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

Porous Flow• Flow through granular medium sand, pebbles, stones porous flow in a dike flow under a caisson... Filters against erosion:Prevent the washing away of underlying materials Nee

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

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

Possible design criteria

Geometrically closed filters

5

F B

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

Filters in unstationary flow

Tests with flow over bed

protection with filter

• 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

1 - 5 0.1 - 1 0.05 – 0.5 0.01 – 2

Blocking of geotextile

Overall stability of filter with

geotextile