U.S. Department of the InteriorU.S. Geological Survey

Climate Change Based Prediction of Coastal Cliff Landslides Near San Francisco, California

Brian D. CollinsUnited States Geological SurveyWestern Earth Surface Processes Team, Menlo Park, California

Climate Change, Natural Resources and Coastal Management WorkshopConcurrent Session C – Nearshore and Open Ocean

Introduction

Coastal bluffs/sea-cliffsSusceptible to a number of forcing agentsWaves, rainfall

Climate change effectsSea level riseChanges in wave climateVariability of storms and precipitation

This researchProcess-based investigation of failure mechanismsEmpirical prediction of triggering event thresholds

Pacifica, California

Coastal Cliff Landslides

December 22, 2002 January 30, 2003

Area of Failure ~10m

Goals and Methods

Predict coastal cliff failures through development of intensity-duration thresholds

Collection of temporally and spatially high-resolution landslide dataset

Correlation with landslide process-based indicators

Develop ability to forecast – predictions from expected weather (short-term) and climate (long-term)

Planning for expected coastal response

Study Area(San Francisco, California)

Pacific Ocean

Study area ~1.5 km

SAF

San Francisco

Bluff N1 Bluff S1(N) Bluff S1(S) Bluff S3

Bluff N4 Bluff N3 Bluff N2

Bluff S2

Copyright © 2002-2007 Kenneth Adelman, California Coastal Records Project, www.californiacoastline.org

South

North

Stairs

Stairs

USA

Qw

Qs

Failure Mechanisms of Weakly Lithified Sandstones

Wave action and vertical toe formation

Failure surfaces and slope profile

prior to failure

Stronger layer

1.5 m (typical crest failure)

Weakly cemented sand

~65°

~24 m

Slope surface after failure (~40°)

Weaker layer

Loose dune sand

Impermeable layers with seepage

Seepageinduced

failure of upper

materials

Moderately cemented

sand

~24 mFailure surfaces(~0.5 m)

Angle of repose slope

Strongly Cemented (UCS 100-400 kPa) Weakly Cemented (UCS 5-30 kPa)

Collins, B.D. and Sitar, N., 2008, Processes of coastal bluff erosion in weakly lithified sands, Pacifica, California, USA, Geomorphology 97, pg.483-501.

Data Collection – 2001-2006

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Copyright © 2002-2007 Kenneth Adelman, California Coastal Records Project, www.californiacoastline.org

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BDC5

Slide 7

BDC5 BCollins, 5/16/2007

Intensity - Duration Relationships

Wave Action: Mean Daily Total Water Level (MD-TWL)Average of the hourly wave run-up elevation

Measured local tide gauge height Off-shore wave buoy data

TWL = η + R2%, where R2% = 0.5Hs-0.22 (Ruggiero et al, 2001)

Cumulative sum of TWL difference from season average (duration index)

Precipitation: 48-hour Rainfall48 hours gives indication of passing stormsCannot take into account full delay of seepageCumulative sum of precipitation (duration index)

Correlation with Wave Action

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The average water elevation on the beach from waves and tides.

The cumulative sum of the difference in MD-TWL to the average of the MD-TWL for the entire season.

Area of Expected Failures

Correlation with Precipitation

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failure non-failure

Area of Expected Failures

Prediction of Future Response from Global Warming (Expected Sea Level Rise)

Prediction of failure events for a 5-year period in 100 years based only on sea level trend.

Sea level # points # days of % Global Trend* exceeding predicted increase emissions(m/100yr) threshold failure of failure scenario*

0 434 54 0 emissions decline0.2 506 61 17 present rate0.4 594 71 37 A2 average0.6 675 81 56 A2 maximum0.72 719 86 65 A1fi maximum

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*Source: California Climate Change Center, derived from IPCC results.

Prediction of Future Response from Global Warming (Potential Changes in Wave Action)

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Implications

Pacifica, California – a site specific model

But… weakest end member in California… a proxy for effects of climate change (canary in the coal mine)

Important to continually monitor sites like this.

Also important to establish new monitoring sites in other locations (more canaries)

Conclusions

Observations give new level of understanding to processes and insight to making predictions

Empirical correlations show promise for defining conditions for failureNewly developed wave-action thresholds.

Ability to forecast landslide events from expected climate change is a new prospect for science and public awareness.

Future direction I – incorporate effect of extreme sea-level events (predictions of increased North Pacific storm activity)

Future direction II – extend methodology to other areas of the west coast

Thank you.

Analysis and prediction of coastal bluff erosionPacifica, San Mateo County, California

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Bluff N1 (Oct 2002-May 2003)

Oct 2002 - May 2003

Prediction Analysis for

Qw Bluffs