LANDSLIDES RELATED TO PRECIPITATION IN EASTERN JAMAICA

RAFI AHMAD

rahmad@uwimona.edu.jm

DAVID J. MILLER

dmiller@uwimona.edu.jm

DEBORAH-ANN C. ROWE

Deb_annrowe@yahoo.co.uk

DEPARTMENT OF GEOGRAPHY AND GEOLOGY

THE UNIVERSITY OF THE WEST INDIES

MONA, KINGSTON 7, JAMAICA

INTRODUCTION:

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 11:15 p.m. EDT on Friday, 27September 2002. From: GoPBI.com

Eastern Jamaica represents extremely steep and rugged topography with the crest of the Blue Mountain Range reaching 2254 m within about 10 km of the coast. Landscape in eastern Jamaica is geologically young (Quaternary). It is seismically most active part of the island and appears to be undergoing neotectonic uplift. Watersheds are relatively small and steep. Rivers flow through steep canyons onto alluvial/debris fans before emptying into the Caribbean Sea. With the exception of alluvial fans, flat areas are rare in the parishes of Portland, St. Thomas, and Upper St. Andrew. Communities have settled on fans and development activities are hindered by a lack of flat land. Landslides are common and are perhaps the most widespread hydrogeologic hazard on human timescale causing land degradation thus affecting land productivity. Bedrock structure strongly influences slope stability . Landslides preferentially occur in locations where the direction of dip of bedding in the shale and sandstone sequence daylights in slope faces. Landslides occurred both in the landscape modified for human use as well as on slopes under dense forest cover.

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, Junction Road, North Western St. Andrew. This site has repeatedly failed since May-June 1991 rainfall. Landslide debris blocked a section of the road in 2001 and 2002. Note the position of electricity transmission line pole which is threatened by this retrogressive landslide.

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 Junction Road in the Stony Hill-Golden Spring area are shown on Figure 3. The retaining wall to hold the landslide debris has been damaged. Mud flows were water saturated, plastic and very slippery when wet. Mud covered the road surface and caused a serious threat to vehicular traffic that negotiates a hairpin bend in this locality.

Mobilization of debris slides into mudflows is shown on Figures 4 to 6 from the Broadgate area, Junction Road, St. Mary. These landslides destroyed two houses and blocked the Junction Road. Landslide debris was carried into the Wagwater River.

While 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 theBuff Bay valley.

In the Swift River, Mabess River, and Spanish River, Portland we observed a continuous spectrum of flow phenomenon ranging from debris avalanches-debris flows-earth flows in the upper reaches to water floods on the relatively flat fan surfaces.

DAMAGE:

Landslide damage to communities and infrastructure was extensive in Eastern Jamaica.  Total economic losses following the 2001 event are estimated at US$ 2.5 Billion (PIOJ, 2002). Landslides and flooding caused land degradation in watersheds, and an extensive damage to road network, standing agricultural crops, electricity transmission lines, water pipelines and facilities, telecommunication lines, schools, residential dwellings, businesses, public service facilities. Most of the damage to private dwellings and public facilities was also due to landslide debris.

Figure 8 shows a view of the breached eastern end of the Yallahs Fording .( Photo taken on1st October 2002.)

Figure 9 shows the Bull Bay River Bridge that was blocked by debris flows several times during 2001-2002. Note the presence of rock blocks on the top surface of the flow, a characteristic feature of debris flows that have approximately 70% of fine and coarse debris by weight.
FROM: THE GLEANER WEBSITE Damage caused by Tropical Storm Lili in Kingston, St. Andrew and St. Thomas / BullbayfloodJ20020929IA
Photographers:Ian Allen and Junior Dowie 9/30/2002

Figure 10. Eastern approach to the Mabess River Bridge in Bybrook, Portland was blocked by debris flows triggered by October 2001 rainfall.

 Electricity and telephone poles damaged by debris flows are seen in Figure 11. This location is near the intersection of Jacks Hill Road and Sunset Avenue in Upper St. Andrew. This slope is frequently affected by debris flows.

AP Photo

Fig. 12. Residents of Bull Bay, Jamaica, take shelter from flood water on Monday, Sept. 30. In Jamaica, flooding from Lili washed away several houses, damaged dozens of others and turned roads into muddy rivers. "This is the worst I've ever seen it," police Sgt. Gladstone Devery said in the eastern town of Bull Bay, where a military helicopter lifted people from rooftops. From: GoPBI.com

Figure 13 shows communities located close to the Bull Bay River were inundated and residents had to be rescued from their houses.

FROM THE GLEANER WEBSITE:

Damage caused by Tropical Storm Lili in Kingston, St. Andrew and St. Thomas / BullbayfloodC20020929IA
Photographers:Ian Allen and Junior Dowie
9/30/2002

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 29^th October 2002.

Figure 15 shows a house and a van partially engulfed by the debris in the Bull Bay community.

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.

Figure 17 Debris flows engulfed houses and church, October 2001, Belcarres, Portland

Figure 18 Debris flow Damage in the Bybrook area, Portland.

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 Chalky River channel in South Eastern St. Andrew was choked by debris. This blockage diverted some of the debris and water east thus aggravating the effects of flooding in the Bull Bay area.

Figure 20 is an upstream view of the silted Chalky River channel. The Bull Bay housing area which was flooded during 25-29 September 2002 is located immediately to the right-hand side of the photograph.

Figure 21 is a view of the Chalky River Bridge, looking north, completely choked by debris flows initiated by 25-29 September 2002 rainfall. It is not known if the bridge and channel were cleared of the debris that were deposited during earlier events.

PERFORMANCE OF CIVIL ENGINEERING STRUCTURES IN THE WAKE OF HEAVY RAINFALL

The flooding processes in Eastern Jamaica represent a classic case of alluvial fan flooding that is dominated by mud floods and debris flow processes that transport large volumes of coarse and fine sediments and organic debris. They carry 40 – 70 % sediment load by weight and behave as visco-plastic flow. These should not be treated as simple pure water floods which are Newtonian flows. The fans themselves are generally built by accumulation of landslide debris.

A survey of damaged bridges and culverts in Eastern Jamaica suggest the inadequacy of these structures to allow for the passage of debris flows.

Figure 22

Blocked culvert on the Buff Bay – Tranquility Road, October 2001. The culvert opening is designed to carry pure water flood, however, the channel acts as a debris flow chute and consequently becomes choked with debris and plant remains resulting in flooding on the road and also upstream.

Figure 23. Culvert on a debris channel, Bybrook, Portland, October 2001. The size of the culvert obstructed the flow of debris.

Figure 24.

Belcarres Bridge, Buff Bay Road.

Debris flows blocked the channel upstream of the bridge in October 2001 resulting in damage to the structure and flooding in the adjoining community.

Figure 25.

Moore Park community bridge, Portland. This structure was blocked and damaged during the passage of debris flows in October 2001.

Figure 26.

A view of the 29^th October 2002 debris flows that blocked the 10 Mile Bridge, Bull Bay.

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 Bull Bay (Figure 27).

CONCLUSIONS

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 and Swift River areas in Portland by some 1.0 to 3.0 m.

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)

• It is possible to reduce the effects of debris flows.

• Mitigation methods reduce the impact but may not necessarily eliminate the associated hazards and risks.

• Mitigation methods are broadly grouped as passive and active measures.

• PASSIVE MEASURES (NON-STRUCTURAL) do not involve any direct engineering intervention to either prevent or modify/control the event and are most affordable.

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., Spanish River and Swift River areas in Portland, and Bull Bay River in St. Andrew. These include removing or converting existing development, discouraging development, and regulating development by limiting the type or amount of development. Experience elsewhere indicates that these are more cost effective than structural mitigation measures.

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.

• ACTIVE MEASURES (STRUCTURAL) aim at controlling the movement of debris across the depositional fan by employing engineering methods and are therefore very costly.

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 JAMAICA.

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.)

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Measure Purpose

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Passive Measures

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)

ACTIVE MEASURES

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.