River mining: sand and gravel resources. (CR/03/161N).
This report is one of a series of Technical Reports on alluvial mining in developing countries, most of which relate to Jamaica (see Preface for details). They are the output from the 'Effective Development of River Mining' project which aims to provide effective mechanisms for the control of sand and gravel mining operations in order to protect local communities, to reduce environmental degradation and to facilitate long-term rational and sustainable use of the natural resource base. The work was carried out under the Department for International Development Knowledge and Research programme, as part of the British Government's programme of aid to developing countries. The project was undertaken in collaboration with key organisations in Jamaica and Costa Rica, who provided field guidance and local support.
Sand and gravel aggregates are extracted from many rivers in Jamaica but little information is available on the distribution of resources and of the volumes extracted. Resource mapping studies were therefore carried out on two major river systems, the lower Rio Minho and the Yallahs, which were the field study areas for the River Mining project. A combination of walkover surveys and studies of aerial photographs were used to rapidly produce aggregate resource maps, showing the distribution and relationships of the major sand and gravel bodies. Groundtruth data were provided by a suite of trial pits dug into the river sediments by the Mines and Geology Division of the Ministry of Lands and Environment. All pits were logged and samples taken for particle size analysis.
The lower Rio Minho flows from the Central Highlands across a broad coastal plain where it has cut a series of terraces into former deposits. The mapping has identified a number of river terrace and floodplain deposits of varying resource potential. The First Terrace deposits are a major sand and gravel resource but the upper parts of the Second Terrace deposits are mostly of clay or silt and have no resource potential. Large amounts of sand and gravel are extracted from the Rio Minho and total production since 1980 when extraction started, is estimated to be about 5.25 million tonnes. Most extraction has been by instream mining, but the mapping has shown that alternative resources of good quality sand and gravel exist in the First Terrace deposits which flank the current river channel.
The Yallahs river drains the southern flank of the Blue Mountains and the river sediments form a lobate fan-delta covering over 10 square kilometres. The delta sediments are composed of coarse sandy gravels, ranging in size from small pebbles to large boulders. The sediments thicken rapidly downstream and are over 150 m thick near the coast. Large amounts of sand and gravel have been extracted from the fan-delta in recent years. It is estimated that about 4.4 million tonnes of aggregate has been produced since 1990. Extraction is concentrated in the main channel which is now incised up to 6 m below the level of the fan-delta surface.
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