landslide susceptibility mapping in maine - maine...

Landslide Susceptibility Mapping in

Maine

An aid for hazard mitigation and loss

prevention

presented by

Mike Foley

Senior Geologist

Maine Geological Survey

Landslide Susceptibility Mapping in

Maine

An aid for hazard mitigation and loss

prevention

Damaging landslides in Maine

Gorham, Maine (1983)

Rockland, Maine (1996)

Norridgewock, Maine (1989)

All underlain by marine clay

Presumpscot Formation

Gorham Landslide - 1983

Rockland Landslide - 1996

No systematic analysis of

landslide hazards

Gorham slide led to initial effort to produce a

landslide inventory (Novak, 1983)

More detailed inventory (Cumberland, Co.) Novak,

1988.

Site investigation report for Rockland Landslide

Berry et al, 1996

Coastal landslide hazard maps (Dickson)

Describes coastal bluffs, shoreline, stability of

coast, and assesses resulting landslide potential

Coastal Landslide Hazard Maps

Events which triggered

development of Landslide

Susceptibility Maps

Abnormally high spring precipitation

(2005 – 2007)

Several large and damaging Maine landslides

Wells – along Merriland River

Cumberland – Range Road

Greenbush – Route 2

Sanford – Branch Brook off of Rte. 99

Cumberland, Maine 2006

Rotational Slide

Wells, Maine - 2005

Earth flow – caused by sapping

Sanford - 2006, Maine

Greenbush, Maine - 2006

Pilot Study Proposed

Derived from USGS Circular 1244

National Landslide Hazards Mitigation Strategy

– A Framework for Loss Reduction

Submitted to Maine Emergency management

Agency (MEMA) for funding

•National losses may exceed $2 billion per year (NRC, 1985)

•Losses in Maine since 2000:

2000 $187,000

2002 $180,000

2005 $330,000

2006 $1,072,000

2007 (to date) $803,000

Does not include Gorham landslide in 1983, Rockland landslides

in 1973 and 1996, and who knows how many others…..

Inland Landslide Inventory and

Hazard Assessment for Emergency

Management

Pilot Study for the Towns of Wells, Cumberland,

Bangor, and Greenbush

-Update Maine’s landslide inventory database

-Develop Methodology to examine/map landslide

susceptibility and causitive risk factors

-Collect data on direct losses caused by landslides

Performed an extensive literature

search to devise a methodology to

produce Landslide Susceptibility

Maps that would fit Maine’s

needs

•Preparation/update of Maine’s landslide database/inventory

•Digitize landslide locations and attributes into geographic information system (GIS).

•Collect all pertinent geological, hydrological, topographical, and environmental data

available for area of study

•Base map creation – Selection of best map scale for project

•Prepare slope map for areas underlain by the Presumpscot Formation

•Create digital data maps: DEM, Slope, Slope Aspect, Curvature, Relief, Surficial

Geology, Orthophoto.

•Analyze data and select statistically significant landslide risk factors for each town.

•Plot areas of increased landslide susceptibility:

•0 risk factor – low landslide risk

•1 risk factor – low-moderate landslide risk

•2 risk factors – moderate landslide risk

•3 or more risk factors – moderate to high landslide risk

•Creation of town-level Landslide Susceptibility Maps

Methodology for landslide

susceptibility analysis & mapping

Developing Landslide

Hazard/Susceptibility Maps

Two principle approaches:

• Rational – incorporates the underlying

physics and mechanics of landslides

• Models are data intensive (soil transmissivity and

soil strength)

• Empirical – assumes that future landslides

are most likely to occur under conditions that

produced past landslides

• Need accurate inventory of previous landslides

• Data on risk factors (slope, aspect, relief, geology)

must be available

Approach used for Maine’s

Landslide Susceptibility Maps

Empirical approach (Risk Factor Analysis):

Collect data on risk factors where available

Overlay maps (GIS layers) of each landslide risk

factor

Classify landslide risk potential

No risk factors = lowest landslide potential

One or two risk factors = moderate landslide potential

Three or more risk factors = highest landslide potential

Landslide risk factors

Characteristics of the site/primary causes

Slope Surficial geologic materials

Slope aspect Bedrock geology

Slope relief Land use

Slope curvature Vegetation

Landform Construction activities

Proximity to surface water

Groundwater levels

Factors in red were used in preparation of landslide susceptibility maps

Landslide Risk Factors

• Geomorphic

-Slope

-Contour shape

-Slope aspect

-Relief (slope

height

• Geologic

-Bedding

-Bedrock depth

•Soil properties •Surficial geologic

materials

-Soil type

Geomorphic Risk factors

Slope: The steeper the slope, the larger the shear

stress on the materials and the more susceptible the slope is to failure

Contour shape: Concave planar topography will

concentrate groundwater flow, raising pore pressures and reducing shear strength of the soil.

Slope aspect: Repeated freeze/thaw cycles

reduces the shear strength of the shallow soil material, increasing the likelihood of shallow soil slumps and creep.

Geomorphic Risk Factors

Relief/slope height: as the thickness of

the potential landslide block increases, the shear

stress on the lower section of block increases,

making the block (slope) more susceptible to

failure. Therefore, thicker sections of surficial

materials will be more susceptible to landslides.

Geologic Risk factors

Bedding characteristics: thin-bedded

glacial marine clay, where clay-rich layers alternate with more permeable layers, will be prone to slumping along the bedding planes.

Depth-to-bedrock: thin deposits on

impermeable bedrock may slump along bedrock surface during rain events where shallow GW pore pressure will be highest. Thicker sections of surficial materials will be more susceptible to failure for the same reasons described in the relief/slope height.

Soil properties

Surficial geologic materials: Cohesive materials such as clays are prone to

landslides along planes of weakness in the

sediment. Less cohesive materials (sands) may

slump if slopes oversteepen or GW pore pressure

increases and reduces internal friction.

Soil type: Soil type is strongly dependent on

the underlying surficial geologic material as

modified by physical and biological activity.

•Select map scale for project – 1:24,000

•Prepare regional map of slope and slope aspect to guide field work

•Aerial photo analysis of town to determine possible landslide locations

•Field checking of predicted and actual landslide locations.

•Detailed description of each landslide.

•Digitize landslide locations into geographic information system.

Detailed town level

landslide mapping

Methodology

Regional map of slope:

Guide to assist field mapping

Shade areas: Slope greater than 5 and 10

degrees

Types of Landslides

Type of Movement

TYPE OF MATERIAL

BEDROCK

ENGINEERING SOILS

Predominantly Coarse Predominantly

fine

FALLS Rock fall Debris fall Earth fall

TOPPLES Rock topple Debris topple Earth topple

SLIDES

ROTATIONAL

Rock slide Debris slide Earth slide TRANSLATIONAL

LATERAL SPREADS Rock spread Debris spread Earth spread

FLOWS

Rock flow

(deep creep)

Debris flow Earth flow

(soil creep)

COMPLEX Combination of two or more principal types of movement

Abbreviated version of Varnes’ classification of slope movements (Varnes, 1978).

Rotational Slide (A)

Rotational Slide (A)

Rotational Slide (A)

Cumberland, Maine

Rotational Slide (A)

Wells, Maine

Winslow - Rotational Slide

Chesterville – Rotational Slide

Earth Flow (H) – Coarse grained

Earth Flow (H) – Coarse grained

Soil Creep (I) - Flow

Soil Creep (I) - Flow

Soil Creep (I) - Flow

Topographic Layer Landslide polygons plotted in red

Landslide polygons

Topographic Layer Landslide polygons plotted in red

Landslide polygons

Landslide Polygons

Use USGS 10 meter digital elevation data (DEM) to identify areas with greater

than 5-percent slope and produce digital data layers (shape files).

•Within these areas, identify areas with one or more of the additional risk

factors:

•southerly slope aspect (135 – 225 degrees (south facing))

•relief greater than 6 meters (20 feet)

•concave slope curvature

•Run Zonal Statistics to correlate between the landslide locations and the

influencing risk factors (Slope, Aspect, Curvature, Relief, Geology)

•Overlay areas underlain by glacial marine deposits or Recent alluvial deposits

•Produce final Landslide Susceptibility Map and Mapped Landslide Sites for

distribution to towns

Creating

Landslide Susceptibility Maps

Determine most statistically significant range of each Risk

Factor where mapped landslides are located.

Risk Factor parameters selected

Slope – 5% slope or greater

Slope Aspect – south facing (135 – 225 degrees

Relief – 10 meters (30 feet) or greater

Curvature – Conceave

Geology – Glaciomarine deposits

Create Risk Factor Grids for each

Used for map compilation and statistical analysis

Intersect Mapped Landslide

Locations to Risk Factor

Grids (DEM Derivatives)

Calculating Aspect from DEM

Digital Elevation Model (DEM)

Slope

Slope Aspect

Curvature (Concave)

Relief (6 meters or greater)

Town Boundary (Clipped)

Determine Risk Factor Statistics

Distribution of landslide location data used to

help determine which risk factors to use to

produce Maine’s Landslide Susceptibility

Maps

Number of risk factors associated with the landslide site

No factors 1 factor 2 factors 3 factors 4 factors

Nu

mb

er

of

ma

pp

ed

la

nd

slid

e s

ite

s(N

ova

k,

19

90

)

0

20

40

60

80

100

120

Number of risk factors associated with the landslide site

No factors 1 factor 2 factors 3 factors 4 factors

Num

ber

of

mapped

landslid

e s

ites

(This

stu

dy)

0

10

20

30

40

50

60For landslide sites mapped in this

study, 91-percent are located in areas

with a slope greater than 5%, and 83-

percent are located in areas with at

least one additional risk factor.

For landslide sites mapped by Novak

(1990), 99-percent are located in areas

with a slope greater than 5%, and 97-

percent are located in areas with at

least one additional risk factor.

Risk Factor Selection

For landslide sites mapped in this study,

71-percent involved glacial marine

deposits and 13-percent involved

Holocene alluvial deposits.

For landslide sites mapped by

Novak (1990), 47-percent

involved glacial marine

deposits and 9-percent involved

Holocene alluvial deposits.

Frequency of mass movementsversus surficial geology unit for this study

Surficial geology unit

af Ha HwsmHwst Pm Pmdi Pmf PmdoPmn Pmrs Pms Pp Pt wa

Num

be

r of

occurr

en

ces w

ithin

this

surf

icia

l geo

logy u

nit

0

20

40

60

80

100

120

71-percent on glacial marine deposits

13-percent on Holocene alluvial deposits

Frequency of mass movementsversus surficial geology unit for Novak inventory

Surficial geology unit

af Ha

Hm

sHst H

w

Hwsm

Pemgp

Pemm

bPm

n

Pmn/

PtPp

Pp/Pm

fbp Pt

Ptd wa

Nu

mb

er

of

occu

rre

nce

s w

ith

in t

his

su

rfic

ial ge

olo

gy u

nit

0

100

200

30047-percent on glacial marine deposits

9-percent on Holocene alluvial deposits

Risk Factor Selection – New Data

York County, Maine

Geologic Factors

91 % of landslides occur on marine glacial deposits

40 % on marine clay (Presumpscot Fm.)

13 % of landslides occur on stream alluvium

18 % of landslides occur on glacial till

Surficial Geology

York County, Maine

Risk Factor Analysis

DEM derivative

90 % of landslides occur in areas with a slope

greater than 5 %

89 % of landslides are located in areas with at

least one additional risk factor

82 % of landslides are located in areas with 2

or more additional risk factors

York County, Maine

Risk Factor Analysis

For landslide sites mapped

by Foley, 88 % are located

in areas with a slope greater

than 5 %, and 92 % are

located in areas with at

least one additional risk factor. No factors 1 factor 2 factors 3 factors 4 factors

N u mb e r o f ris k fa c to rs a sso c ia te d w ith th e la n d s lid e s ite

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

(Fole

y, curr

en

t stu

dy)

Nu

mb

er

of m

ap

ped

la

nds

lid

e s

ite

s

Landslide Susceptibility Map

Landslide Susceptibility Map

•Town level landslide mapping is required

to validate the susceptibility maps and

provide credibility to local and State

officials.

•Landslide susceptibility maps indicate

where landslides are likely to occur, not

whether or when a landslide will occur.

Limitations of the present study:

Based on landslide sites mapped below the glacial marine limit. Extension to other geologic settings would require additional study.

Additional statistical analysis and incorporation of additional risk factors may improve the usefulness of the susceptibility maps. Data for additional risk factors must be available throughout the study area.

Limitations of the present study:

•Identify where landslides have already occurred and

where future landslides may occur.

•Cannot tell when a landslide may occur

•Public education.

•Mitigate the effects of future landslides by adopting

appropriate building codes or land use policies in

landslide prone areas.

Landslide Susceptibility Maps

Applicability

Moving Forward

• Utilize input/reviews of maps from local

and state officials to enhance future

map product.

• Acquisition of LIDAR (Light Detection

And Ranging) data will enhance

quality/accuracy of maps

•Continued updating of Maine’s landslide

inventory database will increase accuracy

and utilization of the maps.

•Ongoing process where Landslide

Susceptibility Maps will continually evolve

into a more useful tool for landslide hazard

mitigation in Maine.

Moving Forward