Extraction and impacts at sea
Sand and gravel are extracted from the seabed by dredging. Specially designed vessels use suction pumps to lift a stream of sand, gravel and sea water through a dredge pipe into the hold where sand and gravel are retained and seawater is discharged into the sea. The usual method is for the vessels to move forward, trailing the dredge pipe behind, thus extracting material from a linear track in the sea floor.
Alternatively vessels may remain stationary while dredging giving rise to a saucer-shaped depression in the sea floor. This operation, termed static dredging, can only be undertaken where the geological deposits are highly localised and thick which limits the locations in which it can be undertaken. However it is employed in some locations on the south coast of England and in the Bristol Channel and Irish Sea.
In trailer dredging, each pass by the dredger leaves a shallow groove in the sea floor (approximately 0.3m) that remains perceptible for several years or sometimes much longer. Repeated passes by the dredger produces more dredging tracks until, over a period, the general level of the seabed within the permitted area is lowered.
The technique is illustrated well on the BMAPA website.
Extraction leads to a variety of impacts on the environment and other uses of the sea that must be mitigated or minimised. Fine grained sediments on the sea floor are carried by marine currents from one place to another and are deposited either temporarily or more permanently for instance in sand banks. All dredging proposals are subjected to an independent hydrodynamic analysis (coastal impact study) to ensure that they will not have any significant effects on sediment circulation patterns by interrupting natural sediment transport pathways or by altering the wave climate 507, 508, 509.
Marine organisms that live above the sea floor and those which move about on the sediment can avoid the dredger by moving aside, but those which burrow in the sediment or attach themselves to gravel or boulders are removed during dredging. Therefore, in general, impacts on organisms that burrow in, or are attached to, the seabed are greater 470, 475, 478, 479. This can affect marine biodiversity 473. Particular care is required if there might be significant impacts on designated sites, especially European Marine Sites M0055, 476, 477, 482 such as Special Areas of Conservation (SACs) (e.g. Sabellaria worm reefs) and Special Protection Areas (important areas identified for particular bird species). Steps are also being taken to develop a network of Marine Conservation Zones, as provided for in Part V of the Marine and Coastal Access Act 2009, some of which may have implications for future dredging M0051.
Impacts on birds may not be immediately obvious but various species depend on other marine organisms as a source of food, therefore care has to be taken that food stocks are not affected adversely M0031, 481.
Commercial species of fish and, especially bottom dwelling, shellfish may also be affected by dredging. While dredging takes place the immediate vicinity cannot be fished therefore the dredging operators liaise with fisheries organisation and provide information on where and when dredging will take place to minimise potential problems M0001, M0002, M0015, M0042, 516. Dredging is prohibited seasonally if it might affect, for example, fish spawning or migratory routes of crabs 484, 497.
The dredger temporarily creates noise at sea but generally this causes little nuisance because there are few people nearby to hear it and, although it could cause brief disturbance to some marine organisms, it is not severe enough to cause severe or protracted disturbance to these, being no noisier than a large merchant vessel travelling at moderate speed. Noise and vibration may be disturbing to, for example, marine mammals but dredging at any particular site is transitory thus reducing the level of disturbance M0023, 522.
The right balance between sand to gravel for various aggregate products can be secured either by separating out the finer material through screens (industrial sieves) at sea and discharging the unwanted fraction back into the water. Water discharged when loading or from on-board screening contains fine grained sediments that form a sediment plume in the water that gradually settles to the sea floor over a distance of the order of a kilometre or two M0004, M0028, 489, 492, 520, 521. Conditions are applied to limit or prohibit screening at sea when necessary.
Extraction can also damage heritage interests such as shipwrecks and marine archaeological sites which may provide Palaeolithic or Mesolithic remains 493. Therefore dredging is prohibited close to such locations and a protocol for recording and reporting finds has been developed for incidental archaeological finds elsewhere M0051. In recent years a significant amount of work has been carried out onsubmerged and buried landscapes and associated elements to reconstruct these prehistoric environments when past sea levels were lower as a context for identifying archaeological potential M0006, M0041, M0044, 484. Guidance has also been issued on marine heritage interests 530, 494.
Dredging is also prohibited close to war graves (wrecks of warships and aircraft) 500, 501. Caution is also needed for the safety of the vessel and crew during dredging because of the potential for picking up unexploded munitions 511.
Safe navigation is an important issue. Dredging is controlled to make sure that it will not conflict with routes taken by shipping and that other vessels are made aware of when and where dredging takes place M0009, 517.
However potential effects of marine aggregates dredging are not considered in isolation. The cumulative effects of dredging and of other activities in the vicinity are, together, taken into account 503, 524, 525.
Dredging conditions require that a layer of sand and gravel should remain undisturbed when dredging ends in the area so that it can be re-colonised by seabed organisms similar to those which inhabited the original sea floor. Dredged areas have traditionally been left, after dredging ends, so that organisms can re-colonise these naturally. In recent years, however, research on the potential for site restoration, to aid recovery, has received increasing attention M0011a, M0011b, M0013, M0030, M0035, M0036, 491, 498, 499, 512, 513. More attention is also being paid to risk assessment, identification of thresholds for impacts that might assist in reviews of operations M0012, M0038, and the sensitivity of the seabed to impacts M0054 .