1. Evaluation of Rainfall Thresholds for Ground Saturation & Slope Stability By Erkan Istanbulluoglu

2. OBJECTIVES Evaluation of;
OBJECTIVES Evaluation 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

3. Project Area The 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 44o25’N latitude, and 115o45’W 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.

4. Preperation for the analyzes in GIS/Arc-view
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.

5. Watershed Contributing Area Per Unit Contour Length

6. Slope of The Watershed

7. Calculation of Threshold Rainfall Depth for Ground Saturation By steady-state subsurface
a, Contributing Area per Unit Contour Length
R, rainfall
For Saturation hw=h and
q=K h Sin q also
q=R a finally
RT= T Sin q / a
T=K h ( Transmissivity )
When R>RT
Q=R a - T Sin q
Subsurface Flow
Ground Surface h
Bed Rock hw
q, subsurface flow q
Schematic view of hillslope subsurface flow process

8. Rainfall Threshold for Slope Stability
Infinite Slope Stability model is used (Pack., et.al. 1998)
FS is the factor of safety, when FS<1, there is non-stability and and a landslide may occur.
C:combined cohesion
f: Friction angle
r: rw/rs
w:Relative 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 R>RT will cause FS<1 at that point.

9. Temporal&Spatial Evaluation of Relative Wetness
hw’ hw
h is the porosity q

10. Watershed Hydrological Parameters The 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). K=1.8m/day h=0.6m C= h=0.3 f=30o

11. Threshold Rainfall Depths in (mm/day)

12. Runoff For a Given Maximum Threshold Event Total Runoff =21302
Runoff For a Given Maximum Threshold Event Total Runoff = m2/day*30m=639079m3

13. Rainfall Thresholds for Slope Stability in (mm/day) when C=0.20

14. Rainfall Thresholds in (mm/day) when C=0

15. Evaluation of Relative Wetness
5 days after saturation
15 days after saturation

16. Relative Wetness

17. Conclusion 1)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 transport 3) Relative Wetness in time forms the initial wetness conditions in a watershed before a rainfall event