The La Conchita LandslidesThe La Conchita Landslidesof 1995 and 2005of 1995 and 2005

Randall W. JibsonRandall W. Jibson

U.S. Geological SurveyU.S. Geological SurveyGolden, ColoradoGolden, Colorado

Santa Barbara County Ventura County

Pacific Ocean

Santa Barbara

Ventura

Ojai

1011 50

150

33

La Conchita

CaliforniaN

180180--mm--high bluffhigh bluff

~35~3500 slopeslope

Monterey Formation in fault contact Monterey Formation in fault contact above Pico Formationabove Pico Formation

•• Monterrey is Miocene siliceous Monterrey is Miocene siliceous shale, siltstone, and sandstoneshale, siltstone, and sandstone

•• Pico Formation is Pliocene Pico Formation is Pliocene siltstone, sandstone, and siltstone, sandstone, and mudstonemudstone

•• Both formations very weakly Both formations very weakly cemented and prone to cemented and prone to landslideslandslides

Entire bluff is ancient landslideEntire bluff is ancient landslide——many scales, types and ages of many scales, types and ages of landslides on the slopelandslides on the slope

Top of bluff in citrus orchards with Top of bluff in citrus orchards with drip irrigationdrip irrigation

0 0.1 0.2 0.3 0.4 0.5 km

N

U.S. Highway 101

400 mm of rain in January, then 400 mm of rain in January, then 200 mm in March200 mm in MarchLandslide began on 4 March Landslide began on 4 March 19951995Landslide moved tens of meters Landslide moved tens of meters in less than 10 minutesin less than 10 minutes350 m long by 120 m wide350 m long by 120 m wide~ 1,300,000 m~ 1,300,000 m33 volume volume Nine houses destroyed, no Nine houses destroyed, no fatalitiesfatalities

1995 La Conchita 1995 La Conchita LandslideLandslide

2005 La Conchita Landslide2005 La Conchita Landslide

Landslide occurred early afternoon on 10 JanuaryLandslide occurred early afternoon on 10 January430 mm of rainfall from 27 December to 10 January (record 430 mm of rainfall from 27 December to 10 January (record 1515--day rainfall)day rainfall)About 200,000 mAbout 200,000 m33 (15%) of the 1995 deposit mobilized and (15%) of the 1995 deposit mobilized and flowed rapidly into the communityflowed rapidly into the community13 houses destroyed, 23 red13 houses destroyed, 23 red--taggedtagged10 fatalities10 fatalities

Much of the 2005 landslide deposit Much of the 2005 landslide deposit was drywas dry……

The video shows dust in the air above the rapidly The video shows dust in the air above the rapidly flowing massflowing mass

The County geologist was on the site within a couple of The County geologist was on the site within a couple of hours and observed that much of the deposit was dry hours and observed that much of the deposit was dry materialmaterial

La Conchita La Conchita slope profileslope profile

Uniform Uniform vertical vertical infiltration of infiltration of a saturation a saturation frontfront

Uniform Uniform vertical vertical infiltration of infiltration of a saturation a saturation frontfront

NonNon--uniform uniform infiltration infiltration from upslope from upslope leading to leading to basal basal rechargerecharge

~Dry

NonNon--uniform uniform infiltration infiltration from upslope from upslope leading to leading to basal basal rechargerecharge

Why did the same landslide mass Why did the same landslide mass mobilize in two very different ways in mobilize in two very different ways in substantially the same conditions?substantially the same conditions?

Subsidiary landsliding from the toe or scarp is commonSubsidiary landsliding from the toe or scarp is common

A wholesale remobilization of a significant portion of the A wholesale remobilization of a significant portion of the 1995 mass as a rapid debris flow is1995 mass as a rapid debris flow is……problematicproblematic

Ve ntura D aily R ainfall

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/200

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10/1

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10/2

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10/3

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11/1

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04

11/2

0/20

04

11/3

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2005

1/19

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Rai

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m)

O jai D aily R ainfall

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/1/1

994

10/1

1/19

94

10/2

1/19

94

10/3

1/19

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11/1

0/19

94

11/2

0/19

94

11/3

0/19

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12/1

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0/19

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12/3

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1/9/

1995

1/19

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1/29

/199

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2/8/

1995

2/18

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2/28

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Rai

nfal

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m)

1994-95

2004-2005

Daily rainfallDaily rainfall

4 Mar 1995 landslide

10 Jan 2005 landslide

Some challenging questions:Some challenging questions:Why did the landslide material not mobilize into a debris flow Why did the landslide material not mobilize into a debris flow in 1995? in 1995? Since only a fraction of the 1995 deposit remobilized in 2005, Since only a fraction of the 1995 deposit remobilized in 2005, could the remainder also mobilize into a rapid debris flow, or could the remainder also mobilize into a rapid debris flow, or is it more likely to remobilize as a slow earth flow? Or will iis it more likely to remobilize as a slow earth flow? Or will it t remain metastable?remain metastable?How do we predict which mode of failure will occur?How do we predict which mode of failure will occur?What are the different rainfall thresholds and antecedent What are the different rainfall thresholds and antecedent conditions required for shallowconditions required for shallow--rapid vs. deeprapid vs. deep--slow slow movement?movement?What monitoring/warning would distinguish between these What monitoring/warning would distinguish between these conditions?conditions?What is going to happen in the next wet period?What is going to happen in the next wet period?