LANDSLIDES RELATED TO
DAVID J. MILLER
DEBORAH-ANN C. ROWE
DEPARTMENT OF GEOGRAPHY AND GEOLOGY
THE UNIVERSITY OF THE
Heavy rainfall from the rain storms of 27 October- 5 November 2001, 22 May – 2 June 2002, 17-24 September ( Isidore) and 27- 30 September ( Lili) 2002 triggered hundreds of landslides on steep slopes of the Port Royal and Blue Mountains in eastern Jamaica. Rainfall patterns obtained from the satellite showed that the pattern of damage was generally consistent with the area of heaviest rainfall.
Fig. 1. The NOAA satellite image of Tropical Storm Lilli taken at on Friday, 27September 2002.
Following the recent rainstorms we visited some of the severely affected drainage basins in the are. Our field data and historical records indicate that previous flooding and debris-flow events of similar magnitude to that of October 2001, and May-September 2002 have occurred throughout this region.
This paper examines the recurrent processes of landslides of flow type and sediment deposition in the eastern section of the island and suggests mitigation measures.
Landslide characteristics include: (a) dominant varieties were debris flows, debris avalanches, debris slides and mud flows originating on mountain-sides with landslide scars being generally located at the steepest section of the slope; vegetation was uprooted (b) landslide debris followed pre-existing depressions/channels, (c) debris flows and debris avalanches originating in small and steep channels caused sediment surges which along with organic matter blocked channels creating temporary landslide dams; these were breached within couple of hours, (e) landslide distribution was irregular suggesting variation in rainfall intensity over short distances, (f) erosion of stream channels was spectacular and contributed significant amount of sediments to total sediment yield. Landslide debris was deposited in several different niches. These included deposits at the base of slopes, channel deposits, debris fans, deposits where mountain streams were blocked by road culverts, and flood plain deposits in the lower reaches of the major rivers.
Most landslides initiated as thin earth (soil) slides or debris slides (soil with pieces of rock), as indicated by shallow sliding surfaces within soil or weathered, and jointed bedrock.
Figure 2 shows a shallow earth slide in decomposed
granodiorite near Temple Hall,
Shallow slides composed of loose soil and rock liquefied into debris flows/ mud flows with the addition of hill slope runoff or the water from within the channels. This process of debris-flow mobilization initiating from shallow slides is a widely recognized process. Large rock slides, and rotational slides of earth or rock, were also observed.
Mud flows in
completely decomposed granodiorite exposed along the
Mobilization of debris slides into mudflows is shown on
Figures 4 to 6 from the Broadgate area,
traveling down steep hillside paths, the debris flows entrain the colluvium
that is resident on hill slopes. Upon entering the main channels, the debris
flows incorporated alluvium from the channel beds and also sediments from collapsing
channel banks. Grain size in debris flows range e from fine-grained clastics to
extremely large boulders as seen in Figure 7 from Belcarres in
damage to communities and infrastructure was extensive in
Figure 8 shows a view of the breached eastern end of the Yallahs Fording .( Photo taken on1st October 2002.)
Figure 9 shows the
FROM: THE GLEANER WEBSITE Damage caused by Tropical Storm Lili in
Photographers:Ian Allen and Junior Dowie
Figure 10. Eastern approach to the
Electricity and telephone poles damaged by debris flows are seen in Figure 11.
This location is near the intersection of
Fig. 12. Residents of Bull Bay, Jamaica, take shelter
from flood water on Monday, Sept. 30. In
Figure 13 shows communities located close to the
FROM THE GLEANER WEBSITE:
by Tropical Storm Lili in
Photographers:Ian Allen and Junior Dowie
Debris and water impact on buildings, roads and other structures may be reduced if structures were built with their length aligned parallel to the direction of flow. This type of construction tends to minimize the width of a house/building exposed to a debris flow. This practice is recommended for all those communities located on debris fans.
Figure 14 is an example of a house with its length parallel
to the flow direction. Although a weak construction, this structure remained
intact following the debris flows of
Figure 15 shows a house and a van partially engulfed by the
debris in the
Figure 16 is an example of the house with its width oriented perpendicular to the flow path. Accumulation of debris and vegetation is seen along the wall facing the flow path.
Debris flows engulfed houses and church, October 2001, Belcarres,
Debris flow Damage in the
Landslides were abundant on steep slopes within all lithologies. Some hillsides , as in Figure 19 were denuded by single or coalescing failures as in the watersheds north of Norbrook in St. Andrew. Several old and stabilized landslide scars are also visible in the background.
Following the 25-29 October 2002 rainfall the
Figure 20 is an upstream view of the silted
Figure 21 is a view of the
PERFORMANCE OF CIVIL ENGINEERING STRUCTURES IN THE WAKE OF HEAVY RAINFALL
flooding processes in
A survey of damaged bridges and culverts in
Blocked culvert on the
Figure 23. Culvert on a debris channel, Bybrook,
Debris flows blocked the channel upstream of the bridge in October 2001 resulting in damage to the structure and flooding in the adjoining community.
A view of the 29th October 2002 debris flows that
blocked the 10 Mile Bridge,
The size of the bridge is inadequate to allow for the passage of debris and plant material. With the channel being completely blocked by tree trunks, branches and debris etc., avulsion took place and debris and water started moving west toward the road and houses located on the right-hand side of the channel in the community.
Design of bridges and culverts in this area must take into consideration the large volume of debris and vegetation that are being carried into the channels. The structures designed for accommodating only pure water floods are unsuitable as they offer a resistance to the passage of debris and vegetation, for example, the current flooding problem at the 10 Mile Bridge on Highway A4.
It appears that
fording is a better alternative to culverts and small-scale bridges as these
structures do not obstruct the flows and are easily cleared by a front end
loader, for example, the fording on Spring Gut east of
The socio-economic impact and losses to infrastructure, private property and agriculture in the wake of 2001-2002 high magnitude rainstorm events were catastrophic. Road network and water supply systems suffered serious disruption.
Landslide deposits have caused severe indirect damage and hazards
manifested in damming of the rivers and sudden debris supply to river channels.
For example, sediments generated by October 2001 event raised the river bed at Bybrook
Excessive coarse sediment supply to channels tends to decrease channel depth and an increase in the frequency of overbank flooding in the lower reaches of these channels, that is fan areas where most of the development takes place.
Mitigation is necessary in order to minimize future losses from events of similar or greater magnitude.
It is considered important to establish rainfall intensity-duration thresholds for triggering of landslides.
DEBRIS FLOW MITIGATION MECHANISMS (Table 1)
Following the identification and assessment of the hazard, a public education programme may be mounted to advice the citizenry of the vulnerability and risk.
Non-structural measures can be
especially cost effective in reducing hazards if the areas in question are
subject to frequent debris flows, e.g.,
A majority of houses/structures on the alluvial fans visited are built with their length oriented perpendicular to the flow direction. The impacts of debris and flood waters are likely to be minimized if houses were oriented with their length parallel to the flow. This would also allow for the construction of V-shaped debris deflection structures.
Recommendations are made to the effect that it is best to avoid problem areas, relocation of existing houses etc. is proposed, or land use regulations controls are applied to prevent further occupation. The final decision, however, rests with the landowners/ state.
An important aspect is to minimize the amount of debris from entering into coastal environments where coral reefs and/or fishing habitats may be adversely affected.
TABLE 1. PROPOSED
DEFENCIVE MEASURES AGAINST DEBRIS FLOW AND FLOODING IN
NOTE:These measures may not be effective in case of a very high magnitude event.
(Modified from the approaches proposed by Hungr, O., and others, 1987, Debris flow defenses in British Columbia, Reviews in Engineering Geology, Vol. VII, p.201- 222.)
1. Hazard mapping and zoning Restrict use of endangered areas; public education
on acceptable risk, and interventions for a safer building practice.
(NOTE: Maps that delineate areas affected by debris flows and floods are available)
2. Warning systems: advance Facilitate evacuation at times of danger
during event or post-event
(NOTE: This requires installation of, e.g., a Tripwire device, rainfall gauges in the watershed)
A. In source areas
3. Revisit landuse practices/ Reduce debris by stabilization of debris sources.
execute an audit / find
innovative uses of sediments
(NOTE: Long-term solution)
4. Reforestation Reduce loss of vegetation cover disturbed by ever-increasing development activities in the watersheds.
(NOTE: Long-term solution)
5. Road construction control Eliminate unstable cuts and fills that could act as
debris sources or initiation points.
B. In transportation and deposition zone
(The zone between the mountain front and the debris deposition area)
6. Open debris deposition basins; Control the extent of depositional area by shaping dykes or walls and diking.
7. Closed retention barriers and Create a controlled deposition space fronted by a
basins; full or partial volume straining structure and a spillway.
8. Raise the height of the bridges Allows for the safe passage of debris under the
channel dredging and widening bridge and through culverts
of culverts; create fording.