in
JAMAICA
prepared
by:
Rafi AHMAD (rahmad@uwimona.edu.jm), Department of Geography & Geology, University of the West Indies at Mona.
HOW DO LANDSLIDES AFFECT US?
LANDSLIDE DISASTER IN THE FELLOWSHIP AREA, RIO GRANDE VALLEY, PORTLAND, 3-4 JANUARY 1998
The experiences of the 1998 Portland landslide disaster forewarn us about what to expect in the wake of sustained rainfall associated with tropical storms. On Sunday, January 4, 1998, at about 11.30 a.m., SHALLOW LANDSLIDES, also called DEBRIS FLOWS, occurred in the Fellowship district, Portland following heavy rainfall, killing 4 persons while another two were seriously injured with fractured ribs, broken leg, and head injuries
Lessons from1998 Portland Landslide disaster:
·
About 4000 m2 of the
slope section broke away following heavy rainfall on 3-4 January
1998. A mass of rock debris, soils and trees
saturated with water quickly traveled down slope and piled up
on top of the shop where some 13 persons were
taking shelter from rain. Landslide debris had the
consistency of wet cement slurry. Villagers had to dig out 8 persons
from under the debris. Two other
houses on the right and left flanks of the landslide were also
seriously damaged.
Agricultural farms and orchards was seriously affected.
Debris flows,
such as described above, are common to Jamaica and are also a well-known cause of widespread
fatalities and destruction worldwide, e.g., December 1999 landslide
disaster in Venezuela.
The original stream course acted as debris chute and was
blocked. A new stream course was formed.
Debris flows and mudflows caused
excessive erosion in the watersheds. For example, it is estimated that some 50,000
- 100,000
m3 of sediments were removed from the slopes along 1.5 km of road
section between Fellowship
and
Berridale. This sediment along with vegetation debris blocked the Sandy
River and caused
flooding.
Main roads in the Rio Grande valley were blocked, often
collapsed due to undermining by landslides·
Water pipelines, telephone lines, and electricity transmission
lines were damaged and or destroyed.
These were triggered by some 865mm of rain that fell
during a period of about
10 hours, a rate of approximately 86.5mm/hour.
A debris flow fan was formed
in the Bluefields area.
The June 1986
rainfall at approximately 300mm in 48 hours (approximately 6.25mm/hr)
induced widespread debris flows in Clarendon and St.Catherine.
Landslide
is a general term used to describe a wide variety of processes that result in
perceptible downward and outward movement of slope forming materials under the
influence of gravity, and also the landform that results. It includes processes
that involve little or no true sliding such as fall of rock and debris and flow
of debris and mud, often on very gentle slopes. Landslides may also occur in
submarine environments along coastlines and on shelf regions where they
are generally triggered by earthquakes, for example, the submarine
landslide that destroyed Port Royal following the earthquake of June 7,
1692. Submarine landslides appear to be the principal causes of localised
tsunami as observed in Jamaica following 1692 and 1907 earthquakes. Landslides
of several different types are among the principal natural processes by which
hillslopes evolve.
TYPES OF LANDSLIDES:
Falls: a mass detaches from a
steep slope or cliff and descends by free-fall, bounding, or rolling.
Topples: a mass tilts or rotates
forward as a unit.
Slides: a mass displaces on one or
more recognizable surfaces, which may be curved or planar.
Flows: a mass moves downslope
with a fluid motion. A significant amount of water may or may not be part of the
mass.
WHY LANDSLIDES?
Gravity,
the agent responsible for landsliding, is always acting on a hillslope to
produce a landslide. The
fact that landslides are not constantly occurring is attributable to the
resisting strength of the soil or rock forming the hillslope.
Every
location on a hillslope can be considered as a part of a continuous tug-of-war
between gravity and the resisting earth materials. A
landslide results when the balance is tipped in favour of gravity.
This
balance can be changed by both natural and man-made
circumstances. This
change may occur quickly as in the cases where a storm increases the water of
the material on a hillslope, or ground shaking due to
earthquakes. Weathering
of rocks may slowly decrease the strength of rocks, which leads to balance being
shifted in favour of gravity.
Land uses can alter the balance between gravity and resisting earth material. A good example would be the occurrence of a landslide during excavation of a road. A slower change in the balance could result where a natural forest is cut to permit planting of crops”.
FACTORS influencing where landslides occur can be divided into two sets, permanent and variable.
Permanent
factors are characteristics
of the landscape which remain unchanged or vary little from a human perspective.
The steepness of a slope or the type of rock, for example, presents changes only
with the passage of long periods of time. Permanent factors such as rock type
and slope steepness can be recognized and identified for specific landslides
long after their occurrence. By examining existing landslides in an area, it is
possible to recognize how permanent factors contributed to these slope failures.
Identifying conditions and processes promoting past instability makes it
possible to use these factors to estimate future landslides.
Variable
factors are landscape
characteristics that change quickly as a result of some triggering event. Ground
vibration due to earthquakes, a rapid rise in groundwater levels, and increased
soil moisture due to intense precipitation are examples of variable factors. It
is often necessary to be present at the time a landslide occurs or shortly
thereafter to assess these factors.
LANDSLIDES: A major social, economic, and environmental issue in Jamaica:
The natural forces and geologic processes that have created the beautiful landscape of Jamaica are the same that make it most vulnerable to natural hazards. The high incidence of geological hazards in Jamaica is due to a combination of several factors related to its geographic and geologic location.
A recurring theme in all of the natural disasters that have affected Jamaica since historic times is that its physical environment
profoundly influences the severity of damage at any site.
Major urban centres on the
island, 11 out 15, are coastal cities and townships which face hazards related to
debris flows and alluvial fan flooding, storm surge, tsunami,coastal or submarine
landslides, earthquakes, liquefaction, and changes in sea level.The abundance of hazardous processes, together with
high urban population densities, many on marginal and ruinate lands, make
Jamaica one of the most environmentally sensitive regions in the world. Natural
hazards have played a major role in shaping the communities that have suffered
through them.
Hazards that affect the island most frequently, however, are landslides and flooding which account for most of the natural disasters that have occurred on the island during the last decade and continue to present common risks to life and property. These hazards are important because of their frequency, associated loss of life, disruption of socio economic activities and their effect on the built and natural environment.
Jamaica’s mountainous
terrain, its geology, its fractured, altered and deeply weathered bedrock, its
abundance of poorly consolidated colluvial sediments on hillslopes, its high
precipitation due to its geographic location in belt of tropical storms, its geophysical
location astride the seismically active plate boundary fault zone, all combine
to make the island particularly susceptible to
landslides.
Over the last four decades
the economy of Jamaica has expanded from an agricultural base to one
that relies heavily on tourism, manufacturing, mining and diverse types of services.
As a consequence, land use in Jamaica is changing rapidly in order to meet the
economic and housing needs of the populace which must utilize hazard-prone geologically
young landforms and steep slopes.
Unfortunately, human actions
often increase the risk of landslides by modifying the landscape, building on
unstable slopes or in the path of potential landslides. It appears that many of us are
completely unaware of our exposure to landslide risks.
However, by understanding
the answers to the following questions and learning from the past disasters and
following the axiom “ present is key to the future”, we may be able to protect
ourselves from the effects of landslides.
WHY SHOULD WE BE
CONCERNED ABOUT LANDSLIDES?
In Jamaica,
the wider Caribbean, and also worldwide landslide disasters cost more money each
year than all other natural disasters combined, and their incidence appears to
be rising. Yet they get little public attention.
View of
Jamaican professionals on landslide costs to
nation:
On January 26-28, 1999 Unit
for Disasters Studies hosted a workshop on issues of landslide hazard mitigation
and loss-reduction strategies. Some 40 Jamaican professionals representing
fields of geology, engineering, water and transportation, disaster response,
development and urban planning, and environmental management were in
attendance.
We decided to take advantage of this
occasion in order to document how well landslide
information was understood by an array of staff support personnel for decision
makers. The questions and responses that follow are anecdotal in nature
and limited in number. A more comprehensive survey is surely warranted, but we
believe that what is captured here reflects the general attitude about this
subject that prevails, not only in Jamaica but seemingly everywhere else in the
world.
Q. WHAT IS THE
YEARLY-AVERAGE COST OF LANDSLIDE DAMAGE FOR JAMAICA AND/OR FOR YOUR
JURISDICTION? All responses are in
Jamaican dollars.
A. $5
billion $3 billion $180 million $80 million
The range in answers covers
about two orders of magnitude. Estimating costs of landslide damage is always
problematic as the direct and indirect costs are rarely collected by a central
agency. From the above range it is understandable why the disparity of
opinions exist regarding the severity of the landslide-hazard
issue.
Q. ARE THE
EFFECTS OF LANDSLIDE DAMAGE INCREASING, AND IF SO WHY?
A.
All
Jamaicans answered YES. The reason for the YES responses include:
The population in Jamaica is reasonably stable. However, the segment of the population that is gaining wealth is moving into hillside settings. Thus, the number of new mountain roads and terraced slopes is increasing, and it is easy to understand why there is a perception that landslide damage would be increasing.
HOW CAN WE MINIMIZE THE EFFECTS OF LANDSLIDES?
Landslide Hazard: as represented by susceptibility, which is
the likelihood of a potentially damaging landslide occurring within a given
area.
Vulnerability: the level of population,
property, economic activity, including public services, etc., at risk in a given
area resulting from the occurrence of a landslide of a given type.
Risk
(specific):
the expected degree of loss due to a particular landslide
phenomenon.
LANDSLIDE RISK MANAGEMENT:
Landslides are not
currently amenable to risk assessment since there is no basis to determine the
probability of landslides occurring within a given time period. Hazard
assessments are possible and can be used in place of risk assessments. Hazard
assessments are estimations of an area's susceptibility to landslides based on a
few key factors. These are each capable of being mapped and allow land areas to
be evaluated on their relative susceptibility to landslides.
Three principles
guide landslide hazard assessment. First, landslides in the future will
most likely occur under geomorphic, geologic, and topographic conditions that
have produced past and present landslides. Second, the underlying
conditions and processes which cause landslides are understood. Third,
the relative importance of conditions and processes contributing to landslide
occurrence can be determined and each assigned some measure reflecting its
contribution . The number of conditions present in an area can then be factored
together to represent the degree of potential hazard present. THE
AIM IS TO
PRODUCE A LANDSLIDE HAZARD MAP THAT IDENTIFIES AREAS WHERE FUTURE LANDSLIDING IS
MOST LIKELY TO OCCUR. THIS MAP
FOREWARNS PLANNERS AND ENGINEERS OF SLOPE INSTABILITY CONSTRAINTS ON THE
MOUNTAINOUS TERRAIN .
GIS-based Landslide susceptibility maps for the Kingston Metropolitan Area, guidelines on their use, and loss-reduction strategies are available at <http://isis.uwimona.edu.jm/uds/index.html >. Other areas in Jamaica where landslide hazard mapping has been completed and maps available in public domain include Clarendon and St.Catherine, South Coast of Jamaica, and RioGrande Valley, Portland.
or for guidelines |
or for guidelines |
The vulnerability of human life, property, and infrastructure to landsliding can be significantly reduced by avoiding slopes prone to landslides, restricting or prohibiting development in areas demarcated as flagged areas on hazard maps,and advising on appropriate engineering design. Agencies including ODPEM, NEPA, Town Planning Department, Parish Councils, and Community Groups can accomplish this by adopting land use regulations and building codes. The existing structures, in particular lifeline structures, in landslide-prone areas are amenable to standard physical mitigation methods.
One of the strategies to reduce
vulnerability to hazards is that the insurance industry uses the available
information in enforcing premiums that reflect different levels of risk from
hazards. Proactive strategies of loss-reduction should
be implemented in the planning stages of all new development
projects.
The lessons learned from the previous disasters should be used to adopt prudent land use policies, institute proper design,
strengthen weak structures,
and design preparedness plans at
national, community, and family
levels. This information
should assist general public, planners and decision makers to take appropriate actions that may help to reduce losses from the
future natural hazards
RESPONSE
TO DEBRIS FLOWS, REPRODUCED AND MODIFIED FROM U.S.GEOLOGICAL SURVEY FACT
SHEETS:
The following response to debris flow hazards is from the guidelines provided by the United States Geological Survey in the Landslide Hazard Fact Sheets, 1999-2000. These guidelines are very useful for Jamaica as we are in the hurricane season.
What are debris
flows?
Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. Debris flows (also referred to as mudslides, mudflows, or debris avalanches) are a common type of fast-moving landslide that generally occurs during intense rainfall on water-saturated soil. They usually start on steep hillsides as soil slumps or slides that liquefy and accelerate to speeds as great as 35 miles per hour (56 km/hour) or more. They continue flowing down hills and into channels and deposit sand, mud, boulders, and organic material onto more gently sloping ground. Their consistency ranges from watery mud to thick, rocky mud (like wet cement), which is dense enough to carry boulders, trees, and cars. Debris flows from many different sources can combine in channels, where their destructive power may be greatly increased.
HOW TO IDENTIFY
HAZARDOUS AREAS?
Debris flows start on steep slopes-slopes steep enough to make walking difficult. Once started, however, debris flows can travel even over gently sloping ground. The most hazardous areas are valley bottoms, stream channels, areas near the outlets of valleys and gorges, and slopes excavated for buildings and roads.
It may be possible for individuals to easily identify the features shown in the following illustration.
A: Valley bottoms, stream
channels, and areas near the outlets of valleys or channels are particularly
hazardous. Multiple debris flows that start high in stream courses commonly
funnel into channels. There, they merge, gain volume, and travel long distances
from their sources.
B: Debris flows commonly begin
in swales (depressions) on steep slopes, making areas down slope from swales
particularly hazardous.
C: Road cuts and other altered
or excavated areas of slopes are particularly susceptible to debris flows.
Debris flows and other landslides onto roadways are common during rainstorms,
and often occur during milder rainfall conditions than those needed for debris
flows on natural slopes.
D: Areas where surface runoff
is channeled, such as along roadways and below culverts, are common sites of
debris flows and other landslides.
What can you
do if you live near steep hills?
These tips may
save your life and property.
Prior
to approaching hurricanes and other storms:
1.
Become familiar with the land around you. Learn whether debris flows have
occurred in your area by contacting UDS at UWI, ODPEM, NRCA, UWA, MGD, or your
Parish Disaster Coordinator. Slopes where debris flows have occurred in the past
are likely to experience them in the future.
2. Support Town Planning Department
and Parish Councils in their efforts to develop and enforce land-use and
building ordinances that regulate construction in areas susceptible to
landslides and debris flows. Buildings should be located away from steep slopes,
streams and rivers, intermittent-stream channels, and the mouths of mountain
channels.
3. Watch the patterns of storm-water
drainage on slopes near your home, and note especially the places where runoff
water converges, increasing flow over soil-covered slopes. Watch the hillsides
around your home for any signs of land movement, such as small landslides or
debris flows or progressively tilting trees.
4. Contact your parish council to
learn about the emergency-response and evacuation plans for your area and
develop your own emergency plans for your family and
business.
During Intense
Storms:
1. Stay
alert and stay awake! Many debris-flow fatalities occur when people are
sleeping. Listen to a radio for warnings of intense rainfall. Be aware that
intense short bursts of rain may be particularly dangerous, especially after
longer periods of heavy rainfall and damp weather.
2. If you are in areas susceptible to
landslides and debris flows, consider leaving if it is safe to do so. Remember
that driving during an intense storm can itself be hazardous.
3. Listen for any unusual sounds that
might indicate moving debris, such as trees cracking or boulders knocking
together. A trickle of flowing or falling mud or debris may precede larger
flows. If you are near a stream or channel, be alert for any sudden increase or
decrease in water flow and for a change from clear to muddy water. Such changes
may indicate landslide activity upstream, so be prepared to move quickly. Don't
delay! Save yourself, not your belongings.
From: | "Rafi Ahmad" <rahmad@uwimona.edu.jm> |
To: | "Ben Oostdam" <boostdam@hotmail.com> Save Address |
Subject: | Fw: Landslides in Jamaica. |
Date: | Tue, 12 Jun 2001 20:11:18 -0500 |