A landslide is defined as a perceptible
downward and outward movement of slope-forming soil, rock, and vegetation
under the influence of gravity. Landslides can be triggered by both natural
and human-induced changes in the environment. These changes may result from
a variety of causes such as:
§Weaknesses in composition or structure of the
rock or soil
§High precipitation
§Changes in ground-water level
§Seismic activity
§Construction or mining activity
§Over-steepening of slopes
§Changes in
surface water runoff
§Heavy loads
on slopes
To see how landslides can be so dangerous, click the
National Geographic movie to the right.
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Landslide Types
There are
several types of landslides and other downhill mass movements. The most
common ones seen in Alabama
include creep, slides, and rockfalls.
Creep
(right)is
an imperceptible, slow downward movement of soil or rocks on slopes.
Along the creep slope, objects can display signs of this slow movement.
Leaning utility poles, trees, retaining walls or offset fences can be signs
that an area is undergoing creep.
Slides (above) are movements of soil
or rock along a distinct surface of rupture which separates the slide
material from the more stable underlying material. At the top of the slide is
a cracked portion of the hill slope referred to as the scarp (above left
photo and center diagram). Near the bottom of the slide (the toe), material
mounds up creating a bulge in the topography (the lower half of the above
right photo).
Rock falls (left) are rapid movements of
bedrock characterized by free-fall, bouncing, and rolling. These are most
common in areas of very steep or near-vertical slopes such as roadcuts.
For more information on landslide types and processes,
click here
to download the U.S. Geological Survey factsheet “Landslide Types and
Processes.” Or click here
to download a more detailed document that goes more in depth (25.8 MB).
Areas Generally Prone to Landslides
Due to differences in geology, slope, and moisture,
some areas are more prone to landslides than others. Areas more susceptible
to slope failure include areas that are:
§Near existing old landslides.
§On or at the base of slopes.
§In or at the base of minor drainage hollows.
§At the base or top of an old fill slope.
§At the base or top of a steep cut slope.
§Developed hillsides where leach field septic
systems are used.
For more general info on landslide types and
processes, click
here to download USGS factsheet 2004-3072.
For more in depth detailed information, click here to download
USGS Circular 1325 (25.8 MB).
LANDSLIDES ON THE MAP
Topographic Maps
Larger landslides can be seen on topographic maps and have
a characteristic curvature. The steepest slope occurs at the main scarp
(see rotational landslide diagram above). Contour lines will appear curved
concave toward the direction of the slide. The contour lines down the slope
will also appear curved, with decreased slope, and convex toward the slide
direction.
In the example to the right, the scarp is the close
East-to-West contours near the center of the image. The material of the
slide is south of the scarp through Stone Hollow and the toe of the slide
is at the end of the “e” of “Stone.”
To read more about maps used for landslide exploration
and research, click here
to download the USGS Circular 1325 The Landslide Handbook—A Guide to
Understanding Landslides Appendix B (Introduction to Landslide Evaluation
Tools - Mapping, Remote Sensing, and Monitoring of Landslides). Or click here to see the full
publicationand other appendices.
Landslide area south of LewisMountain
in the Maysville 7.5-minute topographic quadrangle in Madison County,
Alabama.
Incidence and Susceptibility Maps
Landslide incidence is defined as the number of landslides
that have occurred in a given geographic area. Landslide susceptibility
is defined as the probable degree of slope failure. The map units are split
into three incidence categories according to the percentage of the area
affected by landslides. High incidence means greater than 15 percent of a
given area has experienced landslides; medium incidence means that 1.5 to
15 percent of an area has; and low incidence means that less than 1.5
percent of an area has been involved. High, medium, and low susceptibility
are delimited by the same percentages as the incidence. Susceptibility is
not indicated where it is the same as or lower than incidence.
For more information on the map to the left, click here.
According to the USGS, landslides in the country cause
an excess of $1 billion in damages and about 25 - 50 deaths annually.
To learn more about what you can do if you live near
steep slopes, click
here to download the USGS Fact Sheet FS-071-00 “Landslide Hazards” (o
para Español, aquí).
Landslide Warning Signs
§Springs, seeps, or saturated ground in areas that
have not typically been wet before.
§New cracks or unusual bulges in the ground,
pavement, or sidewalks.
§Soil moving away from foundations.
§Ancillary structures such as decks and patios
tilting and/or moving relative to the main house.
§Tilting or cracking of concrete floors and
foundations.
§Broken water lines and other underground
utilities.
§Leaning utility poles, trees, retaining walls or
fences.
§Offset fence lines.
§Sunken or down-dropped road beds.
§Sticking doors and windows, and visible open
spaces indicating jambs and frames out of plumb.
§Note,
some of the above warning signs may also be indicative of sinkhole or mine
subsidence. Click here to read
more on those topics.
Before a Landslide
§Do not build near steep slopes, close to mountain
edges, near drainage ways, or natural erosion valleys.
§Get a ground assessment of your property.
§Contact local officials, state geological surveys
or departments of natural resources, and university departments of geology.
Ask for information on landslides in your area, specific information on
areas vulnerable to landslides.
§Watch the patterns of storm-water drainage near
your home, and note where runoff water converges, increasing flow in
channels. These are areas to avoid during a storm.
§Learn about the emergency-response and evacuation
plans for your area. Develop your own emergency plan for your family or
business.
§Consider planting ground cover on slopes and/or
building retaining walls to help minimize hazards on your property.
§Stay away from the slide area. There may be
danger of additional slides.
§Watch for flooding, which may occur after a landslide
or debris flow.
§Check for injured and trapped persons near the
slide, without entering the direct slide area and direct rescuers to their
locations.
§Look for and report broken utility lines and
damaged roadways and railways to appropriate authorities.
§Check the building foundation, chimney, and
surrounding land for damage.
§Replant damaged ground as soon as possible since
erosion caused by loss of ground cover can lead to flash flooding and
additional landslides in the near future.
§Seek advice from a geotechnical expert for
evaluating landslide hazards or designing corrective techniques to reduce
landslide risk.A local phone
directory will have contact information for geologic or geotechnical
engineers who can help.
LANDSLIDES IN ALABAMA
Landslides in Alabama
have occurred around the state previously and are related to both geology
and natural slopes as well as human-influenced topography. Geologic units
most prone to landslides include shales and soft clay or sands on moderate
to steeply sloped areas. Weakly cemented rocks are also susceptible to
landslides if on steeper slopes. For a list of geologic units and their
rock types in Alabama, click here, or to
read more in depth about rock types and stratigraphy in Alabama, click
here to download GSA Circular 140 PDF (17.7 MB). To read more about
factors influencing landslide susceptibility, continue reading.
Rock Strength
Rock
strength influences landslide susceptibility. Physical properties of geologic
units (left) influence rock strength and rock strength (right) is
categorizedbased on the below
criteria.
A
(blue). Strongly
Cemented Rocks – crystalline rocks and well-cemented sandstone.
B
(yellow). Weakly Cemented Rocks
and Soils - sandy soils and poorly cemented sandstone.
Rock strength categories A, B, and C are based
on Wieczorek (1985) and match the categories used in
HAZUS software for modeling landslide potential during earthquake shaking.
Slope
Slope
is also a determining factor in landslide susceptibility. While most of Alabama’s topography is relatively low slope (left),
areas such as the Valley and Ridge, Piedmont, and Cumberland
Plateau have a number of steeper slopes. In addition to this,
steep slopes in Alabama
can also be found along river bluffs and roadcuts.
The
map to the left shows slope with warmer colors representing steeper slopes
(based on USGS National Elevation Data). The map to the right shows slope
broken into categories relative to landslide susceptibility modeling.
Warmer colors represent slope categories with higher susceptibility. Blues
have the lowest susceptibility. The palest blue represents areas with the
least slope.
Additional Factors
In
addition to geology and slope, mapping known historical landslides is also
important in helping assess where landslides may occur in the future. Areas
with known historical slides (left) are likely to have future slides.
Points on the map at the left indicate places with historical landslides as
indicated in previous documents and maps by Pomeroy (1982), and Rheams
(1982), and Thomas (1979, 1982).
Rainfall
(right) is also a contributing factor. In general, steep slopes that
receive more annual rainfall are more susceptible to landslides. The map to
the right shows annual rainfall averages (darker blue represents higher
rainfall) from 1961 to 1990 (source
information).
Landslide Susceptibility in Alabama
Click
here for
a pdf poster of landslide susceptibility in Alabama. The poster “Susceptibility to
Landslides in Alabama”
contains geology, rock strength, slope categories, landslide incidence, and
landslide susceptibility maps.
* Please note that the map to the right and
the downloadable poster above is not intended for site planning or site
selection.
Landslide susceptibility is based on estimates of rock
strength and terrain slope. In general, landslide susceptibility increases
with slope and in weaker rocks. Very high landslide susceptibility, classes
VIII, IX, and X, includes very steep slopes in hard rocks and moderate to
very steep slopes in weak rocks. Categories I - X are based on Wilson and
Keefer (1985) where I is the least susceptible and X is the most
susceptible.
Landslide susceptibility data was categorized for use
with HAZUS for earthquake damage and loss modeling. For additional details
on the categories and earthquake-induced landslides and modeling within
HAZUS, see the HAZUS technical manual Chapter 4. To learn more about HAZUS
software, click here.
HISTORICAL ALABAMA LANDSLIDES
A number of landslides have occurred throughout the
state over the past several years. Below are just a few examples of Alabama landslides
and information on clean-up costs and geology by county. The largest
landslide in Alabama
mentioned in historical literature was described in an article in The New York Times on January 30,
1886. This article (“A
Big Landslide”) describes a large landslide (1 mile long as reported!)
along the side of BoganMountain (formerly ScraperMountain) in CherokeeCounty that temporarily dammed up
the ChattoogaRiver. While the
slide is not apparent on the topographic maps,the mountain does have steep slopes with
a relatively high landslide susceptibility.
AutaugaCounty
Near Prattville in AutaugaCounty, County Road 47 was closed by
a landslide in 2005. The problem stemmed from unconsolidated sediments that
move underneath the road when it rains. A temporary repair was implemented
which cost between $150,000 and $200,000 ($173,000 and $232,000 in 2010
dollars), and a more permanent repair is estimated to cost several million
dollars, if feasible.
BaldwinCounty
Currently, in the City of Spanish
Fort in, the bluffs located along the eastern shore of the MobileBay are slowly receding due to wave
erosion causing a substantial threat to a number of homes along the bluff.
The City of Spanish Fort and the State of Alabama applied for
funds to stabilize the bluff to reduce future property and infrastructure
damage that could be caused by a landslide in this area.
BullockCounty
Photo (below) of an active landslide in the backyard of
a neighborhood in Union Springs, BullockCounty, 2010. Scarp
shown clearly in center of photo. The landslide’s scarp is in the Cusseta
Sand Member of the Ripley Formation. Landslide topography can also be seen
on the 1973 Union Springs 7.5-minute topographic map (left).
(Photo
from Francis Ashland, USGS)
ConecuhCounty
Landslide in Conecuh County in 1997. Photo above shows
the scarp and upper portion of the slide.
DeKalbCounty
In 1998, a landslide in DeKalbCounty wiped out a portion of
County Highway 81 on LookoutMountain (above). The
slide moved 117,527 cubic yards of rock and cost $1.7 million to repair.
Other slides on Highway 35 between Rainsville and FortPayne and on Highways 146 and 71
in JacksonCounty have cost between $1 and $2
million each to repair.
EtowahCounty
North of Gadsden in EtowahCounty,
the southbound lane of Interstate 59 slid from its perch on a mountainside down
into the valley below in 1972, resulting in $1.3 million ($7.2 million in
2010 dollars) in repairs and prolonged disruption of traffic.
JeffersonCounty
In Birmingham in 1988,
a landslide destroyed apartment buildings during the construction of an adjacent
FestivalCenter. Estimated damages were over
$10 million ($19.5 million in 2010 dollars).
MarshallCounty
This landslide occurred in the Guntersville Dam 7.5-minute
topographic quadrangle in Marshall County, Alabama. The slide originated in
the Mississippian age Pennington Formation, a geologic formation with weak
rock strength and past history of landslides.
TuscaloosaCounty
Landslide on a deforested slope. Underlying geology is
the unconsolidated sand of the Cretaceous Coker Formation of the Tuscaloosa
Group.
Pomeroy, J.S., 1982, Landslides and related features, Alabama and Tennessee:
U.S. Geological Survey Open-file Report 82-181. Plates of maps used by
Pomeroy in mapping.
Rheams, K. F., 1982,
Inventory of landslides, slope failures, and unstable soil conditions in Alabama: Geological
Survey of Alabama Open-file Report, p. 8-35.
Thomas, R.E., 1979, Landslides and related features, Alabama, Georgia,
and Tennessee:
U.S. Geological Survey Open-file Report 79-944. Plates of maps used by
Thomas in mapping.
Thomas, R.E., 1982, Landslides and related features, Alabama and Tennessee:
U.S. Geological Survey Open-file Report 82-193. Plates of maps used by
Thomas in mapping.
