BED, BANK & SHORE BED, BANK & SHORE PROTECTION - CHAPTER 5 doc

Porous Flow
Filters
Porous Flow
• Flow through granular medium (sand, pebbles, stones)
porous flow in a dike
flow under a caisson
Examples of filters
Function of filters:
• Preventing erosion under a top layer
• Preventing pressure build-up
• Combination
Types:
• Granular material: one or more layer of grain of
varying diameter
• Geotextiles
Filters against erosion:
Prevent the washing away of underlying materials
Needed in case of large gradients on interface soil-water
Drainage filters:
Prevent high exess presures under impervious revetment
• When designing filters, stability and permeability are
important aspects
Influence of filter on flow under
impervious revetment
Gravel filter: 1m thick, 2 m wide
Granular filters
Values of k for various materials
Material d
50
(< 63.10
-3

m)
or d
n50
(m)
Permeability, k
(m/s)
Clay
Silt
Sand
Gravel
Small rock
Large rock
< 2.10
-6

2.10
-6
- 63.10
-6

63.10
-6
- 2.10
-3

2.10
-3
- 63.10
-3

63.10
-3
- 0.4
0.4 - 1
10
-10
- 10
-8

10
-8
- 10
-6

10
-6
- 10
-3

10
-3
- 10
-1

10
-1
- 5.10
-1

5.10

-1
- 1
Possible design criteria
Geometrically closed filters
Filter rules
15
85
5
F
B
d
d
<
60
10
10
d
d
<
15
15
5
F
B
d
d
>
Stability:
Internal stability:
Permeability:

Effect of blocking
Geometrically open filters
• Grains of filter layer much larger than of base layer
• Velocity in filter is always below critical velocity
• Movement of base material cannot be initiated
Perpendicular and parallel flow
Perpendicular flow through
geometrically open filter
Critical parallel gradient for
geometrically open filters
Example of a dynamic load
Flow in a geometrically open filter.
Filter 20 mm
Filter Porosity 0.41
Sand 0.82 mm.
Wave period 2 second
Test no velocity gradient
1 16 mm/s 0.05
2 22 mm/s 0.07
3 28 mm/s 0.10
4 32 mm/s 0.13 threshold
5 35 mm/s 0.13 threshold
6 40 mm/s 0.15
7 42 mm/s 0.18
Filters in unstationary flow
Tests with flow over bed
protection with filter
Geotextile
• Foil and cloth-like synthetic materials
• Used in hydraulic, transport, enviromental engineering

• Two main types: woven, non-woven
Constructing dike with geotextile
(Nam Dinh)
Granular filters or geotextile
Improvisation
Definitions apertures geotextile
and migration of fine particles
Permeability
f
u
k
he
Ρ= =
Δ
Type
O
90
(mm) P (1/s)
Mesh net
Tape fabric
Mat
Non-woven
0.1 - 1
0.05 - 0.6
0.2 - 1
0.02 - 0.2
1 - 5
0.1 - 1
0.05 – 0.5
0.01 – 2

P permittivity parameter
Δh head difference
e geotextile
thickness
k ‘normal’ permeability coefficient
Blocking of geotextile