Geoscience Reference
In-Depth Information
Table 15.11
Suggested competent mean velocities for erosion (after TAC,
2004).
Bed material
Description
Competent mean velocity (m/s)
Depth of
1.5
3
6
15
flow (m)
Cohesive
Low values - easily erodible
0.6
0.65
0.7
0.8
PI
<
10% and C
u
<
50 kPa
Average values
1.0
1.2
1.3
1.5
PI
100 kPa
High values - resistant
10% and C
u
>
<
1.8
2.0
2.3
2.6
PI
20% and C
u
100 kPa
>
>
Granular
Medium sand
0.2-0.6 mm
0.65
1.0
1.4
2.2
Coarse sand
0.6-2.0 mm
0.75
1.1
1.5
2.2
Fine gravel
2.0-6 mm
0.9
1.2
1.6
2.3
Medium gravel
6-20 mm
1.2
1.5
1.8
2.5
Coarse gravel
20-60 mm
1.7
2.0
2.2
2.9
Cobbles
60-200 mm
2.5
2.8
3.3
4.0
Boulders
200 m
3.3
3.7
4.2
5
>
•
The uses of contour drains, silt fences or vegetation buffers are typical control
measures.
Table 15.12
Erosion control measures.
Consideration
Typical erosion control measures spacing
Vegetation buffers
Contour drains
Silt fences
Slope
5%
75 m
50 m
25 m
10%
50 m
40 m
15 m
15%
25 m
30 m
10 m
Typical details
10 m strips of thick
250 mm ditch to divert flow with
0.5 m high posts with
grass vegetation
soil excavated from the formed
filter fabric buried
to trap sediment
ditch placed as compacted
250 mm at the bottom
earth ridge behind
Application
Adjacent to
Temporary protection at times of
Temporary sediment
waterways
inactivity. Diverts water runoff
barrier for small sites
to diversion channels
•
Suitably sized vegetation buffers and contour drains may also be used as permanent
erosion control features.
•
Refer Chapter 16 for added details on silt fences.
15.13 Benching of slopes
•
Benching of slopes reduces concentrated run off - which reduces erosion.
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