Title: The concept of erosion threshold Lecture 14
1The 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)
2Erosion 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
4Erosion threshold vs bed density
5Erosion 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
6The 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)
7The rates of bed erosion(2)
- Parchure Mehta (1985)
- where m3 E0
E
Type I
C
Time(t)
8The 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
9Bed 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 (?)
10Bed 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 (?)