““Research in dam Research in dam breaching"breaching"

Sílvia Amaral

PhD Student (1st year)

December, 14th 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 WorkMain 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 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 (0.80<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

Latera

l

spillw

ay 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

Laboratorial Facility (4/4)Advantages and limitationslimitations

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

D1

D4

D5

D3

D2

corte A-APERFIL LONGITUDINAL PELO DESCARREGADOR DO CANAL C1/ C1-R

ESTRUTURA DECONTROLE DOESCOAMENTO

5

RESERVATÓRIO DE ALIMENTAÇÃO CANAL C1/ C1-R ESTRUTURA TERMINAL

parede existente

13

4

9

B.I.

11

3

7

0 2m1

12.5

(fila sem meios-blocos)

0.67

0.60

0.30

0.08

0.20

brita 10/20

areia grossa

geotêxtil 200g/m2

lintel de betão

PERFIL TRANSVERSAL PELO CANAL C1/ C1-R

0.50

LEGENDA

1 - Laje de betão armado, esp.=0.30m. 3 - Dreno de pé-de-talude do aterro (brita) 4 - Aterro compactado de solos "argilo-silto-arenosos". 5 - Camada de infiltração (areia grossa), esp.=0.30m. 7 - Bloco deflector do canal C1. 9 - Camada de drenagem do canal C1, esp.=0.20m.11 - Bloco de betão prefabricado do canal C1, emédia=0.05m.13 - Peça amovível de sobreposição à última fila de blocos do canal C1.15 - Soleira de guiamento à entrada (1/4 tubo PVC DN400).19 - Tubo de PVC DN110 para recolha do caudal de (9).20 - Tubo de PVC DN110 para recolha do caudal de (3).

B.I. - Blocos instrumentados com transdutores de pressão (3 unid. por bloco).D1 a D5 - Transdutores de pressão intersticial instalados no interior do aterro. - fila de medição da concentração de ar e velocidade do escoamento, utilizando uma sonda de fibra óptica.

1

parede de alvenaria

parede dealvenaria comjuntas abertas

10%

tubos de PVC para passagemde cabos dos blocos instrumentados

1

1.5

fila 32

fila 25

fila 18

fila 10

fila 32

19 20

Instrumentação:

15

0 1m0,5

bloco de betãoprefabricado

parede deperspex

(canal C1-R)

PORMENOR DO CANAL

(Dimensão dos degraus)

areia grossa, esp.=0.05m

brita 10/20, esp.=0.15m

0.25

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.

1xx

xx

xz

2xx1xx2xx

Geotechnical Phenomena

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

xx

xz

1xx2xx

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?