Dredging activities and its implications on man and its environment

Introduction

The process of dredging has created a lot of problem in our country, especially to aquatic lives and the environment in general because of the issue of environmental degradation. Dredging is an excavation activity of operation usually carried out at least partly underwater in shallow sea or fresh water area with the purpose to gather up bottom sediments and disposing of them at different location, (Bertha 2009).

According to Bertha (2009) dredging is the removal of anything forming part of or project from the bed of the sea or of any Inland water; it is also a way of replenish sand on some public beaches, when sand has been cost because of coastal erosion. Dredging is also defined as an excavation activities or operation usually carried out at least partly under water shallow or fresh water area with the disposing of flow at a different location.

Most dredging activities according to Robin (2006) are done to maintain or make deeper navigational channels anchorages or berthing areas of the safe passage of boats and ships unknowingly that its activities have significant implications on aquatic lives and the environment. Where these activities are being carryout, it is used as a means to replenish sand on some places where sand has been lost due to coastal erosion. The process of dredging creates spoils (excess materials) which are carried away from the dredging area. According to him, dredging can also produce materials for land reclamation or other purposes (usually construction of roads and bridges) and has created disturbances in water bodies, health and environment and aquatic ecosystem often with the adverse to impact.

According to Davidson (2008) all methods of dredging release suspended sediments into water column during the excavation and during the flow of sediment from dredgers to barges; increased suspended sediments can affect filter feeding organisms such as shell fish through clogging their breathing organs, similarly young fishes can be damaged if suspended sediment become trapped in their gills and increased fatality of young fish have been observed in heavily turbid water while adult fish are likely to move away from or avoid areas of high suspended solid such as dredging site. The World Health Organization (WHO, 2001) on a research carried out reviewed that, most of the dredging activities carried out in developing countries are close to residential premises which have resulted to collapse of structure cracking of building walls and also claimed lives and properties. The United State clean water act further stated that any discharge of dredging or fill material into water of the United State including wet land is forbidding unless authorized by a permit issued by the Army Corps of Engineer because of the potential impact to the environment. Base on these problems the U.S. clean water restricted only licensed practitioners to participate on dredging activities but because of corruption and bribery; it has been neglected, especially our Country Nigeria the establishment of environmental impact assessment (E.I.A) is no longer effective.

Gerel (1998) stated that the activities of dredging can released heavy metals from machine which can affect nearby building and aquatic lives.

Types of dredging vessels

According to Robbins (2006), there are different types of dredging equipments which are as follows.

1. Bucket dredging: A bucket dredger is equipped device that pick up sediment by mechanical means often with many circulating attached to a wheel or chain.
2. Auger suction: Auger suction is a process that functions like a cutter, suction dredging but the cutting tools is a rotating Archimedean screw set, at right angles to the suction pipe. The first widely used auger dredger were designed by mud eat dredging in the 1980 ^s which was run by national car rentals but is now a division of Ellicott dredges. During the 1980s Auger dredger were primarily used for sludge removal application from waste water treatment plant but today auger dredger are used for wider variety of application including river maintenance and sand mining.
3. Clam shell or grab dredger: A clam shell dredger pick up seabed material with a clam shell bucket which hangs from an on bond, crane or a crane barge, this technique is often used in excavation of bay mud, most of these dredges are crane barges with spuds.
4. Air lift: Lift is a type of small suction dredgers which is use under by divers work by blowing air into the pipe and that air being lighter that water rises inside the pipe dredging water with it.
5. Snag boat dredger: A snag boat dredger is been designed to remove big debris from rivers and canals.
6. Trailing suction hopper dredger (TSHD): A trailing suction hopper dredger is a suction pipe which work and load the dredge material into one or more hopper in the vessel; the hopper are full TSHD sail to a disposal area and either dumps the materials out of the hopper, some dredger also sect of load using drag bucket and conveyors, the largest trailing suction hopper dredger in the world currently.
7. Cutter suction dredger: The cutter suction dredger Is a tube that has mechanism at the suction inlet the cutting mechanism loosens the bed material and transport it to the suction mouth, the dredger material is usually sucked up by wear resistant centrifugal pump and discharge either through a pipe line or through a barge.
8. Police dredge dredger: Some police department, a small dredge sometimes called a drag is used to fined and recover object and bodies from the under water. The bodies may be murder victims or people who committed suicide by drowning or victims of accident, it is sometimes pulled by men walking on the river bank.
9. Water injection dredger: Water injection dredger uses a small jet to water under low pressure to prevent the sediment from exploding into the surrounding water into the sea bed to bring the sediment in suspension, which becomes a turbidity current which flows away down slope is moved by a second bruit of water from the WID or is carried out in a natural currents. Water injection result in a lot of sediment in the water which makes measurement with most hydro-graphic equipment (for instance single bear echo sounder).
10. Fishing dredger: Fishing dredger are used to collect various species of clams scallops oysters or crab from the seabed careless dredging can be destructive to the seabed.

Uses of dredging

According to Bertha (2009), dredging can be used for the following purpose;

1. Land reclamation: Dredging is used to mine sand, clay or rock from the seabed and using it to construct new land elsewhere; this is typically performed by a cutter-suction dredger of trailing suction hopper dredge, the material may also be used for flood or erosion control.
2. Capital dredger: Dredging is carried out to create a new harbour berth or water way or to deepen on existing facilities in order to allow larger ship access because capital works usually involves hard materials or high volume works, the work is usually done using a cutter suction hopper dredge but for rock work drilling and blasting along with mechanical excavation may be used.
3. Construction materials: Dredging sand and gravel from off-shore which are used in the construction of houses and industries principally used for concrete.
4. Beach nourishment: Mining sand off-shore and placing on a beach to replace sand eroded by storm or a wave action; this is done to enhance the recreational and protective function of beaches, which can be eroded by human activities or by storms. This is typically performed by a cutter suction dredger or trailing suction hopper dredge.
5. Harvesting material: Dredging is used to harvest sediment for elements, life gold, diamond or other valuable trace substance.
6. Maintenance: Dredging is used to maintain navigable water ways or channels which are threatened to become skilled with the passage time due to sediment sand and mud possibly making them too shallow for navigation; this is often carried out with a trailing suction hopper dredge, most of the dredging is for the purpose and it may also be done to maintain the holding capacity of reservoirs or lakes.
7. Flood prevention: Dredging can also be carried out to prevent flooding, it can hold to increase channel and therefore increase a channel capacity for carrying water.
8. Contaminant remediation: Dredging is also used to reclaim area affected by chemical spills, storms water surges with (Urban runoff) and other soil contamination disposal becomes a proportionally large factors in these operation removing trash and debris often done in combination with maintenance dredging, this process removes non natural matter from the bottom of rivers and channels harbours.

Implications of dredging on health and aquatic life

According to Walker (2004) young fish can be damage of suspended sediment become trapped in their gills and increased fatalities of young fish have been observed in heavily turbid water, adult fish are likely to move away from or avoid area of high suspended solid such as dredging site unless food supplies are increases as a result in increase in organic materials during dredging operation, the removal of materials from the seabed also remove the animals living on the sediment (benthic animals) with the exception of some deep burrowing animals or mobile surface animal that may survive a grudging event through avoidance dredging may initially result in the complete removal of animals from the excavation site (Bowles 2008). Increase suspended sediments can affect fitter feeding organisms such as shell fish through clogging and damaging feeding and breathing equipment.

Implications of dredging on the environment

According to Bowles (2009), dredging can create disturbance to aquatic life and ecosystem often with adverse implications. In addition dredge spoils may contain toxic chemicals that have adverse implications on the disposal area, furthermore the process of dredging of ten dislodges chemical residing in benthic substrates and infects them into the water column. The activities of dredging can create the following principal implication to the environment; although historically the primary objectives was to minimize dredging operation and economic with little regard to the environment today in most dredging project are evaluated and managed to minimized the adverse environmental implication, while still maximizing economic and environmental benefit, there are existing procedures and regulations in places which are generally considered to effectively avoid and minimize the potential for maintenance dredging and disposal to cause environmental harm.

According to National Research Council (2005), the following action can be taken to avoid or minimize the implications of dredging;

• Managing and information contractors.
• Timing of dredging and disposal operation.
• Reducing the amount of dredging material.
• Promoting and beneficial use.
• Monitory and record keeping.

1. Management and informing contractors: It is important to contractors to be fully briefed by port and harbour management prior to the commencement of dredging and disposal works; contracting procedures may include the requirement of method statement and risk of assessment for operation to be provided by the contractor, when briefing contractors. Special consideration should be given to provision of information on marine and the features for which the site was designated and if appropriate outline are which are particularly sensitive to the effect of dredging at specific times of the year which were identified in the consent process as constraint to dredging operation.
2. Timing of dredging and disposal operation: When planning the timing of dredging operation common sense needs to be applied, in addition to ecological consideration operational factors also used to be addressed such as part recreational and commercial periods and seasonal weather condition.
3. Reducing the Amount of Dredged Material: the amount of dredge materials in ports and harbour has steadily reduced over recent decades as a result of changes in dredging practice and operation such as technological advances, greater dredging accuracy and increase surveying of dredged channel, there are number of operator reducing maintenance dredging in practice by port authorities, two main improvement that would reduce amount of material dredged, firstly allowing over spill of the hopper caused material to be dredge again later. Secondly the estuary was being gradually deepened more than was necessary for safe navigation.
4. Promoting of beneficial use: Between 1989 and 1994 the amount of maintenance dredged material disposed of at sea under license in England and Wales almost halved due to improved port operation and dredged practices and the increased use of beneficial option for the disposal of sediment, the gradual reduction in the amount of material being deposited at sea provide a means of minimizing the over potential effect from the disposed of sediments on the marine environment.
5. Monitoring and recording keeping: In the licensing process for the disposal maintenance dredged material emphasis is been placed on verifications of the effect of dredging and disposal on marine, ecosystem and regimes with increasing demands for pre and post dredge monitoring of disposal sites.

Implications of dredging on the environment

Davidson (2008) stated that the following are the implications of dredging on the environment:

1. Possible contamination of dredging spoil site.
2. Release of toxic chemical (including heavy metals) from bottom sediments into the water column.
3. Short term increase in turbidity which can affect species metabolism and interfere with spawning.
4. Secondary impact to aquatic and benthic organism metabolism and mortality.
5. Tertiary impact to avifauna which may prey upon contaminated aquatic organism.
6. Secondary impact to marsh productivity from sedimentation.

According to Bates (2007), the nature of dredging operation and possible environmental impacts causes the industry to be closely regulated and a requirement for comprehensive regional environmental impact assessment with continuous monitoring. The U.S. clean act required that any discharge of dredge of fill materials into water of the United State including Wet Land is forbidden unless authorized by a permit issued by the Army Corps of engineers as a result of the potential impact to the environment dredging with vessel activity monitored closely using authoritative G.P.S. system.

Factors influencing dredging activities

The following are factors influencing the implications of dredging:

1. Magnitude and frequency of dredging activities.
2. Method of dredging and disposal
3. Channel size and depth.
4. The size density and quality of the material.
5. Intertidal area.
6. Background level of water and sediment quality suspended sediment and turbidity.
7. Tidal range.
8. Current direction and speed.
9. Rate of mixing.
10. Seasonal variability and meteorological condition affecting wave condition and fresh water discharges.

According to Walker (2004) prediction of the potential implications that might be caused by maintenance dredging cannot be made with an degree of confidence if these parameters are not known on a site basis, generally the potential implications of dredging and disposal can be summarised as follow. Removal of sub-tidal benthic level of suspended sediment can give rise to change in water quality which can affect marine flora and fauna both favourably and unfavourably, such as increase of organic matter nutrients or contaminants depending upon the nature of material in the dredging.

According to Murray (2009), the evaluation of the environmental implication of dredging and disposal must take account of both the short terms and long terms implications that may occur both at the site of dredging or disposal (near field) and the surrounding area. The near field implication are simply defined as phenomena occurring between the geographical bounds of the activities or less than approximately 1cm from activities and far field implications as occurring more than approximately 1km from the activity; However, other sources suggest that caution should be used, with adopting an arbitrary distance to distinguish between near and far field due to the site specific nature of the potential effect that arise from dredging.

REFERENCES

Bates, J. (2007). Settlement of suspended sediments; New Delhi: Kohaktar Press.

Bertha, T. (2009). Maintenance dredging; beneficial use schemes, Sydney: Brad-Show Press.

Bowles, P. (2008). Discharge of dredged materials at the disposal sites, New York: Butcher Publishers.

Burkett, M. (2009). World of Boats, Scotland: Eye Mouth Marine Centre.

Copeland, C. (2010). Clean water act: A summary of the law, Report no. ph 30030. Washington DC: Congressional Research Services.

Davidson, M. (2008). Dredging and disposal: contaminated sediments anchorage. New York: Harbour Publications.

Environmental Protection Agency (EPA) (2013). Dredging and dredging materials management. EPA Weekly Report 34(8):69-74

International Association of Dredging Companies (IADC) (2012) environmental implications of dredging.

Murray, P. (2009). Selection of beneficial project through dredging, Journal of Marine Science 72(8):56-69.

National Research Council (2005). Beach Nourishment and Protection. Washington, D.C.: National Academy Press.

Piance, D.F. (2006). Dredging is a source of soil monitoring. Hamsbury, London: Lepto Publishers

Robbins, R. (2006). Surface dredging: Proceedings of Advance Scientific Diving Workshop held at Smith Sonia Institute, Washington DC.

Walker, D. (2004). Prediction of major design parameters of a submersible dredger; India National Conference on Harbour and Ocean Engineering New Dehli.