United States Army Corps of Engineers Engineering Manual EM

Transcription

1 United States Army Corps of Engineers Engineering Manual EM

2 Chapter 2 Open Channel Hydraulic Theory 2

3 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels 3

4 Physical Hydraulic Elements Channel slope Cross-sectional area Wetted perimeter Most efficient section Boundary surface roughness 4

5 Invert Slope Controlled by surrounding topgraphy Can be altered to achieve desired project goals, but Cut/fill can be expensive Effect on hydraulics and costs Need to examine out of reach influences 5

6 Channel Cross-Section Balance capacity with costs Typically: Trapezoidal for rural areas Rectangular for urban reaches V-bottom for minimizing effects of sediment 6

7 Roughness Absolute roughness Theoretical interest only Effective roughness Dimension of length Effective roughness height Composite roughness Most common approach 7

8 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels 8

9 Hydraulic Design Elements Assumptions Friction Losses Friction Coefficients Flow Classification 9

10 Assumptions Familiar with concepts of: Uniform flow Gradually varied flow Conservation of Energy Conservation of Momentum Conversion of friction loss terms Have or have access to Chow (1959) 10

11 Hydraulic Design Elements Assumptions Friction Losses Friction Coefficients Flow Classification 11

12 Friction Losses Three main equations/concepts Chezy Manning Darcy-Weisbach Included Losses due to friction Uniform turbulence and eddy losses Not Included Local turbulence and eddy losses 12

13 Types of Fluid Flow Laminar Flow flow persists as unidirectional movement Molecules flow parallel Movement up and down by diffusion Turbulent Flow highly distorted flow Large scale flow perpendicular to direction of flow Transfer of movement up and down by macroscale processes Turbulence = irregular and random component of fluid motion Eddies = highly turbulent water masses 13

14 14

15 Laminar vs Turbulent Flow Laminar flow velocity constant at a point over time Turbulence Most flows = turbulent Slow settling velocity upward motion of water particles Increases effectiveness of fluid in eroding and entraining particles from the bed; but less efficient transport agent Velocity measured at a point over time tends towards an average value; but varies from instant to instant 15

16 Friction Losses Type of Energy Losses In consideration of the energy losses in fluid flow, they are generally broken down into 2 types: Major losses Minor losses 16

17 Friction Losses Major Head Losses Major head losses are a form of energy considered to take place continuously along the path of flow. These losses are generally called Friction Head Losses. 17

18 Friction Losses Minor Head Losses Minor head losses are those losses generally created by increased turbulence and resistance to flow at points in the stream where the direction of flow is changed or where other obstruction tale place. The most commonly encountered forms of minor head losses are as follows: Hf head losses due to a sudden of gradual enlargement of the cross section of flow. Hc head losses due to a sudden of gradual contraction of the cross section of flow. Hg head losses due to obstruction in the path of flow (gates, valves, metering devices, and so on) Hb head losses occurring at bends and changes in direction of the flow path. 18

19 Friction Losses Total Head Losses The total head loss in a stream of flowing fluid is equal to the sum of all the head losses along its path Total Head Losses = Major Head Losses + Minor Head Losses H = h + h + h + h + h l f e c g b 19

20 Hydraulic Design Elements Assumptions Friction Losses Friction Coefficients Flow Classification 20

21 Recap If depth & velocity remain constant over a length of channel which has a constant cross-section and slope, then the flow is uniform and water surface will be parallel to streambed. The depth is called the normal depth. If depth and/or velocity change over distance then it is varied flow, which can be divided into gradually varied or rapidly varied 21

22 Steady Flow Law of continuity Q = V 1 A 1 = V 2 A 2 = Q = discharge, V = Average velocity, A = cross sectional area Must have continuous, steady-varied flow Flow may also be: Unsteady-varied Unsteady-uniform Steady-uniform 22

23 Uniform flow Gravity (force causing motion) Friction (force opposing motion) For uniform flow the gravity force will equal the friction force and the energy line, the water surface and the streambed will be parallel (have the same slope) 23

24 Analysis of Major Energy Losses Antoine Chezy, in 1775 Chezy Analysis and Formula A French civil engineer developed the Chézy equation, which relates the uniform flow velocity to channel roughness, hydraulic radius, and bed slope. v = C Rs v = average velocity of flow R = hydraulic radius S = slope of the channel C = coefficient depending upon the various characteristics of the channel and their comparison with those of another similar channel 24

25 Chezy s Equation The gravitation force is approximately proportional to the velocity squared V= (ρg/α)rs V is the mean velocity (ft/s) α is a coefficient mainly dependent on channel roughness 25

26 Chezy s C The term (ρg/α) was later replaced with a single coefficient C C has units of (ft 1/2 /s) And varies from: 30 (small rough channels) 90 (large smooth ones) 26

27 27

28 Chezy Where: S f = EGL slope V = velocity C = Chezy coefficient R = Hydraulic radius 28

29 Chezy Hydraulically smooth channels Where: C = Chezy coefficient R n = Reynolds number 29

30 Reynolds Number Re VL = or υ Internal Forces Viscous Forces Where: R e = Reynolds number L = characteristic length ν = kinematic viscosity V = velocity 30

31 Chezy Hydraulically rough channels Where: C = Chezy coefficient k = effective roughness R = hydraulic radius 31

32 Hydraulically Rough or Smooth? Hydraulically rough k s > k c Hydraulically smooth K s < k c 32

33 Hydraulically Rough or Smooth? Where: k c = Equivalent roughness C = Chezy C ν = kinematic viscosity g = gravity V = Velocity 33

34 Darcy-Weibach Formula Henry Darcy, 1857 Showed that f is not dimensionless but dependent upon numerous parameters -the roughness of conduit wall -the velocity of flow -the viscosity and density of fluid -the diameter of pipe 34

35 Darcy-Weibach Formula Julius Weisbach, Developed a formula for pipe flow, based to some extent on the Chezy formula. l v 2 h = fx x f = friction factor f d 2 g d = pipe diameter l = pipe length h f = head losses over pipe length v = velocity 35

36 Analysis of Major Energy Losses Darcy-Weibach Formula Values for f for water flowing in straight smooth pipe are shown in table. 36

37 Darcy Weisbach f Where: S f = EGL slope V = velocity f = Darcy-Weisbach coefficient R = Hydraulic radius 37

38 Reynolds Number Moody Diagram L.F. Moody developed the diagram which expresses the relationships between -Reynolds number -f for various ranges of values of k/d Moody diagram -all curves are concave upward and tend to flatten out as value of R e increase. R e = 400 d k log(3.7 d k ) 38

39 Moody Diagram Friction Factor Reynolds Number 39

40 Open-Channel Flow Formulas Manning Formula Robert Manning, in 1885 Developed Manning formula used for open channel flow conditions. v = 1 2 / 3 1/ 2 R s n v = velocity of flow, m/s R = hydraulic radius, m S = slope of the energy gradient n = a roughness coefficient 40

41 Manning s Equation V= 1/n R 2/3 S 1/2 Q= 1/n AR 2/3 S 1/2 Q is discharge (ft3/s) n is a coefficient known as Manning s n SI units (imperial are multiplied by 1.486) n = sd 1/6 d is the median diameter in (mm) d 50 for which 50% of streambed particles are smaller n = (0.1129R 1/6 )/(1.16+2log(R/d 84 )) d 84 is the diameter (m) for which 84% of streambed particles are smaller 41

42 Manning Where: V = velocity S f = EGL slope n = Manning s n R = Hydraulic radius 42

43 Manning s n Examples 43

44 Manning s n Examples 44

45 Coefficient Relation 45

46 Plate 3 k to C and f Plate 3 46

47 Hydraulically Rough Channels Plate 5 47

48 Hydraulically Rough Channels Plate 4 Stop 15 SEP 08 48

49 Rules of Thumb Most channels are hydraulically rough k values for natural rivers: 0.1 to 3.0 (ft) Typically larger than spherical diameters of bed materials why?? 49

50 Rules of Thumb If field data is available Determine k from: EQ

51 Rules of Thumb If field data is available Determine k from Equation 2-6 Value of k can be assumed for flow levels at or lower than observed condition Appendix C outlines method for determining an effective roughness 51

52 Why use k? Can be assumed to be spherical diameter of average bed material What if losses other than friction? Consider relations: k does not vary with R n varies with R 1/6 52

53 However k must be evaluated for each subsection Use subsections: Differing bed materials Bed forms present Expansions and contractions Form roughness significant 53

54 Why do we need to know all this stuff? Environmental flows Depth of flow Min particle size to prevent erosion The depth of flow is obviously important in calculating possible flood depths. But its not that simple, we actually use this for back water calculations. 54

55 Hydraulic Design Elements Assumptions Friction Losses Friction Coefficients Flow Classification 55

56 Flow Classification Tranquil and rapid flow Pulsating rapid flow Varied Flow 56

57 Tranquil vs. Rapid Flow Distinction centered around specific energy and critical depth 57

58 Depth vs. Specific Energy 58

59 Depth vs. Specific Energy Tranquil Flow d > 1.1dc or Fr < 0.86 Rapid Flow d < 0.9dc or Fr >

60 Depth vs. Specific Energy If in instability region: Evaluate both high and low resistance values Adjust slope (or k) to design in proper zone 60

61 Pulsating Rapid Flow Occurs at Froude numbers much greater than 1 Formation of slugs Typically on steep slopes with shallow depths What is effect? 61

62 Pulsating Rapid Flow Need to determine roughness Concept of limiting Froude number Rather difficult to quantify 62

63 Roughness Where: R = Hydraulic radius n = Manning s n F = Froude number of flow Fs = Limiting Froude number EQ

64 Limiting Froude Number EQ Where: Fs = Limiting Froude number g = Gravity ξ = Flow function ζ = depth/width ratio Z = Channel side slope 64

65 Limiting Froude Number Where: ξ = flow function ζ = depth/width ratio d = flow depth b = bottom width 65

66 Pulsating Rapid Flow Q = 1000 b = 10 Y = 3 z = 2 Fs = 2.12?? z = 0 Fs = 3.4?? Plate 7 66

67 Varied Flow Profiles Prismatic Channels Unvarying cross sections Constant invert slope Straight alignment How to determine profile Direct integration Direct step Standard step Start 11 SEP 67

68 Direct Step Method x = E 2 E So Sf 1 Calculation steps Determine starting depth Assume second depth and compute parameters associated with that depth Solve for step length 68

69 Direct Step Method Compute Upstream direction Subcritical flow Downstream direction Supercritical flow Utilize small changes of y so that: S f = 2 2 nv 2.22R 4/3 S f = fv 2 8gR 69

70 Standard Step Method Calculation steps Assume WS 2 at upstream location Compute associated velocity head Solve for Sf Solve for WS 2 Compare with assumed WS 2 Iterate as needed 70

71 Comparison No Comparison between direct step and standard step methods. Characteristic Ease of computation Type of cross section Independent variable Dependent variable Calculation advances -> Accuracy increases with -> Type of cross-section input Data needs Tools Reliability Direct step method Easy (hours) prismatic flow depth length of channel directly a smaller flow depth increment one typical cross section (prismatic) minimal spread sheet (or programming) Answer always possible Standard step method Difficult (months) any (prismatic or nonprismatic) length of channel flow depth by iteration (trial and error) lesser cross-sectional variability many cross sections (nonprismatic) extensive HEC-RAS (or programing) Answer sometimes not possible (depends on the type of crosssectional data) 71

72 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels 72

73 Flow Through Bridges Energy losses Turbulence Water surface elevations 73

74 Abutment Losses Rapid flow Stay out of channel! Tranquil flow Flow depth between abutments is > yc Preliminary layout 1.0 for expansion loss 0.5 for contraction loss Bradley (1978) Design charts 74

75 Pier Losses Three conditions (Chow 1959) Class A Class B Class C 75

76 Three condition s Energy Class A Momentum Class B Class C Pier Losses Momentum Plate 10 76

77 Pier Losses Plate 11 77

78 Energy Method E1 = E2 + hl 1 2 E2 = E3 = hl 2 3 Where: E1 = Energy at Section 1 E2 = Energy at Section 2 E3 = Energy at Section 3 hl 1-2 = Losses between sections 1 and 2 hl 2-3 = losses between sections 2 and 3 78

79 Momentum Method Where: M = momentum per unit time (lbs) β = momentum correction factor (1.0) Q = discharge (cfs) V = average channel velocity (fps) 79

80 Momentum Method Where: m 1 = hydrostatic force in section 1 m p = hydrostatic force on pier ends m 2 = hydrostatic force in section 2 m 3 = hydrostatic force in section 3 80

81 Pier Losses Plate 12 81

82 Analysis Tools Plate 14 Plate 15 82

83 Analysis Tools Plate 16 Plate 17 83

84 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels Start 22 SEP 84

85 What Is A Transition? Structure that joins two geometrically dissimilar cross-sections Contraction or expansion of flow Minimizes flow disturbance Affects the water surface elevation through energy loss 85

86 Transitions Contraction & Expansion Plan View width 1 b 1 b 2 width 2 Contraction b 1 > b Expansion b 1 < b 2 86

87 Application of Transition Structures Approach to bridge and culvert crossings A placeholder between existing and future improvements To create a choke in the channel 87

88 Analysis of Transition Structures 88

89 Principles of Open Channel Hydraulics Three Governing Principles Conservation of Mass Conservation of Momentum Conservation of Energy E = E Losses 89

90 Energy Equation (Bernoulli s) elevation head y + z + 1 = 2 + y + z h L, 1 2 v p v p 2g γ 2g γ velocity head pressure head Energy loss between sections 1 and 2 90

91 Open Channel Energy Equation Q Channel Bottom z 1 z 2 Datum v 1 2 g v 2 g y 1 + z 1 = + y 2 + z 2 + h L,1 2 91

92 Open Channel Energy Equation HGL y 1 Channel Bottom y 2 z 1 z Datum v 1 2 g v 2 g y 1 + z 1 = + y 2 + z 2 + h L,1 2 92

93 Open Channel Energy Equation v g HGL 2 v 1 2 g y 1 + z 1 = + y 2 + z 2 + h L EGL v 2 g,1 2 y 1 v g Channel Bottom y 2 z 1 z Datum

94 Open Channel Energy Equation 2 v 1 2 g v 2 g y 1 + z 1 = + y 2 + z 2 + h L,1 2 v g HGL EGL h L1-2 y 1 v g Channel Bottom y 2 z 1 z Datum

95 Open Channel Energy Equation Head Losses h = Major Losses + Minor Losses L1 2 Friction between fluid and it s flow boundary Change in velocity or change in flow 95

96 Open Channel Energy Equation Losses due to Transitions are considered Minor Losses h = Major Losses + Minor Losses L1 2 Change in velocity or change in flow 96

97 Open Channel Energy Equation General Equation for Minor Losses h L m = K m V 2g 2 97

98 Transition Types For connecting trapezoidal and rectangular sections Cylindrical quadrant Warped Wedge Rectangular sections Straight-line Abrupt/square 98

99 Types of Transitions Warped Straight Line Cylindrical Quadrant Wedge (longer or shorter than warped?) Abrupt (square) 99

100 Abrupt Transition 100

101 Channel Transitions Subcritical Flow Each type can be used in either direction Cylindrical quadrant used for: Expansions from rectangular to trapezoidal Contractions from trapezoidal to rectangular Straight-line transition or quadrant for rectangular channels 101

102 Channel Transitions Supercritical Flow Cylindrical quadrant Subcritical in trap section to supercritical in rectangular Straight-line for contractions in rectangular sections Special shape for expansions in rectangular channel 102

103 Transition Design Subcritical Flow Rectangular to trapezoidal Wedge Type Plate

104 Plate

105 Transition Design Subcritical Flow Rectangular to trapezoidal Plate 20 6 o maximum change in flow line Water surface profiles determined by step computations < 20 percent change in velocity between steps Adjust to make water surface as straight as practicable 105

106 Transition Design Supercritical Flow Between trapezoidal and rectangular Typically incorporate wedge or straight-line transition Need to minimize/contain standing waves Rule of thumb: 106

107 Recommended Convergence and Divergence Rates Mean channel velocity (fps) 1-10 Wall flare (horizontal to longitudinal) 1: : :20 107

108 Transition Design Supercritical Flow Rectangular contractions: 108

109 Plate

110 Transition Design Supercritical Flow Rectangular contractions: EQ 2-22 EQ 2-23 EQ

111 Transition Design Supercritical Flow Rectangular contractions: EQ 2-25 However, disturbances are minimized when L = L 1 + L 2 so.. EQ 2-26 EQ

112 Transition Design Supercritical Flow Rectangular contractions: Correct design requires choosing a value of Θ so that L = L 1 + L 2 Either solve equations simultaneously or use Plate 22 Plate 23 is a go-by 112

113 Plate 22 Plate

114 EQ Plate

115 Transition Design Supercritical Flow Rectangular expansions Model studies have shown that changes in flow direction are much more gradual in expansions than in contractions Theory and empirical data show that a curved transition helps regulate wave propagation Model testing has indicated that downstream depths can be significantly greater than what theory predicts 115

116 Transition Design Supercritical Flow Rectangular expansions 116

117 Transition Design Supercritical Flow Rectangular expansions EQ 2-25 Where: Z = Transverse distance from channel centerline b 1 = approach channel width X = longitudinal distance from beginning of expansion F 1 = approach Froude number 117

118 Plate

119 Transition Design Supercritical Flow Non-rectangular transitions: No real design guidance, typically need to model 119

120 Transition Design Supercritical to Subcritical Flow Typically expansions from rectangular to trapezoidal Wedge type transition Can be rapid or gradual Jump needs to be contained within transition Scenario?? 120

121 121

122 122

123 123

124 Transition Losses Subcritical flow Design should minimize energy losses and costs Supercritical Flow Can be substantial and drive the design 124

125 Transition Loss Equations Contraction: h = K c V V 2g Expansion: h = K e V 2 1 V 2g

126 Transition Loss Coefficients SHAPE Abrupt (Square) Straight Line Warped Design K c (Contraction) K e (Expansion)

127 Design Considerations Define purpose of structure Define project constraints Right-of-Way Site conditions Economic feasibility Location of transition structure Design will be case by case 127

128 General Guidelines for Design 1. Define design parameters Design discharge, Q (cfs) Geometry of existing and proposed channel section 2. Determine location of transition structure 3. Length of transition structure 4. Determine transition head loss, H t 5. Determine existing WSE at beginning and end of preliminary location of transition 6. Based on initial water surface elevation calculation (output), refine design of transition 128

129 Key Points to Remember Transitions in Subcritical Flow are analyzed using the Energy Equation Transition losses are energy losses associated with a change in velocity If possible, avoid design of transitions in supercritical and unstable range of flows Design of transitions is case by case and comes with experience 129

130 References 1. Chow, V. T., Open Channel Hydraulics, McGraw Hill Book Co., Inc. New York (1959) 2. Ippen, A. T., Mechanics of Supercritical Flow, Transactions, ASCE, Vol. 116 (1951) 3. Ippen, A. T. and Dawson, J. H., Design of Channel Contractions, Transactions, ASCE, Vol. 116 (1951) 4. Rouse, H., Bhoota, B. V., and Hsu, En-Yun, Design of Channel Expansions, Transactions, ASCE Vol. 116 (1951) 5. Knapp, R. T., Design of Channel Curves for Supercritical Flow, Transactions, ASCE, Vol. 116 (1951) 6. Morris, Henry M. and Wiggert, James M., Applied Hydraulics in Engineering, Second Edition, John Wiley & Sons 7. U. S. Army, Office, Chief of Engineer, Hydraulic Design of Flood Control Channels, Engineer Manual EM (1970) 8. Mostafa, M. Gamal, Open Channel Transitions in Subcritical Flow, Final Report No. ERC FR, (1978) 9. Orange County Flood Control District Design Manual, O.C.F.C. D., Orange County, California 10. Majaj, Nadeem H., Appendix A Basics of Hydraulics, Basin Analysis Software User s Manual, Hydraulic Solutions, Inc. (2003) Note: Graphics and text depicting the Energy Equation were obtained from another slide presentation on the web. Author was not named. 130

131 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels Start 6 OCT

132 Flow in Curved Channels Superelevation Limiting curvature Bend losses Shear stress 132

133 Flow in Curved Channels Superelevation Secondary currents Shift in maximum velocity 133

134 Flow in Curved Channels Superelevation VW 2 y = C gr EQ Where: y = rise in water surface C = coefficient V = average velocity W = channel width g = gravity r = radius of curvature 134

135 Flow in Curved Channels Superelevation Subcritical Increase wall height along outside of curve to account for rise in water surface elevation Can have waves on inside of channel bend 135

136 Flow in Curved Channels Superelevation Supercritical Effect propagated downstream requiring increased wall height Mitigation Spiral transition curves Spiral banked curves Limiting curvature 136

137 Flow in Curved Channels Spiral transition curves Induce curvature and superelevation at a gradual rate Gradually increases from infinity to a maximum value at plan view center point VW L s = 1.82 gy EQ

138 Flow in Curved Channels Spiral banked curves Used in rectangular channels Rotate channel bottom about centerline Maximum banking twice y from EQ 2-31 Inside bend depressed by y Outside bend elevated by y Permits wall heights to be equal on both sides of channel 138

139 Flow in Curved Channels Limiting curvature Subcritical flow Rc >3 times channel width Supercritical flow Use Equation

140 Flow in Curved Channels r min 2 4V W EQ gy Where: r min = minimum radius of curvature V = average velocity W = channel width at design water surface y = flow depth 140

141 Flow in Curved Channels Bend losses Scobey (1933) recommended increasing n by for each 20 o of curvature per 100 feet of channel Maximum increase of Minor losses in HEC-RAS, assumed to be negligent Effective roughness due to secondary currents Recent experiments indicate that losses can be significant for values of r c /W < ~4 141

142 Curved Channels and Shear Stress 142

143 Shear Stress Distribution Upstream Bend Baseline Test 16cfs 12cfs psf psf 8cfs psf Flow Direction psf psf psf psf psf 143

144 Design Guidance: EM-1601 (1970) 144

145 Design Guidance: HEC-15 (1988) R c = Centerline Radius of Curvature B = Channel bottom width K b = τ b / τ o τ b = Maximum bend shear stress τ o = Average approach shear stress 145

146 Data Information Ippen,, A. T., et. al. (1962) ; 16 data points USBR (1964) ; 1 data point Yen, B.C. (1965) ; 5 data points CSU / USBR (2002) ; 8 data points Characteristics (for all sets) Trapezoidal channels Preston Tube utilized for shear measurements Rigid boundary

147 Bend Geometry Ranges Variable Range Side Slope 1 to 2 Bottom Width 1 to 10.2 ft Top Width 1.67 to ft Centerline Radius of Curvature 5 to 65 ft Centerline Radius of Curvature / Top Width 1.22 to 6.73 Centerline Radius of Curvature / Bottom Width 2.5 to 10.83

148 HEC-15 Method Maximum Shear in Channel Bends Ippen (1962) USBR (1964) Yen (1965) CSU (2002) Ippen (1962) Ippen Ippen USBR (1962) (1962) (1964) USBR USBR (1964) Yen (1964) (1965) Yen Yen (1965) (1965) CSU CSU (2002) CSU (2002) (2002) Original HEC-15 Curve Upper Envelope Original HEC-15 Curve Upper Envelope Curve Kb Kb = = τb τb / / τo τo Upper Envelope Curve K b = 4.89(r c /b) Original HEC-15 Curve Original HEC-15 Curve K K b = 2.86(r c /b) b = 2.86(r c /b) Best Fit on All Data K b = 3.16(r c /b) R 2 = Centerline Radius of Curvature, r c / Bottom Width, b

149 EM-1601 Method Maximum Shear in Channel Bends Ippen (1962) USBR Ippen (1964) (1962) USBR Yen (1964) (1965) Yen (1965) CSU (2002) CSU (2002) Original EM-1601 Equation Kb = τb / τo Upper Envelope Equation K b = 3.53(r c /w) Best Fit on All Data Best Fit on All Data K K b = 2.63(r b = 2.63(r c /w) c /w) R 2 R 2 = 0.80 = 0.80 Original EM-1601 Equation K b = 2.65(r c /w) Centerline Radius of Curvature, r c / Top Width, w

150 Comparison of Methods Shear Stress Correction Factor for Channel Bends Linear Fit Fit to to Data Data Line Line of of Equal Fit Fit Kb = τb / τo (CSU/USBR Best Fit Equation) Kb = τb / τo (CSU/USBR Upper Envelope Equation) y = 1.44x y = 1.09x 65 % Maximum Variability K b = K b τ b = / τ b o / τ o (HEC-15) Envelope) 22 % MaximumV ariability

151 Reality 151

152 3-D problem 152

153 Application Boundary shear stress has been determined to be a fundamental part of channel migration (Brown, 1988) 153

154 Theory - Overview Boundary Shear Stress in a stream bend can be expressed as τ =τ +τ b viscous turbulent τ viscous = viscous boundary shear stress in a stream bend τ turbulent = turbulent boundary shear stress in a stream bend Start 8 OCT 08

155 And in further detail as τ o τ lat τ b = total boundary shear stress in a stream bend = average longitudinal viscous shear stress lat = average lateral viscous shear stress = average additional viscous shear stress due to channel geometry τ g Theory - Overview Viscous Turbulent τ =τ +τ +τ +τ +τ +τ b o lat g zx yx zy τ zx = average vertical turbulent shear stress component in longitudinal inal direction τ yx = average lateral turbulent shear stress component in longitudinal nal direction τ zy = average vertical turbulent shear stress component in lateral direction

156 Theory - Illustration Viscous Flow Direction Top of Bank Turbulent τ =τ +τ +τ +τ +τ +τ b o lat g zx yx zy Cross Section Centerline τ zy τ lat θ τ o τ g τ zx τ yx τ b Toe of Bank 156

157 Theory - Illustration Flow Direction Top of Bank Cross Section Centerline τ zy τ lat θ τ o τ g τ zx τ yx τ b Toe of Bank 157

158 Theory Viscous Shear Stress τ =γrs o f Where: τ o = average longitudinal viscous shear stress γ = unit weight of water R = hydraulic radius S f = friction slope

159 Theory Viscous Shear Stress V 2 z x lat z y y ρ τ =ρv V +γs z r Where: τ lat = average lateral viscous shear stress γ = unit weight of water r = centerline radius of curvature ρ = density of water S y = lateral water surface slope V x = average velocity in the x-direction V y = average velocity in the y-direction

160 Theory Viscous Shear Stress τ =τ τ g s o Where: τ g = average additional viscous shear stress due to channel geometry τ o = average longitudinal viscous shear stress τ s = average longitudinal boundary shear stress due to super elevation

161 Theory - Illustration Flow Direction Top of Bank Cross Section Centerline τ zy τ lat θ τ o τ g τ zx τ yx τ b Toe of Bank 161

162 Theory - Illustration τ zx + τ zx z τ yx y δz τ xx τ xx + τ xx τ yx + τ yx δy δx τ zx x 162

163 Theory Turbulent Shear Stresses ( ) 2 u' ( u'v' ) ( u'w' ) τ ρ ρ ρ xx τ xy τxz τ τ τ = ρ ρ ρ = ( ) ( ) 2 ( ) yx yy yz v'u' v' v'w' τ 2 zx τ zy τzz app ρ w'u' ρ w'v' ρ w' normal to the x-plane applied in the x-direction normal to the y-plane applied in the x-direction normal to the z-plane applied in the x-direction ( ) ( ) ( ) normal to the x-plane applied in the y-direction normal to the y-plance applied in the y-direction normal to the z-plane applied in the y-direction normal to the x-plane applied in the z-direction normal to the y-plane applied in the z-direction normal to the z-plane applied in the z-direction

164 Theory Turbulent Shear Stresses ( ) 2 u' τ ρ xx ( ) ( ) 2 τyx τ yy = ρ v'u' ρ v' = τ 2 zx τ zy τzz app ρ w'u' ρ w'v' ρ w' normal to the x-plane applied in the x-direction normal to the y-plane applied in the x-direction normal to the z-plane applied in the x-direction ( ) ( ) ( ) normal to the y-plance applied in the y-direction normal to the z-plane applied in the y-direction normal to the z-plane applied in the z-direction

165 Theory Turbulent Shear Stresses τ ( ) yx = ρ v'u' = τ ( ) ( ) zx τ zy ρ w'u' ρ w'v' normal to the y-plane applied in the x-direction normal to the z-plane applied in the x-direction app normal to the z-plane applied in the y-direction

166 Theory - Illustration Flow Direction Top of Bank Cross Section Centerline τ zy τ lat θ τ o τ g τ zx τ yx τ b Toe of Bank 166

167 Illustration 167

168 Illustration 168

169 τ o Example Data Model discharge was 20 cfs. was calculated to be psf Cross Section Piezometer C C C C C C C C τ p (lb/ft 2 ) τ g (lb/ft 2 ) % of τ o 0% 28% 68% 23% 14% 5% 23% 49% τ lat (lb/ft 2 )* % of τ o 0% 13% 31% 13% 10% 4% 9% 22% * These values were estimated. XS # Piezo % Depth τ yx % of τ o τ zx % of τ o τ zy % of τ o (psf) (psf) (psf) 1 C % % % 2 C % % % 3 C % % % 4 C % % % 5 C % % % 6 C % % % 7 C % % % 8 C % % % 169

170 Illustration of Example Piezo C Cross Section 5 In this particular example, τ b, would have a magnitude of τ zy τ lat θ τ o τ g τ zx τ yx psf with an angle θ of 82.4 degrees, which is a τ b total of 68% larger than τ o. 170

171 Summary of Theory Total boundary shear stress in a stream bend can be theoretically calculated, however: All terms except, τ o, require a detailed set of data The dataset would require: Direct shear measurements Water surface elevations throughout and across the bend Turbulent three-dimensional velocities Detailed channel geometry All of which are not generally available 171

172 Shear Stress Distribution Upstream Bend Baseline Test 16cfs 12cfs psf psf 8cfs psf Flow Direction psf psf psf psf psf 172

173 HEC-15 Method Maximum Shear in Channel Bends Ippen (1962) USBR (1964) Yen (1965) CSU (2002) Upper Envelope Original HEC-15 Curve Upper Envelope Curve Upper Envelope Curve K b = 4.89(r c /b) Kb = τb / τo Original HEC-15 Curve K b = 2.86(r c /b) Best Fit on All Data K b = 3.16(r c /b) R 2 = Centerline Radius of Curvature, r c / Bottom Width, b

174 HEC-15 Method 3.5 Maximum Shear in Channel Bends (HEC-15) Ippen (1962) USBR (1964) Yen (1965) Upper Envelope 8 cfs Maximum Kb P iezo d T(P reston)/t(hec-ras Approach) 12 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 20 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 8 cfs HEC c fs HEC c fs HEC cfs HEC-15 8 cfs Maximum Kb Outside of Bend T(P reston)/t(hec-ras Approach) 12 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Outside of Bend T(P reston)/t(hec-ras Approach) 20 cfs Maximum Kb Outside of Bend T(P reston)/t(hec-ras Approach) New Upper Envelope Curve New All Data For Best Fit Upper Envelope Curve P ower (New All Data For Best Fit) P ower (New Upper Envelope Curve) Kb = τb / τo Old Upper Envelope Curve (Black Dash Line) K b = 4.89(r c /b) New Upper Envelope Curve (Pink) K b = (r c /b) R 2 = Old Best Fit on All Data (Black Solid Line) Kb = 3.16(rc/b) R 2 = 0.59 New Best Fit on All Data (Green) K b = 2.842(r c /b) R 2 = Centerline Radius of Curvature, r c / Bottom Width, b

175 EM-1601 Method Maximum Shear in Channel Bends Ippen (1962) USBR (1964) Yen (1965) CSU (2002) Kb = τb / τo Upper Envelope Equation K b = 3.53(r c /w) Best Fit on All Data K b = 2.63(r c /w) R 2 = Centerline Radius of Curvature, r c / Top Width, w

176 EM-1601 Method 3.5 Maximum Shear in Channel Bends (EM1601) Ippen (1962) USBR (1964) Yen (1965) 8 cfs Maximum Piezo d Kb T(Preston)/T(HEC-RAS Approach) 12 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 20 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 8 cfs Kb (HEC-15) 12 cfs Kb (HEC-15) 16 cfs Kb (HEC-15) 20 cfs Kb (HEC-15) 12 cfs Maximum Kb Piezo d T(preston)/T(HEC-RAS Approach) 20 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 8 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) New Upper Envelope Curve Old Best Fit Power (Upper Envelope) Power (Old Best Fit) Power (All Data) Power (New Upper Envelope Curve) Kb = τb / τo Old Upper Envelope Equation (Black Dash Line) K b = 3.53(r c /w) New Upper Envelope Curve (Pink) K b = (r c /w) R 2 = New Best Fit on All Data (Green) y = (r c /w) R 2 = Old Best Fit on All Data (Black Solid Line) Kb = (rc/w) R 2 = Centerline Radius of Curvature, r c / Top Width, w

177 Reality 3.5 Maximum Shear in Channel Bends (EM1601) Ippen (1962) USBR (1964) Yen (1965) 8 cfs Maximum Piezo d Kb T(Preston)/T(HEC-RAS Approach) 12 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 20 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 8 cfs Kb (HEC-15) 12 cfs Kb (HEC-15) 1.5 < K 16 cfs Kb (HEC-15) 20 cfs Kb (HEC-15) 12 cfs Maximum Kb Piezo d T(preston)/T(HEC-RAS Approach) 20 cfs Maximum Kb Piezo d T(Preston)/T(HEC-RAS Approach) 8 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) 16 cfs Maximum Kb Outside of Bend T(Preston)/T(HEC-RAS Approach) New Upper Envelope Curve Old Best Fit Power (Upper Envelope) Power (Old Best Fit) Power (All Data) Power (New Upper Envelope Curve) b < 3.25 Kb = τb / τo Old Upper Envelope Equation (Black Dash Line) K b = 3.53(r c /w) New Upper Envelope Curve (Pink) K b = (r c /w) R 2 = New Best Fit on All Data (Green) y = (r c /w) R 2 = Old Best Fit on All Data (Black Solid Line) Kb = (rc/w) R 2 = Centerline Radius of Curvature, r c / Top Width, w 1.05 < K b <

178 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels 178

179 Special Considerations Freeboard Distance from WSE to top of channel wall or containment Provides factor of safety against Changes in hydrology Urbanization Sedimentation Variations in channel/overbank roughness 179

180 Special Considerations Freeboard Identify local areas where WSE may be indeterminate Bridge piers Hydraulic jumps Transitions Drop structures 180

181 Freeboard Amount Concrete lined channels Rectangular sections 2 ft Trapezoidal sections 2.5 ft Riprap channels 2.5 ft Earthen channels 3 ft Consequence of damage! 181

182 Special Considerations Sediment Transport EM-1601 provides limited information and is generally out of date However.. Plate

183 Plate

184 Plate 27 Estimate relative effects of channel characteristics on bed-load movement Estimate equilibrium sediment discharge Estimate scour and/or deposition Estimate size of a detention basin 184

185 Open Channel Hydraulic Theory Physical Hydraulic Elements Hydraulic Design Aspects Flow Through Bridges Transitions Flow in Curved Channels Special Considerations Stable Channels 185

186 Stable Channels Lanes balance 186

187 187

188 Stable Channels Lanes balance Scour of channel bed and banks are typically main concerns Revetments designed through analysis of tractive force: Limiting shear stress Limiting velocity 188

189 Limiting Velocity 189

190 Limiting Velocity Plate

191 Chapter 3 Riprap Protection 191