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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