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- Landslides are a common occurrence and recurring problem for much of
the mountainous island of Jamaica.
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- Disruption of transportation routes;
- Stranded communities, loss of income, closed schools, substantial
property damage & community facilities, trauma of evacuation, and
disruption of social fabric of communities;
- Destruction of domestic water supply;
- Loss of productive agricultural areas, especially coffee farms and
farm-to-market access roads;
- Landslides added sediment to many river drainages raising channel levels
thus increasing flood hazard;
- Many areas remain exposed to landslides from future periods of sustained
heavy rainfall.
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- Extensive landslide damage in Jamaica is mostly due to transported
landslide debris and are prominent along debris chutes and deposition
areas, often far removed from the landslide source.
- Much of the relatively flat land in Jamaica that adjoins mountain fronts
is built by landslide debris brought over the years by debris chutes (or
innocent looking dry gullies). These are the hazardous areas, or debris
fans, where most of our human settlements are located.
- Modifications of hillslopes for urbanization and road construction have
also resulted in numerous landslides.
- Rainfall-triggered landslides on both modified and unmodified slopes are
most common and occur frequently throughout the mountainous terrain of
the island.
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- The distribution and characteristics of 2001-2002 rainfall-triggered
landslides determined by UWI Mona’s Unit for Disaster Studies Landslide
Project can be used as a guide to future landslide activity triggered by
rainstorms in Jamaica.
- OUR DATA SUGGESTS THAT HAZARDOUS AREAS INCLUDE:
- Hillslopes underlain by colluvium and fractured & weathered bedrock,
- Moderate to steep hillslopes, and areas directly at the base of these
slopes,
- Drainage channels acting as debris chutes downslope from landslide prone
hillslopes,
- Alluvial fans at the mouth of main drainage channels, and
- Alluvial fans at the mouth of debris chute drainages along hillslope
fronts.
- Attempts should be made to prevent housing and infrastructure
development in the above areas using landslide inventory maps.
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- For hazardous areas that are already inhabited, methods of predicting
and delivering landslide warnings can be developed.
- One of the easiest, and most accurate ways to predict the timing of
rainfall-triggered landslides, is by using a rainfall threshold.
- THIS IS THE SUBJECT OF THIS PRESENTATION.
- When such a threshold has been exceeded, or is expected to be exceeded
by an approaching storm, a landslide warning can be issued for hazardous
areas.
- Two thresholds should be developed:
- For debris flows that commonly develop from shallow landslides during
intense bursts of rainfall.
- For deep-seated landslides that are usually triggered by prolonged
rainfall.
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- Landslide times of occurrence based on fieldwork, sequential remote
sensing imagery (Jamaica needs to make an investment in this – imagery
for earth science research), and local sources,
- Rainfall data from nearby continuously recording rain gauges, preferably
through a network of telemetric network.
- Since the most hazardous landslides are of flow type, gauges should
have at least hourly resolution.
- Data collecting capabilities of the National Meteorological Service
need to be substantially strengthened if we were to mitigate natural
hazards.
- Rainfall amounts for storms that did not trigger landslides are equally
important.
- A systematic programme for developing a network of rain gauges,
especially in collaboration with frequently affected communities, is
considered very important.
- However, funding for this type of proactive response which prevents
hazards from becoming disasters is rarely available.
- Is response to hazard mitigation in developing countries is shaped by
Precipitation and Politics?
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- Rainfall Thresholds determine whether debris flows are triggered during
rainstorms.
- Subthreshold storm rainfall (below left) simply raises groundwater
levels in hillside soils.
- Rainfall conditions above the threshold (right) raise groundwater
levels high enough to trigger failure in soil types that can liquefy.
- The resulting mass, which flows suddenly and rapidly downslope, can
destroy buildings and claim lives within moments after movement starts.
- (Modified from S.E and B. Rogers. 1996)
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- In Jamaica, two-thirds of the area is located in mountainous zones.
- Because of the scarcity of relatively flat lands, residential areas have
been built at the hillsides and on the hills.
- Severe erosion (aided by landslide processes) on steep hills that
consist of relatively easily erodible geological materials has resulted
in abundant colluvial (debris) accumulations. These are called debris or
alluvial fans.
- Colluvium (debris)- usually weathered material lying on the surface of
a hill or slope which is transported across and deposited on a low-angle
slope or on a footplain. Being the result of wash and gravity-induced
mass movement processes over different distances, a colluvium contains
grain sizes related to the bedrock of the source area.
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- Rainfall records from National Meteorological Service of Jamaica have
been used. Data for 23 storms, 1951- 2002, have been used. These
included landslide producing storms and also those that did not. As far
as possible, rain gauges located close to landslide sites and with an
hourly resolution were used.
These may represent maximum landslides at a particular site. A
summary of those data are given in Tables 2-4.
- Fieldwork, newspaper archives, agency reports form the basis for
landslide data.
- At this stage of research, our landslide producing data are obviously
incomplete. Although there were many storms that produced or did not
produce landslides in Jamaica, this paper includes only those storms
that produced verifiable landslides, 2-3m deep, and in tens-hundred.
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- During heavy rainfall the colluvium is easily weakened, which often
leads to debris flows that present a great threat to areas downstream.
- Debris flows initiation of requires three fundamental conditions and at
least one triggering condition.
- Fundamental Conditions (subjective):
- Abundant debris
- A steep slope angle
- A lot of surface and subsurface water
- Triggering conditions:
- Heavy rainfall
- Highly variable topography
- Abrupt changes in vegetation
- Slope failures
- Debris (rock, soil, woody debris) is mobilized from hillslopes and
channels by the addition of sufficient moisture.
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- Tables 1-3 suggest that in our data set:
- Only 6 of 36 events were hurricanes; remainder were tropical
disturbances and cold fronts.
- A majority of the landslide triggering storms occurred during the
hurricane season (June through November).
- Storms which triggered landslides also occurred in May, the time period
between the last of the cold fronts and the first of the tropical waves.
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- Using data for 19 storms, 1951-2002, a threshold relation between
rainfall intensity-duration and landsliding was established.
- A threshold fitted to the lower boundary data points reflects the
approximate minimal rainfall conditions necessary to trigger shallow
landslides in eastern Jamaica.
- This rainfall threshold relation is defined for storms that had
durations between 1-168 hours and average rainfall intensities between
2-93 mm/h.
- The threshold relation indicates that for rainfall of short duration
(about 1 h), higher than 36 mm/h, are required to trigger landslides.
- Low average intensities of about 3mm/h appear to be sufficient to cause
landslilding as storm duration approaches approximately 100 h.
- There is a relation between landslide characteristics and the position
of the landslide-triggering storm on the threshold line. Storms near the
short-duration/high intensity end of the threshold line trigger mostly
shallow landslides by causing an excess pore pressure in shallow
colluvial zones. Such landslides were typically associated with
2001-2002 type storms.
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- In contrast, storms near long-duration/low intensity end of the
threshold have triggered the largest, deepest landslides in eastern
Jamaica, e.g., Flora, Gilbert, 2001 rainfall.
- We are trying to establish the effects of antecedent soil-moisture
conditions. It appears that these
may not be very significant in May or beginning of the hurricane season.
It is at the end of the hurricane season that increased soil moisture
becomes important.
- Given hourly rainfall intensity resolution and accurate timing of
landslides, it is possible to establish fairly robust intensity-duration
relationships, e.g. Hurricane Gilbert analysis.
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- The earthquakes of June 1692 and January 1907 and flood rains of June
1979 created hundreds of landslides which caused severe deforestation
and erosion on the island.
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- In Eastern Jamaica, rainfall threshold relation is defined for storms
that had durations between 1-168 hours and average rainfall intensities
between 2-93 mm/h.
- The threshold relations reported here are reasonable first
approximations.
- The threshold relation indicates that for rainfall of short duration
(about 1 h): Intensities > 36 mm/h, are required to trigger
landslides. These storms trigger mostly shallow landslides by causing an
excess pore pressure in shallow colluvial zones. Such landslides were
typically associated with 2001-2002 type storms.
- Low average intensities of about 3mm/h appear to be sufficient to cause
landslilding as storm duration approaches approximately 100 h. These
triggered the largest, deepest landslides in eastern Jamaica, e.g.,
Flora, Gilbert, 2001 rainfall.
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- Thresholds provide a key element of early landslide and flood warning
systems.
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- Grateful thanks are extended to the Staff of the National
Meteorological Service of Jamaica, Faisal Butt, Michelle (from the
National Meteorological Service) and Debbie (Deborah-Ann Rowe).
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Notes