“Research in dam breaching" Sílvia Amaral PhD Student (1 st year) December, 14 th 2009

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 Framework of the study Unfortunately these studies failed to produce detailed phenomenological information on the breaching processes Description of the geotechnical discrete failure episodes Interaction between hydrodynamic erosion and geotechnical failure (Wahl, 2004)

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 Theoretical Work to guide the empirical work Laboratorial Work to collect data for the empirical characterization of the main hydrodynamics and geotechnical phenomena Computational Work

Laboratorial Work Main Objectives Provide empirical data that can be used to access the most important parameters that influence breach formation and flow hydrograph shape  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

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)

Laboratorial Facility (1/4) Main characteristics  1 storing tank – with approximately 90 m 3 of maximum stored volume;  1 pumping circuit with a flow controller with 200 l/s with a maximum capacity (2 pumps l/s each);  1 pool representative of a reservoir – V máx ≈ 50 m 3 ;  Prepared to perform earth dam breach tests with 6,65 m wide embankments, variable heights (0.80<h<1,30m) and variable upstream and downstream bank slopes (between 1V:1.4H-1V:3.0H);  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

Laboratorial Facility (2/4) Pictures Reservoir inlet Frontal View zone of the embankment 6.65 m

Laboratorial Facility (3/4) Pictures Lateral spillway Slope to guide sediments to the collection basin Instrumentation car Perspective View

Laboratorial Facility (4/4) Advantages and limitations  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) wouldn’t behave like a real dam  Direct measurement of the flow discharge – laser visualization fro breach evolution, flow elevation measurements and synchronized velocity measurements  Pore pressure measured directly – geotechnical instabilization should depend on reduction of suction head – this may be directly assessed.  Variable input discharge – Allows for virtually increasing the size of the reservoir

limitations 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

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

Instrumentation and methods  Synch flow elevation and velocity measurements velocity: UVPs elevation: level acoustic probes laser camera UVPs acoustic probes

Pilot Facility (1/2) Our goals are: i)To win experience with the collecting and acquisition data equipment; ii)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. 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 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 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

Pilot Facility (2/2)

Data Interpretation o 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?

Data Interpretation o Reduction of the specific weight of the bank material is the solution. o o What about the CLAY CORE? o

Geotechnical Phenomena o Scale issues on geotechnical similitude.

Geotechnical Phenomena o Reducing the specific gravity will help… Tests on centrifuge?

Main uncertainties o Instrumentation - placement of pore pressure probes - synchronization of instrumentation o Bank material – pumice? plastic? (advantages/disadvantages) o Boundary conditions – infinite reservoir? test several reservoir sizes?