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Jamaica Source: http://www.jamaica-gleaner.com/gleaner/20070719/
Debris flows and debris floods are in fact the movement of sediment as a singular body by water which collectively is called sediment flows. Sediment flows are commonly misidentified as water floods. The Unit for Disaster Studies Public Education Programme on hydro-geologic hazards of Department of Earth Sciences, University of the West Indies, has shown that many of the recent disastrous flood events in the region were actually sediment flows, and not water floods as commonly believed.
Debris flows comprises of poorly sorted rock debris comprising of 70 per cent to 90 per cent of the total weight. Flow of the mass is gravity induced, meaning it generally flows downhill, and its properties vary depending upon the sediment size; which may range between clay and boulders. The actual process of formation debris flows as a unified mass is the intermediate between land sliding and water flooding, where particles which make up the flow ranging from clay as the smallest sand particles to boulders ranging in diameter from 20 cm to greater than one metre in diameter. Many debris flows become diluted downstream as the ratio of water to sediment changes. The debris flow then becomes a debris flood where the percentage of sediment mass ranges somewhere in the range of 40 per cent to 70 per cent of the flow. In contrast, water floods are turbulent flows carrying relatively small amounts of sediments of around one per cent to 40 per cent of the total weight, have low densities, with generally stratified and well sorted deposits. Mass transport phenomenon These water-related disasters are regarded as a mass transport phenomenon; they differ from landslides where sediments are moved along structural planes as a result of support failure precipitated by shifting of the earth's crust, or human activities on slopes simulating what have been described as hyper-concentrated flows (debris flows of greater than 90 per cent mass) or sediment slurries which can easily move on gentle slopes. Our contribution to the problem Demand for land-use in Jamaica has grown with increasing rapidity over the last 40 years as the socio-economic demands of the population have grown and the concomitant change in the development agenda of government in response to these demands. The high incidence of hydro-geological hazard events in the last decade is perhaps indicative of the many miscalculated and underinformed decisions made by physical planners during our recent history. Policy makers sought to meet the heavy demand for housing, increased industry and social amenities which are now standard fare within the global market place. The cost of such choices appears to be the degradation of the physical environment through deforestation and improper land settlement tenure, among others. The frequency of water-related sediment flood disasters in Jamaica, is approximately one in every four years during the last 50 years, making it abundantly clear that these events are widespread and costly. Jamaica's vulnerability to multiple hazards is one of the main threats to the sustainable development of the country. However, the impact from these natural hazards could be minimised as in many cases the causes of the events are relatively well understood. Sediment flow damage Caribbean Despite the fact that direct and indirect losses from sediment disasters in the Caribbean run into millions of US$ annually, this subject has not received due attention. Sedimentfloods cause death and injury and significant damage to lifeline structures, residential dwellings, water supply, road network, power lines, agriculture, and businesses throughout the Caribbean region. Direct and indirect losses from recurring sediment disasters in Jamaica alone are in the order of millions of dollars for every event; from 1996 to 1998 almost US 26.7 million dollars in damage was done. In 2004 and 2005, several Caribbean islands including Haiti, Dominica, Grenada, Jamaica, St. Lucia, and Trinidad and Tobago were affected by sediment floods. Knowledge equals accurate mitigation Hydrological processes and channel dynamics in the relatively small and steep mountain watersheds of the Caribbean are not comparable to those operating in large river basins. Since flooding processes are markedly different in the two environments, the response and management for two scenarios is also different. It is critical that physical planners and persons involved in disaster mitigation in Jamaica and the Caribbean make the distinction between sediment floods and water floods, as protective measures for water floods may not be effective for debris flows. Indeed, the inadequate design of many mitigating structures intended to minimise water related damage suggest that most sediment flows are recorded and treated as water floods and hence most mitigating structures are designed to contain water floods. This is unfortunate as mitigation strategies for sediment flows are significantly different than those for water floods. Also, most sediment floods are avoidable thereby economic losses may be significantly reduced. Rainfall thresholds One possible solution to the recurrent water-related challenges faced by Jamaica and the region is the development of rainfall thresholds for the initiation of debris flows to aid in the early detection and prediction of hydro-geologic hazard events with a fair degree of reliability. This will allow for mitigating actions to be taken which will preventor reduce damage to communities and infrastructure. In addition, mitigation measures such as reforestation on hill slopes, grouting of slopes, groynes placed in river beds to assist with river training and discouraging the removal of the toe of many of our hill sides, could be implemented to arrest water-related damage in particularly vulnerable features. The level of damage suffered in Jamaica and throughout the Caribbean from sediment flows suggests that previous mitigation methods employed throughout the region are inadequate. Physical planners, disaster mitigation professionals and the general public must seek to become more informed of the nature of disasters which face our region. They must also insist that decision makers involved in physical planning and development use the knowledge available to inform their actions so that mistakes of omission or commission made in the past are not repeated in the future. Given the vulnerability of the small island states of the Caribbean, it is also critical that adequate funding is allocated to fund research in ensuring that we have a clear understanding of the dynamics of our physical environment and our interaction with it. -V.H. This article was adapted from the following research : Debris Floods and Debris Flows in the Caribbean, Rafi Ahmad (Unit for Disaster Studies, Department of Geography and Geology, The University of the West Indies) The presentation for this paper is available at: http://mona.uwi.edu/cardin/virtual_library/docs/1133/1133.pdf What We Call Flood in Jamaica is Important, Rafi Ahmad (Unit for Disaster Studies, Department of Geography and Geology, The University of the West Indies) This presentation is available at: http://mona.uwi.edu/cardin/virtual_library/docs/1131/1131.pdf The MIAS is a non-profit organisation of the University of the West Indies, Pure and Applied Sciences Department, offering analytical, technical and, web services and specialised science projects. If youhave any question or comments about these articles please email : mias@uwimona.edu.jm or contact the MIAS Analytical Services Division at 970-2042 or 512-3067 for enquires on services offered. |
