Rafi Ahmad

Morphology and sedimentation in Caribbean montane streams" examples from Jamaica and Puerto Rico

Abstract This paper presents a summary description of the morphology, sedimentation, and behaviour of the montane streams of eastern Jamaica and eastern Puerto Rico. The area is located within a 200 km wide seismically active zone of Neogene left-lateral strike-slip deformation which defines the plate boundary between the Caribbean and North American Plates. Tropical storms, occasionally strengthening up to hurricane force, affect the region periodically. This is an area of steep, mass-movement-scarred hillslopes which supply a large amount of coarse sediment to the rivers. From the description presented, we have constructed a model for the rivers of this region controlled by both neotectonics and periodic large floods. The drainage density is low with a near-rectangular stream network. The gradients are steep with boulder accumulations in the channels, their location at times related to the presence of large past landslides on hillslopes. Narrow, steep and confined channels occur in the mountains, but in wider sections and lower down near coastal plains, flood depositional forms appear in coarse valley alluvium. Small-scale deviations from the general pattern occur locally, controlled by variations in lithology, neotectonism, seasonality in flow, etc. This model for Caribbean montane streams differs considerably from the standard descriptions of alluvial rivers for which a number of detailed studies are available.

Geomorphology and the urban tropics: building an interface between research and usage

Abstract The developing countries, located almost entirely within the tropics, are currently undergoing urbanization at a rapid pace. Many of these cities are not more than a few centuries old, having been established to function as regional trading posts or administrative centers by either colonial or regional powers. It is doubtful that the site conditions were taken into consideration, and consequently, many of these cities were established in hazardous or environmentally sensitive areas. As these cities developed over time, they spread across a wide range of terrain conditions much of which are unsuitable, such as floodplains, coastal swamps, steep slopes, or sand dunes. For a number of these cities, which are located near active plate margins and tropical cyclone belts, such problems are magnified. Increased demand for water has required subsurface extraction, leading to problems of subsidence and quality. We examine the range of these cities with reference to (1) their site-related problems, (2) the nature of geomorphological information required for specific ameliorating actions, and (3) the level of management required for city maintenance. Management requires interfacing geomorphology with engineering practices and urban planning. We present case studies of cities ranging from those with limited problems and possible engineering and land-zoning solutions (Singapore) to cities where the hazards (either natural or anthropogenic) are so acute and widespread that a practical solution is difficult to achieve (Kingston, Bangkok). Precise geomorphological and geological information and long-term data sets are not available for most of the cities. Furthermore, it is necessary to present the information in formats appreciated by the engineering and planning communities. Often a set of specialized maps are extremely useful. We complete the discussion with several examples and suggestions for collection of geomorphological information and inventory preparation for communicating geomorphological data to city engineers and planners.

Uranium and boron distributions related to metamorphic microstructure-evidence for metamorphic fluid activity

The distribution of uranium and boron in polymetamorphosed, granulite facies schists and gneisses has been studied using particle track methods. The concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into: (1) the behaviour of U and B in metamorphism, (2) the activity of a fluid phase in the metamorphic processes and (3) the nature of chemical processes during schistosity development. A low concentration of primary U occurs in micro-inclusions of apatite and zircon (many are metamict) in the granulite facies M1 assemblage. This assemblage which lacks B, except for zoned sillimanite, has undergone a localized retrograde metamorphism RM1 characterised by hydrous alteration products containing abundant U and B. The RM1 metamorphism is attributed to fluids generated during granulite facies dehydration reactions. A schistosity S2 defined by M2 fibrolite aggregates overprints the M1 events. It is associated with (1) intragranular U concentrated in M2 apatite and titanium bearing minerals and (2) abundant intergranular U within the fibrolite aggregates. High B contents also occur with the fibrolite. The S2 schistosity appears to develop in a metamorphic environment containing a fluid enriched in U and B.

Producing landslide-susceptibility maps for regional planning in data-scarce regions

In many of the lesser developed areas of the world, regional development planning is increasingly important for meeting the needs of current and future inhabitants. Expansion of economic capability, infrastructure, and residential capacity requires significant investment, and so efforts to limit the negative effect of landslides and other natural hazards on these investments are crucial. Many of the newer approaches to identifying and mapping relative landslide susceptibility within a developing area are hindered by insufficient data in the places where it is most needed. An approach called matrix assessment was specifically designed for regional development planning where data may be limited. Its application produces a landslide-susceptibility map suitable for use with other planning data in a Geographical Information System (GIS) environment. Its development also encourages collecting basic landslide inventory data suitable for site-specific studies and for refining landslide hazard assessments in the future. This paper illustrates how matrix assessment methodology was applied to produce a landslide-susceptibility map for the Commonwealth of Dominica, an island nation in the eastern Caribbean, and how with a follow up study the relative landslide-susceptibility mapping was validated. A second Caribbean application on Jamaica demonstrates how this methodology can be applied in a more geologically complex setting. A validated approach to mapping landslide susceptibility which does not require extensive input data offers a significant benefit to planning in lesser developed parts of the world.

Landslide damage to the board river water supply pipeline, Bromley Hill, Jamaica: Case study of a landslide caused by hurricane Gilbert (1988)

This case study highlights the importance of geotechnical investigations in the site selection, design and maintenance of life-line structures in multiple hazard prone areas.The Boar River Pipeline, carrying part of Kingston’s water supply, was damaged by a debris flow originating on the eastern slope of Bromley Hill, following the passage of hurricane Gilbert over Jamaica on September 12, 1988. Regional and site-specific geotechical investigations indicate that landslides are ubiquitous in the region, especially on east-facing slopes, and that the slide which damaged the pipelnine is a relatively shallow failure. The slide may stabilize itself in the due course of time provided there are no extreme weather events. Artificial measures of slope stabilization are recommended as a short-term solution. However, since the entire slope is involved in sliding and favourable conditions for future failures exist in the area, the better approach would be to avoid the failed slope altogether and relocate the pipeline.RésuméL’exemple traité dans cet article met en évidence l’importance des études géotechniques pour le choix des sites destinés à recevoir des structures durables, ainsi que pour leur conception même et leur entretien.La conduite de la Boar River qui achemine une partie de l’approvisionnement en eau de Kingston, a été endommagée par un coulée de débris qui s’est déclenchée sur la pente orientale de la Bromley Hill, suite au passage du cyclone Gilbert sur la Jamaïque le 12 septembre 1988. Des études géotechniques à l’échelle de la région et du site montrent que les glissements de terrain sont très répandus dans la région, en particulier sur les pentes orientées vers l’Est, et que le glissement qui a endommagé la conduite est relativement superficiel. Le glissement pourrait se stabiliser de lui-même avec le temps pourvu qu’el ne se produise pas d’événements météorologiques extrêmes. Des mesures de stabilisation de pente sont recommandée comme solution à court terme. Cependant, comme la pente tout entière est affectée de glissements et que des conditions favorables au déclenchement de nouvelles ruptures existent dans tout le secteur, la meilleure solution serait d’éviter l’ensemble de la pente et de réimplanter la conduite dans un autre site.

Recognizing the importance of tropical forests in limiting rainfall-induced debris flows

Worldwide concern for continuing loss of montane forest cover in the tropics usually focuses on adverse ecological consequences. Less recognized, but equally important to inhabitants of these affected regions, is an increasing susceptibility to rainfall-induced debris flows and their associated impacts. The same high rainfall rates that sustain tropical forest cover can often serve as the triggering mechanism for debris flows. The natural rate of debris flow occurrence on steep slopes subject to episodic, intense rainfall is dependent on the stabilizing effect of tropical forests. Either loss or significant reduction in forest cover can weaken this natural defense. Information from postdisaster observations and research on the November 1988 storm event in southern Thailand provides a case study illustrating the potential impacts of increased debris flow susceptibility resulting from conversion of forest cover to rubber tree crops. Development resulting in the loss of tropical forest cover may be accompanied by local increase in population, property development, and infrastructure. Consequently, the potentially disastrous consequences of increased debris flow occurrence are amplified by the greater vulnerability of local populations. Preserving the tropical forest cover is an obvious and often difficult means of retaining this natural protection. Effective policy should capitalize on the values of tropical forests as part of the strategy for retaining adequate forest cover. Policy should also seek to avoid creating pressures that foster forest removal or their conversion to other types of land cover in steep terrain. Areas where tropical forests were converted to other cover types can be restored to secondary forests to avoid a permanent state of increased debris flow susceptibility. Restoration of secondary tropical forests can successfully re-establish the forest characteristics that limit debris flow occurrence. Experience in Central America and the Caribbean demonstrates that successful restoration is possible but requires a significant commitment of both time and resources. In addition to the cost and technical difficulties involved, the increased susceptibility to debris flow occurrence persists through many years until successful restoration is achieved. Both retention of existing tropical forests and restoration of forest cover where loss has occurred are often justified by the reduced risk of debris flow impacts to vulnerable populations and infrastructure.

Urban steeplands in the tropics: an environment of accelerated erosion

The developing countries at present have a high urban growth rate that is likely to continue for at least another quarter-century. In addition, many of these urban centres are located in the Neogene plate boundary zones and are subject to multiple earthquake and volcanic hazards. Slope failures and accelerated surface and channel erosion are particularly severe in cities near active plate margins, and in areas affected also by tropical cyclones. We discuss two extreme cases: Singapore and Kingston (Jamaica). Singapore is located in a stable environment and the urbanization related problems of flood and slope instability have been reduced by proper building and drainage practices at a considerable cost. In Kingston, the external disturbances are repetitive, large-scale, and very difficult to control. The fast-growing cities in the tropics need to be carefully monitored, especially when located in an unstable physical environment.