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The concept of erosion threshold Lecture 14

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The concept of erosion threshold. Lecture 14. Partheniades and Mehta (1982) ... Erosion thresholds in Venice Lagoon. Sederode (HR Wallingford) Mini Flume (GSCA) ... – PowerPoint PPT presentation

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Title: The concept of erosion threshold Lecture 14


1
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)
2
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

3
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
4
Erosion threshold vs bed density
5
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

6
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)
7
The rates of bed erosion(2)
  • Parchure Mehta (1985)
  • where m3 E0

E
Type I
C
Time(t)
8
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

9
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 (?)
10
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 (?)
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