The concept of erosion threshold Lecture 14

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The concept of erosion thresholdLecture 14

• Partheniades and Mehta (1982)
• Type I erosion (asymptotic)
• Type II erosion (continuous)
• ?b - bed shear strength
• ?c - critical shear stress
• ?o - fluid bed shear stress

concentration(C)
No erosion
erosion
stress(?o)
?o gt ?c
?o lt ?c
Threshold (?c )
Time(t)
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Erosion types

• Type I erosion
• floccule or aggregate erosion
• decreases with time
• benign erosion
• transitional flows
• Type II erosion
• failure along planes of weakness
• continuous with time
• chronic erosion
• turbulent rough flows

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SEA CAROUSEL Sample hose and power cable Shaft
end-coder (lid rotation) Electronics
housing Rotating lid of flume Digital motor
drive MINI FLUME Deployment frame controller
pack for motor drive Springs at top of
flume Battery pack for motor
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Erosion threshold vs bed density
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Erosion thresholds in Venice Lagoon

• Sederode (HR Wallingford)
• Mini Flume (GSCA)
• Sea Carousel (GSCA)
• Big differences between devices
• Differences in size
• Differences in operation
• Differences in threshold concept
• Differences in applied stress
• Differences in location
• Differences in time

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The rates of bed erosion(1)
E

• Owen (1970)
• where m 1.07 - 2.04
• Thorn Parsons(1980)

Type I
C
C
Type II
E
Time(t)
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The rates of bed erosion(2)

• Parchure Mehta (1985)
• where m3 E0

E
Type I
C
Time(t)
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The process of bed erosion (type I)

• erosion takes place where
• ?o gt ?c
• erosion rate f(?o - ?c)
• C(t) increases asymptotically
• bed erosion stops when
• ?b ?o
• erosion depends on
• consolidation profile

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Bed erosion and the Mohr-Coulomb plot(1)

• ?c is force/unit area to fail the bed
• ? is the geostatic and hydrostatic pressures
  within the sediment column
• the ratio ?c(z) ?(z) is constant ( 0.25)

?
critical shear stress(?c)
Effective stress (?)
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Bed erosion and the Mohr-Coulomb plot(2)
critical shear stress(?c)

• ?c is measured with depth for (E 0)
• i (1 - porosity)
• p is often unknown (assumed zero)
• ? 140 for normally consolidated bed (linear)

?
Exercise A sediment column has a friction angle
of 200 and a cohesion of 2 Pa. What is (1) the
buoyant effective stress at 0.01 m depth for a
bed of average porosity 0.5 and excess pore
pressure of 5 Pa ? (2) what is the erosion
threshold at that depth ? Assume sediment density
and water densities of 2650 and 1026 kg/m3
?c(0)
(cohesion)
Effective stress (?)