Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 128

Table 4.2 In ltration rates used in hydrologic soil groupings
Class Hydrologic

soil group

Rate/hour Remarks

Inches Millimeters

Very
low

D Less than

0.1

Less than
2.5

High clayey soil

Low C 0.1 0.5 2.5 12.5 Shallow soils, clay soils, soils low

in organic matter

- -

Medium B 0.5 - 1.0 12.5 - 25.0 Sandy loams, silt loams

High A Greater

than 1.0

Greater
than 25.0

Deep sands well aggregated soils

Table 4.3 Permeability ratings used in hydrologic soil groupings
Group Hydrologic soil group Rate/hour

Inches Millimeters

Very slow D Less than 0.05 Less than 1.30

Slow C 0.05 - 0.20 1.31 - 5.00

Moderately slow B and C 0.20 0.80 5.01 20.00

- -

Moderate B 0.80 2.50 20.01 50.00

- -

Moderately rapid B 2.50 - 5.00 50.01 - 130.00

Rapid A 5.00 10.00 130.01 250.00

- -

Very rapid A Greater than 10.00 Greater than 250.0

soil texture normally included in this group is sand, loamy and loamy-sand and
sandy-loam. These soils have a transmission rater greater than 0.76 cm/h. Soils
having moderate in ltration rates and moderately low run-off potential character-

ized by Hydrologic soil group-B, when thoroughly wetted and consisting chieflyof
moderately deep to deep and moderately well to well drained soils with moderately
coarse textures (Tables 4.2 and 4.3 ). These soils have a moderate rate of water
transmission.
The USDA soil textures normally included in this group are silt-loam and loam.
These soils have a transmission rate between 0.38 and 0.76 cm/h. Soils having slow
in

ltration rates when thoroughly wetted and consisting chiefly of moderately deep
to deep and moderately well to well drained soils with moderately ne to moder-

ately coarse textures includes Hydrologic soil group-C having moderately high run-
off potential (Tables 4.2 and 4.3 ). These soils have a moderate rate of water
transmission. The USDA soil textures normally included in this group is sandy clay
loam. These soils have a transmission rate between 0.13 and 0.38 cm/h. Soils
having very slow in ltration rates and high run-off potential when thoroughly

wetted and consisting chiefly of clay soils with a high swelling potential with a
permanent high water table comprising Hydrologic soil group-D (Tables 4.2 and
4.3 ). These soils have a clay pan or clay layer at or near the surface with shallow
soils cover of nearly impervious material.