Avijit Gupta

Evaluation of part of the Mekong River using satellite imagery

Abstract Large rivers of the world are difficult to investigate. This is essentially due to the logistical problems involved when attempting to study rivers that are about a thousand kilometres or more in length and drain basins of appropriate dimensions. This creates difficulties in river management and also in determining the possible impact on the river system of specific development projects or basin land-use changes. We demonstrate the capability of satellite images, a huge number of which are currently archived in various centres around the world, to provide at least a coarse-scale solution to these problems. A significant amount of work can be done even without the application of any special remote sensing technique. About 14,000 km 2 of the middle Mekong Basin in Lao PDR has been studied. The Mekong is the 12th longest river in the world and is ranked eighth in terms of mean discharge. The Mekong is worth investigating for several reasons. (1) It has a fascinating and complicated physiography. (2) Little published information exists for the basin, at least for the part selected for the case study. (3) Various development plans have been drawn up for the Mekong with possible environmental impact on the basin and the river. The images are used in conjunction with the limited geomorphic and hydrologic data available in order to (1) provide a concise account of the local geomorphology, (2) map environmental degradation and sediment transfer in parts of the basin and (3) determine the nature of possible environmental impact associated with certain proposed development projects on the river. This is carried out by searching through the SPOT satellite images archived at the Centre for Remote Imaging, Sensing and Processing (National University of Singapore), selecting and studying clear scenes, and comparing the 1996, 1998 and 1999 images of the study area to record changes over time. Primarily, the study is a demonstration in combining remote sensing and geomorphology for environmental management of a large river system. It is also a plea for transforming the satellite images accumulating in various archives to useful geomorphological databases. Given the scale of large rivers, satellite images are excellent tools for their investigation.

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.

Geology and Landforms of the Mekong Basin

Publisher Summary The Mekong flows in a pan-shaped basin shaped by regional geology. The upper basin in China is a steep narrow valley and its geometry is determined primarily by Himalayan orogeny. The drainage basin widens south of the Chinese border in Lao PDR and Thailand but remains mountainous. A large part of the Mekong Basin comprises pre-Tertiary metamorphosed terrestrial and marine sedimentary rocks associated with suture zones, and intruded granites. Downbasin from Vientiane to Savannakhet, a shallow Quaternary alluvium, mainly fine to coarse sand with some clay and gravel, covers Paleozoic and Mesozoic rocks in the valley of the Mekong. Mesozoic sandstones and evaporites reappear in the Mekong valley further south beyond this Quaternary cover, locally overlain by shallow alluvium. In Cambodia, the basin widens to about 500 km with an alluvium of variable thickness as surface cover. Permian carbonates, Triassic sedimentary rocks, and Neogene basalts form the divides, and with granite emerge from underneath the alluvial cover elsewhere to form low isolated hills. The Mekong Lowland stretches north–south along the river from the northern mountains to the delta. The lowlands widen from Vientiane as the river angles cross-basin toward the eastern divide marked by the northern Annamite Range.

The changing geomorphology of the humid tropics

Abstract It is debatable whether tropical geomorphology is intrinsically different from the geomorphology of other places. The traditional geomorphology of the humid tropics had certain expectations with regard to landforms and processes. A number of case studies over the last three decades, however, using new concepts and field and laboratory techniques, have changed such expectations. This paper primarily examines the research themes of the last ten years or so, identifies the major trends, and attempts to predict areas where interest is expected to concentrate in near future. The following topics among the current research trends are highlighted: geochemical erosion, movement on tropical slopes, rivers in the tropics, tropical landforms, hydrology and sedimentation in the rainforest, denudation efficiency in the humid tropics, the Quaternary evolution and changes, and the applied side of tropical geomorphology. Clearly, the landforms and geomorphological processes in the tropics are dependent only partially on climate, but to greater extent on neotectonism, lithology, episodic and catastrophic events such as tropical cyclones, and above all on anthropogenic alterations of the landscape. A wide variety of landforms and processes are found in the humid tropics, and generalisations are extremely difficult. In the next century geomorphological studies in the tropics are likely to develop along certain lines. Environmental investigations will form a major component along with the study of postulated sea level rise. Current major research trends will undoubtedly continue, but increasingly through a multidisciplinary and multipeople approach. Large-scale investigations involving pantropic correlation of past events, such as synchronised large floods, will grow. Use of new research tools such as satellite imagery, various dating facilities, etc. will become common and expected. It seems likely that the arrival of so much information will modify mainstream geomorphological and sedimentological theories or will provide these with a tropical version.

Urban Hydrology and Sedimentation in the Humid Tropics

Over the last twenty-five years a considerable amount of research has been done on the modification of the physical environment by urbanization. A large storage of data on changes in local hydrology, sedimentation patterns and channel morphology has been created, a conceptual model of the time-sequence and intensity of such changes has been built, and the information and concepts have been used to ameliorate environmental problems arising from urbanization.

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.

First Satellite Image of a Moving Pyroclastic Flow

Pyroclastic flows that emerge from the bases of collapsing eruption columns or lava domes are transient dynamic phenomena, rarely documented due to their very brief existence, which may be only minutes. Such flows are commonly reconstructed from their deposits. They have been photographed on the ground, but to the best of our knowledge, the pyroclastic flow described in this article is the first such flow in action that has been recorded on a satellite image. This provides an excellent opportunity to see the flow in action.

Environmental study of the middle Mekong Basin using multi-temporal SPOT imagery

The middle Mekong Basin in Lao PDR and Thailand with an area of about 13,969 sq km was investigated with SPOT data acquired from 1996 to 1999. The following digital image processing techniques were implemented in this study: generation of DEM, slope, aspect and orthorectified XS images, mosaic of the XS orthoimages, atmospheric correction and unsupervised change detection to create the thematic change map. Two significant changes were shown clearly by comparison of the images of 1996 and 1998/9: (1) marked depletion of vegetation from slopes, especially southern slopes, and (2) the bed of the river is choked with sediment, which is progressively silted up over time. This study demonstrated the application of remote sensing data for investigation of large river systems and for planning their management.

Satellite Observations of Coastal Changes in ACEH After the 2004 Tsunami

The December 2004 Sumatra earthquake triggered a massively destructive tsunami that affected several coastal areas in the Indian Ocean. In this study, very high resolution IKONOS satellite images acquired before and after the tsunami were used to study the changes of the coastal morphology in the Northwestern coast of Sumatra. The coasts were eroded during the tsunami. At some stretches of the coast, the coastline receded by as much as 500 m. About 13 months after the tsunami, new beaches were formed at many areas along the coastline. The widths of the newly formed beaches exceeded those of the pre-tsunami coast. In areas where anthropogenic modifications have occurred before the tsunami, the coastline remained eroded after 13 months and no evidence of recovery was observed.