Water Power and Dam Construction. Issue October 2010
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WWW.WATERPOWERMAGAZINE.COM OCTOBER 2010 21
CONSTRUCTION
the joints, and then a round stick machine is used to roll twice on
the joint. The contraction joints are lled with non-woven fabrics,
causing the construction joints to be weakly connected induced
joints that are not only good for temperature stress releases but
also enhance the integrity of the dam.
The nonlinear time domain seismic analysis of a dam with 10
transverse joints shows that by this measure the integrity of the dam
can be increased signicantly. Compared with the completely cut-
off contraction joints, the maximum displacement of the dam along
the river direction was reduced from 8.49cm to 7.64cm, and the
maximum displacement of the dam across the river direction was
reduced from 5.02cm to 2.09cm. The loads considered in the calcu-
lation include self-weight, hydrostatic pressure, sediment pressure,
uplift pressure, wave pressure, and the design earthquake effect.
Figure 5 shows the construction process of the contraction joints.
By this special joint, the temperature cracks can be avoided, and the
speed of construction can also be increased.
Earthquake-resistant steel bars design
The parts of the dam where the shape changes during strong ground
shaking are the weak regions. The maximum dynamic tensile stress
is generally 2 MPa and may reach 3 MPa in some areas; particularly
on the surface and the corner edges of the dam. According to the
180-day strength of the C9020 concrete, the dynamic tensile strength
that approaches 3.15MPa meets the requirements in most parts of the
dam (Hong, 2006).
In addition, in zones of high dynamic tensile stresses steel reinforce-
ment is provided. Two layers of steel bars with a diameter of 28mm
(steel bars with a diameter of 36mm are embedded at the bracket
support) are provided with a spacing of 20cm. The layout of the
reinforcement is shown in Figure 6.
The stilling basin arrangement
The main bedrocks at the dam site are basalt and amygdaloidal
basalt, although single rock has high strength, developed joints and
fractures which can cause low integrity. The bedrock in the riverbed
has joints lled with chlorite, therefore the scour resistance of the
rock is low. In addition, there are some loose accumulation bodies,
B
2
and B
20
, distributed over the downstream side of the dam on the
left bank. B
2
is in the plant bedrock mass and the excavation slope
at the end of the stilling basin. B
20
is in the zone from which energy
is dissipated to release, and its edge extends to the river.
Operation of the spillway on the right bank will cause scouring
in the impact area of the water jets and a mound would be created,
which can affect the safe operation of a hydropower station.
Setting up a high lip at the end of the crest spillway
The maximum head for the Jinanqiao hydropower station is 111m.
The maximum ow velocity at the bottom in the stilling basin is
40m/sec. According to the hydraulic model tests, the ski-jump with
a height of 6m causes energy dissipation by air entrainment, and
the velocity of the ow at the bottom is reduced to 16m/sec in the
stilling basin.
Strengthening measures of the dam foundation
There are some factors that negatively affect the sliding resistance
in the dam foundation. For example, the deformation modulus
and the strength of the jointed rock mass are low for the plant-dam
foundation. Because there is a faulted bed plane on the epidote
quartz that is distributed over the left bank of the dam foundation,
the deep and shallow-layer sliding stability needs to be studied. In
the nonlinear nite element analysis, stability safe factors for deep-
layer sliding are 1.12 and 1.18 for usual static loads and seismic
loads, respectively. The stability of the dam foundation satises
the Chinese Code requirements. The horizontal displacement at
the dam crest is less than 5cm for usual load combinations includ-
ing self-weight, hydrostatic pressure, sediment pressure, uplift
pressure and wave pressure and is less than 8.5cm in the seismic
load combinations.
The physical and mechanical properties of the fractured jointed
rock mass are reduced because of the unloading and reloading
effects in the excavation process. In order to increase the deforma-
tion modulus and strength of the fractured/jointed rock mass, the
high-pressure consolidation grouting method is used. The maxi-
mum grouting pressure is 2 MPa, and ordinary cement and ground
ne cement are chosen for grouting.
Treatment of the accumulation body on the left bank
There are three collapse accumulations distributed around the dam.
The collapse accumulation body B
1
on the left bank is distributed
upstream of the dam. It is about 490m away from the dam, and
its volume is about 246×10
4
m
3
. The distribution elevation of the
front edge is 1400m. The second collapse accumulation body B
2
is
distributed downstream of the dam. It is about 250m away from the
Rock with
strong clay
material
Divide line of
resist body
1#slide mode
Right
slide
surface
2#slide mode
t
1b
Rock with
strong clay
material
1#slide mode
Right
slide
surface
Divide line of
resist body
2#slide mode
t
1b
Powerhouse
Rock with
strong clay
material
Divide line of
resist body
1#slide mode
Right
slide
surface
2#slide mode
t
1b
Rock with
strong clay
material
1#slide mode
Right
slide
surface
Divide line of
resist body
2#slide mode
t
1b
Powerhouse
Figure 3: View of dam construction from downstream, powerhouse is on the
right and crest spillway on the left (25 February, 2009)
Figure 4: Sliding failure modes of dam and powerhouse. (a) Dam and pow-
erhouse are separated and are treated individually in the sliding stability
analysis; (b) Dam and powerhouse are combined
22 OCTOBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
CONSTRUCTION
dam, and its volume is about 84×10
4
m
3
. The distribution elevation
is about 1370-1495m. The third collapse accumulation body B
20
is also distributed downstream of the dam. It is about 450m-650m
away from the dam and its volume is about 226×10
4
m
3
; in addi-
tion, its maximum depth is 65m, and the lowest part of the leading
edge extends to the river.
According to the analysis of the anti-slide stability, the main treat-
ment measure of B
1
is monitoring and protecting the slope. The meas-
urements of B
2
and B
20
are wall cutting and unloading. Prestressed
anchor rope and concrete bafe are used to protect the slope toe of
B
2
. For B
20
, a concrete retaining wall is built at the foot of the slope
to protect the downstream canal.
CONCLUSIONS
The following innovative design and construction measures
ensure the construction quality of the Jinanqiao hydropow-
er station: (i) The grouting construction joints are designed
between the plant and dam parts to enhance the earthquake-
resistant ability of the dam; (ii) In order to improve the lateral
stability of the dam, the depth of the construction joint is only
2/3 of the dam thickness in each roller-compacted layer, and the
joints are lled with non-woven fabrics. This treatment causes
the construction joints to be weakly-connected induced joints,
which are not only good for temperature stress release but also
enhance the integrity of the dam; (iii) Steel bars are embedded in
high tensile stress zones to ensure dam safety.
Yong-Wen Hong, Senior Engineer, General Design Engineer,
Jinanqiao Hydropower Station. Department of Engineering
Mechanics, Hohai University, Nanjing 210098, China;
HydroChina Kunming Engineering Corporation, Kunming
650051,China. E-mail: hong_yw@khidi.com.
Cheng-Bin Du, Professor, Dept. of Engineering Mechanics, Hohai
Univ., Nanjing, 210098, China. E-mail: cbdu@hhu.edu.cn
This work was supported by the funds from the National
Basic Research Program of China (973 Program, Grant No.
2007CB714104) and the open research fund is supported by the
China Institute of Water Resources and Hydropower Research
(Grant No. 2008538613). The authors are grateful to Dr Martin.
Wieland, Chairman of ICOLD Seismic Committee (Pöyry Energy
Ltd., Zurich, Switzerland) for his helpful suggestions.
IWP& DC
References
Hong Y. W.(2006). “Design of RCC Gravity Dam of Jinanqiao Hydropower
Station.”Water Power,32(11),54-56.
Yuan, J. W., Du C. B., Hong Y. W.(2008). “Static and dynamic anti-sliding
stability of the powerhouse section of a gravity dam under combined effects
of powerhouse and dam.” J. of Hohai University, (Sci&Tech),36 (2), 29-33.
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Figure 5: Construction of the contraction joint over 2/3 of the dam thickness
(10 January, 2008)
Figure 6: Layout of steel reinforcement in typical dam sections. (a) Dam section at
power intake; (b) Over-flow section of dam; (c) Non-overflow section of the dam
WWW.WATERPOWERMAGAZINE.COM OCTOBER 2010 23
DESIGN
A
RESEARCH project to produce small, standardised
pump-turbines rather than larger, site-specic unique
designs for hydropower schemes is being undertaken as
an in-house, spin-off project by Gravity Power, LLC, the
venture-backed US rm focused mainly on developing a modular,
deep shaft technology for the energy storage market.
Design development of the smaller-than-typical pump-turbine concept
is nearing completion and preparations are being made to have a scale
model produced and tested by the Laboratory for Hydraulic Machines,
Ecole Polytechnique Federale de Lausanne (EPFL), in Switzerland.
Gravity Power, LLC, says that the standardised production of
the pump-turbines would support the modular approach of its core
technology: the deep shaft, piston-like acting Gravity Power Module
(GPM) system. Typically, clusters of GPM units would be construct-
ed, each unit with its own pump-turbine.
Beyond their use in GPM systems though, the company believes
there could be opportunities elsewhere in the pumped storage market
for such small, standardised units.
Following the tests by EPFL, the model pump-turbine will be
transported to the US and installed for the trials as part of an insitu,
scaled GPM unit to be constructed near its ofces in Santa Barbara,
California. The company is seeking a patent for the GPM system, vari-
ations of which could be on land or sea, it envisages.
As the young company continues its R&D effort and financing
rounds, it is receiving a lot of interest in the potential of the GPM system
from China, India and South Africa, says Executive Vice President, Chris
Grieco. Gravity Power, LLC, was launched a year ago as a spin-out busi-
ness from LaunchPoint Technologies, Inc, a US venture engineering and
hi-tech incubator company.
GPM CONCEPT – REVISED
The concept of the land-based GPM system is to have a pair of under-
ground vertical shafts – a main shaft and a narrower, return ow
shaft – that form a water-ow circuit. In the sealed system, a concrete
mass – the ‘piston’ – drifts down under gravity to push out water from
the main storage shaft and up the return shaft to the pump-turbine,
which is mounted at the ground surface, to generate electricity. The
ow is discharged back into the main shaft, on top of the ‘piston’.
At periods of cheaper electricity on the market, the cyclical GPM
system would see the unit work in reverse mode, pumping water back
down, injecting it below the concrete mass and so raising the ‘piston’
to a desired elevation. The GPM system would then be primed with
potential energy, awaiting the next call to generate.
Initially, the GPM concept was envisaged as only a single excavated
shaft that would hold both the main storage tube (with stored water
and the concrete ‘piston’), and at least one return pipe. The modular
concept would see clusters of such single shafts sunk to deliver the
required power capacity, and the storage volumes, that would be
sought by different markets and client (see IWP&DC March 2010).
However, earlier this year the concept layout of the GPM system
was modied – the single shaft system was completely replaced with an
arrangement based instead on a pair of excavated shafts, one large and
the other small, for the storage and return ow functions, respectively.
For commercial operations, the size of the largest shaft (storage) – up
to 6m diameter or up to 10m diameter – is dependant on project pur-
pose. Neither these nor their depths were changed by the revisions.
For ancillary services to an electricity market, such as grid support
and power provision, the shaft would be up to 6m diameter and the
return shaft 2m wide. The shafts would be 500m deep.
Much larger shafts and depths are envisaged for the ‘peaking plant’
GPMs, which would supply steady energy for relatively longer periods
and a cluster could have total installed capacities of easily more than 1GW.
A GPM unit in such a cluster would have a main shaft of 10m diameter, a
3m wide return pipe and depths of 1km-2km are envisaged.
PUMP-TURBINE RESEARCH & DEVELOPMENT
The R&D effort for the new pump-turbines is focused solely at this
stage on mathematical modelling with computational uid dynamics
(CFD) analyses, and the in-house research team is led by the com-
pany’s chief scientist, Dr Jingchun Wu. In the near future the design
will be tested in an external hydraulic laboratory and then take part
in eld trials run by the company.
Dr Wu formerly managed Hitachi’s team on turbines and pump-tur-
bines, and worked on hydropower plants designed by Yangtze Water
Conservancy Committee, China. More recently, he undertook pump
design work for LaunchPoint Technologies, and the work is unrelated
to the R&D efforts at Gravity Power, LLC, or the GPM concept.
As part of the research into the small, standardised pump-turbine,
Gravity Power, LLC has had discussions with leading hydroturbine man-
ufacturers to seek optimal production solutions, and explore potential
collaboration, the company says. But details have not been disclosed.
While the pump-turbine is not integral to the GPM concept it
is a strategic complementary offering in the company’s package.
Therefore, as GPM systems would operate at constant head either
pump or turbine mode, the company says the pump-turbine units
can be designed to run at its sweet spot for best combinations of
efciency, cavitation risk, etc.
With patents sought for the GPM concept, and being explored
separately for the advanced shaft boring technology, it is not how-
ever anticipated that the pump-turbine R&D will necessarily result
in bonus intellectual property (IP) for the company. The main dif-
ferential from a commercial perspective, therefore, is volume sales:
the company says no-one has had an approach to pump-turbines
involving design and production of different standard units to sell not
merely as a few or a dozen but in the hundreds, or more.
The key, therefore, is marrying CFD into a series of manufacturing-
style standardised designs for a selection of pump-turbines. The CFD-
based design is almost done, the company says, and EPFL will soon
receive the data to build and then test the model. As further R&D is
undertaken, the company says that experience of the industrial pump
sector will also be drawn upon to help look at high-volume ows for the
pump-turbine systems.
Following the hydraulic tests by EPFL, the model will be among the
key components of the GPM installed in Santa Barbara to examine
their operational performance. The insitu trial will see the shafts exca-
vated by auger to about 60m depth, which is the limit of the available
conventional drill. The installed power capacity of the scaled GPM is
to be less than 100kW.
The company anticipates that it will take about 18 months, once it
gets underway with the hydraulic model tests in Switzerland and site
preparation in California, to nish the design, insitu installation and
scaled GPM trails.
A US rm looking at a shaft-based system as a new approach to pumped storage has
modied the concept and is also designing small, standardised pump-turbines for mass
production. Report by Patrick Reynolds
Standard approach to turbine R&D
IWP& DC
24 OCTOBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
INSPECTION
E
NGINEERS, geologists, and seismologists have spent recent
years evaluating and verifying the earth conditions under
and around Martis Creek dam near Truckee, California, US,
to better understand its seismic and seepage challenges and
ensure that the dam continues to operate effectively and safely.
In the most recent effort, the Sacramento District of the US Army
Corps of Engineers (USACE) and its consultants sought to better
assess the causes of seepage and hydrologic deciencies and further
assess subsurface anomalies found during previous subsurface inves-
tigations. The team combined advanced remote sensing technology,
namely light detection and ranging (LiDAR), with more traditional
geomorphic and paleoseismic trenching techniques to gain a clearer
denition of these anomalies. The team knew LiDAR could help strip
away the vegetation to produce a so-called bare earth model in hopes
this would allow them to visualise these features more effectively. In
doing so, they discovered something unexpected: a previously unrec-
ognised fault extending across the North Tahoe Basin, now named
the Polaris Fault.
DAM DESIGN AND CONSTRUCTION
USACE named Martis Creek Dam one of its highest risk dams in its
inventory due to its history of excessive seepage and the consequences
of ooding to downstream communities if it failed. Owned and oper-
ated by the USACE Sacramento District, the dam and reservoir are
located approximately 4.8km east of Truckee on Martis Creek, a
tributary to the Truckee river.
Constructed in 1972, the 34m high zoned, rolled earthll dam has
a maximum storage capacity of 25,163m
3
x10
6
at the spillway crest
and 42,678m
3
x10
6
at the maximum spillway design ood pool eleva-
tion. The dam’s function is to provide ood control and future water
supply, but heavy seepage coincident with higher pool elevations
during test reservoir llings have prevented USACE from allowing
the dam to achieve its full design potential.
The dam consists of three zones:
r 1) An upstream impervious zone.
r 2) A downstream zone (Zone 2) identied as random ll.
r 3) A vertical and horizontal drain.
The original dam design did not include a seepage cutoff barrier
but instead designed and constructed an upstream clay blanket within
the reservoir and upstream toe of the embankment. The embankment
foundation was stripped to a nominal depth of about 30cm on the
abutments, and the soft alluvium in the valley oor was removed
to a maximum depth of about 1.8m and an average depth of about
0.9m (USACE, 1972), to reach a low permeability foundation layer
referred to as the Blue Silt zone.
Between the upstream impervious zone and clay blanket, the poten-
tial for excessive seepage was thought to be resolved. These seepage
control measures were apparently inadequate as uncontrolled seepage
was judged excessive during test reservoir llings at various times
from construction through to 1995. Modications to seepage control
measures were made from 1972 to 1995 but were not completely
successful.
PREVIOUS INVESTIGATIONS
Geologic and foundation investigations were performed during the
dam design and again in the 1980s to understand the cause of seep-
age deciencies. During the most recent round of studies, detailed
mapping and geomorphic reconnaissance coupled with hollow-stem
auger and sonic boreholes at various locations around and on the
dam were performed. The US Geological Survey (USGS) and subcon-
tractor have also conducted geophysical investigations using seismic,
resistivity, self-potential and time-domain electromagnetic methods.
Boreholes were drilled to conrm and improve the existing strati-
graphic model of a relatively continuous and simple sequence with a
Blue Silt sandwiched between more granular, sandy layers above and
below. But several boreholes in the left abutment of the dam either
found the Blue Silt to be missing or at a distinctly different elevation
than surrounding layers. When USACE rst mapped a fault trace near
the dam using LiDAR imagery, changes to the drilling programme
and the USGS geophysical survey helped conrm the fault.
LIDAR STUDIES
While the scientists and engineers focused on characterising the dam’s
foundation, it became evident that better quality base mapping was
required. This terrain model would also be useful for the hydrologists
to evaluate the true capacity of the watershed and Martis Creek dam’s
capacity to store and discharge this volume of water. They were also
studying the consequences of a failure of the dam known as a breach
condition to see how this might affect a large population in Reno and
Sparks downstream along this narrow Truckee river drainage.
The hydrologists needed a good quality terrain model to reect the
ground surface digitally for use in hydrology and hydraulic computer
models. To do this, they obtained LiDAR imagery from a local water
purveyor and began studies based on this imagery.
LiDAR is a technology that uses focused beams of light (laser
pulses) and measures the time it takes the light pulse to go from the
sensor to an object where it reects back to the sensor. Using the
speed of light, the travel time is converted to a distance. With airborne
systems whose ying height is known, the elevation of the ground
can be calculated. Modern LiDAR scanning systems are capable
of emitting more than 150,000 pulses per second. When combined
with aerial photography, LiDAR is a powerful tool for mapping and
studying the earth’s surface.
As the team interacted to collectively resolve the dam’s issues, a
very important discovery was made by USACE. Its geologists used the
bare-earth data to produce a three-dimensional terrain model using
specialised LiDAR software. This data represents those data points
that were the latest returns recorded, meaning they had travelled the
farthest and are the ones most likely to have been reected off of
the ground surface. By focusing on these data points, vegetation is
effectively removed and the modeled surface is that closest to repre-
senting the ground. The result was amazing. When the intensity data
(effectively a measure of the strength of the return pulse) was looked
at, painted stripes and numbers on the runway at the airport near the
dam could even be seen.
In the terrain model produced they observed several linear features
that are indicative of faults in the vicinity of the East Martis Creek Dam.
Martis Creek Dam in the US is classied as a high-risk structure due to its history of
excessive seepage and ooding risks associated with dam failure. During dam inspections,
LiDAR technology helped to reveal a previously unrecognised active fault which could be
the seismic source for future earthquakes. By Bruce Hilton, Ronn Rose and Lewis Hunter
Fault finding at Martis Creek Dam
WWW.WATERPOWERMAGAZINE.COM OCTOBER 2010 25
INSPECTION
Once focused in on that area, a sharp vegetation lineament was also
observed in the aerial photography. This nd was signicant because
the recent seismic hazard assessment for the dam had not indicated
any Quaternary-age faults in the immediate area, and the lineaments
identied in the LiDAR imagery projected towards the dam.
This result led the team to acquire the remaining LiDAR data from
the utility district and analyse them for more evidence of faulting.
With these new data the broader context of what the team was look-
ing at became apparent.
A linear feature that cuts across a broad alluvial terrace below
the dam was identied. This feature continued to the north where
it crossed the Truckee river and offset at least two generations of
terrace risers. Projecting south towards the dam, a subtle feature was
identied cutting across the terrace riser below the spillway and pro-
jecting between the spillway and left abutment of the dam. Features
indicative of the fault included aligned hills, ridgelines, depressions
and mole-tracks (characteristic sequences of small ridges and depres-
sions). Some of these features have only 0.3-0.6m of relief, but the
LiDAR was able to depict them.
Each feature was carefully digitised in GIS and set the stage for
extensive eld geomorphic studies to see and understand these fea-
tures on the ground surface. By comparing aerial photos with the
LiDAR data, the team was able to ll in many of the gaps in the
LiDAR lineament with features that are not reected in the topogra-
phy but result in changes in vegetation cover.
Once the team was able to ground map and examine these
numerous features along the lineament, they selected a few can-
didate sites for trenching. These paleoseismic trenches are not
the usual trenching and logging. They are logged in tremendous
detail searching for rotated cobbles and smeared clay threads
that may represent faulting. Among these candidate sites, the
team selected the best sites based on right-of-entry and access
along with the locations most likely to reveal strong evidence of
the ground disturbance along this lineament. One site was where
a strong vegetation contrast was visible in both aerial imagery
and on the ground. The second site was higher near Martis Creek
and was coincident with a well-dened scarp strongly evident in
the LiDAR data.
EXPLORING LINEAMENT INTO THE GROUND
In the fall of 2008, the team began trenching at the two locations
simultaneously. A team of geologists excavated and logged the sub-
surface condition in detail at both paleoseismic trench sites. Early in
the logging at both sites, the evidence was clear.
In both cases, well-dened shears in the ground offset otherwise
continuous layers of the fan deposits, and both coincided precisely
with the surface feature that had been mapped at that location. The
lineament was clearly a fault, or perhaps two different strands of the
same fault that are parallel.
Not all faults are considered a problem. Historically, geologists
have based the interpretation of the hazard of fault rupture to be
consistent with their age or activity. Younger faults are thought to be
active, and older faults are thought to be dormant or inactive. In the
case of USACE and at the time these studies were performed, their
regulations dene this activity as 35,000 years. If the fault appears
to have moved recently the fault needs to be considered as a location
where ground rupture may occur and is also recognised as a potential
source of an earthquake.
The fan deposits were sampled and examined to attempt to deter-
mine an age, but no good quality results could be obtained. Samples
were taken and tested using mass radiocarbon dating techniques and
yielded less than 1000-year ages but could not be relied on because
of the averaging that occurs within the samples contaminated by pos-
sibly younger roots and soils. Earth deposits are also qualitatively
dated-based on specialised methods of assessing their colour, climate,
and presence of clay and mineralisation that are strong indicators
of age. In both trenches, the fault was interpreted to be possibly as
young as 10,000 years, or Holocene.
Two distinct earthquake events can be identied in Trench 1, with
the most recent event rupturing the basal contact of the uppermost
soil unit. Apparent vertical offset during this event is about 12.5cm,
whereas the largest earthquake event produced an estimated 0.3m
of apparent vertical offset. The horizontal offset could not be deter-
mined for either event, but is likely to be signicantly larger than
the vertical offset. The fault exposure in Trench 2 offsets volcanic
bedrock and the overlying fan deposits also in an apparent down-to-
the-west sense of displacement.
The fault that had been previously mapped across the North
Tahoe Basin is now known to be an active fault capable of ground
rupture and needs to be considered as another seismic source for
future earthquakes. The fault was named the Polaris fault after a
nearby historic locality along the Truckee river, north of Martis
Creek dam.
Studies continue to further assess the Polaris fault regarding its true
age, recurrence interval (frequency of earthquakes), and relationship
to other nearby fault systems. This will require identication and log-
ging of several additional paleoseismic trench sites so that multiple
earthquake ruptures can be directly observed and dated. These stud-
ies are currently progressing.
Bruce Hilton is a principal engineering geologist for
Kleinfelder. BHilton@Kleinfelder.com.
Ronn Rose is an engineering geologist with the
Sacramento District of the US Army Corps of Engineers.
His current position is that of the district dam safety
programme manager.
Lewis Hunter is the senior geologist for the Sacramento
District, US Army Corps of Engineers.
IWP& DC
40
F1
F2
F3
60
70
80
100
90
10
30
N24ºW80ºSW
(Across trench)
N22ºW78ºSW
(Across trench)
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Log of paleoseismic trench wall showing fault zone.
26 OCTOBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
SPILLWAYS
U
LLEY reservoir is located to the south of Rotherham
town centre and was originally built to provide a water
supply to the town. It was constructed between 1871 and
1875 and comprises an earth embankment with a puddle
clay core. The embankment is 205m long and the maximum height
is 16m retaining a reservoir of 580,000m
3
. It originally had two
spillways which were located one on each mitre, discharging into
the natural watercourse at the toe of the embankment.
The area was subject to coal mining and in 1943 a new spillway was
built downstream of the left mitre. The arisings were placed on the
crest of the embankment to raise it to counter the effect of mining sub-
sidence. In 1969, it was realised that the 1943 works had not raised
the clay core so works were undertaken to add ‘plastic concrete’ to the
top of, and keyed into, the clay core. As a result of the various works,
the operational top water level was progressively lowered.
The integrity of the dam was threatened during heavy rain on 25
June 2007 when storm ows in one of the spillways damaged the
spillway walls. The spillway that failed was located on the left hand
mitre and had a chute gradient of 1 in 5 followed by a level section
which discharged into the natural watercourse. The invert comprised
a series of steps made up of stone slabs with a small lip along the
downstream edge and incorporating a low ow pipe within the lip.
The spillway had stone masonry walls made up of tapered units to
provide an appearance of ashlar masonry. The stability of the tapered
units depended on a good packing of mortar behind the visible face.
When originally constructed, the mortar would have been a lime
mortar and over the years this washed out leaving an open structure
which was less stable than when originally constructed.
During the incident, a single spillway took all of the ow from the
reservoir. The water in the chute did not ow smoothly as it was dis-
rupted at each lip. The effect was to increase turbulence and increase
the air content of the ow causing the ow to bulk as it reached
Rehabilitation work at Ulley dam in Rotherham, UK, was ofcially opened on 25 June
2010, exactly three years to the day since heavy rain caused the original overow structure to
collapse and left the Victorian dam in danger of failure. At a cost of almost US$6M, innovative
engineering works have included the addition of a new spillway in the centre of the dam.
New spillway for Ulley dam
Below: Remedial works (copyright Arup)
WWW.WATERPOWERMAGAZINE.COM OCTOBER 2010 27
SPILLWAYS
the bottom of the chute. Photographs taken before failure showed
the water to be splashing outside the chute and well above the side
walls. The capacity of the downstream level section was less than the
volume being delivered down the chute. Water levels in the spillway
rose at the base of the chute until the walls were overtopped.
PROGRESSIVE FAILURE
It is not clear what happened next. It is possible that the turbulent
ow within the spillway was enough to destabilise the masonry walls,
or that overtopping resulted in the ground outside the walls to be
washed away and the walls being pushed over. It is known that the
start of the problem was at the toe. A wall progressively failed work-
ing upstream along the chute and the unprotected embankment ll
behind the walls was washed away by the water. The spillway was
founded on natural bedrock which did not erode and this contributed
to containment of the scouring process.
The supervising engineer called for a section 10 inspection to be
carried out under the Reservoirs Act and the remedial works were
designed to address the conclusions and recommendations of the sec-
tion 10 report. In the interests of safety, areas of focus included work
to the spillways, core, rip-rap and drawdown facility plus other more
minor matters. The design process took each structural element in
turn and determined what was present on site and compared it to
what was needed to allow the reservoir to operate safely as a category
A reservoir.
SPILLWAY DESIGN
The rst step was to determine the design ood and hence the design
outow to be accommodated by the spillway. A ood study was under-
taken to assess design ows and also generate the level rise in the res-
ervoir to inform design decisions on the top level of the waterproong
(the puddle clay core). There was no information about the clay core
and a geotechnical investigation was undertaken to obtain the material
parameters of not just the core but the shoulder material as well.
The site is within a country park and a decision was taken that
the rehabilitated reservoir had to be able to operate without a wave
wall. There had not previously been a wavewall and to introduce one
would have detracted from the setting as a country park. The crest
was also identied as being very narrow and it was agreed to increase
the crest level by 0.5m to provide a safer working platform. The spill-
way width was determined to limit the rise in water level during a
PMF event to below the reduced crest level taking into account wave
run-up. This produced a weir length of 20m and maintained the nal
water level at the same level as existed prior to the June 2007 incident.
The new spillway has to accommodate a PMF of 125m
3
/sec.
Several options were evaluated for the location of the new spill-
way. There were complex issues associated with putting it in either
abutment. On the left, it would clash with the existing 1943 spillway
which still provided a useful function for removing modest storm dis-
charges. On the right, there was steep sidelong ground which would
have presented major geotechnical challenges to insert a new spill-
way. Also on the right, the width of land available was limited which
would have required a less efcient weir arrangement. The decision
was made to line the new spillway up with the receiving watercourse.
This avoided any bends, provided an efcient weir and removed the
water quickly away from the toe of the embankment by being the
shortest route from the water body to the receiving watercourse.
By placing the spillway in the embankment, the existing crest foot-
path was severed and a new timber clad footbridge was provided to
maintain public access. The spillway is of reinforced concrete con-
struction with joints to allow a limited amount of rotation to take
account of any future long term embankment movements. The view
was taken, based on monitoring information, that only nominal con-
solidation was likely to occur given the embankment was built in
1875 and load was being removed to construct the spillway. The
joints incorporate waterbars with dedicated underdrains at each joint
to enable any seepage to be monitored and prevent seepage permeat-
ing into the earth ll.
The clay core had been raised in 1969 using a ‘plastic concrete’. The
geotechnical investigation showed that this concrete had hydrated in
places and was therefore brittle and unable to act with the underlying
clay core as an effective waterproong barrier. Also, by hydrating, it
developed the ability to bridge over the core; four small seepages were
observed in the scour hole during the incident and the ‘plastic concrete’
in the corresponding point in the core had hydrated. In addition, the
quality of construction of the ‘plastic concrete’ was not perfect because
trench sheets had been left in place which had corroded.
The geotechnical investigation comprising boreholes and lab test-
ing found that the original clay core was in better condition than
expected and that the shoulders provided a good lter. It also found
that the embankment is founded on broken rock which acts as a
large drain under the embankment and reduces the hydraulic gradi-
ent across the core. The designers considered the risk of hydraulic
fracture and piping failures.
Denitive answers were difcult to nd using the laboratory test
results for particle size distribution and permeability but, based on
published sources for other embankments, it was possible to identify
a probability of failure using an event tree. This was found to be low.
In light of this, a decision was taken not to touch the clay core but to
just replace the ‘plastic concrete’, using a bentonite slurry. The ‘plastic
concrete’ was excavated from the core and replaced by the bentonite.
To ensure an efcient watertight barrier, the bentonite was keyed into
the clay. Also, it was important to ensure that the interface between
the bentonite and the new spillway would remain watertight. This
was achieved by sloping the outer face of the spillway structure and
incorporating a waterbar in the outer spillway face.
The drawdown system had used the original pipe work. There was
corrosion to the iron pipes and an analysis of the system showed that
the drawdown capacity was limited. The new design requirement
was to be able to achieve a drawdown rate of 1m per day during Q10
inow conditions. This means that for 90% of the time, a 1m per day
rate could be achieved.
Various options were considered. Since the reservoir was not used
for water supply, then there was only an operational requirement for
very limited drawdown of the reservoir. However should there be an
emergency, then the reservoir would need to be drawn down signi-
cantly. There was also the problem that the reservoir is now home to
many sh and there are environmental constraints on emptying the
reservoir. The design team wanted to limit any effect on the water-
proong system from the construction of a new draw-off pipe and this
led to the decision to retain the existing pipe route through a down-
stream tunnel and avoid new penetrations through the waterproong.
The downstream portal of the tunnel was appropriately located to
allow the drawdown pipe to discharge into the new stilling basin.
On the upstream side of the crest, the decision was made that for
the normal operational range of water levels, ows could be control-
Above: Remedial works included the construction of a new spillway, a new
scour pipe, erosion protection to the embankment as well as improvements to
the clay core of the dam wall (copyright Environment Agency)
28 OCTOBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
SPILLWAYS
led using a gravity system but for the rare occasions when a greater
drawdown might be required, this could be achieved by using a
siphon system. The designed drawdown was one pipe which can be
operated under either gravity or siphon conditions, depending on
reservoir water levels.
The original rip-rap was limited in extent and new provision was
based on the ood study output. Wave calculations were used to
identify a stone size for the armour stone which in turn was bedded
on a graduated layer of lter stone. The stone used for the rip-rap is a
dense Pennine sandstone obtained from near Hudderseld.
The rehabilitation scheme started in September 2009 and was com-
pleted in June 2010. The detailed investigations and design work,
including the building of a working model on the new spillway, was
carried out by consultants Ove Arup. Ringway contractors carried
out the spillway work.
The authors are Dave Crook, Supervising Engineer and
Jon Horrocks, Associate Director, Arup, 13 Fitzroy Street,
London W1T 4BQ, England.
www.arup.com
Above, top: The start of the problem occurred at the toe of the dam where a
wall progressively failed and unprotected embankment fill behind the walls
was washed away by the water (copyright Environment Agency)
Above: After the incident: Rotherham Borough Council used around 2600
tonnes of limestone to shore up the erosion on the dam wall to make it safe
on a temporary basis (copyright Arup)
IWP& DC
WWW.WATERPOWERMAGAZINE.COM OCTOBER 2010 29
SPILLWAYS
MONTICELLO DAM
Monticello Dam is located on Putah Creek near Winters in Solano
County, California, US. It is a medium concrete-arch dam which
impounded Putah Creek to cover the former town of Monticello and
ood Berryessa Valley. This created Lake Berryessa, the second larg-
est lake in California. Water from the reservoir is supplied mostly
to the North Bay area of San Francisco. The Monticello dam power
plant was built at the dam in 1983 and has three generators.
The dam is noted for its classic uncontrolled spillway which has
an outside diameter at the lip of 22m, slowly narrowing to 8.5m
at the exit. Technically known as the morning glory spillway it is
locally, somewhat more colourfully, known as ‘The Glory Hole’.
When the dam reaches capacity, the spillway swallows water at a
rate of 1382m
3
/sec.
The dam and spillway were constructed between 1953 and 1957.
Other notable glory hole spillways include one at Whiskeytown Lake,
near Redding also in Northern California.
LADYBOWER DAM
Ladybower reservoir is situated at the bottom of a cascade of three
reservoirs on the River Derwent in the Upper Derwent Valley in
Derbyshire, UK. Construction of the dam started in 1935. Filling
of the reservoir began in March 1943 but the works weren’t fully
completed until 1945.
The reservoir has a surface area of 2.1km
2
with a direct catchment
of 124.3km
2
and an indirect catchment of 72km
2
from the River
Noe diversion.
Ladybower dam was constructed as an earthwork embankment
with a puddle clay core and upstream stone protection. The length of
the dam is 380m and it was originally constructed to a height of 43m.
However, in 1999, it was raised to a height of 46m to compensate for
past settlement and to meet the latest ood estimation standards.
When the reservoir is full, surplus water spills into 2x24m diameter over-
ows which discharge into the river. The largest recorded overow was on 4
Dec 1960 at 0.6m above the overow, equating to 96m
3
/sec.
IWP&DC presents a pictorial on the
morning glory type spillway, as featured at
Monticello dam in the US and Ladybower
dam in the UK
Glorious spillways
Overflow at the Labybower reser-
voir in 1960. Inset: The overflow
spillway at Ladybower (1999)
Above, top: Aerial view looking upstream at Monticello Dam and Lake Berryessa
on Putah Creek near Winters, California (Photo: J.C. Dahilig May 1975).
Above: The morning glory type spillway passing 6m
3
/sec at Lake Berryessa
formed by Monticello dam. As part of the Solano project constructed by the
US Bureau of Reclamation in northern California, it is designed to irrigate
approximately 304km
2
of land and furnish municipal and industrial water to
the principal cities of Solano County. (Photo: J.C. Dahilig March 1975).
Below: The first bucket of concrete being placed at Monticello Dam on 9
August 1955 (Photo: E.S. Ensor)
IWP& DC
30 OCTOBER 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
SPILLWAYS
M
EDITERRANEA delle Acque (MdA) operates
the Val Noci reservoir in Italy as a drinking water
supply. A recent estimation of the ood hydrograph
has shown that the peak discharge exceeds the exist-
ing discharge capacity of the three spillway structures. An addi-
tional morning glory spillway was therefore planned to increase
the hydraulic capacity. The Laboratory of Hydraulics, Hydrology
and Glaciology (VAW) of ETH Zurich, Switzerland, was commis-
sioned to investigate the design of the new spillway in a physical
model with a scale factor of 25.
The model tests of the initial design showed that mean minimum
pressures may drop below vapour pressure during the transition
from free to pressurised ow condition in the vertical shaft, result-
ing in a phase change from water to vapor. As this may cause struc-
tural damage due to strong vibrations as well as limited spillway
capacity in the prototype, VAW optimised the design by widening
the vertical shaft and installing a throttle at the end of the bottom
bend. Furthermore, overow piers were added to the morning glory
crest to ensure radial inow conditions and avoiding air-entraining
vortex formation. The required discharge capacity of 75m
3
/sec is
ensured and extreme pressure uctuations are limited. However,
in the upper section of the vertical shaft, sub-pressures may locally
drop below vapor pressure during the transition phase from free to
submerged inow.
PROJECT OVERVIEW
Water is stored by a 56m high masonry gravity-arch dam with crest
elevation at 540.5masl, harnessing the valley of the Noci River with a
catchment basin of 7.6km
2
and creating 3.4Mm
3
of live storage (gure
1). The dam project dates back to 1922. Excavations began in 1923
and the construction works for the dam ran from 1925 to 1931.
Three spillway structures (gated chute spillway, middle and bottom
outlet) with a total spilling capacity of 240m
3
/sec at the maximum
expected ood level (MFL) of 538.97m asl were used for ood evacu-
ation prior to the design ood update (gure 2). The chute spillway
is located at the right abutment and is equipped with a radial gate of
10m wide and 4m high. The discharge capacity of the fully opened
chute spillway totals 190m
3
/sec at MFL. The middle outlet consists
of a bell-shaped valve (Verrina valve) with a capacity of 34m
3
/sec at
MFL, conveying the water through the former diversion tunnel. The
bottom outlet with a discharge capacity of 16m
3
/sec at MFL consists
of a breglass cased steel pipe with an inner diameter of 1m, operated
by a cone valve situated at 491m asl.
A new estimation of the design ood hydrograph performed by
the Italian National Hydrographic Service in 2003 assessed a maxi-
mum inow into the reservoir of 280m
3
/sec and a total ood volume
of about 3.8Mm
3
for an event with a return period of 1000 years.
Because this discharge exceeds the existing total spilling capac-
ity, MdA planned to erect an additional morning glory spillway.
The concept aimed at increasing the spillway capacity using a reli-
able spillway structure which operates automatically and requires
minimum maintenance while minimizing impact on the existing
structures.
The new morning glory spillway is located at the right valley
ank, close to the gated chute spillway, and uses the existing middle
outlet tunnel for a time and cost saving design. The project includes
a new stilling basin to ensure adequate energy dissipation at the
tunnel outlet.
The routing of the design ood hydrograph through the reservoir
with the chute spillway, the bottom outlet and the morning glory
spillway in operation (the middle outlet is not considered), predicts a
peak rate of 75m
3
/sec released through the morning glory spillway at
a reservoir water level of 538.9m asl (design discharge).
The Laboratory of Hydraulics, Hydrology and Glaciology (VAW)
of ETH Zurich, Switzerland, was commissioned to investigate the
designed morning glory spillway. The physical model tests aimed at
evaluating the discharge capacity, the approach ow conditions and
the ow conditions in the vertical shaft in order to test the overall
hydraulic behaviour of the new structure and to improve its operat-
ing efciency and safety. The investigation also included tests on the
modied middle outlet tunnel and the new stilling basin. The model
Mattia Pinotti and Adriano Lais give an indepth look at physical model investigations which
were carried out on the morning glory spillway at Val Noci dam in Italy
Investigating Val Noci
Val Noci Reservoir
Middle outlet
Morning glory
spillway
Service
building
Spillway tunnel
Gravity - arch
dam
Gated chute
spillway
Middle outlet
tunnel
Bottom
outlet
Stilling
basin
Figure 1: Val Noci reservoir and dam
Figure 2: Plan view of the Val Noci dam with existing spillway structures
(grey) and projected morning glory spillway (red)