16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford1 Geomorphic dam-break floods: Near-field and...

16 May 2002 Spinewine - IMPACT Workshop 1 - Wallingford 1

Geomorphic dam-break Geomorphic dam-break

floods:floods:Near-field and far-field Near-field and far-field

perspectivesperspectives

Benoit Spinewine, Yves ZechUniversité catholique de Louvain


Presentation outlinePresentation outline

• Introduction and motivation• Near-field perspective

Key aspectsKey aspects: inertial effects, coupled description, vertical movements

Experimental workExperimental work: dam-break & breaching; past, present & future…

Theoretical frameworksTheoretical frameworks: strengths & limitations

• Far-field perspective Key aspectsKey aspects: 2D-H, bank erosion, frictional resistance, debulking Experimental workExperimental work: channels with erodible bed & banks Numerical modelNumerical model: bank failure mechanism, 1D & 2D-H models

• Conclusions


IntroductionIntroduction

• Problems to be solved Geomorphic changes induced by rapid flood waves Mobilised sediments affect wave dynamics

• Motivation Lessons from the past: real cases of catastrophic

failures Increased risks in the future

• Approaches Near-field vs. far-field Theoretical, experimental and numerical Collection of data from real cases



• Real cases Lawn Lake dam failure (1982)

U.S. Geological Survey



• Real cases Lawn Lake dam failure (1982) Lake Ha!Ha! Break-out flood (1996)








• Conclusions


Near-field perspectiveNear-field perspective

• Introduction Phenomenological descriptionPhenomenological description

• Complex initiation of movement, including vertical and pressure-related effects

• Intense scouring of the sediment bed• Bulking of sediments into the flow• Remobilisation of sediments trapped in reservoir

Key aspects of near-field modellingKey aspects of near-field modelling• Account for sediment inertia• Multi-phase approach• Debris-flow-like rheology• Erosion viewed as a phase change• Vertical movements and non-hydrostatic pressures

2D-V model ??



• Experimental work Idealised dam-break experiments

• Chen & Simons (CSU, 1979) Sudden removal of a

submerged barrage




• Chen & Simons (CSU, 1979)• Capart & Young (NTU, 1998)

Sudden dam-break wave Flat sediment bed Light pearls (s = 1.048) Particle tracking




• Chen & Simons (CSU, 1979)• Capart & Young (NTU, 1998)• Spinewine, Zech & Capart (UCL, 2000)

Heavier PVC pellets(s = 1.54)

Flat bed Upward-moving gate



• Louvain experiments 10 cm of water Mosaics assembled from different runs




• Chen & Simons (CSU, 1979)• Capart & Young (NTU, 1998)• Spinewine, Zech & Capart (UCL, 2000)• Fraccarollo & Larcher (UDT, 2000)

PVC pellets (s = 1.54) Downward-moving gate




• Chen & Simons (CSU, 1979)• Capart & Young (NTU, 1998)• Spinewine, Zech & Capart (UCL, 2000)• Fraccarollo & Larcher (UDT, 2000)• Leal, Ferreira et al. (IST, 2000)

Natural sand (d50 = 1 mm) Various water and sediment levels



• Experimental work Idealised dam-break experiments Future work during IMPACT project

• Explore various water and sediment levels Preliminary results obtained at UCL laboratory (hs= 6; hw=

10 cm)




• Explore various water and sediment levels• Explore extended grain size distributions (multimodal,

graded)





graded)• Explore changes in geometry

Widening, contraction, chute, sloping channel





graded)• Explore changes in geometry• Improve quantitative measurements

Velocity profiles within water and sediment phases 3D imaging (concentration estimate, flow topography)





graded)• Explore changes in geometry• Improve quantitative measurements• Other types of ‘near filed’ fast transients involving

intense sediment movements Dike-breaching experiments



• Experimental work Dike-breaching experiments



• Theoretical frameworks

Traditional alluvial hydraulics & why it fails…• Slowly evolving alluvial flows• Solid transport as a passive phase

Key features for geomorphic flood routing :• Transients shock-capturing schemes (also

sediments!)• Bulking of sediments couple water and sediment

flows



• Extended shallow water framework (Capart, 2000)

Layered flow description Basic assumptions on velocities & concentrations Erosion & deposition viewed as a phase change of

bed material (transition from solid-like to fluid-like behaviour)




Two continuity equations

One momentum equation

euhxt

h

euh

xt

hs

s

bws cr 1

w

b

bssss

uee

r

x

zhrhghgrhguhrh

xuhrh

t

21

2212

212

w

b

bssss

uee

r

x

zhrhghgrhguhrh

xuhrh

t

21

2212

212

et

zb




Semi-empirical closure relation for the erosion rate

• Sediment mobility parameter m• Relaxation time tr

Physically-based relation expressing a shear-stress imbalance across the bed interface (Fraccarollo-Capart 2002)

• in the flow region, s is given by a Chezy-type relation

• In the bed region, b derives from a Coulomb yield criterion

)(1 2

smr

b humtt

ze

)()1(

1bs

mwb ure




Numerical results• Finite volumes• Roe method for evaluation of convective fluxes




Limitations• Equal velocities in the clear water and slurry layer

Pure water

transport layer

dep

th

velocity




Limitations• Equal velocities in the clear water and slurry layer

Let the two moving layers flowing at different velocities uw & us

Unlocked, but still coupled velocities (not independent)

One more momentum equation for the slurry layer

h w

h s

z b

u w

u s

w

s

b




Limitations• Equal velocities in the clear water and slurry layer• Equal concentration in the slurry layer and static bed

???




Limitations• Equal velocities in the clear water and slurry layer• Equal concentration in the slurry layer and static bed• Inherently limited by shallow-water assumptions

2D-V model ???








• Conclusions


Far-field perspectiveFar-field perspective

• Introduction Phenomenological descriptionPhenomenological description

• Lateral effetcts: bank failures and channel widening• Progressive debulking of the sediment phase• Deposition in downstream reaches• Formation of alluvial fans• Blocking of tributaries, formation of natural dams

Key aspects of far-field modellingKey aspects of far-field modelling• 2D-H modelling for reproducing actual valley

topography• Geometrical singularities (bends, widening, chutes, …)• Discrete bank failure criterion



• Erodible channel dam-break wave experiments

Gate

erodible channel

rigid bank

Gate

erodible channel

(a) (b )

h = 0.2 m0

1 60

6 0

2 00 1 60

4 5°1 60

6 0

2 56 1 34 1 05

5 0°

h = 0.14 m0

(a) (b )

Gate

erodible channel

rigid bank

Gate

erodible channel

(a) (b )

h = 0.2 m0

1 60

6 0

2 00 1 60

4 5°1 60

6 0

2 56 1 34 1 05

5 0°

h = 0.14 m0

(a) (b )

• Le Grelle & Jonard (UCL, 2001)

Half-channel experiments Top-view and side-view Time evolution

• Paquier & Poncin (UCL, 2000) Full-channel experiments Post-flood valley topography



• Erodible channel dam-break wave experiments• Paquier & Poncin (UCL, 2000)

Full-channel experiments Post-flood valley topography

« Panel » digital imaging initial post-flood

Flow



• Erodible channel dam-break wave experiments

• Le Grelle & Jonard (UCL, 2001)

Half-channel experiments Top-view and side-view Time evolution

(b )

(a)

side

top

Flow



• Numerical modelling Bank failure mechanisms

• Traditional models: continuous process, excess bank shear stress

• Not applic. to sudden surges with abrupt changes of water levels !



• Numerical modelling Simple model accounting for discrete mass failure events

• Distinct angles of repose for submerged and emerged banks• Failure triggered whenever and wherever critical angles

exceeded• Easily extended to 2D by invoking conic failure surfaces

8 5 °

3 7 °

5 0 °

q s

c e

c s h

i



• Numerical modelling Simple model accounting for discrete mass failure

events• Distinct angles of repose for submerged and emerged

banks• Failure triggered whenever and wherever critical angles

exceeded• Easily extended to 2D by invoking conic failure surfaces• Extension: distinguish between critical and residual angles

Failure triggered when critical failure is exceeded Residual angle is adopted after failure

z z s 0

u n s t a b l e m e t a s t a b l e s t a b l e

s u b m e r g e de m e r g e dh w 0

( )( ) + ( )x x y y 0 0

2 2 1 / 2



• Numerical modelling 1D model with separate hydr. and sed. routines (Spinewine

et al 2002)

• Hydrodynamic: finite volume, Roe• Sediment transport: non-equilibrium, bed and submerged

banks• Bank failure

Two angles of repose (emerged and submerged) Failed material uniformly distributed over wetted cross-section



• Numerical modelling 2D-H model, coupled phases (Capart & Young 2002)

• 2D extension of the set of extended shallow water equations

Two layers, two velocities

• Bank failure Distinct failure and residual angles Biconic failure surfaces Failed material assumed to

liquefy instantaneously After failure, the flow operator

simply takes over

( a )

( b )

( c )

( d )



• Numerical modelling Comparisons of 1D simulations with half-channel

experiments• (blue = experimental, red = numerical)• Elevation profiles at successive times



• Numerical modelling Comparisons of 1D simulations with half-channel

experiments• (blue = experimental, red = numerical)• Width evolutions in time at a given x-section








• Conclusions


ConclusionsConclusions

• Conclusions Geomorphic impacts divided in near-field and far-field

features Near-field: bulking of sediments, inertia, vertical effects,

… Far-field: decreasing velocities, lateral effects,

debulking Modelling

• Near-field : extended shallow water framework, 2DV• Far-field : 2DH extension, bank failures

Experiments

• Perspectives

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford1 Geomorphic dam-break floods: Near-field and...

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