an overall length of 88m. Thompson Falls is a run-of-river
project with a total generating capacity of 94MW, generating enough
power to satisfy the energy needs of over 70,000 households.
In 1979, the Federal Energy Regulatory Commission (FERC) reli-
censed the Thompson Falls dam (FERC Project Number 1869) to
the Montana Power Company (now PPL Montana). A major order
amending the licence was issued in 1990 allowing for construction
of an additional powerhouse and generating unit. That project was
completed in 1995. In 1999 PPL purchased the power generating
assets of the Montana Power Company and the licence was trans-
ferred to PPL Montana.
THE NEED FOR A FISH LADDER
The Clark Fork River at Thompson Falls was originally natural
rapids, passable to migratory fish. After the dam was built, bull trout
(Salvelinus confluentus) and other fish species were prevented from
swimming upstream to reach spawning tributaries. Fish were com-
monly observed jumping on the bedrock outcroppings downstream
of the dam when small amounts of flow cascaded over the rocks.
Unable to pass the dam, the fish would typically fall back down-
stream of the main dam without successfully spawning.
In 1998, the bull trout was federally listed as a threatened species
under the Endangered Species Act and critical habitat was defined in
2005. The fish passage project was developed as a direct result of an
Endangered Species Act consultation.
Because bull trout are present in the Clark Fork river upstream and
downstream of the Thompson Falls hydro project, a draft Biological
Evaluation (BE) was prepared by PPL Montana and submitted to the
United States Fish and Wildlife Service (USFWS) and FERC in 2003.
The purpose of this BE was to assess the impacts that Thompson Falls
dam and powerhouse may be having on bull trout and to make recom-
mendations about conservation measures to reduce those impacts.
Section 7 of the ESA requires federal agencies to ensure on-going
project actions are not likely to jeopardise the continued existence
of federally listed threatened or endangered species, or to destroy or
adversely modify designated critical habitat.
The draft BE concluded that the Thompson Falls project was likely
to adversely affect bull trout due to the lack of upstream adult fish
passage, potential for delay or mortality during downstream passage
and potential water quality impacts from increases in total dissolved
gas (TDG) during high spill time periods. PPL Montana sought con-
servation measures and has worked cooperatively with the TAC
over the last eight years to clarify the regulatory issues, plan research
activities, and develop conservation measures appropriate to address
bull trout issues at the project.
Initially, the TAC managed research to determine the numbers and
types of fish attempting to pass the project. Fish were trapped at the
main dam using a 13m long, 0.6m wide aluminium, Denil-type fish
ladder with a fish trap constructed at the top. The installation took
advantage of the cascading flows in the area near the left abutment
of the dam. Fish captured in the trap were enumerated, tagged, and
released. Bull and westslope cutthroat trout (Oncorhynchus clarki)
were radio-tagged, trucked above the dam, and released. Signals from
the radio tags were tracked so that biologists could assess trout move-
ments and determine the location of spawning areas.
Data collected during this research indicated that large numbers
of a wide range of species were attempting to migrate past the main
dam. In addition, bull trout were found to spawn in tributary streams
over 48km upstream of the main dam. While this method of collect-
ing fish downstream was effective for research purposes, the small
Denil ladder was insufficient for providing permanent fish passage.
It was effective only during time periods when little or no spill was
passing over the dam. In addition, the Denil ladder had to be removed
from the tailrace every spring prior to high flows, or else risk being
washed away, a difficult and labour-intensive project.
A fish behaviour study was implemented to determine the best
location for a permanent fish passage structure. Trout were collected
in the tailrace, radio tagged, and released. Their movements as they
approached the dam were tracked. Experiments with spillway open-
ing plans were conducted to determine if fish behaviour could be
influenced by modifying hydraulic conditions in the tailrace. The
conclusion of these studies was that fish approached the main dam
when migrating upstream, and could be attracted to the right bank
by opening panels on the spillway in a specified order.
The new fish passage ladder consists of a full height pool and weir
fishway on the right abutment of the main dam. The design incorpo-
rates a sequence of 48 concrete pools, each approximately 1.8m long
by 1.5m wide by 1.2m deep and separated by aluminium weir plates.
A 0.3m water differential is controlled between the pools by either a
0.6m wide notch at the top of each weir or an orifice near the bottom
of each weir that will pass approximately 0.17m
3
/sec. Biologists will
have the option to switch between the two weir designs to help iden-
tify the best configuration for attracting bull trout up the ladder.
Because the fishway has a nominal 0.17m
3
/sec of flow passing
from pool to pool, a larger flow is required to entice fish to enter the
ladder. The ladder has an auxiliary water system (AWS) to increase
flow in the downstream ladder pools and create a total discharge of
1.7m
3
/sec at the entrance pool. To further assist in attracting fish to
the ladder, a 0.6m
3
/sec high velocity attraction AWS is also being
constructed. The additional AWS discharges directly to the tailrace
in front of the ladder to create an area of turbulence similar to the
bottom of a waterfall; to draw the fish into the ladder.
LADDER DESIGN
The ladder design includes a sampling loop at the upstream end of
the fish ladder. The fish sampling facilities under construction include
a fish trapping mechanism which directs fish into a holding pool.
Once enough fish are in the holding pool, a horizontal fish crowder
corrals the fish to one side of the pool and into a fish lock. Water is
then pumped into the lock, and fish are lifted 4.6m up to a sampling
platform where they can be sorted and handled. Non-intended fish
such as invasive species can be separated from the native population,
and other fish can be released upstream of the dam.
PPL Montana will collect and record species, numbers, condition,
and other pertinent data for fish passed at the project. Collection of
this data will help fine-tune the operation of the ladder. Because water
used to entice fish through the ladder is therefore not used for power
generation, it is in the utilities’ best interest to use as little water as pos-
sible to attract fish to the ladder. By experiment and analysing data,
PPL is better able to determine the optimal amount of water needed to
successfully operate the ladder. In addition, data collection will also be
used to enhance fisheries management in the area by allowing manag-
ers to quantify the numbers, species, and timing of fish passing through
this ladder. As part of this data collection and analysis, all collected bull
trout will be tagged with passive integrated transponders (PIT tags) to
gather project passage data. The ladder will be equipped with PIT tag
Above: Fish ladder below heated enclosures with insulating blankets and
heaters on recent concrete placements
26 MARCH 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
FISH PASSAGE
readers that record when a tagged fish enters the ladder and when it
enters the holding pool at the sampling loop.
In addition to bull trout, PPL Montana anticipates that the ladder
will provide upstream passage to a wide array of other migratory fish
species attempting to pass the project to reach upstream spawning
areas. This project is one component of an overall programme to
restore ecological connectivity for fish in the Clark Fork river basin.
Once upstream of Thompson Falls, fish will have access to miles of
free flowing river.
PPL Montana will operate the fishway during low and no spill
periods from 1 July to 15 May annually. Fishway dewatering and
maintenance will occur from about 15 November to 28 February
because bull trout are not typically migrating in the mainstem of the
Clark Fork river at this time.
Under the terms of the FERC licence for the project, PPL Montana
was required to consult with the State Historic Preservation Office,
conduct a cultural resources survey of these areas, and file a report
with the FERC documenting the survey prior to any land-clearing,
land-disturbing or spoil-producing activities. Therefore, PPL Montana
conducted an intensive archaeological survey of the proposed project
area. The survey found the fish ladder project will affect constitu-
ent elements of the main channel dam. As a result, PPL Montana
included these features in the Historic American Engineering Record
(HAER) documentation, which was prepared as part of the overall
cultural resource management study associated with the proposed
fish ladder project.
The permits required for this construction project included a
Clean Water Act Section 404 permit issued by the US Army Corps of
Engineers, a biological opinion and incidental take statement from
the US Fish and Wildlife Service, and an order from the FERC. The
environmental concerns about construction included how to manage
debris from demolition and fill material to prevent it from entering
the river channel.
This project was developed through a collaborative process with
the TAC members and was designed by GEI. The construction con-
tractor is COP Construction from Billings, Montana. Construction
supervision is being done by Morrison-Maierle, with assistance from
GEI Consultants.
L. Brent Mabbott, is a senior fish biologist for PPL
Montana. Email: lbmabbott@pplweb.com
Ginger Gillin is a senior environmental scientist at GEI
Consultants. Email: ggillin@geiconsultants.com
Chad Masching is a civil design engineer for GEI
Consultants. Email: cmasching@geiconsultants.com
www.geiconsultants.com
IWP& DC
Above: General layout plan of the fish ladder
Left: Rock excavation with viewing area overlooking fish ladder
• Impermeable
• Flexible
• Resistant to settlements
• Installed quickly,
also underwater
• Durable
• Applicable to all types
of structures
• Environmentally friendly
• Efficiently monitored
GEOMEMBRANES ARE:
Messochora, Greece 2010 - Exposed PVC geomembrane to water-
proof a 150 m high CFRD. Hydropower.
CARPI SYNTHETIC GEOMEMBRANES
HAVE STOPPED LEAKAGE IN 100+ DAMS
Since 1963
1,200+ projects completed. Design and supply of waterproofing systems for
dams, canals, hydraulic tunnels, reservoirs. Dry and underwater installation.
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Corso San Gottardo 86 - 6830 Chiasso - Switzerland
Ph. ++41 91 6954000 Fax. ++41 91 6954009
www.carpitech.com e-mail: info@carpitech.com
C
28 MARCH 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
PLANNING & PROJECTS
I
N the last twenty years concrete face rockfill dams (CFRDs)
have been built in increasing number. However, because the
number of these dams is still relatively small, experience on
the actual performance of these dams under strong earthquake
loading is still very limited. We have to realize that not all seismic
problems pertaining to large dams have been solved. Every time
there is another strong earthquake, upgrading the design criteria
and design concepts may become necessary. This applies in par-
ticular to more recent types of structures such as CFRDs.
In the subsequent sections some of the relevant earthquake aspects
of CFRDs are briefly discussed. Before an attempt is made to solve
a particular problem, it is important to take an integral look at the
phenomenon first, otherwise important aspects may be overlooked.
This is particularly true for new types of dams.
An overview on the seismic aspects of CFRDs is given in the General
Report on seismic aspects of dams presented at the 21st ICOLD
Congress (Wieland, 2003). At that time, the information on the per-
formance of CFRDs under seismic loading was still very scarce.
The study of the damage of the concrete face of Zipingpu CFRD
in China caused by the Wenchuan earthquake will have implications
on the seismic design of high CFRDs in highly seismic regions. At the
moment the world’s highest CFRD, Shuibuya dam in China, has a
height of 233m and it is planned to use this technology for dams with
heights up to 300m. Before such dams can be built a more thorough
understanding of the behaviour and safety of CFRDs under strong
earthquake ground shaking is needed. This can be achieved by sophis-
ticated dynamic analyses, shaking table tests, and strong motion instru-
mentation will eventually provide useful data. In the past the earthquake
vulnerability of CFRDs was assumed to be lesser than that of other dam
types; however, today this opinion is questioned as the seismic hazard is
a multiple hazard, which, depending on the site, does not only include
ground shaking but also fault movements, rockfalls and others.
BEHAVIOUR OF ZIPINGPU CFRD
The 156m high Zipingpu CFRD is one of the largest CFRDs in China and
one of the key projects for water supply and irrigation for the Chengdu
basin. The dam was completed in 2006 and represented the latest CFRD
technology in China. The dam was designed for an intensity of VIII
(Chinese scale), with a design peak ground acceleration of 0.26 g.
The dam was subjected to very strong ground shaking during the
12 May 2008 Wenchuan earthquake (magnitude 8.0). The dam was
located 17km from the earthquake epicenter in Wenchuan County.
The intensity in the epicentral region was XI and at the dam site in
the range of IX to X. This intensity is far beyond the value accepted
in the original design. During the earthquake the reservoir level was
low and its volume was 300Mm
3
. Under normal operation condi-
tions the reservoir volume is 1100Mm
3
. The crest of the dam and
the concrete face were damaged. A description of the Wenchuan
earthquake and the damage to large dams and the Zipingpu CFRD
was published by Chen H. (2008), Chen S. (2008), and Xu (2008).
From the six strong motion instruments installed in the dam only the
three located on the crest provided data, the other instruments were
out of order during the earthquake. On the crest, the recorded peak
accelerations were over 2g; however, as the concrete structure of the
crest behaved differently from the dam body and since the concrete
part also separated from the rockfill, the dynamic behaviour of the
concrete elements of the crest was quite different from that of the
rockfill. This was reflected in the high frequency components of the
accelerograms recorded by the instruments, which were attached to
the concrete elements on the crest.
Of greatest interest was the damage to the concrete face and water-
proofing system which consisted of damage of the vertical joints and
the offset of the horizontal joint between the second and third stage
slab construction (Figures 1 to 3).
After the earthquake the maximum settlement at the dam crest
was 735mm and the horizontal deflection in downstream direc-
tion was 180mm. The cross-canyon deformation of both abut-
ments was 102mm. Because of the low water level in the reservoir
at the time of the earthquake, it is difficult to estimate what the earth-
quake behaviour of the dam, the concrete face, and the waterproofing
system would have been if the reservoir were full.
Zipingpu dam will be a milestone in the development of CFRDs.
The lessons and experience from this project will provide an excellent
reference for dam engineers in seismic areas of the world.
The damage observed at Zipingpu CFRD is acceptable for very
strong ground shaking such as the safety evaluation earthquake
(SEE). Besides the satisfactory behaviour of the dam, it has to be
pointed out that bottom outlets and spillway gates must be operable
after the SEE, i.e. the performance criteria applicable to these compo-
nents are stricter than those for the dam where according to ICOLD
guidelines (ICOLD, 1989) structural damage is accepted as long as
the dam is stable and no uncontrolled release of water takes place that
would lead to catastrophic flooding in the downstream area.
GENERAL SEISMIC FEATURES OF CFRDS
Since the rockfill is essentially dry, earthquake shaking cannot gener-
ate excess pore water pressures. With the absence of pore water pres-
sures and the high shear strength of compacted rockfill, this dam type
is considered inherently resistant to seismic loading (Cooke, 1991).
Possible failure modes that were considered are (1) sliding of shallow
material along planar or nearly planar surfaces and (2) wedge failure
or deep-seated rotational failure (Seed et al., 1985).
However, there are certain features with these dams, which should
not be overlooked (Wieland and Brenner, 2008):
UÊÊa) Vulnerability of perimetric joint: The perimetric joint connecting
face slab and plinth is a critical element in the dam. It is protected
with filters, thus the washing out of foundation material can be
prevented in the case of leakage.
CFRDs in highly seismic regions
Until the Wenchuan earthquake of 12 May 2008 no large concrete face rockfill dam (CFRD)
was subjected to strong ground shaking. Quite a few dam engineers were of the opinion that
CFRDs are inherently safe against earthquakes, because any leakage in the concrete face would
not lead to the failure of the dam. Even if such a state could be achieved excessive leakage
could not be tolerated and the reservoir would have to be drawn down if severe damage of
the concrete face and/or the water proofing system has to be assumed. In this paper the main
measures to be considered to achieve a safe CFRD in highly seismic regions are discussed
WWW.WATERPOWERMAGAZINE.COM MARCH 2010 29
PLANNING & PROJECTS
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ASSESSMENT OF CONSEQUENCES OF SEISMICALLY
DAMAGED CONCRETE FACE
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Above from top: Figure 1 – Concrete face of Zipingpu CFRD after repair:
detail of vertical joint; Figure 2 – Damage of vertical joint of concrete face
of Zipingpu CFRD (looking towards crest) (Photograph courtesy Xu Zeping,
China); Figure 3 – Damage at the longitudinal joint concrete face of Zipingpu
CFRD (Photograph courtesy Xu Zeping, China)
30 MARCH 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
PLANNING & PROJECTS
the soil. The damage patterns of a concrete face shown in Figures
1 to 5 are more likely to be expected in the uppermost parts of the
dam. At greater water depth local joint damage in the concrete
face due to compression and shear but also joint opening, must be
expected, leading to increased leakage losses. Widening of exist-
ing cracks within the concrete slab elements should also to be
expected all over the concrete face.
Due to the fact that in many concrete slabs the reinforcement
is placed in the middle plane of the slab the flexural stiffness will
reduce significantly once cracks have formed. Also the crack width
will be larger at the surface as in the case where the reinforcement
would be placed in two separate layers near the surfaces of the
slab. Although a large concrete cover is needed from the durability
point of view, standard reinforcement of the elastically supported
slab elements would be beneficial. At the vertical joint additional
reinforcement is needed to prevent spalling (high stresses normal
to the joint surface) due to impacting slabs and for proper transfer
of the shear forces.
After strong ground shaking, leakage is expected to increase signifi-
cantly. It should be shown in the seismic design of the concrete face
(as a worst case) that the complete failure of a slab element does not
have a negative effect on the safety of the dam and that such a failure
scenario can be accepted.
SEISMIC DESIGN FEATURES FOR CFRDS
For the design of a high CFRD in a highly seismic region the follow-
ing measures are recommended, which improve the performance and
the seismic safety of the dam:
UÊÊÊ(1) Flat slopes (reduce seismic deformations).
UÊÊÊ(2) Generous freeboard (accounts for seismic deformations and
impulse waves in the reservoir).
UÊÊÊ(3) Wide crest (improves safety of crest region of dam and increases
resistance against overtopping from impulse waves).
UÊÊÊ(4) Proper material selection in dam body with proper zoning (rockfill
shall allow free draining of water leaking through the concrete face).
UÊÊÊ(5) Provision of geogrid and other techniques for strengthening the
downstream slope of the crest region of CFRD.
UÊÊÊ(6) Provision of a bottom outlet (to lower the reservoir if the con-
crete face or waterproofing system is damaged).
UÊÊÊ(7) Concrete panel with smaller width to account for non-uniform
deformations of the concrete face.
UÊÊÊ(8) Arrangement of reinforcement of the face slab to improve load
bearing behaviour in-plane and out-of-plane and ductility.
UÊÊÊ(9) Arrangement of proper joint system including horizontal joints
and selection of the joint width to account for the reversible nature
of the seismic response.
UÊÊÊ(10) Water-proofing system of face slab and the plinth joint to
account for static and seismic joint movements.
UÊÊÊ(11) Well-compacted rockfill, etc.
Moreover, the general design features for embankment dams given in
ICOLD Bulletin 120 (ICOLD, 2001) will contribute substantially to
the satisfactory behaviour of a CFRD under strong ground shaking.
As joint leakage is accepted after the SEE, a different design earth-
quake may be specified for the water proofing system. In general, it is too
conservative to design the water proofing system for the SEE. The dam
must be designed for the SEE and it may experience inelastic deforma-
Left: Figure 4 – Zipingpu Dam: Damage at crest (downstream edge) (top and
middle), and damage at joint of upstream parapet wall (bottom left)
Below: Figure 5 – Cogoti CFRD in Chile before (top) and after the 14
October 1997 Punitaqui earthquake (middle); cracks on dam crest (bottom)
(Photographs courtesy G. Noguera, Chile)