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Effect of intensity of heavy rainfall on infiltration of rainwater into slope through numerical simulations

Yohei USUKI Osaka University, Osaka, JapanKazuhiro ODA Osaka University, Osaka, Japan Keigo KOIZUMI Osaka University, Osaka, JapanKyohei UMEMURA Osaka University, Osaka, JapanTakayuki ONISHI Osaka University, Osaka, Japan

The 14th International Conference of the International Association for Computer Method and Advances in Geomechanics

September 24, 2014

○

Contents

2. Numerical method for analysis

3. Analytical model

5. Conclusion

4. Analysis results

1. Introduction

Four Cases (except for Case-S)

Case-S

Background 1/4 ［Climate in Japan］

Slope disasters happen every year In Japan

TyphoonRainy season

L

It rains frequently over the entire area

Background 2/4 ［Infiltration of rainwater into slope］

Bedrock

Surface runoff

InfiltrationIt’s a key component for slope disastersthat amount of rainwater infiltrate into slope

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Background 3/4 ［Two factors of rainfall］

Bedrock

Intensity DurationRainfall

Long spell of rain

Background 4/4 ［Typical heavy rainfall］

High intensityShort duration

Middle intensityLong duration

Time

Inte

nsity

Typical heavy rainfall

Inte

nsity

Time

Localized torrential rain

Purpose

Consideration of effect of rainfall intensity on infiltration of rainwater

Purpose

Plain slope modelChange : Soil types only

Localized torrential rainLong spell of rain

Slope

Rainfall

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

5. Conclusion

Four Cases (except for Case-S)

Case-S

4. Analysis results

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Numerical method for analysis

Analytical code : Hydrus–2D

reproduce water behavior in unsaturated soils

th

hCxhK

xp

jij

i

)(

Richards’ Equation

h ：Total head

ph ：Pressure head

)(hC ：Specific moisturecapacity

ijK ：Hydraulic conductivity

: gradient of water characteristic curve

Water characteristic curve

Van Genuchten model

nm /11

eS ：Effective saturation

：Water content

s ：Saturated water content

r ：Residual water content ：Material parameter

m ：Suctionn ：Material parameter

k ：Hydraulic conductivity

sk ：Saturated hydraulicconductivity

l ：Material parameter

2/1 })1(1{ mme

les SSkk

)/( rsreS

Water characteristic curve

Hydraulic conductivity

mnm

})(1{

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

5. Conclusion

Four Cases (except for Case-S)

Case-S

4. Analysis results

Analytical model 1/7 ［ Model slope ］

Rainfall

Free drainage

Impermeable

Unit : m

1.0

1.9

7.9

1.5

4.0

1.6

40.0°

An experiment withfull-scale model

at Tsukuba in Japan

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Analytical model 2/7 ［ Model slope ］

1.0

1.9

7.9

1.5

4.0

1.6

40.0°

0.3m each

Observation points(output VWC)

Rainfall

Free drainage

Impermeable

Unit : m

Analytical model 3/7 ［ Model slope ］

Soil condition : Homogeneous

Initial condition (VWC) : θ 0.15

ChangeSoil type

Fix

Rainfall

Free drainage

Impermeable

Unit : m

1.0

1.9

7.9

1.5

4.0

1.6

40.0°

Analytical model 4/7 ［ Localized torrential rain ］

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

Time (h)

Localized torrential rain

Max :119(mm/h)

Observed in Kumamoto on July 12 (2012)

Total :456(mm)

3 4 5 9 10

Analytical model 5/7 ［ Long spell of rain ］

120

100

80

60

40

20

00 6 12 18 24 30 36

Rai

nfal

l (m

m/h

)

Time (h)

Long spell of rain

Max :42(mm/h)

Observed in Shiga on September 16 (2013)

Total :502(mm)

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Analytical model 6/7 ［ Analytical cases ］

SandSandy LoamLoamSandy Silt ClaySilty Clay Loam

Soil Types

Permeability

High

Low

Parameters are predicted bybasic soil data

/Case-S 0.05 0.43 0.15 2.68 8.25 10 0.5Case-SL 0.07 0.41 0.08 1.89 1.23 10 0.5Case-L 0.08 0.43 0.04 1.56 2.89 10 0.5Case-SSC 0.07 0.40 0.02 1.36 1.20 10 0.5Case-SCL 0.09 0.43 0.01 1.52 1.94 10 0.5

Analytical model 7/7 ［ Water characteristic curve］

/Case-S 0.05 0.43 0.15 2.68 8.25 10 0.5Case-SL 0.07 0.41 0.08 1.89 1.23 10 0.5Case-L 0.08 0.43 0.04 1.56 2.89 10 0.5Case-SSC 0.07 0.40 0.02 1.36 1.20 10 0.5Case-SCL 0.09 0.43 0.01 1.52 1.94 10 0.5

0.01 0.1 1 10 100Suction (m)

0.5

0.4

0.3

0.2

0.1

0Volu

me

wat

er c

onte

nt

Case-SCase-SLCase-LCase-SSCCase-SCL

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

5. Conclusion

Four Cases (except for Case-S)

Case-S

4. Analysis results

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

5. Conclusion

Four Cases (except for Case-S)

Case-S

4. Analysis results

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120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Amount of rainfall

Rainfall

Amount of rainfall (Localized torrential rain)

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Amount of infiltrated rainwater

RainfallCase-SLCase-LCase-SSCCase-SCL

Amount of infiltrated rainwater in each four Cases

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Amount of surface runoff

RainfallCase-SLCase-LCase-SSCCase-SCL

Surface runoff in Case-SL

Rainfall – Infiltrated rainwater = Surface runoff

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Four cases and Localized torrential rain

RainfallCase-SLCase-LCase-SSCCase-SCL

Much Surface runoff occur (Localized torrential rain)

Surface runoff in Case-SL

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120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

0 6 12 18 24 30 36Time (h)

Four cases and Long spell of rain

RainfallCase-SLCase-LCase-SSCCase-SCL

Rainfall Intensity Surface runoff

Little surface runoff

120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

0 6 12 18 24 30 36Time (h)

To reveal the reason surface runoff occur

RainfallCase-SLCase-LCase-SSCCase-SCL

Distribution of volume water content in Case-L

At the time surface runoff start

Distribution of volume water content in Case-L

0.43

0.15

0.20

0.25

0.30

0.35

Case-L

0.429

Local saturated area is observed at the surface of slope

The reason surface runoff occur

Hortnian surface runoff

Infiltration capacity Rainfall intensity＜

Local saturated area

Infiltration of rainwater

Bring surface runoff

Intercept

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Maximum infiltrated rainwater

Max infiltrated rainwater (mm/h) /

Case-SL ‐ 44.2Case-L 10.5 10.4Case-SSC 4.5 4.3Case-SCL 0.9 0.7

120100

80604020

00 6 12 18 24 30 36

Rai

nfal

l (m

m/h

)

Max infiltrated rainwater (mm/h)

Infiltration capacity

Time (h)

Maximum infiltrated rainwater

Max infiltrated rainwater (mm/h) /

Case-SL ‐ 44.2Case-L 10.5 10.4Case-SSC 4.5 4.3Case-SCL 0.9 0.7

120100

80604020

00 6 12 18 24 30 36

Rai

nfal

l (m

m/h

)

Max infiltrated rainwater (mm/h)

Infiltration capacity

Time (h)

Amount of rainfall that infiltrate into slope

almost can be predicted by Ks (mm/h)

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

5. Conclusion

Four Cases (except for Case-S)

Case-S

4. Analysis results

120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

0 6 12 18 24 30 36Time (h)

Case-S + Long spell of rain

RainfallCase-S

Little surface runoff occur in Case-S (Long spell of rain)

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120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Case-S + Localized torrential rain

RainfallCase-S

Surface runoff occur (Localized torrential rain)

Case-S + Localized torrential rain

Infiltration Capacity Rainfall intensity＞

Not Hortonian surface runoff

Ks = 297(mm/h)

Max : 119(mm/h)

Change of VWC with time at each depth

Case-S + Localized torrential rain120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

Volu

me

Wat

er C

onte

nt

0.300.250.200.150.100.050.00

0.350.400.45

Surface －0.3m －0.9m－0.6m

0 1 2 6 7 8 113 4 5 9 10

Change of VWC with time at each depth

Surface －0.3m －0.9m－0.6m

Unsaturated

Case-S + Localized torrential rain120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

0.300.250.200.150.100.050.00

0.350.400.45

0 1 2 6 7 8 113 4 5 9 10

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Case-S + Localized torrential rain120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

0.300.250.200.150.100.050.00

0.350.400.45

0 1 2 6 7 8 113 4 5 9 10

Change of VWC with time at each depth

Surface －0.3m －0.9m－0.6m

UnsaturatedSaturated

θ Case-S + Localized torrential rain120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

0.300.250.200.150.100.050.00

0.350.400.45

0 1 2 6 7 8 113 4 5 9 10

Change of VWC with time at each depth

Surface －0.3m －0.9m－0.6m

Saturated

θ

From Bottom to Surface

The reason surface runoff occur in Case-S

The slope is completely saturatedfrom bottom to surface

Rainfall can’t infiltrate into slope more than drainage discharge

There are two types of surface runoff

Case-S + Localized torrential rain

Contents

1. Introduction

2. Numerical method for analysis

3. Analytical model

Four Cases (except for Case-S)

Case-S

4. Analysis results

5. Conclusion

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Conclusion

1. Hortnian surface runoff occurs when rainfall intensity exceeds infiltration capacity

2. It can be predicted that amount of rainwater infiltrate into the slope by saturated hydraulic

conductivity of the slope soil

3. Surface runoff sometimes occur due to saturation of the entire area of slope

Considered the effect of rainfall Intensity on Infiltration of rainwater through numerical simulations

Acknowledgement

This study was found byGrant-in Aid for Scientific Research (24510254),

for which I wish to express my gratitude here.

Thank you for your kind attention

Background 3/6 ［Factors of infiltration of rainwater］

Intensity Duration

Rainfall

Angle of slopeSoil type

Thickness ofsurface layer…

Slope

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Angle of slopeSoil type

Thickness ofsurface layer…

Slope

Background 4/6 ［Rainfall］

Intensity Duration

Rainfall

Background 6/6 ［Slope］

Angle of slopeSoil type

Thickness ofsurface layer…

Slope

There are so many key factors that these effects on infiltration of rainwater can’t

be considered at the same time

About observation points

Indicate how water conditioninside of slope is in each time

Change of VWCwith time at each depth

output VWC at the point

Consider the reason surface runoffoccur in Case-S

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

Change of VWC with time at each depth

Case-S + Localized torrential rain Saturated

From Bottom to Surface

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The characteristic of seepage flow /

Case-S + Long rain Case-SL + Long rain120

100

80

60

40

20

0

0.300.250.200.150.100.050.00

0.350.400.45

0 6 12 18 24 30 36 0 6 12 18 24 30 36

120

100

80

60

40

20

0

VWC changes in small step with rainfall intensity in Case-S

0.300.250.200.150.100.050.00

0.350.400.45

The way of expansion of Saturated area is different

Background 1/5 ［Land Use in Japan］

Slope disasters happen every year In Japan

Slope failure ・ Debris flow ・ Landslide...

Mountains66.7%

Agriculture12.1%

Curtilage5.0% Almost occupied

by mountains

A lot of slopes

Land Use in Japan

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

The characteristic of seepage flow /

Case-SL + Guerilla rainstorm

From Surface to Bottom

120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

0 6 12 18 24 30 36

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

The characteristic of seepage flow /

Case-SL + Long rain

From Surface to Bottom

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120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

0 6 12 18 24 30 36

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

The characteristic of seepage flow /

Case-SL + Long rain

VWCs From Surface to 0.6m depth become Saturated at almost same time

The characteristic of seepage flow /

Case-SSaturated area expands

from Bottom to Surface

Case-SLSaturated area expands

from Surface to BottomCase-SThe way of expansion of Saturated area

is the converse

The characteristic of seepage flow /

Case-SL

Case-SSaturated area expands

from Bottom to Surface

Case-SLSaturated area expands

from Surface to Bottom

The way of expansion of Saturated areais the converse

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Amount of infiltrated rainwater 1/4

RainfallCase-S

Amount of infiltrated rainwater in Case-S

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Amount of infiltrated rainwater 5/7

Hortnian Surface Runoff

Infiltration Capacity Intensity of Rainfallvs

Deep red shows θ

Distribution of VWC(when Surface runoff start)

Local Saturated area

Infiltration of rainwater

Generates Surface runoff

Decrease

Like Guerilla Rainstorm

＜ 120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

Amount of infiltrated rainwater 1/4

RainfallCase-S

Rainfall – Infiltrated rainwater = Surface runoff

Purpose 1/2

The relationship between Rainfall and Slope disasters

Key Point

For Example

High Intensity Rainfall : Intensity > Infiltration

Debris flow

Focus

Infiltration of Rainwater into slope

120

100

80

60

40

20

00 1 2 6 7 8 11

Rai

nfal

l (m

m/h

)

3 4 5 9 10Time (h)

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

The characteristic of seepage flow /

Case-S + Guerilla rainstorm

All VWCs become constant value

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120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

0 6 12 18 24 30 36

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

The characteristic of seepage flow /

Case-S + Long rain

Shallow surface area isn’t saturatedBut : Inside has much moisture

One dimension seepage flow 1/2

Seepage flow direction= Gravity direction

Homogeneous soil

The characteristic of Seepage flowIn Case-S and Case-SL

Amount of infiltrated rainwater 7/7

Max Infiltrated rainwater (mm/h) /

Case-S - 297.0Case-SL 44.2 44.2Case-L 10.5 10.4Case-SSC 4.5 4.3Case-SCL 0.9 0.7

120100

80604020

00 6 12 18 24 30 36

Rai

nfal

l (m

m/h

)

Max Infiltrated rainwater (mm/h)

Focus

Case-S Case-SL

The Cases that rainwater infiltrate plenty into the slope

Next discussion : Seepage flow in the slope

120

100

80

60

40

20

0

Rai

nfal

l (m

m/h

)

Time (h)

Volu

me

Wat

er C

onte

nt

Surface －0.3m －0.9m－0.6m

0 6 12 18 24 30 36

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0.35

0.40

0.45

One dimension seepage flow 1/

Case-S + Long spell of rain

Surface isn’t saturated