REALTIME FORECASTING OF SHALLOW LANDSLIDES USING RADAR-DERIVED RAINFALL Ryohei Misumi, Masayuki...

REALTIME FORECASTING OF SHALLOW LANDSLIDES USING RADAR-DERIVED RAINFALL

Ryohei Misumi, Masayuki Maki, Koyuru Iwanami,

Ken-ichi Maruyama and Sang-Goon Park

National Research Institute for Earth Science and Disaster Prevention, Japan

By

Shallow landslide at Izumozaki-town, Niigata Prefecture on 13 July 2004

Outline

X-band polarimetric radar

Distributed rainfall-runoff model

Real-time forecast of landslides

Soil water

Rainfall estimation in high accuracy and resolution

WeightBed rock

Soil layer

Slip surface

Sliding force

Shear resistance

Normal situation

What is shallow landslide ?

Bed rock

Soil layer

Saturated Zone

Soil particles

Water During heavy rainfall

Increase of pore pressure

Decrease of normal stress

Rain

Estimation of rainfall intensity using the specific differential phase (KDP)

not affected by rain attenuation

immune to beam blockage

less sensitive to beam filling and drop size distribution

＋＋

Z KDP

Disdrometer

+

R-KDP

R-Z

R-Z (corrected)

Catchment50m grid

Rainfall

Distributed rainfall-runoff model

Evaporation

Infiltration

Direct runoff

Base flow

Maximum storage capac

ity

SmaxWater

storage

S

50m grids

tan

'tan)])((1[

cossin

' w

z

h

z

cF

γγ

γ

Infinite slope stability analysis

F: Factor of safety

γ, c', Φ': Soil properties

γw: Density of water

h: Level of groundwater

z: Depth of soil

β: Inclination of slope

maxS

S

z

h

80km

Tsurumi river basin

MP-X radar

Real-time evaluation

Meso-β scale depression on 9 July 2005

22:59

Rainfall rate

Water storage

Predicted landslides

Meso-β scale depression on 9 July 2005

23:29

Rainfall rate

Water storage

Predicted landslides

Meso-β scale depression on 9 July 2005

23:53

Rainfall rate

Water storage

Predicted landslides

3 km

Actual landslide

Predicted grid

Landslide on 9 July 2005

Evaluation with past events

Upper Abukuma River basin (151.07km2)

Rainfall event in August 1998

0 4321 5

02JST on 27 August

(2 hours before severe landslides)

Distance (km)

Dis

tanc

e (k

m)

Slope failure

Predicted grids

Welfare facility

Predicted factor of safety (F) and occurrence of landslides

Factor of safety

Occur Not occur

F < 1 205 grids 1809 grids

F > 1 56 grids 5318 grids

Correct forecasts: 74.8 %

False alarm ratio: 89.8 %

Conclusions

An accurate forecast of shallow landslides is still difficult.

Increase of landslide potential in a few-km scale may be predictable using rainfall observation with the X-band polarimetric radar (lead time is about 2 hours).

The lead time would be greatly extended if a reliable QPF is available.