Water Power and Dam Construction. Issue January 2010

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WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 21

DESIGN

it as much as possible. In addition, braking (or retarding) mounds are

used widely where there are dense, wet-snow avalanches.

Deﬂecting dams need to be well positioned on steep terrain in suf-

ﬁciently long run-out zones, which should provide plenty of distance

and space to spill the diverted snow ﬂows. Optimally, they will divert

an avalanche without slowing the ﬂow by much to both keep deposits

to a minimum and keep the dam mostly clear for the next event.

Where space is limited, such as at the foot of slopes, and there is the

risk of run-out from dense avalanches or the dense parts of mixed ﬂows,

then it may be possible to build catching dams instead. The internal fric-

tion angle of the snow, so far as can be known, should be greater than the

slope of the terrain. The storage volume upstream of the dam may need

to be big enough to take more than one avalanche per season.

The effectiveness of catching dams, however, depends on the scale

of events as well as the size and location of the structures. It is not

unknown for them to be overtopped by large, fast avalanches.

Braking mounds are built uphill from dams. They are used to

protect against wet-snow events and are thought to have little effect

against faster, dry snow avalanches. The main aim of braking mounds

is to help slow and break avalanche ﬂows by dissipating some of the

kinetic energy.

Properly built, traditional avalanche dams comprising loose materi-

als (rock, gravel, sand) are usually 10m-25m high and can be 50m-

500m in length, or even longer, resulting in volumes in the order of

104m

-105m

The deﬂecting dam at Seljalandsmuli in Isafjordur, in Iceland, is

650m long and its braking mounds are 7m high, resulting in total

construction volume of 360,000m

. Elsewhere on the island, above

Neskaupstadur, the defence structures comprise two rows of 10m

high mounds and a steep 17m high by 400m long catching dam.

In addition to loose deposits, dams and mounds may also be built using

reinforced earth or reinforced concrete (RC) where there are steep faces.

In Norway, for example, there is use of a 8m high, buttressed RC wall at

Odda and a 10m high dam of six RC shells anchored to bedrock.

APPROACH ES TO ANALYSIS

Traditional analytical principles are based on simple or dynamic point-

mass calculations, or dynamics of the leading edge of the avalanche.

The weakness in the point-mass approach is seen as its simplifying an

avalanche mass so much that the transverse width is neglected, which

leads to lateral and longitudinal interactions being ignored.

Modelling an avalanche is also a challenge when the mass might

not strike a dam perpendicularly and this is the design case for deﬂect-

ing dams. The simpliﬁed approach also means there is no objective

method to calculate momentum loss within an avalanche upon

impact with a dam.

Advances in theoretical, experimental and ﬁeld research over the

last decade have led to improvements in design criteria for deﬂecting

and catching dams. The criteria are the more dynamic concepts of

supercritical overﬂow and ﬂow depth downstream of ‘shock’ – sharp

gradients or discontinuities in ﬂows. Though incomplete, and based

on some subjective and partly justiﬁable considerations, note research-

ers, the criteria represent a new basis for avalanche dam design.

The updated approach takes the dynamic equations for the grav-

ity ﬂow of a shallow, free-surface layer of granular material on a

slope, producing a range of Froude Numbers (Fr), and uses them in

a depth-averaged calculation. Only the dynamics of the dense core of

the avalanche are modelled, Froude numbers for natural dry-snow

ﬂows is approximately 5-10, which is well within the supercritical

range as deﬁned by Fr>1.

Changes in upstream ﬂow depth and velocity are of greater impor-

tance to how the dense core interacts with obstacles than previously

thought. Sharp gradients or discontinuities, in depth-averaged analy-

sis, are believed to be an approximation to real physical responses to

changes in shallow, supercritical ﬂows. But, the shocks are generally

local effects in avalanches.

Some parameters and processes, however, are not often repre-

sented in the depth-averaged analysis, such as: variable terrain and

dam shapes; initial ‘splash’ impacts; shearing in overﬂow at the dam

crest; and, overﬂow of other, non-core layers (ﬂuidised, powder), and

related dynamic and air pressure effects in the interstitial air.

Further processes not represented include: impacts that transfer

snow from the core into suspension; transfer between the avalanche

and snow cover, including deposition near the dam; and, snow drift.

Another parameter that is not well understood in terms of avalanche

dynamics is terrain friction. It is expected to be relatively unimportant

for dry-snow events as there are rapid, sequential impacts of avalanche

layers, but where there is prolonged contact, such as at long deﬂecting

dams with acute diversion angles, it may play a greater role.

But a cautionary note is sounded for this analysis, for the utility

of the hydraulic equations can only be taken so far in the analogy;

snow avalanches are not true ﬂuids but inelastic granular materials.

Therefore, a supplemental approach to help gain further, empiri-

cal information is to run small-scale tests, although modelling is,

of course, limited by how well the dimensional similarity can be

achieved between the natural, ﬂowing snow mass and the reduced-

scale experiment.

Tests that have been done include ﬁeld chutes with snow and labo-

ratory experiments with granular materials. They have, for instance,

given some measurable understanding of the effectiveness of braking

mounds with data indicating that, despite experimental uncertainties,

they do substantially slow supercritical granular ﬂows. The results are

little affected by the scale of chutes, velocities and materials.

Suspension layer

Fluidizer layer

Dense-flow layer

Snow cover

Below, top: Two deﬂecting dams and a catching dam at Flateyri, north western

Iceland. The deﬂecting dams are 15–20m high and the small catching dam

between them is 10m high. The dams were built after a catastrophic avalanche in

1995 that killed 20 people. (Photo: © Mats Wibe Lund) Courtesy of the EC

Bottom: A schematic ﬁgure of a dry-snow avalanche showing the dense core,

the ﬂuidised (saltation) layer and the powder cloud. The depth-averaged quanti-

ties u

and h

apply to the dense core in the sloping (x,z)-coordinate system.

The ﬁgure is adapted from Issler (2003). Courtesy of the EC

22 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

DESIGN OF DAM S

Once the location for a dam has been decided, the forgoing analyses

are important to help determine the Design Avalanche with which to

help estimate the appropriate size and shape of a dam or mound.

In applying design criteria, the sensitivity of results to various param-

eters and awareness of where uncertainties lie in theory, analysis methods

and data should all be borne in mind. The design process can use the

avalanche analysis calculations for an initial, simpliﬁed estimate of dam

height and to help develop early ideas of geometry. To that end, the pro-

fusion of possible design variable should be omitted, initially.

Afterward, more complex design modelling might be undertaken

by gradually, and increasingly, including the variables to build the

numerical simulations that are based on depth-average equations and

shock-capturing algorithms. The design work can help establish the

dam height, length, width and geometry of a dam. For catching dams

it is also important to estimate the storage capacity available on the

upstream slope, on top of the existing snow cover.

Design work will also establish the ‘critical dam height’, deﬁned as

the maximum height of the obstacle (above snow cover) that changes

the avalanche from supercritical to subcritical ﬂow state as the snow

passes over the crest. By adding the upstream ﬂow depth then the

minimum physical height of the dam to arrest the ﬂow can be esti-

mated. Designs based on such a shallow ﬂuid dynamics approach of

the avalanche core should be viewed as the minimum requirement,

say researchers.

The design, though, assumes no loss of momentum in the ava-

lanche during its impact with the dam. Yet the reduction in kinetic

energy has been shown to be signiﬁcant in tests, such as chute experi-

ments with granular materials, and snow. The tests with glass beads

show that where the upstream faces of dams have angles of at least 60

degrees to the terrain they have similar efﬁciency in dissipating energy

to those built perpendicular to the slope. Angles of 30 degrees were

found to be less efﬁcient.

However, when seeking a basis for assessing momentum reduction

in a design then there is considerable uncertainty in applying results

from chute tests to full-scale dams when examination the reduced

in velocity up the faces. Test data seem to overestimate the reduc-

tion. Whatever value of the momentum reduction factor is adopted

in supercritical ﬂow analysis, it is catching dam design that has most

sensitivity to the parameter, according to chute tests.

DESIGN OF BRA KING MOUN DS

Designs of braking mounds draws upon limited ﬁeld evidence, so far,

to describe their effectiveness but laboratory experiments show that,

at least using granular materials, they can reduce avalanche speed and

also shorten run-out distances. However, there are no accepted design

guidelines for the structures.

Experiments with braking mounds have only got underway seri-

ously in recent years but they are viewed as having a similar effect to

bafﬂe blocks and piers in stilling basins at impounding dams.

Researchers suggest that, measured above snow cover, mounds

should be up to three times as thick as the dense core of a wet-snow

avalanche. They further suggest the mounds should be designed with

steep upstream and sides faces, and the aspect ratio kept close to

unity. A row of short mounds is taken to be more effective for energy

dissipation than fewer, wider structures. Subsequent rows should

have their mounds in staggered patterns, though data suggest they

are less effective in reducing velocity than the ﬁrst, uphill line.

Aside from their safety functions, of course, the avalanche defence

structures need to ﬁt well into the local landscape. They can have

large environmental and visual impacts, notes the EC report, and so

integration is important when designing such structures to be near

settlements, in particular.

VERIFICA TION

The EC report has a data set with 22 events, mostly from Norway, of

the run-up of snow avalanches to man-made dams (six) and natural

obstacles. Avalanche events ranged in volumes estimate at 15,000m

to 800,000m

, the impact velocities were calculated to be 20m/sec-

70m/sec and the vertical run-up the structures was 7m-90m.

Many of the obstacles are on rather steep terrain. While the run-up

for many of the events fall within the ranges predicted by supercriti-

cal ﬂow analyses there are several that were higher. However, the

veriﬁcation exercise also showed the uncertainty in the estimates from

only moderate ranges in ﬂow depth (1m-3m) and velocity (+/- 15%)

producing large variations in run-up of the dense core. The greatest

sensitivity is to velocity.

The report notes that ‘the rather wide spread of the data points

compared with the assumed uncertainty of the theoretical predictions

clearly indicates an incomplete understanding of the dynamics of the

impact process.’ Damage may occur considerably higher than the

run-up of the dense core, and some ﬁeld data marks are believed to

have been caused by the impacts of ﬂuidised layers or powder clouds.

There are virtually no observations of the interaction of powder snow

avalanches with man-made dams, however, though inferences are

drawn from impacts on natural obstacles.

FURTHER RESEARCH

Further research is needed on a number of fronts to take forward the

understanding and design of avalanche dams and braking mounds.

The EC report identiﬁes the following key areas for study: momen-

tum reduction from impacts; maximum deﬂecting angle; the effect of

terrain slope on the design; entrainment and deposition effects; and,

the effects of the ﬂuidised layer and powder cloud, which are ignored

in analysis focused on the dense core.

In addition, study is needed into inconsistencies in ﬁeld data, or

at least greater variability than anticipated. More complete physical

descriptions of lateral and longitudinal interactions within the ava-

lanche snow mass during impact also need to be developed.

For deﬂecting dams, a larger data set is needed to judge if empirical

adjustments of calculated run-up can help improve estimates. There is

also a view among researchers, which can be examined further, that

less ﬂexibility should be allowed in selecting model parameters as this

will help reduce subjectivity.

A copy of the EC report “The Design of Avalanche

Protection Dams: recent practical and theoretical

developments” can be downloaded from:

ftp://ftp.cordis.europa.eu/pub/fp7/environment/docs/

avalanche-protection.pdf

IWP& DC

Left: A photograph of the snow hitting a 60cm high catching dam in the 34m long

chute at Weissﬂuhjoch (Hákonardóttir and others, 2003d). A part of the dam broke

during this experiment as seen on the photograph. Courtesy of the EC

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 23

DESIGN

MERSON Process Management has been contracted by

Korea South East Power Company (KOSEP) to install its

Ovation expert control system at two pumped storage

power plants in Korea.

For the ﬁrst project, Emerson is modernizing the controls at the

MuJu pumped storage plant, located in MuJu-Gun, Jeonlabuk-Do,

south of Seoul, Korea. The two-unit plant, with a maximum installed

capacity of 600MW (2 X 300MW), has been in operation since 1991.

The fast-track project required equipment delivery within ﬁve months

of the contract award. Installation took place during a scheduled two-

month outage, with the plant scheduled to resume generating electric-

ity at the end of November, as the magazine goes to press.

Emerson is responsible for engineering, procurement, installation,

testing and commissioning of the Ovation system, which will perform

data acquisition, as well as monitoring and control of the electrical

control system, hydro turbine governor and balance of plant proc-

esses. The contract also includes partial discharge monitoring for high

Muju pumped storage plant

Location 150 Bukchang-ri Juksang-myeon Muju-gun Jeollabuk-do

Land 1,340,000

Construction Dates Unit 1 : February 1995, Unit 2 : April 1996

Facility Capacity 600MW (two 300MW units)

Yecheon pumped storage plant

Location Seon-ri Yongmoon-myeon Yecheon-gun, Gyeongbuk(higher area)

Songweol-ri Hari-myeon Yecheon-gun, Gyeongbuk(lower area)

Land 2,313,678

Construction Dates Unit 1 : September 2010, Unit 2 : December 2010

Facility Capacity 800MW (2 x 400MW units)

voltage motors and generators.

Emerson is supplying eight workstations and seven controllers

for the MuJu plant. When it is up and running, the Ovation system

will monitor and control a total of 4541 I/O points at the hydroelec-

tric facility. Emerson is also remodelling the facility’s control room,

which in addition to controlling operations at MuJu, will also be used

to remotely monitor and control operations at the Yecheon facility,

located more than 160km away.

YECHEON PROJECT

For the second project, Emerson will install its Ovation system at the

new two-unit Yecheon project in Yecheon Kun Kyungsangbukdo,

southeast of Seoul in the Yecheon resort area. Yecheon Unit 1 is

slated for commercial operation in September 2011; Yecheon Unit 2

is expected to come online three months later. The 800MW plant (2

X 400MW), will have the largest capacity among existing pumped

storage power plants in Korea and will play a major role in stabilizing

grid operation at peak times.

At Yecheon PSPP, the Ovation system, which will monitor and

control a total of 3720 I/O points, will be responsible for data acqui-

sition, as well as for monitoring and controlling the electrical control

system and balance of plant processes. Emerson will also provide

partial discharge monitoring for high voltage motors and generators,

and Web closed-caption TV.

Under the contract, Emerson will provide a total of eight worksta-

tions and six controllers.

For further information on the services and products

supplied by Emerson Process Management, please visit

www.emerson.com

Under

control

Emerson Process Management will install its

Ovation expert control system at pumped

storage power projects in Korea

Plan of the Yecheon project. Courtesy of

Emerson Process Management

IWP& DC

24 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

HE new Loch Coire nan Arr dam is an embankment dam

about 175m long with a maximum height of 6.5m, built

at the outlet from a natural loch in Scotland. It has been

developed by the owners, Lighthouse Caledonia, for aug-

menting the fresh water supply to their salmon smolt facility at

Russel Burn, Kishorn. It will increase the reliability of production

whilst allowing hydro power development in the future.

Loch Coire nan Arr is located in the west Highlands of Scotland,

within the southern reaches of the Applecross mountains just north

of Skye. The location of the loch is at an elevation of about 125m

AOD within Coire nan Arr, a large glacial bowl (corrie) formed in

Precambrian Torridonian Sandstones. This is generally a very tough

sandstone and very resistant to weathering. The corrie is formed with

high cliffs and very steep slopes rising over 600-700m above the loch

and the ﬂoor is covered in glacial moraine and tills with extensive

peat bogs in ﬂatter areas. The area is designated as a Site of Special

Scientiﬁc Interest (SSIS) under UK legislation and Special Area of

Conservation (SAC) under European legislation, as well as being of

outstanding landscape value.

LOCH DEVELOPMENT

During the mid and late 1990s, the natural Loch Coire nan Arr was

raised by a progression of small structures to augment river ﬂow for

water supply to the smolt facility some 1.5km away near the shore of

Loch Kishorn. Abstractions for supply are from an intake lower on

the Russel Burn near the smolt facility, and water is released from the

loch as required to maintain the required river ﬂow at the intake.

The original dam developed into a concrete, steel and timber

retaining wall structure extended on the left side by a low embank-

ment formed of locally-won glacial moraine materials. The dam had

a maximum height of 2.6m, and an overall length of about 80m. All

the structures were founded at a shallow depth on the glacial depos-

its. In this form, the loch had a surface area of about 0.2km

and a

storage capacity above natural ground level of some 370,000m

The principal outlet was a 750mm diameter pipe set in the dam

wall at the level of the outlet of the loch. Later, in around 2002,

outlet ﬂow capacity was augmented by a further 750mm diameter

outlet pipe, constructed at a lower level through the right abutment.

It was controlled by a sluice gate at the downstream end, some 125m

downstream of the dam wall.

The reservoir had its ﬁrst formal inspection under Section 8 of the

Reservoirs Act in 2004, and some inadequacies were identiﬁed, nota-

bly a lack of ﬂood handling capacity and a vulnerability to seepage.

The total freeboard available was only 0.4m and the dam had been

overtopped on a number of occasions, such that the embankment at

the left side had been rebuilt several times.

Because of its form of construction, the dam did not lend itself to

being modiﬁed easily for upgrading at the then storage level or being

raised. Work ceased until 2006 owing to change of ownership of the

facilities but the new owner Panﬁsh Scotland (and the subsequent

owner Lighthouse Caledonia) also conﬁrmed a need for additional

water storage for their operations and wanted to make provision for

hydro power development. In addition, planners and environmental

bodies, notably Scottish Natural Heritage (SNH), were keen for the

appearance of the dam to be improved. The proposal was therefore

put forward to:

U Construct a new dam downstream of the original structure.

U Incorporate a larger spillway.

U Raise the storage level by 1.5m.

U Allow the sensitivity of the area to be taken into account.

Initially, project management, outline design, planning and environ-

mental issues and contract negotiation were by HGA Consultants,

Inverness, with AECOM Edinburgh providing the All-Reservoirs

Panel Engineer and technical support. During the course of the prepa-

ration of the detailed design in 2008, AECOM took on responsibility

for all the above aspects.

The ﬁnal dam is 6.5m high and 175m long, creating a reservoir with

a storage capacity of some 690,000m

. The spillway is designed to pass

the 1 in 1,000 year ﬂood with full wave surcharge allowance.

The design and construction of a replacement dam on difficult foundations was recently

completed at a salmon smolt facility in Scotland. RM Doake from AECOM explains how

the design process was shaped by working in such an environmentally sensitive area

Finding a replacement

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 25

DESIGN

GEOTECHN ICAL STUD IES

The site is on glacial moraine and tills, with areas of peat bog, but

no geotechnical information was initially available for the region of

the new dam footprint other than from what could be observed on

the surface. Geotechnical investigations were therefore developed in

2007 based on a provisional line for the new dam. Bedrock appeared

likely to be at shallow depth in the stream bed and left bank region,

but under thicker deposits on the right bank. The investigations were

organised by HGA and Applied Geology, with the scope of the inves-

tigations speciﬁed by AECOM. Provision was made for 8 boreholes

and 17 trial pits, with allowance for a spare borehole and trial pit.

Discussions with site investigation contractors highlighted the dif-

ﬁculties of drilling through the moraine materials, which contained

very large boulders while it was clear that the ﬁner materials contained

very little clay. The only method considered practicable for obtaining

reasonable sample recovery from boreholes was by sonic drilling.

The drilling and insitu testing was carried out in July 2007 by Boart

Longyear. Boreholes were drilled 3m below drift deposits to prove bed-

rock. In-situ testing included SPTs, undisturbed sampling, and falling

head permeability tests. Laboratory testing by specialist laboratories

Enverity and Geolabs included PSD, Atterberg limits, moisture contents,

dry density relationship tests and 300mm shear box strength tests.

A bedrock of siltstones and sandstones was encountered over the

whole site generally at a depth of around 5m and shallower only

locally near the river with 1m at the shallowest. The overburden is

typically a layer of variable orange-brown sandy gravels and boul-

ders about 0.5-1.5m thick overlying dense – very dense grey-brown

silty-sandy gravels and boulders. Local peat pockets also occur,

and boulders could reach sizes in terms of metres. All materials

were derived from the local Torridonian Sandstones. Although the

upper layer was described as ‘made ground’ it is considered that the

layer is only locally associated with the construction of the original

dam, and that it generally represents moraine of the last phase of

glaciation. The ﬁner and denser underlying layer would represent

more highly worked glacial till or moraines heavily consolidated

by the later glaciation.

The in-situ permeability tests in the glacial materials generally

showed permeabilities generally in the order of 1x10

to 1x10

m/sec.

Assessments of permeability by Darcy’s equation, based on the D10

size, indicated permeabilities in the order of 1x 10

m/sec, although

it was considered that this method would not take adequate account

of the denseness and amount of ﬁnes. The laboratory tests showed

that the main overburden materials have grading envelopes generally

very close to the critical curve for stability against internal erosion,

given by the criterion of H/F=1 where F is the percent by weight

passing a particular size D and H is the percentage between D and

4xD. However, some samples showed greater potential for internal

erosion, with stability indices (minimum H/F ratios) down to 0.43.

Characteristic tests showed the materials were non-plastic, and effec-

tive stress shear strengths were around c’ = 0, Ø’ = 37˚.

A wide range of alternative forms of construction was considered to

identify a practical and economic approach. The nature of the glacial

materials is such that seepage control in the foundation is difﬁcult and

the material is not suitable for the watertight element of an embank-

ment. No other more cohesive materials were available locally.

The potential for very large boulders within the foundations ruled

out a sheet pile cut-off. A diaphragm wall approach might have been

more practical as such boulders could be excavated, but it would have

been extremely difﬁcult to form the bentonite slurry panels in places

particularly on the steep right abutment.

The alternative of an open cut excavation to rock was considered

for construction of a concrete cut-off wall or installation of a geosyn-

thetic polymer barrier (GBR-P) membrane. However, because of the

depth, the water bearing and cohesionless nature of the materials and

closeness to a live reservoir, the open cut approach was ultimately

considered to be too problematic and risky.

The most viable option was to control seepage by limiting the

hydraulic gradient across the foundation. The quantity of seepage

is not critical in itself as any ﬂow contributes to the required releases

for compensation and river regulation. Seepage through the founda-

tion materials could potentially cause internal erosion by suffusion

(migration of ﬁne particles through pores between the particles of the

coarser fractions) and/or piping (progressive erosion by end move-

ment within a tunnel or crack).

The usual approach to determining critical hydraulic gradients is to

consider upward ﬂow against gravity; however, gravity has little effect in

the horizontal ﬂow conditions applicable here. For suffusion, the critical

gradient for movement of ﬁnes in sandy gravel material under horizontal

ﬂow has been reported as 0.17 for stability indices (H/F)

min

of 0.36-0.5.

Critical gradients for piping are in the order of 0.14 for ﬁne sand with a

coefﬁcient of uniformity of 3 and 0.37 for gravel. The foundation materi-

als under consideration are broadly graded (coefﬁcient of uniformity gen-

erally about 30) and a gradient of 0.14 was adopted as being adequately

conservative to cover both mechanisms of internal erosion.

The method adopted for controlling the seepage gradient is by

laying a geosynthetic clay barrier (GBR-C) membrane directly on the

prepared foundation surface below the embankment. The GBR-C

membrane was anchored in a trench at the upstream end and cov-

ered generally by a 0.5m thick layer of sandy gravel protective layer,

with a 0.2m thick sand ﬁlter extending over the downstream end

Finding a replacement

Main photo left: Completed main embankment works and landscaping with

spillway rock armour being placed. Above: First operation of the spillway

26 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DESIGN

and the adjoining foundation surface. The sand ﬁlter was designed

to be effective against ﬁner material possibly transported by suffu-

sion, while the membrane protective layer was also designed to act

as a ﬁlter/drain against the bulk foundation materials where the

layer extended further below the rest of the downstream shoulder

of the embankment.

The downstream end was terminated at a location set back from

the toe of the downstream shoulder of the embankment. This would

ensure that there would be adequate overburden pressure to resist uplift

if migration of ﬁnes led to blockage of the end ﬁlter and a build-up of

pore pressures in this region. Because of the length of seepage path

required, this layout meant that the upstream end of the membrane

extended upstream of the embankment construction particularly in the

river bed region. The dam alignment had to be developed to keep the

membrane at a practical distance from the original dam.

The glacial materials were too sensitive to moisture content to

use for ﬁll, and in any case it would have been difﬁcult to obtain

permission for borrow pits because of the environmental sensitiv-

ity of the area. However, a new quarry had recently been opened

nearby to supply armour stone and rockﬁll for construction of a new

ferry terminal on the island of Raasay. The quarry is in Torridonian

Sandstone and the rock is highly suitable for such purposes.

The embankment layout adopted is a rockﬁll embankment formed

over the impermeable foundation membrane, sealed with a double-

sided textured geosynthetic polymer barrier (GBR-P) membrane on

the upstream face, and with a landscaped downstream shoulder zone

of spoil material from the excavations and surface stripping. The

GBR-P membrane is protected on both sides by thick non-woven

geosynthetic textiles (GTX) and the upstream surface protected by

stone cover layers and rip-rap.

The GBR-P face membrane is joined to the GBR-C foundation

membrane at the toe of the upstream slope by sandwiching the end of

the GBR-P between the foundation membrane and a second overlap-

ping layer of GBR-C. The GBR-P membrane is anchored at the crest

of the embankment by a concrete beam. The slope of the upstream

face is 2.5H:1V, and that of the downstream face of the rockﬁll is

1.5H:1V. The downstream face of the embankment has been land-

scaped to a ﬂat slope between 2.5 and about 4H:1V with mixed local

peaty material and spoil from the foundation excavations.

The original alignment of the left wing of the dam had been

straight, but a deep pocket of peat had been found here during the

site investigation. Detailed investigation of peat depths in the region

of the left abutment at the start of construction showed the peat was

more extensive and deep than had been anticipated. A ridge upstream

appeared to consist substantially of coarse moraine material and was

very exposed to wave action, and had already been rejected as a suit-

able alignment. A revised alignment was therefore adopted to termi-

nate on a low ridge downstream, after trial pits conﬁrmed suitable

foundation material occurred at a shallow depth

Foundation preparation consisted of stripping off surface peaty

deposits and loose orange-brown coarse granular moraine deposits

too close to the top level of the dense grey-brown ﬁner glacial till.

Some areas of more uniform silty sand occurred in pockets towards

the river bed region, and these were removed to expose broadly-grad-

ed till. The surface of the till was then excavated down to a sound

surface and the surface given a ﬁnal preparation immediately before

laying of the GBR-C. A sound, smooth surface was required for form-

ing a good interface with the GBR-C, and trials were carried out at

the start of laying to establish the best method of achieving this, given

the inevitable disturbance caused by mechanical cleaning and plant

trafﬁcking and the occasional need to make-up around protruding

boulders. It was found that a light rolling with a self-propelled vibrat-

ing roller reworked loose surface material satisfactorily and produced

a uniform smooth surface.

Control of the reservoir level during construction was by the

bottom outlet, supplemented as necessary by a new 600mm diameter

scour pipe. This had been connected early to the original higher outlet

pipe intake so that the scour could be available throughout the main

construction period. Constraints on the draw-down to store ﬂood

inﬂows were created by Lighthouse Caledonia’s requirements for a

reliable supply to their smolt facility. The loch overﬂowed only once

during construction, at a stage when there were no safety considera-

tions and contingency measures and pre-planning ensured that the

part-complete works were not signiﬁcantly damaged.

OVERFLOW DESIGN

The depth to bedrock in the river region was too great for easy foun-

dation of a concrete spillway here and a location further from the

river would cause excessive permanent disruption of the environmen-

tally sensitive area. A concrete spillway was in any case regarded as

Below: View of the completed dam; Right: Coire nan Arr from Kishorn.

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 27

DESIGN

undesirable visually, although the impact might have been reduced

by some form of masonry facing. Given the ready availability of large

rock, the spillway has therefore been formed by a rock-armoured

chute over the central section of the dam.

The rock armour design is similar in concept to the major spill-

ways constructed for the Khasab dams in Oman as presented in an

ICOLD paper by EH Taylor with which the author was involved

brieﬂy at the time of construction. The design followed the method

of Hartung and Scheuerlein as simpliﬁed by J Knauss, presented in

an ICOLD paper and also quoted in the latest edition of the Rock

Manual, CIRIA C683. For ease of construction and to provide

a margin for variations in the achieved standard, the layout was

developed to allow the rock to be placed with ‘natural packing’ (or

dumped embankment) according to the design method, although

placing was speciﬁed to be in accordance with ‘dense’ criteria of

current marine revetment speciﬁcations in order to be conserva-

tive. Again for ease of speciﬁcation and control, the rock size was

selected to be an industry standard 300-1000kg “heavy” grading

as set out in the Rock Manual. These criteria lead to the slope of

the spillway chute being 4H:1V. Such a ﬂat slope was in any case

considered desirable for ease of construction and tolerance of any

damage that might occur from local effects.

The spillweir size was determined for the 1 in 1000 year event,

requiring a 27m crest length and 1.2m ﬂood surcharge. With a nomi-

nal 0.4m wave surcharge allowance, the total freeboard allowance is

1.6m. The rock armour of the chute is sized for the limiting event of

the reservoir just overtopping the embankment, i.e a head of 1.6m on

the spillweir (a unit discharge of 3.2m

/sec).

The water ﬂowing in the chute inﬁltrates into the underlying rock-

ﬁll embankment and discharges generally into the stilling basin apron

area. This water could affect the stability of the rockﬁll if the induced

water table in the rockﬁll becomes too high. A thick non-woven geo-

synthetic (GTX) textile is placed between the rock armour and the

rockﬁll as a separation layer, and this layer also acts to limit the rate

of inﬁltration of the water. A window in the GTX layer has been

provided at the base of the chute to allow free drainage of the water

from the rockﬁll. This window is covered by a geosynthetic grid as a

high-permeability separation layer.

ENVIRONM ENTAL ASP ECTS

The principal environmental considerations when planning the works

have been to minimise disturbance of the area during construction,

to provide a shape and form that is as unobtrusive as possible, and

to reinstate the site, particularly the drainage systems and peat bog

areas, as close as possible to the original.

The original dam has been partly demolished and buried in an

upstream platform adjacent to the upstream face of the new dam.

Normally, the downstream face of a dam is made regular to allow

easy visual detection of any settlement or movement occurring, but

in this case a monitoring system on the concrete crest beam pro-

vides an adequate indication of the behaviour of the structural part

of the embankment. The downstream face of the dam has therefore

been landscaped to an irregular proﬁle with boulders to replicate

the moraines found elsewhere in the corrie. Vegetation is being re-

established by recycling the original topsoil and heather and grass

vegetation, supplemented by sowing of carefully selected grass seed.

Temporary drainage ditches formed around parts of the site have

been reinstated using the material removed.

OPERATIO N

The contract was negotiated with RJ McLeods Ltd, Dingwall,

Highland, and awarded under a NEC3 contract. Work commenced

on site in December 2008 and the dam was completed in May 2009.

The reservoir was ﬁrst ﬁlled above the storage level of the original

dam in May 2009. Seepage discharges initially showed some ﬁnes

and colour, but such bedding in soon ceased and all discharges are

now clear. Electrical piezometers in an array of the foundation show

consistent seepage conditions, with a signiﬁcant part of the total head

drop from the reservoir occurring upstream of the foundation mem-

brane. Total seepage ﬂows have been moderate.

The spillway ﬁrst operated in August 2009, and has performed well

on several occasions with heads of up to 0.3m.

Richard M Doake, Consultant, AECOM, 1 Tanﬁeld,

Edinburgh EH3 5DA, Scotland

Acknowledgements:

H Dalgety, Freshwater Manager Lighthouse Caledonia

P Nichol, Site Agent RJ McLeod

IWP& DC

Left from top to bottom: Laying of the GBR-C membrane; Rockﬁll embank-

ment under construction with foundation works in the river bed; Start of

construction of rockﬁll embankment, original dam on the left.

Landell Mills- Afghanistan Dam Engineer

Dam Engineer (3 year fixed term position)

Landell Mills Limited (www.landell-mills.com), a development

consultancy from the UK, is looking for a dam engineer for a

World Bank funded project in Afghanistan. The position will be

based in Kabul at the Ministry of Energy and Water on a project

to help the Ministry develop proposals for new water resource

projects across the country (mainly dams for hydro-power and

water storage) that are of a standard to be eligible for funding by

the international community. Tasks will entail:

• Preparing terms of reference for pre-feasibility and feasibility

studies, and follow-on design contracts for dam projects;

• Assisting the Ministry staff to oversee contractorsʼ work;

• Reviewing contractor reports, and providing recommendations

for improvement, so that; proposals are of the standard to be

submitted for funding from the international community;

• Preparing design manuals for contractors to follow;

• Provide on-the-job training to Ministry staff.

The position will be for three years, starting in April

(although slightly later starting dates will be accepted). While

this is a full-time position, the firm is negotiable on the amount

of leave/R&R time taken. Interested candidates should send

their CVs to kristenf@landell-mills.com

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The latest issue of

Dam Engineering

is on its way to subscribers.

This issue contains the following papers:

Field measurments of suspended sediments in a dam reservoir southwest of Iran by A Najaﬁ-Jilani

& M Monshizadeh

Field investigations have been carried out, in two six-month periods, on

an arch dam reservoir located in southwest Iran. The main goal of these ﬁeld

measurements was to study the characteristics of turbid ﬂow, which transports the main

suspended load of sediment into the dam reservoir.

The Jinnah Barrage rehabilitation project - prospects and concerns by Dr Zulﬁquar Ali Chaudhry.

A project to rehabilitate the world’s largest irrigation network is currently underway in Pakistan.

Asphalt concrete core of the Meijaran dam in brief by Alireza Shariﬁ Soltani & Siavash Litkouhi.

TThe lack of clay borrow sources

at the Meijaran dam site, along with a high annual precipitation rate in the area, led consulting engineers to replace the

clay core with an asphalt concrete core (ACC) at the rockﬁll dam.

A new approach for the determination of forces on a sheet pile cut-off wall by A K Singh & N K Samadhiya.

Exact seepage forces

have been computed, and active and passive earth pressures acting on a rigid cut-off wall have been determined.

Dam Engineering

is an academic journal, published quarterly by

International Water Power & Dam Construction.

It contains refereed papers on subjects related to all areas of dams and hydro power. At present there is no restriction on the

length of submitted papers.

For information on submitting papers, or for more details about subscribing to

Dam Engineering

, contact: Tracey Honney,

Dam Engineering, Progressive House, 2 Maidstone Road, Foots Cray, Sidcup, Kent DA14 5HZ

tel: +44 20 8269 7767 , fax: +44 208 269 7804, email: thonney@progressivemediagroup.com

New Issue of

Dam Engineering

WWW.WATERPOWERMAGAZINE.COM JANUARY 2010 29

DAM SAFETY

HE issue of reservoir-triggered seismicity (RTS) received

a great deal of attention worldwide following the earth-

quake which jolted Wenchuan County in Sichuan

Province on 12 May 2008, resulting in the death of over

60,000 people. A major question raised was whether the earth-

quake was related to the impoundment of the nearby Zipingpu

reservoir – or even the Three Gorges reservoir. There are a number

of different views on this issue and the authors believe the ongoing

discussion will emphasize the importance of the seismic safety of

dams, while promoting further research on the subject.

GENERAL CONCEPTS OF RE SERVO IR

TRIGGERE D SEISMIC ITY

Seismic events have occurred near large dam sites or in reservoir areas,

and may have been triggered by changes in the physical environment

as a result of impounding and operation of reservoirs.

For example, seismicity was observed following the 1929 impound-

ing of the Marathon reservoir in Greece (dam height of 60m).

Earthquake activity was also observed in 1935 after the impounding

of the Hoover dam in the US (dam height of 220m). Since then, over

100 large dams may have experienced RTS. There are still disputes

however as to when RTS has actually occurred.

In the ICOLD Bulletin on Reservoirs and Seismicity – State of

Knowledge (Bulletin 137, 2009) prepared by ICOLD’s Committee

on Seismic Aspects of Dam Design, 39 cases of RTS are presented.

Considering this, the number of RTS cases worldwide is very small

compared to the total number of reservoirs worldwide – with RTS

suspected in a higher portion of large dams. Most RTS events are

small magnitude events. However, there are a few cases with mag-

nitudes exceeding 5. So far, there are four major RTS events with a

magnitude over 6.0. They are: (i) 103m high Koyna gravity dam in

India (M=6.3); (ii) 120m high Kremasta embankment dam in Greece

(M=6.3); (iii) 105m high Hsinfengkiang buttress dam in China

(M=6.1); (iv) 122m high Kariba arch dam in Zambia (M=6.25). The

highest observed earthquake magnitude was 6.3.

The complicated mechanisms of RTS are not well understood and

may differ from case to case. The main reasons for this are the very

limited knowledge of the rheology of crustal material and groundwa-

ter movement under high pressures and high temperature conditions

in the hypocenter region. Therefore, in the absence of instrumen-

tal data, it is difﬁcult to establish and calibrate a physical model to

describe this complicated process. At present, this is studied using

statistical methods, computer simulations, and increased monitoring

of areas where RTS has been observed. The following two types of

earthquakes associated with reservoirs can be distinguished:

UÊÊ(i) Earthquakes of non-tectonic nature with shallow focus, which

are mainly related to stress adjustments in the foundation rock,

collapse of karst caves and mining tunnels, and mass movements.

These events of relatively small magnitude often occur shortly after

reservoir impounding or following sudden reservoir water level

ﬂuctuations. Generally, on the basis of existing case histories, these

earthquakes have magnitudes of less than 5 and are not harmful to

dams and the reservoir area.

UÊÊ(ii) Earthquakes of tectonic nature (referred to as RTS) caused by

seismogenic faults passing through or adjacent to the reservoir area.

The initial stress state must already be very close to failure so that a

minor change in stress or minor change in strength properties in a

fault plane caused by the water in the reservoir could trigger seismic

events. The epicenters of the foreshock with small magnitude of this

kind of earthquake are usually clustered around seismogenic struc-

tures. The magnitude of RTS events may gradually increase until

the main shock occurs. Following the main shock, the aftershocks

normally last for a certain period. As the process of reservoir water

inﬁltration into the rock stratum takes time, there is normally a time

lag between the main shock and when the reservoir water reaches

its highest level. However, depending on the local conditions, dif-

ferent foreshock, main shock and aftershock patterns are possible.

Following the 12 May 2008 Wenchuan

earthquake in China, Chen Houqun, Xu

Zeping and Li Ming discuss the question

of whether large reservoirs can trigger

strong earthquakes

The relationship

between large

reservoirs and

seismicity

Zipingpu dam

30 JANUARY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

DAM SAFETY

The magnitude of the main shock is the main concern for dam

engineers. Until now, the four RTS earthquakes with magnitudes

over 6.0 belong to this foreshock-main shock-aftershock type. This

kind of RTS event is a major concern for engineers.

In the studied literature, these two types of earthquakes are also con-

sidered as endogenous and exogenous reservoir earthquakes.

The accumulation of strain energy of tectonic earthquakes origi-

nates from the seismogenic structure of the earth. It is the potential

seismic source irrespective of the existence of a reservoir. Compared

with the huge amount of energy released by a strong earthquake,

the impact of reservoir impounding on the strain energy of the seis-

mogenic faults is very small. The impounding of a reservoir can only

trigger an earthquake when the accumulated strain energy of the seis-

mogenic fault is close to the critical stress state.

The total seismic energy includes the following parts: fault rupture

energy, wave radiation energy, and heat caused by friction and inelas-

tic processes. The intensity of earthquake ground shaking depends

on the energy transferred by earthquake wave radiation. The energy

of seismic waves is measured in terms of magnitude. Theoretically,

the maximum magnitude of RTS events is determined by the upper

bound magnitude of the seismogenic faults. No earthquake can be

triggered by a reservoir with a magnitude higher than that of the

spontaneous earthquake. As for the determination of the upper

bound magnitude, the commonly applied methods in China include:

(1) the method based on historic earthquakes and tectonic structure,

(2) the method based on paleo-earthquake and statistics of active

fault parameters and, (3) the method of synthetic structure analogy.

It is still not entirely possible to reliably predict strong earthquakes,

and the present methods to determine the upper bound magnitudes of

seismogenic faults are imperfect. The Wenchuan earthquake occurred

at the section of the Longmenshan Fault where the historically recorded

seismic activity is relatively low. However, it has the most signiﬁcant

compression deformation and stress concentration. The Longmenshan

Fault has the lowest long-term deformation rate compared with the

other major faults of the Qinghai-Tibetan plateau, and the recorded

maximum magnitude in this area was 6.5. In general, the understand-

ing of the seismotectonic processes of low-activity faults – which have a

long-time energy accumulation that could ﬁnally lead to a strong earth-

quake – is limited. After the Wenchuan earthquake, seismological and

geophysical experts reanalyzed the related data and increased the upper

bound magnitude of the Yinxiu-Beichuan fault area from 7.0 to 8.0.

Although seismological and geophysical experts have conducted

many studies and investigations and have carefully determined potential

seismic sources and their upper bound magnitudes, you cannot exclude

the possibility that the maximum historically recorded magnitude could

be exceeded. This is an important problem in the assessment of seis-

mogenic faults, irrespective of whether there are reservoirs or not.

Although the role of the water in the reservoir hydrogeology for RTS

still needs further studies, it is widely accepted at present that the inﬁl-

tration pore pressure and the added weight of the reservoir water are

the main factors affecting any seismicity patterns. Obviously, even for

a dam storing a reservoir with a depth of over 100m, the impact of the

added weight of the reservoir water on the crustal stress ﬁeld near the

hypocenter – which may be located at a depth of more than 10km – is

negligible in comparison with the deadweight stresses of the rock mass.

But when water from the reservoir inﬁltrates into the rock mass at the

depth of several kilometers, the pore pressure will reduce the effective

stress and the shear strength parameters in the fault will be reduced.

Thus the total shear strength of the fault is reduced and may initiate

a crack propagating along the faults and thus trigger an earthquake.

Such a model is well understood by dam engineers, although the water

inﬁltration process is still unclear and the mechanical properties of the

crustal materials at a depth of say 10km, as well as the initial stress state

originating from the underlying tectonic processes, is unknown. Based

on the concept of water inﬁltration the following factors for triggering

RTS events may have to be considered [4]:

UÊÊAs the impact of reservoir impounding on the crustal stress ﬁeld

at the source of earthquake is very small when compared with the

stress ﬁeld of the seismogenic fault, the reservoir impounding can

only trigger an earthquake which is already at critical condition.

UÊÊFor RTS to occur, certain seismic and hydro-geological conditions

must be satisﬁed, which include: (1) the existence of seismic struc-

tures in or adjacent to the reservoir area, (2) the seismic structure is

close to failure before reservoir impounding, and (3) the existence

of the hydrogeologic condition for the inﬁltration of reservoir water

into the deep rock stratum.

Obviously, there are very few large reservoirs that possess such con-

ditions. This is the reason why only a few cases of tectonic reservoir

earthquakes with high magnitude has occurred at the many high dam

projects constructed around the world.

In fact, a reservoir-triggered earthquake (RTE) has no substantial

difference from a spontaneous earthquake. The magnitude of its main

shock could not exceed the upper limit magnitude of the spontaneous

earthquake of the fault. The process of reservoir water inﬁltration causes

a time lag between the reservoir impoundment and the occurrence of the

main shock of RTS. The focal depth of RTE is relatively shallow.

The area affected by reservoir water inﬁltration cannot exceed

the ﬁrst dividing ridge between valleys where the ground water

table exceeds the reservoir level. This could determine the spatial

distribution of RTS.

As for the type of fault, from the analysis of the impact of reservoir

water on the stress state of the fault plane, the strike slip and normal

faults are more prone to be triggered compared with thrust fault [7].

Figure 1: Water level in the reservoir (top) and seismic activity observed in

Zipingpu reservoir are (N: number of events; M: magnitude)

890

820

750

Water level /m

160

Frequency /N

4.00

3.00

2.00

1.00

0.00

-1.00

Magnitude /M

2004 2005 2006 2007 2008

Year

Yinxiu Town, Wenchuan County,

epicenter of the earthquake