Surface Run-off, Soil Erosion and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 137

4.4 Sub-watershed Wise Analysis of Curve Number
(AMC-III), Potential Retention (S) and Run-off

The determined Curve Number (CN) under antecedent moisture condition-III
(AMC-III) for sub-watershed I, II, III, IV, V and VI are 85.02, 73.52, 87.36, 87.76,
85.57 and 89.85 respectively. The potential retentions (Table 4.10 ) are very low for
the sub-watershed VI (28.69 mm), IV (35.46 mm) and III (36.75 mm) which
indicates that these 3 sub-watersheds are signi cant run-off contributors irrespective

of areal coverage. In terms of surface run-off, sub-watershed-I contribute
4,52,349.40 m 3 , sub-watershed-II 50,641.50 m 3 , sub-watershed-III 1,90,278.00 m 3 ,
sub-watershed-IV 1,07,109.00 m 3 , sub-watershed-V 66,174.00 m 3 and sub-water-
shed-VI 3,48,948.30 m 3 . Estimated LPIV is very high for Sub-watershed I and VI
(Table 4.10 ). On the basis of the estimated potentialities of run-off and landslide
potentiality index value (LPIV) an instability scale is made which reveals that sub-
watershed I is very much prone to soil erosion and slope instability, followed by VI,
IV and III. Considering the greater contributing area, sub-watershed I not only
became the most contributor of the run-off of the entire Shivkhola Watershed but
also became a spatial unit of high magnitude landslide potentiality. Running water
on steep slope in the form of turbulent, concentrated flow or sheet wash dislodge
materials from slope surface and thus deserves due importance.

4.5 Periodic (Monthly) Relationship Between Surface
Run-off and Slope Susceptibility

The month wise estimated surface run-off (Table 4.11 ) states that the month of July
experiences the higher amount of runoff and which is followed by August, June and
September mainly during the rainy season. The potentiality to soil erosion, soil
saturation and landslip of the sub-basin I and VI are greater because these two
contribute considerable surface run-off to the Shivkhola watershed. The time span
between October and March is attributed with lower amount of surface runoff
throughout the watershed. This time span is called as during which

the potentiality to soil erosion, soil saturation and soil slip are very low, whereas
that are more frequent during the wet period (July, August and September). On the
basis of the surface runoff from the whole Shivkhola watershed, a year is divided
into two distinct periods:—(i)

' the dry period '

— (ii) less susceptible period (October April) and (ii) more

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susceptible period (June September). Thus all the required constructions and other

preparedness are to be completed during less susceptible period.

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