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Research in dam breaching"
Sílvia Amaral PhD Student (1st year) December,
14th 2009
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Framework of the study
Although overtopping is the most common cause of
failure in recent dams, there is still an evident
need of reliable prediction tools to assess the
flood impacts in river floodplains following dam
failure Dam failure by overtopping have been
object of several laboratory studies (Vaskinn et
al. 2004) that have provided useful data like
discharge hydrographs for model validation
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Our believes
We believe that the improvement of the current
ability for reliable prediction of breach
formation by overtopping and its evolution in
earth embankments can only be achieved by
synthesizing hydrodynamic and geotechnical
phenomena into detailed conceptual models
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Laboratorial WorkMain Objectives

• Improving current ability to perform a more
  reliable prediction of breach formation by
  overtopping and its evolution in earth dams
• advancing the state-of-the-art in the
  characterization of the hydrodynamic and
  geotechnical phenomena involved in the evolution
  of a breach in earth dams

Provide empirical data that can be used to access
the most important parameters that influence
breach formation and flow hydrograph shape
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Methodology

• The laboratorial work will encompassing breach
  simulations in homogeneous and zoned earth dams

• The empirical data will be provided by the
  large-scale (0.70 m and 1.4 m tall) dam breach
  tests whose laboratorial conditions will be
  closely controlled
• the morphological time evolution of the breach
• strain and pressure fields in the body of the
  embankment
• flow discharge (direct measurement)

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Laboratorial Facility (1/4)Main characteristics

• 1 storing tank with approximately 90 m3 of
  maximum stored volume
• 1 pumping circuit with a flow controller with 200
  l/s with a maximum capacity (2 pumps - 100 l/s
  each)
• 1 pool representative of a reservoir Vmáx 50
  m3
• Prepared to perform earth dam breach tests with
  6,65 m wide embankments, variable heights
  (0.80lthlt1,30m) and variable upstream and
  downstream bank slopes (between 1V1.4H-1V3.0H)
• a 14,5 m length flume downstream the dam toe
• with a constant width of 6,65 m in the first 11,5
  m and a
• convergent width in the last 3 m (between 6,65
  and 1,70m).
• a settling basin, located at the end of the flume
  with 1,7 m width, 4,5 m length and a maximum
  water/sediments height of 0,60 m

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Laboratorial Facility (2/4)Pictures
6.65 m
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Laboratorial Facility (3/4)Pictures
Perspective View
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Laboratorial Facility (4/4)Advantages and
limitations

• Pore pressure measured directly
• geotechnical instabilization should depend on
  reduction of suction head this may be directly
  assessed.

• Embankments compaction energy defined by
  geotechnical engineers
• experimental studies have not attended to some
  geotechnical aspects as compaction energy of the
  embankment layers (one, to this date)

• 20 cm granular bed under the embankments
  compacting against a rigid surface modifies the
  layer compaction characteristics in a way that
  the first 2 compactions layers (near the rigid
  surface) wouldnt behave like a real dam

• Direct measurement of the flow discharge laser
  visualization fro breach evolution, flow
  elevation measurements and synchronized velocity
  measurements

• Variable input discharge Allows for virtually
  increasing the size of the reservoir

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Laboratorial Facility (4/4)Advantages and
limitations

• Embankments dimensions (0,70-1,40m height)
  Taller dams would be desirable

• Synchronization of all equipment
• such work can only be performed within a
  multidisciplinary research group

• Variable input discharge limited to the pump
  capacity and to small kinetic head

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Instrumentation and methods

• Direct measurement of the breach evolution
• underwater camera collecting a footage of the
  trace generated by a 0.2 w laser

laser
camera
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Instrumentation and methods

• Synch flow elevation and velocity measurements
• velocity UVPs
• elevation level acoustic probes

laser
acoustic probes
UVPs
camera
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Pilot Facility (1/2)

• Before performing the experimental campaign of
  tests on the main facility it is envisaged that a
  2,9 m high homogeneous embankment, already
  existent at LNEC, should be induced to fail by
  overtopping

• Our goals are
• To win experience with the collecting and
  acquisition data equipment
• To perform a preliminary dam breach test to help
  refining the main facility similarity conditions
  and choosing the main parameters of dimensional
  analysis and defining the experimental procedure.

• It will allow to win sensitivity to some
  parameters and
• To use the knowledge acquired in the improvement
  of the main facility characteristics and
  measuring methods

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Pilot Facility (2/2)
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Data Interpretation

• Scale issues, how to deal with breaking of
  hydraulic and geotechnical similitude when the
  scale of the grain is not the scale of the
  embankment?

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Data Interpretation

• Reduction of the specific weight of the bank
  material is the solution.

• What about the CLAY CORE?

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Geotechnical Phenomena

• Scale issues on geotechnical similitude.

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Geotechnical Phenomena

• Reducing the specific gravity will help Tests
  on centrifuge?

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Main uncertainties

• Instrumentation - placement of pore pressure
  probes
• - synchronization of instrumentation

• Bank material pumice? plastic?
  (advantages/disadvantages)

• Boundary conditions infinite reservoir? test
  several reservoir sizes?