16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford1 Geomorphic dam-break floods: Near-field and far-field perspectives Benoit Spinewine, Yves Zech Université catholique de Louvain

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford2 Presentation outline Introduction and motivation Near-field perspective Key aspects Key aspects: inertial effects, coupled description, vertical movements Experimental work Experimental work: dam-break & breaching; past, present & future… Theoretical frameworks Theoretical frameworks: strengths & limitations Far-field perspective Key aspects Key aspects: 2D-H, bank erosion, frictional resistance, debulking Experimental work Experimental work: channels with erodible bed & banks Numerical model Numerical model: bank failure mechanism, 1D & 2D-H models Conclusions

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford3 Introduction 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford4 Introduction Real cases Lawn Lake dam failure (1982) U.S. Geological Survey

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford5 Introduction Real cases Lawn Lake dam failure (1982) Lake Ha!Ha! Break-out flood (1996)

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford6 Presentation outline Introduction and motivation Near-field perspective Key aspects Key aspects: inertial effects, coupled description, vertical movements Experimental work Experimental work: dam-break & breaching; past, present & future… Theoretical frameworks Theoretical frameworks: strengths & limitations Far-field perspective Key aspects Key aspects: 2D-H, bank erosion, frictional resistance, debulking Experimental work Experimental work: channels with erodible bed & banks Numerical model Numerical model: bank failure mechanism, 1D & 2D-H models Conclusions

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford7 Near-field perspective Introduction Phenomenological description Phenomenological 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 modelling Key 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 ??

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford8 Near-field perspective Experimental work Idealised dam-break experiments Chen & Simons (CSU, 1979)  Sudden removal of a submerged barrage

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford9 Near-field perspective Experimental work Idealised dam-break experiments Chen & Simons (CSU, 1979) Capart & Young (NTU, 1998)  Sudden dam-break wave  Flat sediment bed  Light pearls (s = 1.048)  Particle tracking

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford10 Near-field perspective Experimental work Idealised dam-break experiments Chen & Simons (CSU, 1979) Capart & Young (NTU, 1998) Spinewine, Zech & Capart (UCL, 2000)  Heavier PVC pellets (s = 1.54)  Flat bed  Upward-moving gate

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford11 Near-field perspective Louvain experiments 10 cm of water Mosaics assembled from different runs

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford12 Near-field perspective Experimental work Idealised dam-break experiments 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford13 Near-field perspective Experimental work Idealised dam-break experiments 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 (d 50 = 1 mm)  Various water and sediment levels

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford14 Near-field perspective Experimental work Idealised dam-break experiments Future work during IMPACT project Explore various water and sediment levels  Preliminary results obtained at UCL laboratory (h s = 6; h w = 10 cm)

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford15 Near-field perspective Experimental work Idealised dam-break experiments Future work during IMPACT project Explore various water and sediment levels Explore extended grain size distributions (multimodal, graded)

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford16 Near-field perspective Experimental work Idealised dam-break experiments Future work during IMPACT project Explore various water and sediment levels Explore extended grain size distributions (multimodal, graded) Explore changes in geometry  Widening, contraction, chute, sloping channel

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford17 Near-field perspective Experimental work Idealised dam-break experiments Future work during IMPACT project Explore various water and sediment levels Explore extended grain size distributions (multimodal, graded) Explore changes in geometry Improve quantitative measurements  Velocity profiles within water and sediment phases  3D imaging (concentration estimate, flow topography)

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford18 Near-field perspective Experimental work Idealised dam-break experiments Future work during IMPACT project Explore various water and sediment levels Explore extended grain size distributions (multimodal, graded) Explore changes in geometry Improve quantitative measurements Other types of ‘near filed’ fast transients involving intense sediment movements  Dike-breaching experiments

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford19 Near-field perspective Experimental work Dike-breaching experiments

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford20 Near-field perspective Experimental work Dike-breaching experiments

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford21 Near-field perspective 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford22 Near-field perspective 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)

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford23 Near-field perspective Extended shallow water framework (Capart, 2000) Two continuity equations One momentum equation

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford24 Near-field perspective Extended shallow water framework (Capart, 2000) Semi-empirical closure relation for the erosion rate Sediment mobility parameter m Relaxation time t r 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford25 Near-field perspective Extended shallow water framework (Capart, 2000) Numerical results Finite volumes Roe method for evaluation of convective fluxes

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford26 Near-field perspective Extended shallow water framework (Capart, 2000) Limitations Equal velocities in the clear water and slurry layer Pure water transport layer depth velocity

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford27 Near-field perspective Extended shallow water framework (Capart, 2000) Limitations Equal velocities in the clear water and slurry layer  Let the two moving layers flowing at different velocities u w & u s  Unlocked, but still coupled velocities (not independent)  One more momentum equation for the slurry layer

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford28 Near-field perspective Extended shallow water framework (Capart, 2000) Limitations Equal velocities in the clear water and slurry layer Equal concentration in the slurry layer and static bed  ???

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford29 Near-field perspective Extended shallow water framework (Capart, 2000) 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 ???

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford30 Presentation outline Introduction and motivation Near-field perspective Key aspects Key aspects: inertial effects, coupled description, vertical movements Experimental work Experimental work: dam-break & breaching; past, present & future… Theoretical frameworks Theoretical frameworks: strengths & limitations Far-field perspective Key aspects Key aspects: 2D-H, bank erosion, frictional resistance, debulking Experimental work Experimental work: channels with erodible bed & banks Numerical model Numerical model: bank failure mechanism, 1D & 2D-H models Conclusions

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford31 Far-field perspective Introduction Phenomenological description Phenomenological 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 modelling Key aspects of far-field modelling 2D-H modelling for reproducing actual valley topography Geometrical singularities (bends, widening, chutes, …) Discrete bank failure criterion

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford32 Far-field perspective Erodible channel dam-break wave experiments 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford33 Far-field perspective Erodible channel dam-break wave experiments Paquier & Poncin (UCL, 2000)  Full-channel experiments  Post-flood valley topography « Panel » digital imaging initialpost-flood Flow

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford34 Far-field perspective Erodible channel dam-break wave experiments Le Grelle & Jonard (UCL, 2001)  Half-channel experiments  Top-view and side-view  Time evolution side top Flow

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford35 Far-field perspective Numerical modelling Bank failure mechanisms Traditional models: continuous process, excess bank shear stress Not applic. to sudden surges with abrupt changes of water levels !

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford36 Far-field perspective 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford37 Far-field perspective 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford38 Far-field perspective 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford39 Far-field perspective 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

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford40 Far-field perspective Numerical modelling Comparisons of 1D simulations with half-channel experiments (blue = experimental, red = numerical) Elevation profiles at successive times

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford41 Far-field perspective Numerical modelling Comparisons of 1D simulations with half-channel experiments (blue = experimental, red = numerical) Width evolutions in time at a given x-section

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford42 Presentation outline Introduction and motivation Near-field perspective Key aspects Key aspects: inertial effects, coupled description, vertical movements Experimental work Experimental work: dam-break & breaching; past, present & future… Theoretical frameworks Theoretical frameworks: strengths & limitations Far-field perspective Key aspects Key aspects: 2D-H, bank erosion, frictional resistance, debulking Experimental work Experimental work: channels with erodible bed & banks Numerical model Numerical model: bank failure mechanism, 1D & 2D-H models Conclusions

16 May 2002Spinewine - IMPACT Workshop 1 - Wallingford43 Conclusions 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