Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 129

The USDA soil textures normally included in this group are clay loam, silt clay
loam, sandy clay, silt clay and clay. These soils have a very low rate of water
transmission between 0.0 and 0.13 cm/h. Some soils are classi

ed in group D
because of a high water table that creates a drainage problem; however, once these
soils are effectively drained, they are placed into another group. The following
parameters were used to assess run-off curve number and all these parameters are
related with hydrologic soil group.

ltration rate: See Table 4.2 .
(ii) Soil permeability: These are tentatively suggested rates through saturated
undisturbed areas under about 0.5 in. (12.7 mm) head of water.
(iii) Draining parameter of the soil:Inwell drained condition water is removed
from the soil readily but not rapidly. Well drained soils are commonly
intermediate in texture and moderate to rapid in

(iii) In

ltration and permeability
rate with hydrologic soil group A and B, although soils of other textural
classes may also be well drained (Tables 4.2 and 4.3 ). In moderately well
drained situation water is removed from the soil somewhat slowly so that
the pro

cant part of the time. Moderately well
drained soils commonly have a slowly permeable layer within or immedi-
ately beneath the solum or some combination of these conditions consisting
hydrologic soil group of B and C (Table 4.3 ). The soil remains wet for a
large part of the time in poor drained condition. The water table is com-
monly at or near the surface during a considerable part of the year.
(iv) Effective depth of the soil: This refers to depth of soil material that is
readily penetrated by plant roots, and is important to the moisture regime
of the soils. The effective depth

le is wet for a small but signi

figures are those observed during eld

le examination. In case of those soils which are lying over disinte-
grated and weathered rock material (murrum), the total depth of such a
layer has been converted into effective soil layer by assuming percent soil
material in the layer. The total depth is then calculated by adding this
assumed depth to overlaying thickness of soil layers.
(v) Average clay content within the soil at various depths: This pertains to
the average clay content in the whole pro

pro

le depth (Table 4.4 ). To derive

this figure clay content of each horizon is multiplied by thickness of the

horizon. Sum of all the horizons is then divided by total thickness of the
pro les. Example — Hydrologic soil group-A have less than 10 % clay and

Table 4.4 Average clay contain according to thickness
Horizon Thickness (cm) Clay (%) Avg. clay contain

1 15 20 300 1845/75 = 24.4 %

2 20 23 460

3 25 29 725

4 15 24 360

Total 75 1,845

Hand book of Hydrology ( 1972 )