Terrain assessment, drainage and erosion - Geotechnical Investigation and Design Tables - page 192

Table 15.9 Slope erodibility with grades.

Erosion potential Grade %

High 10% >

Moderate 10

−

5%

Low 5%

<

15.10 Typical erosion velocities based on material
•

The definition of erosion depends on its application, ie whether internal or surface
erosion. Surface erosion against rainfall is also different from erosion in channels.

•

The ability of a soil to reduce erosion depends on its compactness.

•

The soil size (gradation characteristics), plasticity and cohesiveness also affect its
erodibility.

•

Fine to medium sand and silts are the most erodible, especially if uniformly graded.

•

The table is based on Hjulstrom's Chart (Figure 15.1) based only on particle size
for stream flow velocities. However the state of the soil (compactedness) and the
relative proportion of materials also influence its allowable velocity.

Table 15.10 Typical erosion velocities.

Soil type Grain size Erosion velocity (m/s)

particle size only

Cobbles, cemented gravels, conglomerate. 60 mm 3.0 >

Soft sedimentary rock
Gravels (coarse) 20 mm to 60 mm 2.0

Gravels (medium) 6 mm to 20 mm 1.0

Gravels (fine) 2 mm to 6 mm 0.5

Sands (coarse) 0.6 mm to 2 mm 0.25

Sands (medium) 0.2 mm to 0.6 mm 0.15

Sands (coarse) 0.06 mm to 0.2 mm 0.25

Silts (coarse to medium) 0.006 mm to 0.06 mm 0.5

Silts (fine) 0.002 mm to 0.006 mm 1.0

Clays 0.002 mm 3.0 <

•

Hard silts and clays (C u 30%) is expected to

have a higher allowable velocity than that shown. Conversely, very soft materials
of low plasticity may have a lower velocity.

> 200 kPa) and high plasticity (PI >

•

Very dense sands and with high plasticity material mixed is expected to have a
higher allowable velocity.

15.11 Typical erosion velocities based on depth of flow
•

In channels, the depth of flow also determines its erosion velocity.

15.12 Erosion control
•

Erosion control depends on the size and slope of the site.