Give reason : Sand particles are separated from gravel at construction sites using sieving .
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Sand/Gravel Extraction
Introduction
Sand and gravel is dredged from the seabed in various parts of the world for, among other things, land reclamation, concreting aggregate, building sand, beach nourishment, and coastal protection. The largest producer in the world is Japan at around 80–100 Mm3per year, Hong Kong at 25–30 Mm3 per year, the Netherlands and the UK regularly producing some 20–30 Mm3per year, and Denmark, the Republic of Korea, and China lesser amounts.
Reclamation with Marine Dredged Sand and Gravel
In recent years, some very large land reclamations have taken place for port and airport developments particularly in Asia. For example, in Hong Kong some 170 Mm3 was placed for the new artificial island airport development with another 80 Mm3 used for port developments over the period 1990–98. Demand projections indicate a need for a further 300 Mm3 by 2010. Also, in Singapore the Jurong Island reclamation project initiated in 1999 was projected to require 220 Mm3 of material over 3 years. However, in 2003 Singapore estimated that it needed 1.8 billion cubic meters of sand over the following 8 years for reclamation works including Tuas View, Jurong Island, and Changi East. The planned Maasvlakte 2 extension of Rotterdam Port in the Netherlands will reclaim some 2000 ha and require 400 Mm3 of sand with construction due to take place between 2008 and 2014.
Regulation
National authorities generally carry out the regulation of sand and gravelextraction and they may have guidance on the environmental assessment of the practice. There is currently no accepted international guidance other than for the Baltic Sea area under the Helsinki Convention and the North-East Atlantic under the OSPAR Convention that has adopted the ICES guidance.
Impacts
The environmental impacts of sand and gravel dredging depend on the type and particle size of the material being dredged, the dredging technique used, the hydrodynamic situation of the area and the sensitivity of biota to disturbance, turbidity, or sediment deposition. Screening of cargoes as they are loaded is commonly employed when dredging for sand or gravel to ensure specific sand:gravel ratios are retained in the dredging vessel. Exceptionally, this can involve the rejection of up to 5 times as much sediment over the side of the vessel as is kept as cargo. When large volumes of sand or gravel are used in land reclamation, the runoff from placing the material may contain high levels of suspended sediment. The potential effects of this runoff are very similar to the impacts from dredging itself. The ICES Cooperative Research Report No. 247 published in 2001 deals with the effects of extraction of marine sediments on the marine ecosystem. It summarizes the impacts of dredging for sand and gravel. An update of this report is due for completion in 2007.
Physical impacts
The most obvious and immediate impacts of sand and gravel extraction are physical ones arising from the following.
•
Substrate removal and alteration of bottom topography. Trailer suction dredgers leave a furrow in the sediment of up to 2 m wide by about 30 cm deep but stationary (or anchor) suction dredgers may leave deep pits of up to 5 m deep or more. Infill of the pits or furrows created depends on the natural stability of the sediment and the rate of sediment movement due to tidal currents or wave action. This can take many years in some instances. A consequence of significant depressions in the seabed is the potential for a localized drop in current strength resulting in the deposition of finer sediments and possibly a localized depletion in dissolved oxygen.
•
Creation of turbidity plumes in the water column. This results mainly from the overflow of surplus water/sediment from the spillways of dredgers, the rejection of unwanted sediment fractions by screening, and the mechanical disturbance of the seabed by the draghead. It is generally accepted that the latter is of relatively small significance compared to the other two sources. Recent studies in Hong Kong and the UK indicate that the bulk of the discharged material is likely to settle to the seabed within 500 m of the dredger but the very fine material (<0.063 mm) may remain in suspension over greater distances due to the low settling velocity of the fine particles.
•
Redeposition of fines from the turbidity plumes and subsequent sediment transport. Sediment that settles out from plumes will cover the seabed within and close to the extraction site. It may also be subject to subsequent transport away from the site of deposition due to wave and tidal current action since it is liable to have less cohesion and may be finer (due to screening) than undredged sediments.
Introduction
Sand and gravel is dredged from the seabed in various parts of the world for, among other things, land reclamation, concreting aggregate, building sand, beach nourishment, and coastal protection. The largest producer in the world is Japan at around 80–100 Mm3per year, Hong Kong at 25–30 Mm3 per year, the Netherlands and the UK regularly producing some 20–30 Mm3per year, and Denmark, the Republic of Korea, and China lesser amounts.
Reclamation with Marine Dredged Sand and Gravel
In recent years, some very large land reclamations have taken place for port and airport developments particularly in Asia. For example, in Hong Kong some 170 Mm3 was placed for the new artificial island airport development with another 80 Mm3 used for port developments over the period 1990–98. Demand projections indicate a need for a further 300 Mm3 by 2010. Also, in Singapore the Jurong Island reclamation project initiated in 1999 was projected to require 220 Mm3 of material over 3 years. However, in 2003 Singapore estimated that it needed 1.8 billion cubic meters of sand over the following 8 years for reclamation works including Tuas View, Jurong Island, and Changi East. The planned Maasvlakte 2 extension of Rotterdam Port in the Netherlands will reclaim some 2000 ha and require 400 Mm3 of sand with construction due to take place between 2008 and 2014.
Regulation
National authorities generally carry out the regulation of sand and gravelextraction and they may have guidance on the environmental assessment of the practice. There is currently no accepted international guidance other than for the Baltic Sea area under the Helsinki Convention and the North-East Atlantic under the OSPAR Convention that has adopted the ICES guidance.
Impacts
The environmental impacts of sand and gravel dredging depend on the type and particle size of the material being dredged, the dredging technique used, the hydrodynamic situation of the area and the sensitivity of biota to disturbance, turbidity, or sediment deposition. Screening of cargoes as they are loaded is commonly employed when dredging for sand or gravel to ensure specific sand:gravel ratios are retained in the dredging vessel. Exceptionally, this can involve the rejection of up to 5 times as much sediment over the side of the vessel as is kept as cargo. When large volumes of sand or gravel are used in land reclamation, the runoff from placing the material may contain high levels of suspended sediment. The potential effects of this runoff are very similar to the impacts from dredging itself. The ICES Cooperative Research Report No. 247 published in 2001 deals with the effects of extraction of marine sediments on the marine ecosystem. It summarizes the impacts of dredging for sand and gravel. An update of this report is due for completion in 2007.
Physical impacts
The most obvious and immediate impacts of sand and gravel extraction are physical ones arising from the following.
•
Substrate removal and alteration of bottom topography. Trailer suction dredgers leave a furrow in the sediment of up to 2 m wide by about 30 cm deep but stationary (or anchor) suction dredgers may leave deep pits of up to 5 m deep or more. Infill of the pits or furrows created depends on the natural stability of the sediment and the rate of sediment movement due to tidal currents or wave action. This can take many years in some instances. A consequence of significant depressions in the seabed is the potential for a localized drop in current strength resulting in the deposition of finer sediments and possibly a localized depletion in dissolved oxygen.
•
Creation of turbidity plumes in the water column. This results mainly from the overflow of surplus water/sediment from the spillways of dredgers, the rejection of unwanted sediment fractions by screening, and the mechanical disturbance of the seabed by the draghead. It is generally accepted that the latter is of relatively small significance compared to the other two sources. Recent studies in Hong Kong and the UK indicate that the bulk of the discharged material is likely to settle to the seabed within 500 m of the dredger but the very fine material (<0.063 mm) may remain in suspension over greater distances due to the low settling velocity of the fine particles.
•
Redeposition of fines from the turbidity plumes and subsequent sediment transport. Sediment that settles out from plumes will cover the seabed within and close to the extraction site. It may also be subject to subsequent transport away from the site of deposition due to wave and tidal current action since it is liable to have less cohesion and may be finer (due to screening) than undredged sediments.
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