SEBRA Protecting the Island’s Riverbeds


If present climatic conditions persist for the next 20 years, coupled with continued increase in mining activities, there will be a limited supply of sand available to the construction industry.
This according to Sedimentologist at the University of the West Indies, (UWI), Mona, Geology and Geography Department, Dr. Simon Mitchell. He explains that current extraction rates show no new material being deposited in the Rio Minho,which is the longest river system and the biggest catchment area in the island for rainfall.
Speaking to JIS News on the effects of the extraction of sand and gravel from the river, Dr. Mitchell says a model of the Rio Minho system has been made examining the geology, land use, mining activities, river flow (gradient) and rainfall, all of which control the amount of sediment in the river.
“We zeroed in on the Rio Minho and the Yallahs Rivers looking at what is currently being extracted, how the systems are working and the sustainability for the future,” he explains.
He informs that a detailed geological mapping of central Jamaica has been completed showing the geology of the area and the type of rocks contributing to the sediment in the river. “Therefore, the grain size of the sand has economic value and is environmentally important to the industry,” he notes.
Dr. Mitchell adds that the nature of the grain size influences the clarity of the water with large grains making it clear and vice versa. “The coarser the material is the more energy the river has to have impacting on how much sediment the river can carry,” he says.
The Mines and Geology Division’s Sedimentary Basin Resource Assessment Project (SEBRA), funded by the Environmental Foundation of Jamaica (EFJ) at a cost of some $8.5 million, is aimed at producing a “sediment budget,” on the Rio Minho and Yallahs Rivers to evaluate the rate of accretion of sand and assess the rate of depletion to ascertain the right balance for quarrying.
The SEBRA Project has so far revealed that if mining were increased in the Rio Minho, the resource would be adversely affected as, due to the re-vegetation of hillsides, the volume of sediments is not being transported into the rivers as it was former years.
According to Commissioner of the Mines and Geology Division, Coy Roache, the aim is to prevent depletion of Rio Minho and Yallahs reaching the level of the Rio Cobre and Hope River. The experts explain that no further mining can be carried out in the Rio Cobre and Hope Rivers because these water bodies have been so over-exploited and over-quarried that even after ten years of no mining activity, they still did not contain any sand.
Dr. Mitchell states that there are major problems being encountered with the amount of material being extracted from the rivers versus how much new material is being deposited into them. However, he notes that the recent passage of Hurricane Ivan has probably produced a great amount of new material, thereby stemming the problem in the short term.
Nevertheless, calculating how much sand can be removed from the river and maintaining some form of balance in the environment is the constant challenge facing sand quarry operators and everyone who use the river system. Additionally, Dr. Mitchell notes that there are other aspects to the issue such as the source of material. The economic material for the construction industry comes from a quarter of the rock now being produced.
According to Dr. Mitchell, “The country needs to seriously begin looking down the road at alternative sources because if Jamaica is going to sustain itself in building projects such as Highway 2000 and other future road projects then the maintenance of current supplies will be crucial”. He adds, “the flood plains of these rivers contain deposits hundreds of years old which have the same characteristics as the river gravel and so these areas, which are usually agricultural lands, can be mined and then returned to agricultural practice”.
This will no doubt provide more resource, Dr. Mitchell says, without causing environmental damage as this extraction is commonly practised in many First World countries.
With regard to the significance of the results of the SEBRA Project to the construction industry, Dr. Mitchell notes that it identifies areas along the river system, containing the right grain size, that are more appropriate for sand extraction. “It demonstrates the likely future or potential of new material being deposited in the rivers as we are on the threshold of seeing extraction outstripping supply,” he says.
He adds that the findings, “become critical as they point to an increased cost if other materials are to be used, for example, material dredged off-shore, the removal of over burden and the crushing of rocks for sand”.
In the long-term the island is perhaps at the end of simple and cheap river sand extraction methods and the expensive alternatives will no doubt impact on the construction industry.
The SEBRA Project also seeks to make more information available to the public on the importance of sand and gravel as an economic resource, stemming the increasing growth of illegal/unregulated sand/gravel quarries along rivers, and the adverse effects of unregulated sand/gravel quarrying on host communities and the environment.

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