Rock Scour: Past, Present and Future George W. Annandale, D.Ing, P.E. Engineering and Hydrosystems Inc. Denver, Colorado
Outline Rock Scour Process Jet Hydraulics Scour Resistance of Rock Methods of Analysis Past Present Future Plunge Pool Design
Bartlett Dam, Arizona
Bartlett Dam, Arizona 30m Scour in Granite
Turbulent Jet Scour Process Analysis Design
Fluctuating Pressures and Resonance 6 4 excitation at fissure entry end of fissure middle of fissure Pressure [m] 2 0-2 1 11 21 31 41 51 61 71 81 91-4 -6 Time [5 msec/unit] Impacting high velocity jet Fissure length = 10 m Sinusoidal pressure excitation at entry of fissure Resonance conditions at middle of fissure Resonance conditions at end of fissure Bollaert 2002
Rock-Water Interaction H 1 2 3 4 5 6 Aerated jet impact Macro-turbulent energy dissipation Interface pressure fluctuations Pressure propagation-hydrojacking Uplift of rock entities Downstream displacement β q,v 1 h t y 2 3 5 d m 6 4 p Bollaert 2002 Scour Process Analysis Design
Hydraulics Fluctuating Pressures Entrained Air C = 1000 m/s 100 m/s Resonance f = c / 4L approx 25 Hz Scour Process Analysis Design
Rock Breakup Processes Brittle Fracture Fatigue Failure Removal of Intact Rock Blocks Scour Process Analysis Design
Brittle Fracture / Fatigue Close-ended Fissures impacted by Pressure Fluctuations Brittle Fracture or Fatigue Failure Scour Process Analysis Design
Brittle Fracture and Sub-Critical Failure Stress Intensity K I Fracture Toughness K I,insitu Scour Process Analysis Design
Removal of Intact Rock Downward Force Friction Fluctuating Uplift Force Scour Process Analysis Design
Santa Luzia Dam 76m Drop 134m 3 /s ~7 m Scour Process Analysis Design
Outline Rock Scour Process Jet Hydraulics Scour Resistance of Rock Methods of Analysis Past Present Future Plunge Pool Design
Analysis Techniques Rigorous Mathematical Modeling Semi-Empirical Methods Empirical Methods Increased Understanding Increased Complexity Increased Value Scour Process Analysis Design
Past: Empirical Methods Veronese (1937) Ys = 1. 90H 0.225 q 0.54 Yildiz and Uzucek (1994) Y s = 1.90H 0.225 q 0.54 cosα Mason and Arumugan (1985) Y = s K q x y H h v z g d w Scour Process Analysis Design
Near-Prototype Testing Scour Process Analysis Design
Empirical Methods 2 Yildiz Mason Prototype Identity Line Linear (Mason Prototype) Predicted Erosion Elevation (m) 1.5 1 0.5 Linear (Yildiz) y = 1.0983x R 2 = 0.5345 y = 0.6856x R 2 = 0.4358 0 0 0.5 1 1.5 2 Experimental Erosion Elevation (m)
Current: Semi-Empirical Quantify Relative Magnitude of Erosive Capacity of Water Quantify Relative Magnitude of Ability of Rock to Resist Scour Scour Threshold Relationship based on Field Data and Near-Prototype Validation Scour Process Analysis Design
Essence of Erosion Process Fluctuating pressures Jacking Dislodgment Displacement Scour Process Analysis Design
Fluctuating Pressures and Resonance 6 4 excitation at fissure entry end of fissure middle of fissure Pressure [m] 2 0-2 1 11 21 31 41 51 61 71 81 91-4 -6 Time [5 msec/unit] Impacting high velocity jet Fissure length = 10 m Sinusoidal pressure excitation at entry of fissure Resonance conditions at middle of fissure Resonance conditions at end of fissure
Erosive Power of Water 320 P = γ. Q. E Std. Deviation of Pressure Fluctuations (Pa) 300 280 260 240 220 200 180 160 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 Rate of Energy Dissipation (W/m 2 ) Annandale 1995 Scour Process Analysis Design
Estimation of Stream Power Q H P= ρgqh/a A Scour Process Analysis Design
Turbulent Jet? Scour Process Analysis Design
Plunging Jet Footprint? Scour Process Analysis Design
Rock Resistance Principal Elements Geo-mechanical Index Scour Threhold
M s - Intact Material Strength Water Jets Perfect Rock Perfect Clay Water jet likely to scour perfect clay easier than perfect rock Intact Material Strength of latter is greater Therefore greater resistance Scour Process Analysis Design
K b - Block Size Large Blocks Small Blocks or particles More Difficult to Erode Easier to Erode Scour Process Analysis Design
Block Size and Shape Removal of blocks by flowing water is easier than removal of elongated blocks. Flow direction Elongated slabs of rock Equi-sided blocks of rock Scour Process Analysis Design
Friction Scour Process Analysis Design
Friction Scour Process Analysis Design
Friction + Effects of Gouge Scour Process Analysis Design
Orientation Intersection between plane of discontinuity and horizontal plane (also known as the strike) Dip Dip Direction Dip Plane of discontinuity Scour Process Analysis Design
Orientation Flow penetrates underneath rock and removes it from bed. Increased difficulty to remove rock by flowing water. Rock dipped in direction of flow Rock dipped against direction of flow. Scour Process Analysis Design
Mass Strength Erodibility of Rock Factors Block Size Primary Inter-block Shear Strength Relative Dip and Dip Direction Secondary Scour Process Analysis Design
Erodibility Index Block Size Number Ground Structure Number K = M s. K b. K d. J s Mass Strength Number Joint Shear Strength Number Scour Process Analysis Design
Erodibility Index Erosion Threshold 10000.00 1000.00 Scour No Scour Scour-CSU Threshold Stream Power KW/m 2 100.00 10.00 1.00 0.10 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 Erodibility Index Scour Process Analysis Design
Seismic Velocity Erosion Threshold 10000.00 Seismic Velocities (p-wave) Scour Stream Power KW/m 2 1,200 ft/sec 1000.00 100.00 10.00 2,000 ft/sec CASE 590M Refusal 2,500 ft/sec 3,000 ft/sec 3,500 ft/sec 3,600-3,800 ft/sec No Scour Excavation Class 1 1.00 2 3 4 5 6 7 A D3 and D5 D5 and D6 D7 and D8 D9, D10 and D11 0.10 Extremely Hard Hand Pick and Very Hard Ripping and Power Tools Easy Ripping Hard Ripping Spade Ripping Blasting 0.01 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.0 Erodibility Index Scour Process Analysis Design
Gibson Dam Montana Scour Process Analysis Design
Gibson Dam Scour Process Analysis Design
Gibson Dam Scour Process Analysis Design
Gibson Dam 10000 1000 Stream Power at lower abutment EROSION Concrete Stream Power KW/m2 100 10 Erosion threshold line Fractured rock where scour was observed Stream power at upper abutment Competent rock where no scour was observed 1 NO EROSION 0.1 0.01 0.1 1 10 100 1000 10000 100000 Erodibility Index Scour Process Analysis Design
Erodibility Index Simulated Rock Scour Process Analysis Design
Erodibility Index Granular Material Scour Process Analysis Design
Erodibility Index Failure of Simulated Rock Scour Process Analysis Design
Erodibility Index Method Simulated Rock: Verification Erosion Threshold for a Variety of Earth Materials 10000.00 1000.00 Scour-SCS No Scour-SCS Scour-CSU Threshold Stream Power KW/m 2 100.00 10.00 1.00 0.10 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 Erodibility Index Scour Process Analysis Design
San Roque Philippines Scour Process Analysis Design
San Roque Philippines Scour Process Analysis Design
Future: Computer Modeling Simulate Fluctuating Pressures Air Entrainment - Resonance Rock Failure Brittle Fracture Fatigue Failure Direct Removal of Rock Blocks Scour Process Analysis Design
Experimental installation Lausanne, Switzerland Scour Process Analysis Design
Pressure Fluctuations Scour Process Analysis Design
Outline Rock Scour Process Jet Hydraulics Scour Resistance of Rock Methods of Analysis Past Present Future Plunge Pool Design
Plunge Pool Design Options Plunge Pools: Energy Dissipaters Pre-formed Self-formed formed Hardened Scour Process Analysis Design
Plunge Pool Design Approach Plunge Pool Scour Assessment Jet Modification Plunge Pool Pre-Forming Plunge Pool Boundary Modification Rock Modification Lining Is it a Problem & to What Extent? L/Lb > 2 Scour Analysis; How Deep? Mass Strength & Block Size Scour Process Analysis Design
Plunge Pool Pre-Forming Minimum Depth q H Y required Yrequired 0.113 = H q 2g ( ) 2 5 Puerta 2004 Scour Process Analysis Design
Plunge Pool Pre-Forming Appropriate Pool Depth Scour Process Analysis Design
Erodibility Index Erosion Threshold 10000.00 1000.00 Scour No Scour Scour-CSU Threshold Stream Power KW/m 2 100.00 10.00 1.00 0.10 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 Erodibility Index Scour Process Analysis Design
Plunge Pool Scour Assessment Hydrology & Hydraulics Material Properties: Geology and Geotechnical Ele vat ion Available Stream Power Ele vat ion Threshold Required Stream Power Stream Power El ev ati on Scour Depth Calculation Stream Power Plunge Pool WSEOriginal Riverbed Available Stream Power Stream Power Maximum Scour Elevation ThresholdRequired Stream Power Scour Process Analysis Design
Plunge Pool Boundary Modification Rock Anchors Lining Scour Process Analysis Design
Rock Anchors Mass Strength Block Size Tensioned Scour Process Analysis Design
Lining Jet Mass Strength Block Size Concrete Lining Scour Process Tensioned Anchors Analysis Design
Concrete Lining Design Weight Brittle Fracture Fatigue Scour Process Analysis Design
Example Scour Process Analysis Design
Scour Assessment: Validation Stilling Pool Elevation = 690' Bull Run Dam No. 2: Erodibility Index Jet Erosive Power 20000cfs 25100cfs 30000cfs 40000cfs General Stratigraphic Column highly weathered basalt 620 610 A B Approximate Current Stilling Pool Level (Bottom) Flow 3 vesicular basalt pillow lava Elevation (ft) 600 590 580 From "A" to "B" is the Probable Range of Material Resistance for Flow 3 After Calibration Sedimentary Interbed Flow 4 claystone, sandstone, tuff: cemented and non-cemented vesicular basalt altered/weathered basalt 570 Flow 5 560 550 Fault Zone Resistance 540 0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0 1600.0 1800.0 Power per Unit Area (kw/m^2) vesicular basalt 40000 cfs Discharge 30000 cfs Discharge 25100 cfs Discharge 20000 cfs Discharge Calibration Max Rock Resistance Min Rock Resistance Fault Zone
Scour Assessment NW General Cross Section Showing Scour Potential: Bull Run Dam No. 2* SE 40' 40' WSE ~ 695' Approximate Jet Centerline Jet Spread (~14 ) Flow 1 No Significant Scour for 30,000cfs Event (If Material Resistance is Closer to Line "B" Flow 2 ~ 677' Stilling Pool Level 1964 Conduits 3 & 5 Approximate Current Stilling Pool Level ~ 627' Probable Scour from 40,000 cfs Event Probable Scour from 30,000cfs Event (Line "A") ~613' Sedimentary Interbed Flow 3 ~ 597' ~ 594' Flow 4 ~ 573' *General profile (i.e. ground surface, flow locations, etc.) taken from Shannon & Wilson, Inc. report (July 1978) Cross Section C - C`. Flow 5
Scour Assessment: Backroller Protective Concrete Slab Beneath Spillway d = Diameter of Backroller; As the Amount of Scour Increases, so does the Diameter d Backroller Flow Length = p *d ~5' of Scour Observed Along Fault Zone Beneath Spillway Associated with 1964 Event
Scour Assessment: Backroller Protective Concrete Slab Beneath Spillway Probable Scour from 40,000 cfs Event Probable Scour from 30,000 cfs Event Existing Scour Hole (25,100 cfs - 1964) 5' 7' 22'
Mitigation Design Flow Length of the Macroturbulent Eddy = p *d Approximate Jet Centerline Jet Spread (~14 ) Jet Thickness d = Depth of Pool = Diameter of Eddy d
Optional Protection Measures Pre-Forming + Maintain Plunge Pool Elevation WSE = 690 ft Concrete Wall with Rock Bolts Excavation Flow 5 ~ 572 ft
Optional Protection Measures Lining + Increase Plunge Pool Elevation WSE = 695 ft Concrete Slab with Rock Bolts
Optional Protection Lining + Riprap + Increase Plunge Pool Elevation WSE = 695 ft Riprap with D50 ~ 3.5 ft Concrete Slab with Rock Bolts Covering Jet Impingement Zone and Fault Zone
Outline Rock Scour Process Jet Hydraulics Scour Resistance of Rock Methods of Analysis Past Present Future Plunge Pool Design
Reviewed Rock Scour Analysis Methods Past: Empirical Present: Semi-Empirical Quantify Rock and Erosive Capacity Scour Threshold for Rock Erodibility Index Method Field and Near-Prototype Validation Future: Computer Simulation Rock: Brittle Fracture, Fatigue and Block Removal Hydraulics: Air, Pressure Fluctuations and Resonance
Plunge Pool Design Self Formed Pre-Formed Hardened Example