Review on sediment scour, transport, and deposit in coastal environments, including under tsunami forcing Jørgen Fredsøe, Techn. Univ.

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1 Review on sediment scour, transport, and deposit in coastal environments, including under tsunami forcing Jørgen Fredsøe, Techn. Univ. Denmark

2 At which location are the flow velocity largest? Carrier et al 2003: at the moving shoreline. Max inshore direction:initial waveform depression. Offshore directed if initial wave have a dominant elevation characteristic.

3 Soil properties?? Sand in the sea and in the beach. Brought further onshore:???

4 I: Run up: onshore sediment transport II: Sedimentation III: Draw down :offshore sediment transport

5 River flow: Flow velocities lower reaches: flood: m/s, Sand Upper reaches, Mountain streams: 4-6 m/s, stones

6 Rivers: Engelund-Hansen: θ smaller than 2-3

7 But coarser fractions can be mobilized

9 Waves D=40 m T=15 sec H=20m (e.g. thenorth Sea) : U(max)=3.7 m/s layer). θ about 5. (thin boundary

10 Tsunami: T=13 min, Amplitude=0.75 m in 2000 m waterdepth! = H s 0 / L " Slope: 1/15:! =53, Vertical run-up=3.5 m, U(max)= 0.42 m/sec S=1/120:! =6.6, run-up=9.9 m, U=9.5 m/sec

Observed or estimated flow velocities Hokkaido 1993: 10-18 m/s (Tsutsumi

11 Observed or estimated flow velocities Hokkaido 1993: m/s (Tsutsumi et al, ASCE, 2000) (evaluated by considering deformation of railway tracks)

12 Morphology on land: Flat Dikes Dunes

13 Planform view longshore variations in dune crest level etc

15 Picture by Barbara Keating 2004 Indian Ocean Tsunami Incised Erosional Channels (30 m long & 1 m deep)

16 What special is there about a Tsunami from a hydrodynamic/sediment-transport point of view, compared to coastal or river environment. Magnitude of flow velocity Duration (transient problem) The magnitude of run-up Flow reversal Groundwater flow Sediment properties.(fine sediment+large V)

17 What to focus on? 1.Basic concepts of sand transport mechanisms 2.Boundary layers (bed friction) and turbulence in waves. 3.Net transport in a Tsunami 4.Scour in waves

18 PART 1: Non-cohessive sediment transport modeling

19 Some simple physical considerations on transport of bed load, suspended load and sheet flow

20 2,, U u!

21 1 2! F = " c $ # U & U % d d D f B 2 ' ( 4 2! F W g s d 6 = µ = " ( # 1) 3 µ s d d

22 \\MEKFSA\E- Home\jf\strøm1\Introd uktion\bedload transport.mov

23 1 2! Fd = " c $ D # U f U % B d 2 ' & ( 4 2! F = W µ = " g( s # 1) d µ 6 3 s d d U U b f #! $ c = " U f & 1% 0.7 ' (! )

24 Tau is transferred to the grains as drag, so τ decreases to the critical! =! + nf b c D " =! b # g( s $ 1) d! " = " + µ c 6 d p

25 ! p q = d 3 U b 6 2 b d qb " b = = 8(! #! ) 3 c ( s # 1) gd 3/ 2

26 c c! ( D z b ) Z = f b z D! b w Z =!U

27 Suspended sediment Requirement for a bed particle to go into suspension ws U! " f

28 Settling=diffusion mixing length=l, vertical velocity fluctuation= v cw +! dc 1 v = 2 dy 0

29 Problems Bed concentration Influence of coherent structures in the turbulence

30 Bagnold 1954

32 2.5! µ ' = µ (1 "! /! ) m m φ=solid fraction, =0.64

33 Bagnold-1954! # $ du % = 0.013s &" d ' ( dz ) 2 c = 0.65 (1 + 1/!) 3

34 Bed concentration of suspended sediment! s # $ du % = 0.013s &" d ' ( dz ) 2! =! + nf +! b c D s 0.013! = s"# 2 s 2 b $

37 Wilson 1966: The moving solid particles appeared generally to be travelling in a dense layer immediately above the bed supported by intergranular collisions rather than by fluid turbulence Sheet flow

39 Break: Sheet flow movie

41 Collisions between particles create a stress field (a particle pressure P or σ) Jenkins(1987) kinetic theory for rapid grain flow Jenkins and Hanes (JFM 1998): turbulent velocity fluctuations neglected Hsu, Jenkins and Liu (Proc Royal Soc 2004): incl description of turbulence

42 A simple sheet flow model(engelund 1981) Based on Bagnolds expression Assumes the particles to move with max concentration in a layer of considerable thickness - compared to the grain diameter

44 (" # " ) cg + = 0 s f d! dz or d! ( ) " = # c( s # 1) g dz

45 0 c ( s 1) gl 0 " = # du U = f,! dz z = d! z! 0 # 2 % du & = 0.013s '" 0 ( d ) dz * % U f & = 0.013s '" 0d ( + 1.3sU ) $ d * 2 2 f

46 q = c LU B b 0 U B = KU f K = 10.5 c 0 =.33

47 L d = 1.3s! " = 4.5s! b 3 2

48 s=2.670: eq3 s=1.138: eq4 " = 12(! #! ) " = 5(! #! ) c c

49 Hsu et al, 2004

50 Sediment mixtures

51 1 2 ' ( 4 2! Fd = " c $ # D U f & U % B d 2! F W g s d 6 = µ = " ( # 1) 3 µ s d d

52 Bed forms: not in sheet flow

53 Conclusions: Sediment transport in a tsunami will usually be as sheet flow and in suspension. Models are availible for the sheet flow in current Coarse sediment will be transported as bedload, but might be easier mobilized due to presence of fine sediment The transition from the sheet flow layer to the suspended sediment is not fully descriebed, and the bottom boundary condition for the suspended sediment needs clarification Transport of mixtures needs to be studied

54 PART 2: SEDIMENTTRANSPORT IN WAVES Needed for sediment transport : friction, level of turbulence

55 Potential flow and Wave boundary layer

56 Flow velocities in the wbl

57 Turbulence level in wbl

58 Friction factor Boundary layer thickness fw = 1 2 ρu 2 10

59 Vertical distribution of sediment % c % # % c $ = & wc +! ' % t % y ( % y ) cbed = cb( " )

60 Distribution of suspended sediment: comparison with lab measurements

61 Distribution of suspended sediment: comparison with field measurements

62 Sediment in broken waves

65 Distribution of sediment in a spilling breaker

66 Lin and Liu, 1998: VOF (RIPPLE)+k-ε Normalized turbulence intensity

67 On or off shore transport Waves: Asymmetry in nearbed velocities Drift (time averaged) Streaming (time averaged) Percolation Wave breaking: Undertow (timeaveraged)

68 Wave asymmetry Larger flow velocities onshore than offshore Sed tr proportional to U**3

69 Ribberink 2006: wave asymmetry

70 Adaptation length or time

71 Percolation

72 Web search Tsunami AND turbulence Boundary layer under a solitary wave

73 Net transport in a Tsunami (erosive or depositive) 1:the very much idealized case

74 Run up U=5m/s : δ =1m after 60 sec.

75 Draw down: boundary layer much thicker Result: larger friction on- than off-shore (?)

76 Puelo and Holland, Coastal Eng 2001

77 Larger flow velocities during draw down than in the Run-up.

78 Flow in the sand Weak seapage Liquefaction Fluidization

79 Austin et al 2006: Gravel beach

80 Seepage Ground water flow pattern in the beach during falling sea level.

82 Fluidization

83 Seepage

90 Seepage destroy the velocity profile

92 Liquefaction: reshaping the grain skeleton

95 Momentary liquefaction

96 Is it a bore?? or a fast rising tide- or a surge?? more sediment onthan off-shore

97 ! = H s 0 / L "

98 Petti &Longo, Coastal Eng 2001

99 Effect of external generated turbulence on sediment-transport (Sumer et al. 2003)

100

101

102

103 Scour Part 1: SCOUR IN DUNES: Important for a Tsunami: timescale

104 Dunes

105

106

107 Breach 1d

108 Breach 2d

109 Need for research Wave boundary layers in runup and drawdown, and under breaking waves Impact of sheet flow on near bed turbulence and the bed condition for suspended sediment in the sheet flow regime Transport of mixtures in the sheet flow

110 Part 2 Scour arond structures in a Tsunamilike environment

111 roads, bridges, buildings, etc.

112 Culverts Bridge abutments

113

114 Types of scour (Melville and Coleman)

115 Degradation

116 (Melville and Coleman)

117

118 The wood increases the effective width of the pier and decreases the effective clear span for the flow. In addition it spoils the smooth lines of the pier

119

120

121 Melville and Coleman

122 Scour in a current: Bridge scour:

123 d = K K K K K K K s yb I d s! G t yb=depth-size I=flow intensity d=sediment size s=shape θ = pier alignment G=channel geometry T=time

124 Scour depth variation with velocity

125 Sediment gradation

126 Variation with shallowness

127

128

129

130 Scour movie by Roulund

131 Stone-protection

132 foredrag\copy of Riprap1.avi

133 Sacrificial piles: deflects the high velocity flow

134 Vanes: inducing secondary currents which interfear with the horseshoe vortex

135 Collar:Shields sediment bed from the downflow and the horseshoe vortex

136 Unprotected pile, where vortex is well-defined before shedding Pile with splitter-plate, where vortex grows initially and finally breaks down

137

138 Marine environment

139

140

141 Scour around a cube under wave action (Univ. Sydney)

142

143 Difference in scour mechanism in between current and waves: Flow attack from two directions KC-dependence (maybe no vortex shedding)

144 Wave scour depends on KC KC = 2! a D

145 Tsunami scour Does the flow last long enough time so the scour will develops fully? : Time-scale!

146 Additional problems: liquefaction: reshaping the grain-sceleton Momentary fluidization: the effective stresses between the grains disappear due to groundwater flow.

147 Two very idealized cases: The vertical cylinder (pile) The horizontal cylinder (pipeline, seewage pipe etc).

148 Pipeline, placed on the ground Pressure variation in a current

149 Onset of scour: Seepage flow

150

151

152 Onset of Wave scour depends on KC KC = 2! a D

153 Tunnel erosion (piping)

154 Scour development, current

155

156

157 Tsunami?? Final scour depth

158 T =! " * 5/3 / 50 T * = g( s " 1) d D 3 3 T

159 Example: Diameter=30 cm grain size=.5 mm Waterdepth=5 m Flow velocity=.6 m/s T=1.4 hour Waves: waveperiod=10 sec H=2m depth=10m: T=5min Flow velocity=6m/s T= 3 sec.

160 Vertical structure: the single pile Waves Pile D (a) Scour hole at a vertical circular pile under waves S Pile Waves Vortex shedding (b) For 6 < KC < 100, vortex shedding is the dominant mechanism for scour at piles under waves

161

162 Shape-effect

163 Current T T * 2.2 * =! " = h / D / 2000 g( s " 1) d D 2 3 T Waves T " # KC $ = 10 % & '! ( * 6 3

164 D=3m d=.5 mm V=.6 m/s h=5 m :T=50 hours V instead 6 m/s: T=8 sec,

165

166 1 S / D 0.1 Sumer et al. (1992) Eq. (8) D (cm) Protection type 4 Unprotected 6 Unprotected 4 Splitter-plate Experimental results of unprotected piles correspond closely with the curve proposed by Sumer et al. (1992) Reduction of scour depth for protected piles is prominent 6 Splitter-plate KC 1 Sumer et al. (1992) Eq. (8) D (cm) Protection type S / D Unprotected 6 Unprotected 4 Threaded pile (b = 2 cm) 6 Threaded pile (b = 2 cm) 4 Threaded pile (b = 3 cm) Threaded pile (b = 3 cm) KC Variation of nondimensional scour depth S/D with KC number for unprotected and protected piles under waves

167 Fluidization

168

169

170 Sand gravel

171

172 Need for research Is scour different in the sheet flow regime Scour in super-critical regime Timescale for scour at large Shields parameters Scour around structures with large horizontal dimensions compared to waterdepth

Simulation of tsunamiinduced boundary layers and scour around monopiles

Simulation of tsunamiinduced boundary layers and scour around monopiles David R. Fuhrman Isaac A. Williams Bjarke Eltard Larsen Cuneyt Baykal B. Mutlu Sumer Boundary layer model Fuhrman, D.R., Schløer,