Publications scientifiques

Abstract: Operator Choices, Both In Acquiring The Video And Data And In Processing Them, Can Be A Prominent Source Of Error In Image-Based Velocimetry Methods Applied To River Discharge Measurements. The Large Scale Particle Image Velocimetry (Lspiv) Is Known To Be Sensitive To The Parameters And Computation Choices Set By The User, But No Systematic Comparisons With Discharge References Or Intercomparisons Have Been Conducted Yet To Evaluate This Operator Effect In Lspiv. In This Paper, An Analysis Of A Video Gauging Intercomparison, The Video Globe Challenge 2020, Is Proposed To Evaluate Such Operator Effect. The Analysis Is Based On The Gauging Reports Of The 15 To 23 Participants Using The Fudaa-Lspiv Software And Intents To Identify The Most Sensitive Parameters For The Eight Videos. The Analysis Highlighted The Significant Impact Of The Time Interval, The Grid Points And The Filters On The Lspiv Discharge Measurements. These Parameters Are Often Inter-Dependent And Should Be Correctly Set Together To Strongly Reduce The Discharge Errors. Based On The Results, Several Automated Tools Were Proposed To Reduce The Operator Effect. These Tools Consist Of Several Parameter Assistants To Automatically Set The Orthorectification Resolution, The Grid And The Time Interval, And Of A Sequence Of Systematic And Automatic Filters To Ensure Reliable Velocity Measurements Used For Discharge Estimation. The Application Of The Assisted Lspiv Workflow Using The Proposed Tools Leads To Significant Improvements Of The Discharge Measurements With Strong Reductions Of The Inter-Participant Variability. On The Eight Videos, The Mean Interquartile Range Of The Discharge Errors Is Reduced From 17% To 5% And The Mean Discharge Bias Is Reduced From -9% To 1% With The Assisted Lspiv Workflow. The Remaining Inter-Participant Variability Is Mainly Due To The User-Defined Surface Velocity Coefficient Alpha.

Abstract: Protection Systems Are Usually Implemented In Mountains Aiming To Resist Natural Dangerous Phenomena. As Any Other Critical Infrastructure, Protection Systems Should Always Withstand And Operate Efficiently As They Guarantee The Safety Of People And Protect Socio-Economic Assets. However, The Efficacy Of These Systems Decreases With The Increase Of The Deterioration Level Of The Interdependent Components, Which Constitute Them. To Provide Desirable Operation Over Their Lifetime, The Management Of Protection Systems Is Of Paramount Importance. A Major Key Issue In Such Critical Infrastructure Management Is To Optimize The Cost Effectiveness Of Maintenance Actions While Maintaining A Sufficient Protection Efficacy. This Study Proposes A Decision-Aiding Model To Assess Different Maintenance Strategies Applied To A Protection System Against Debris Flows. The Model Is Constructed Using Physics-Based Stochastic Petri Nets. It Incorporates (1) A Stochastic Deterioration Model, Which Is A Surrogate Model Of A Physics-Based Model Developed For Building Deterioration Trajectories Of The System And (2) Maintenance Model That Permits Assessing The Cost And Efficiency Of Maintenance Strategies. This Study Addresses The Case Of A Debris Retention System, In Which The Progressive Filling Of Its Basin By Debris Materials Is Modeled. This Is Followed By Assessing Several Maintenance Strategies Concerning The Cleaning Of The Basin. A Simple Sensitivity Analysis Is Also Carried Out In Order To Check The Effect Of The Uncertainty That Invades The Model'S Inputs On Maintenance Decisions. A Numerical Analysis Is Performed Using Real Data Of The Retention System Located In The Claret Torrent In France And Subjected To Debris Flows Over A Period Of 50 Years.

Abstract: Documenting And Interpreting Channel Responses To Catastrophic Floods Help Understanding How Rapid Fluvial Metamorphosis Can Propagate In A Catchment Under Sediment Cascading Effects. The Recent Example Of The October 2020 Storm Alex In Se France (Similar To 500 Mm Of Rain In 24 H) Provides A Unique Opportunity To Investigate Major Geomorphic Responses Along 70 Km Of Two Confined Alpine Valleys (V & Eacute;Subie And Roya) And To Link Them To Sediment Wave Initiation And Propagation. Gis-Based Analysis Of Remote Sensing Data (High-Resolution Ortho-Imagery And Airborne Lidar Data) Acquired Before And After The Flood Allowed Combining Channel Changes With Sediment Erosion And Deposition Along A 35-Km Reach Of The V & Eacute;Subie, Including The Most Impacted Portions Of The Valley. In The Roya, The Analysis Was Restricted To 2D Morphological Changes Reconstructed With The Sequence Of Ortho-Imagery. Archives Of Aerial Imagery Were Also Used To Integrate The Storm Impact In The Historical Trajectory Of The Rivers. The Reconstruction Of Geomorphic Responses Shows A Quasi-Continuous Fluvial Metamorphosis Along The Investigated Stream Networks, With Dramatic Active Channel Widening And Aggradation, Having No Antecedent Analogs During The Last 70 Years In Both Valleys. The Different Glacial Imprints Between The Two Valleys Are Considered A Key Factor Explaining The Exacerbated Channel Response In The V & Eacute;Subie, Where A Braided Channel Emerged Along A 35-Km River Length. Many Evidences Strongly Support That The Fuelling Effect Of Alluvial Storage Is A Key Element Of The Sediment Cascade At The Origin Of The Braided Channel Formation. This Regional Case Study Allows Us To Discuss The Critical Role Of Floodplain And Terrace Erosion In The Formation Of The Post-Flood Braided Channel, And To Compare The Geomorphic Impact Of The Storm With Similar Reported Cases In The Literature.

Abstract: A Depth-Averaged Model For Turbulent Open-Channel Flows With Breaking Roll Waves On A Sloping Smooth Bottom Is Derived Under An Assumption Of Independence Between The Wall Turbulence And The Roller Turbulence. The Model Includes Four Variables – The Water Depth, The Average Velocity, And Two Variables Called Enstrophy, The Shear Enstrophy And The Roller Enstrophy – Which Take Into Account The Deviations Of The Velocity With Respect To Its Depth-Averaged Value Due To Shear Effect And Roller Turbulence, Respectively. The Four Equations Of The Model Are The Mass, Momentum, Energy And Shear Enstrophy Balance Equations, With The Mathematical Structure Of The Euler Equations Of Compressible Fluids, With An Additional Transport Equation And With Source Terms. The System Is Hyperbolic. The Roller Enstrophy Is Created By Shocks. A Non-Zero Value Of The Roller Enstrophy Indicates A Breaking Wave And A Turbulent Roller. The Model Is Solved By A Fast And Well-Known Numerical Scheme, With An Explicit Finite-Volume Method In One Step. The Model Is Used To Simulate Periodic And Natural Roll Waves With A Good Agreement With Existing Experimental Results. There Is No Parameter To Calibrate In The Model, Which Gives It A Predictive Character.

Abstract: There Is A Paradox In The Relationship Between Bedload Transport Rates And Flow Variables: Laboratory And Field Studies Have Reported On How Bedload Transport Rates Depend On Flow Variables Through A Power Law, But None Of The Empirical Laws Fitted To The Data Has Managed To Provide Accurate Predictions Of Bedload Transport Over A Wide Range Of Flow Conditions. Inferring Bedload Transport'S Scaling Behavior From Data Has Remained A Stubborn Problem Because The Data Are Very Noisy. It Is, Therefore, Difficult To Progress On This Problem Without Some Informed Speculation About How Bedload And Flow Interact. Ralph Bagnold Proposed An Original Theory To Resolve This Problem. This Paper Reviews And Updates Bagnold'S Model By Separating The Effects Of Flow Resistance And Efficiency (Energy Transfer From Water To Bedload) On Dimensionless Transport Rates Phi. Both Variables' Contributions To Transport Rates Can Be Parameterized Separately For The Three Transport Regimes That Bagnold Defined (No Transport, Transitional, And Sheet Flow). We Also Consider Two Possible Control Variables: The Dimensionless Shield Stress Tau* And A Dimensionless Number Related To Stream Power. In The Transitional Regime, The Dimensionless Bedload Transport Rate Scales As Phi Proportional To Tau*3, Whereas In The Sheet-Flow Regime, It Varies As Phi Proportional To Tau*5/3. We End Up With Two Bagnold Equations: One Based On Physical Principles And Involving Shields Stress Tau*, Flow Resistance F, A Density Ratio, And A Bed Slope; The Other Based On Non-Linear Regression And Stream Power. Compared To A Large Set Of Laboratory And Field Data, Predictions From Bagnold'S Model Show Reasonable Accuracy When The Bed Is Plane.

Abstract: We Derive A Model For Bingham Fluid Flows Down An Inclined Plane With A Consistent Asymptotic Method In The Shallow-Flow Approximation. The Variables Are Expanded Up To The First Order Of Accuracy Both In The Sheared And Pseudo-Plug Layers. The Divergence Of The Strain Rate, Which Is Obtained In Classical Approaches, Is Here Avoided By A Specific Regularization Of The Rheology Allowing To Implement A Regular Perturbation Method In The Whole Fluid Domain. Unlike Classical Regularization Methods, The Material Is Here Characterized By A True Yield Stress. Below The Yield Point, The Behavior Is Perfectly Rigid. An Alternative Tensor Expression Of The Constitutive Law Is Proposed. In Particular, The Assumption Of An Alignment Between The Yield-Stress Tensor And The Strain-Rate Tensor Is Removed. The Model Is Derived By Averaging The Mass, Momentum And Energy Balance Equations Over The Depth. This Yields A Hyperbolic Model Of Three Equations For The Fluid Depth, The Average Velocity And A Third Variable, Called Enstrophy, Related To The Variance Of The Velocity. The Model Features New Relaxation Source Terms And Admits An Exact Balance Energy Equation. The Velocity Field In The Depth Is Consistently Reconstructed Using Only The Variables Of The Depth-Averaged Model Without Any Derivative. The Physical Relevance Of The Enstrophy Is Related To The Shape Of The Velocity Profile. The Linear Stability Of A Uniform Solution Is Investigated For This Model, Showing A Stabilizing Effect Of The Plasticity. Roll Waves Are Simulated Numerically Using A Classical Godunov'S Scheme. The Model For A Newtonian Fluid Is Presented As A Particular Case.

Abstract: Granular Jumps Commonly Develop During Granular Flows Over Complex Topographies Or When Hitting Retaining Structures. While This Process Has Been Well-Studied For Hydraulic Flows, In Granular Flows Such Jumps Remain To Be Fully Explored, Given The Role Of Interparticle Friction. Predicting The Length Of Granular Jumps Is A Challenging Question, Relevant To The Design Of Protection Dams Against Avalanches. In This Study, We Investigate The Canonical Case Of Standing Jumps Formed In Granular Flows Down Smooth Inclines Using Extensive Numerical Simulations Based On The Discrete Element Method. We Consider Both Two- And Three-Dimensional Configurations And Vary The Chute Bottom Friction To Account For The Crucial Interplay Between The Sliding Along The Smooth Bottom And The Shearing Across The Granular Bulk Above. By Doing So, We Derived A Robust Scaling Law For The Jump Length That Is Valid Over A Wide Range Of Froude Numbers And Takes Into Account The Influence Of The Packing Density. The Findings Have Potential Implications On A Number Of Situations Encountered In Industry As Well As Problems Associated With Natural Hazards.

Abstract: This Article Presents The Development And Calibration Of A Numerical Model Simulating The Response Of A Novel Rockfall Protection Structure Subjected To Localized Dynamic Loading. This Structure Is Made Of Piled-Up Concrete Blocks Interconnected Via Metallic Components Whose Dynamics Response Under Projectile Impact Is Examined Via Real-Scale Experiments. The Corresponding Numerical Model Is Developed In A Python Based Open Source Software Siconos Which Implements The Non-Smooth Contact Dynamics (Nscd) Method. The Geometrical Features And Mechanical Properties Are Incorporated In The Model Via Specific Developments Pertinent To The Modelling Requirements In Siconos. Some Parameters Peculiar To The Numerical Model Are Calibrated Against The Spatial-Temporal Measurements From Two Full-Scale Impact Experiments. The Bayesian Interface Statistical Learning Method Aided By The Polynomial Chaos Expansion Based Meta-Model Of The Nscd Model Is Deployed For The Calibration. The Additional Understanding Of The Model Dynamics Through The Byproducts Of The Meta-Model Is Highlighted. In The End, The Nscd Model Is Successfully Calibrated Against The Spatial-Temporal Response Of The Experimental Structure With More Than 90% Accuracy For Impact Energies Up To 1 Mj.

Abstract: Draix-Bleone Critical Zone Observatory Was Created In 1983 To Study Erosion Processes In A Mountainous Badland Region Of The French Southern Alps. Six Catchments Of Varying Size (0.001 To 22 Km(2)) And Vegetation Cover Are Equipped To Measure Water And Sediment Fluxes, Both As Bedload And Suspended Load. This Paper Presents The Core Dataset Of The Observatory, Including Rainfall And Meteorology, High-Frequency Discharge And Suspended-Sediment Concentration, And Event-Scale Bedload Volumes. The Longest Records Span Almost 40 Years. Measurement And Data-Processing Methods Are Presented, As Well As Data Quality Assessment Procedures And Examples Of Results. All The Data Presented In This Paper Are Available On The Open Repository 10.17180/Obs.Draix (Draix-Bleone Observatory, 2015), And A 5-Year Snapshot Is Available For Review At 10.57745/Beyqfq (Klotz Et Al., 2023).

Abstract: Flexible Barriers Have Been Recently Proposed As A Promising Alternative For Trapping Woody Debris Driven By The Flow In Torrents And Rivers Before They Reach Elements At Risks. Small-Scale Experiments In Similitude With The Real-Scale Have Been Conducted In View Of Addressing The Interaction Between The Flow And The Barrier. A Particular Attention Was Paid To The Identification Of The Parameters With Influence On The Loading Experienced By The Barrier, Varying The Woody Debris Mixtures Characteristics, Water Discharge, Flume Inclination And Woody Debris Supply Mode. This Investigation Emphasised The Intricacy Of The Relation Between The Barrier Loading And The Characteristics Of The Trapped Logs And Of The Logs Accumulation. The Barrier Loading Revealed Inversely Proportional To The Woody Debris Accumulation Permittivity, Which Quantifies Its Capacity To Let The Water Seep Through. Permittivity Depended On The Way The Accumulation Built Up And On The Evolution Of Its Characteristics With Increasing Discharge And Trapped Logs Volume. Finally, The Loading Exerted By The Flow On The Barrier Was Derived From The Barrier Elongation, Revealing That It Could Be Modelled As A Hydrostatic Load With A Reduction Factor Of 0.5.

Abstract: Following The Projected Increase In Extreme Precipitation, An Increase In Extreme Snowfall May Be Expected In Cold Regions, E.G., For High Latitudes Or At High Elevations. By Contrast, In Low- To Medium-Elevation Areas, The Probability Of Experiencing Rainfall Instead Of Snowfall Is Generally Projected To Increase Due To Warming Conditions. Yet, In Mountainous Areas, Despite The Likely Existence Of These Contrasted Trends According To Elevation, Changes In Extreme Snowfall With Warming Remain Poorly Quantified. This Paper Assesses Projected Changes In Heavy And Extreme Snowfall, I.E., In Mean Annual Maxima And 100-Year Return Levels, In The French Alps As A Function Of Elevation And Global Warming Level. We Apply A Recent Methodology, Based On The Analysis Of Annual Maxima With Non-Stationary Extreme Value Models, To An Ensemble Of 20 Adjusted General Circulation Model-Regional Climate Model (Gcm-Rcm) Pairs From The Euro-Cordex Experiment Under The Representative Concentration Pathway 8.5 (Rcp8.5) Scenario. For Each Of The 23 Massifs Of The French Alps, Maxima In The Hydrological Sense (1 August To 31 July) Are Provided From 1951 To 2100 And Every 300 M Of Elevations Between 900 And 3600 M. Results Rely On Relative Or Absolute Changes Computed With Respect To Current Climate Conditions (Corresponding Here To + 1 Circle C Global Warming Level) At The Massif Scale And Averaged Over All Massifs. Overall, Daily Mean Annual Maxima Of Snowfall Are Projected To Decrease Below 3000 M And Increase Above 3600 M, While 100-Year Return Levels Are Projected To Decrease Below 2400 M And Increase Above 3300 M. At Elevations In Between, Values Are On Average Projected To Increase Until + 3 Circle C Of Global Warming And Then Decrease. At + 4 Circle C, Average Relative Changes In Mean Annual Maxima And 100-Year Return Levels, Respectively, Vary From – 26 % And – 15 % At 900 M To + 3 % And + 8 % At 3600 M. Finally, For Each Global Warming Level Between + 1.5 And + 4 Circle C, We Compute The Elevation Threshold That Separates Contrasted Trends, I.E., Where The Average Relative Change Equals Zero. This Elevation Threshold Is Shown To Be Lower For Higher Return Periods, And It Is Projected To Rise From 3000 M At + 1.5 Circle C To 3350 M At + 4 Circle C For Mean Annual Maxima And From 2600 To 3000 M For 100-Year Return Levels. These Results Have Implications For The Management Of Risks Related To Extreme Snowfall.

Abstract: At The End Of February 2018 The Mediterranean Area Of Montpellier In France Was Struck By A Significant Snowfall That Turned Into An Intense Rain Event Caused By An Exceptional Atmospheric Situation. This Rain-On-Snow Event Produced Pronounced Damage To Many Buildings Of Different Types. In This Study, We Report A Detailed Back Analysis Of The Roof Collapse Of A Large Building, Namely Irstea Cevennes. Attention Is Paid To The Dynamics Of The Climatic Event, On The One Hand, And The Mechanical Response Of The Metal Roof Structure To Different Snow And Rain Loads, On The Other Hand. The Former Aspect Relies On Multiple Sources Of Information That Provide Reliable Estimates Of Snow Heights In The Area Before The Rain Came Into Play And Substantially Modified The Load On The Roof. The Latter Aspect Relies On Detailed Finite-Element Simulations Of The Mechanical Behavior Of The Roof Structure In Order To Assess The Pressure Due To Snow And Rain Loading, Which Could Theoretically Lead To Failure. By Combining The Two Approaches, It Is Possible To Reconstruct The Most Probable Scenario For The Roof Failure Before Its Full Collapse. As An Example Of Building Behavior And Vulnerability To An Atypical Rain-On-Snow Event In The Mediterranean Area Of France, This Detailed Case Study Provides Useful Key Points To Be Considered In The Future For A Better Mitigation Of Such Events In Non-Mountainous Areas.

Abstract: The Glacial And Torrential Basin Of Taconnaz (Mont-Blanc Massif, France) Dominates The Chamonix Valley. It Is One Of The Major Paths For Snow Avalanches In The Alps, Often Triggered By Serac Falls From The Taconnaz Glacier. On 24 November 2018, The Basin'S Multi-Risk Nature Was Further Accentuated By A New Type Of Hazard With A Rockfall Triggered At C. 2700 M A.S.L. It Travelled Down Over A Distance Of 1.85 Km And Stopped 165 M Away From The Construction Site Of A Micro-Hydroelectric Power Station. We Studied The Triggering Conditions At The Permafrost Lower Limit, The Effects Of The Supra-Glacial Path On The Flow Patterns, And The Fate Of The Scar And The Deposit On Torrential Activity. By Comparing A Pre-Event Structure From Motion Model With A Post-Event Lidar Model, We Estimated The Volume Of The Scar To Be 42,900 M3 (& Plusmn;5%). A Numerical Model Was Employed To Simulate The Rapid Runout. It Revealed The Complexity Of The Flow, Attributed To The Sequestration Of A Part Of The Deposit In Crevasses, The Incorporation Of A Significant Volume Of Ice Resulting In A Transition From A Dry Granular Flow To A Mud-Like Flow, And The Presence Of Numerous Deposit Zones. Subsequent Monitoring Of The Area After The Event Allowed For The Documentation Of The Scar'S Evolution, Including A Landslide, As Well As The Progressive Degradation And Evacuation Of The Deposit By The Torrent Without Producing Debris Flow. The Study Of The Triggering Factors Indicated Glacial Retreat As The Probable Main Cause, Assisted By The Melting Of Ice Lenses Left By The Permafrost Disappearance. Finally, We Present Replicable Methods For Managing Risks At The Site Following The Event. This Event Improves The Understanding Of Cascading Processes That Increasingly Impact Alpine Areas In The Context Of Climate Change.

Abstract: A Mathematical And Numerical Framework Is Proposed To Compute The Displacement And Merging Dynamics Of Sliding Droplets Under The Action Of A Constant Shear Exerted By A Gas Flow. An Augmented Formulation Is Implemented To Model Surface Tension Including The Full Curvature Of The Free Surface. A Set Of Shallow-Water Evolution Equations Is Obtained For The Film Thickness, The Averaged Velocity, An Additional Quantity (With Dimension Of A Velocity) Taking Into Account The Capillary Effects And A Tensor Called Enstrophy. The Enstrophy Accounts For The Deviation Of The Velocity Profile From A Constant Velocity Distribution. The Formulation Is Consistent With The Long-Wave Expansion Of The Basic Equations With A Conservative Part And Source Terms Including The Effect Of Viscosity, In The Form Of A Viscous Friction And The Effect Of The Shear Stress. The Model Is Hyperbolic With Generalised Diffusion Terms Due To Capillarity. Finally, Our Model Is Completed With A Disjoining Pressure Formulation That Is Able To Account For The Hysteresis Of The Static Contact Angle. In This Formulation, The Advancing Or Receding Nature Of The Contact Line Is Assessed By The Accumulation Or Reduction Of Mass Of The Droplet At The Contact Line. Simulations Of Sliding Water Droplets Are Performed With Periodic Boundary Conditions In A Domain Of Limited Size. Hysteresis Of The Static Contact Angle Causes A Slowdown Of The Drops And A Delay In The Sequence Of Coalescence Of The Drops.