Evaluation of Rainfall Thresholds for Ground Saturation

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Evaluation of Rainfall Thresholds for Ground
Saturation Slope Stability
By Erkan Istanbulluoglu
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OBJECTIVESEvaluation of Threshold Rainfall
Depths Required for Ground Saturation Slope
Stability Runoff Temporal Spatial
Watershed Wetness Parameter Threshold Rainfall
Depths for Slope Stability In a Mountainous
Forested Watershed
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Project AreaThe study area is in the Silver
Creek drainage, a tributary to the Middle Fork of
the Payette River in southwestern Idaho.
Coordinates of the center of the project area are
approximately 44o25N latitude, and 115o45W
longitude.Soil textures are loamy lands to
sandy loams and depth to the bedrock is usually
less than 1 m. Shallow soil less than 20 cm deep
are common on ridges.Hillslopes are steep,
ranging from 15 to 40 degrees. Two principal
vegetation habitat types are Douglas-fir/white
spirea and Douglas-fir/nineninebark, both in both
in ponderosa pine phase.Annual precipitation
890mm, most occurring in winter and 65 of the
precipitation is in snow form.
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Preperation for the analyzes in GIS/Arc-view
DEM and DRG of Middle Fork of the Payette
River is downloaded from USGS. Location of
Silver Creek is found in those maps and outlet
coordinate is determined. Pitt-filling, Flow
directions, slope and contributing area per unit
contour length are calculated by using
TARDEM.
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Watershed Contributing Area Per Unit Contour
Length
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Slope of The Watershed
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Calculation of Threshold Rainfall Depth for
Ground Saturation By steady-state subsurface
a, Contributing Area per Unit Contour Length
R, rainfall
For Saturation hwh and qK h Sin q also qR a
finally RT T Sin q / a TK h (
Transmissivity ) When RgtRT QR a - T Sin q
Subsurface Flow
Ground Surface
h
Bed Rock
hw
q
q, subsurface flow
Schematic view of hillslope subsurface flow
process
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Rainfall Threshold for Slope Stability
Infinite Slope Stability model is used (Pack.,
et.al. 1998) FS is the factor of safety, when
FSlt1, there is non-stability and and a landslide
may occur. Ccombined cohesion f Friction
angle r rw/rs wRelative wetness (hw/h) FS set
equal to 0 and solved for R, and a threshold
value for R (RT) is derived as seen on the
right. Any RgtRT will cause FSlt1 at that point.
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TemporalSpatial Evaluation of Relative Wetness
hw
hw
h is the porosity
q
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Watershed Hydrological ParametersThe parameters
required are K, h, h, f are lumped over the
watershed refering to previous studies by Megehan
et.al,(1991) Megehan and Molitor (1975). The
lumping of similar parameters was done by Barling
et al.,(1994) Dietrich et.al.,(1992) Dietrich
et.al.,(1993) and Wu and Sidle (1995).
K1.8m/dayh0.6mC0-0.20h0.3f30o
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Threshold Rainfall Depths in (mm/day)
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Runoff For a Given Maximum Threshold EventTotal
Runoff 21302.664m2/day30m639079m3
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Rainfall Thresholds for Slope Stabilityin
(mm/day) when C0.20
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Rainfall Thresholds in (mm/day) when C0
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Evaluation of Relative Wetness
5 days after saturation
15 days after saturation
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Relative Wetness
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Conclusion1)Threshold rainfall depths can be
used for evaluating max and min T/R in the
watershed.2)Runoff rates can be used for erosion
and sediment transport3) Relative Wetness in
time forms the initial wetness conditions in a
watershed before a rainfall event