Investigation into the health and environmental implications of dredging activities

Introduction

Dredging is an excavation activity usually carried out underwater in shallow seas or fresh water areas with the purpose of gathering up bottom sediments and disposing of them at different location “Jan De Nul Group” (2008). Some of these sediments include mud, sand, gravel, pebbles, rocks and other deposit from the bottom of the creeks, lagoons, sand water ways etc. This technique is often used to keep waterways navigable, it is also used as a way to replenish sand beaches on some public beaches where sand has been lost because of coastal erosion using dredger.

However, researchers have investigated into the health and environmental implications of dredging activities and the diverse use of the marine environment leading to human induced changes both in marine environment, inhabitants and landscapes making necessary the development of marine policy that considers all members of the user community and addresses the current multiple issues as it affects dredging, (Jonathan, Atkins, Darye, Burdon & Brunn, 2011).

The implication of dredged materials disposal largely depends on the nature of the material (inorganic, organically enriched, contaminated) and the characteristics of the disposal area (accumulative or dispersive areas) “Scottish Office of Agriculture, Environment and Fisheries Department” [SOAEFD] (1996). The potential impacts of the disposal of dredgings on the marine environment also leads to the environmental degradation due to the disposal of heavy contaminated sediments (Jonathan et al., 2005).

The evaluation of the environmental effects of dredging and disposal must take account of both the short term and long term effects that may occur both at the site of dredging or disposal (near field) and the surrounding areas (far field). According to International Association of Drilling Contractors (IADC) and Clearwater economic development Association (CEDA) (1988) guide, illustrates various environmental effects of dredging that might be released. Near fields effects are simply defined as phenomena occurring within the geographic bounds of the activity or less than 1km from the activity. However, to distinguish between near and far field effects caution should be taken due to the site specific nature of the potential effects that arises from dredging.

According to (IADC/CEDA, 1978)environmental effects of dredging that may occur as a result of bathymetry and hydrodynamic processes such as alterations to coastal or estuary morphology for example, alteration of sediment pathways and changes to siltation patterns which may affect coastal habitats and species, in addition to marine ones, alterations to water currents and water columns which might affect navigation and conservation interest and reduction of water quality.

Dredging is a worldwide excavation activity that involves removing sediments from a sea, river or lake bed and depositing it at a new location. Uses are vast which includes construction of ports, dykes, waterways, marine infrastructure, flood and storm protection extraction of mineral resources to provide materials for the construction industry e.g. for road construction and its environmental remediation of contaminated sediments, land reclamation by (Brunn, 2005; CEDA, 2011; Tillin, 2011; WODA, 2013).

Conceptual framework

According to Scottish Office of Agriculture, Environment and Fisheries Department [SOAEFD] (1996), dredging activities and sediments are pathways contaminants. It was found that a low environmental risk according to biological parameters is normally associated with low contamination.

Additionally, mechanical dredgers (including clamshell and mechanical shovel) posed a lower environmental risk than hydraulic dredgers (cutter suction dredger). Nevertheless the environmental risk according to chemical parameters remained high at both sites categories, regardless of the technology used. The impacts of dredging vary according to chemical, biological and physical parameters of the aquatic environment. Further descriptions of dredging impacts and parameters increased or decreased as a result of dredging as seen in this conceptual model that the impacts of dredging are highly dependent on the levels of contamination of dredged sites and technologies used. Furthermore the increase in chemical parameters that occurs during dredging and disposal shows that the sediments expose the ecosystem to contaminations. Increase in the levels of organic and inorganic compounds heighten the risk of contaminants exposure that can negatively affect the flora or fauna (SOAEFD, 1996).

The change in physical parameters further re-enforce this point. While it has been noted that some positive changes can occur during the various stages of dredging, this review treats those with more anecdotal and suggests that the impacts are largely detrimental to the environment (Abel et al., 2008).

Current legislative actions aiming to preserve the environment from dredging harmful effects and their related problems. Another important issue relating to dredging is its high cost. The cost of dredging varies according to the technology and equipment used, estimated time, types of dredged materials, distance from excavation to disposal site, time and distance of mobilization and demobilization and disposal method. The high cost has always been the main problem for port operators who are responsible for dredging and maintaining deep channels but also need to spend funds to expand or build new terminals in order to cater for growing trade activities (Anderson & Barkdoll, 2010; Williams, 2008). Although operational costs are perceived as the biggest issue by a number of dredging stakeholders a few papers have discussed or analyzed the cost of dredging.

For example, Lee (2011) attempted to create a framework for dredging cost analyzing the construction operation process, type of river section and the combination of equipment employed for river dredging. This analysis was based on historical data of river dredging projects conducted in South Korea (Lee et al., 2011). Despite the fact that developing countries were estimated to become the largest dredging markets in the world over the next few years, stiff competition from foreign dredging contractors heightens the need to lower costs for local dredging contractors (George, 2011; Thicker, 2007). This, together with poor facilities and limited dredging and environmental negligence in developing countries, dredging operators in developing countries has a great effect on health. For example Malaysia, face an even greater challenge of limited funds (Barrow, 2005; Bartelmus, 1986). Although the maritime industry in Malaysia has been treated as a priority by its government (Ministry of Finance Malaysia, 2010; Mohammed, 2010; Tun, Abdul & Razan, 2010) this nation is facing a challenge in effectively monitoring the impact of dredging activities.

The sensitivity of its environment, which is deteriorating makes it more critical to investigate the impacts of dredging at national level (Spairding, 2001). Environmental management tools that have been previously applied in the dredging industry for auditing and monitoring data collection and strategic monitoring and planning (Barrow, 2005; Bartelmus, 1986).Examples of tools used for auditing and monitoring includes Environmental Impact Assessment (EIA), Lifecycle Analysis (LCA) and Risk Assessment Analysis (RAA)(Guin & Heijungs, 2000; Horne 2009; Kiker, 2007; Linkol and Seager, 2011; Morisey, 1993; Staerdan et al., 2004)

Another set of environmental management tools focuses on the data collection with one example being the use of Geographical Information System (GIS). A combination of strategic monitoring, planning and the above is gaining support as an integrated environmental management approach that aims to achieve sustainable development and maximize benefits for society, the economy and the ecosystems by integrating and balancing the issues of resource exploitation, social and economic activities and environmental preservation (Wang, 2006).

A number of applications of these tools have been developed usually coupled with Multi-Criteria Decision Analysis (MCDA), which aims to create structured and defendable decisions (Kiker, 2007). In general a significant body of research has reviewed the environmental impacts of dredging and so many management tools have been identified attempting to control its adverse effects. Nevertheless, these tools are subjected to individual weaknesses that could limit their effectiveness. It has been noted that environmental management tools and practice which enables the integration of the conflicting issues during dredging decision making should be put to practice in order to make a sustainable decision and prevent its adverse impacts. Furthermore the sources, pathways and impacts of dredging should be taken into consideration when identifying measures for reducing dredging impacts (Eisma, 2006; Osteand Hin, 2010; Raaymakers, 1994; Vellinga, 2002). The concept of integrated environmental management has an all encompassing definition; Wang, 2006).

According to Wang (2006) he defined this concept as “a process that aims to achieve sustainable development and maximizing benefits for human society and ecosystems by balancing resource exploitation socio-economic activities and environmental protection through co-operation and co-ordination of administrative entities and stakeholders”. Hence integrated environmental management could provide a structured framework to accommodate different views of stakeholders and identify the most suited scale of actions towards addressing multi-criteria and conflicting issues as faced by many countries (Antunes & Santos, 1999).

Successful applications of this concept have been seen in the Integrated Coastal Zone Management which is among the tools of the integrated environmental management (IEM) (Antunes & Santos, 1999; Paccho et al., 2007). However the focus of previous research has generally been on developed countries with fewer attempts made, made addressing how these tools can be applied in developing countries. Developed and developing countries have very different primary concerns. In developing countries, the desire for the economic growth and development often takes precedence over environmental issues and concerns, while developing countries often have the economic strength to put greater emphasis on environmental issues (Vellingo, 2002) understanding the conceptual model is a first step to help develop this framework. Source pathway receptor linkages as described in the conceptual model, offer different opportunities for reducing avoiding or mitigating environmental impacts.

These measures can be applied by controlling the levels of contaminants from point and diffuse sources managing the pathways by using appropriate artificially increased metals and organic concentrations (Garret, 2000; Holt, 2000). Sediments can therefore also release contaminants bound on sediments particle surfaces and interior matrices can be released when sediments are disturbed (Burton 2002; Fluck et al., 2000; Garret, 2010; Salmons & Brills, 2004). Transportation of contaminants by sediments is dependent on several factors, primary particle size (Jain & Ram, 1997).

Sediment particles are classified into different sizes, namely fine particles size up to 2mm (clay) particle size up to 16mm (silt), particles size between 63mm and 64mm (rock) (Nittrouer et al., 2007; Tinsker, 2004; Verbreek, 1984). Furthermore, contaminants in sediments may be transported in different forms whether in dry, gaseous, state dry particulate or wet deposition (Lair, 2009). Ocean and wetland systems, bodies, currents and waves can be attributed to sediment transportation (Nielsen, 2009; Office of Naval Research, 2008). Sediments Quality Guidelines (SQGS) have been used to screen potentially contaminated sediments before dredging even though this is not a regulatory requirement (Burton & Wenning, 2005).

Currently in the U.S, Ireland, the U.K, Belgium and Canada, SQGs are used to determine the sediments level of contamination at a dredging site, although still not because of regulatory requirements (Pan, 2009; Praveena, 2008; Suedal et al., 2008).The National Oceanic and Atmosphere Administration (NOAA, 2006) SQGs are utilized to evaluate the quality of dredged sediments in order to help protect both the environment and humans from contaminated exposure (Burton, 2002). This means that if the sediment exceed the guidelines values, it becomes necessary to consider an alternative technological means to handle them (O’Connor, 1998). Along with SQGs/Water Guidelines Values (WGVs) are used to monitor the chemical parameters of the water columns affected by dredging operations. WGVs can be determined from two perspectives; Water quality in aquatic water systems and quality of water intended for portable uses (MacGillivray, 1994).

They are usually derived from either studies on human or animal toxicity, but the latter is widely used. The easiest way to understand the environmental impacts of dredging is through a traditional source pathway target assessments of risks with the sources covered under sediments characteristics earlier and with the pathways of contaminants mainly associated with transport of sediments and therefore depending on dredging technologies. Indicated examples of impacts, that could be due to the activities of dredging are namely Physical Impacts (PI), Chemical Impacts (CI) and Biological Impacts (BI).

Understanding the nature and extent of sediment contamination requires investigating the sources of pollution. Industrial effluents and sedimentary rocks represent point and diffuse sources for contaminated sediments respectively, from each source contaminants can dissipate into underground water, be released through precipitation or be transported by sediments into surface water and finally absorbed and retained in sediment on seas or river beds (De Nobili et al., 2002; Jain & Ram, 1997, Moss et al., 1996). Similarly contaminants pathways into the environment are through media including sediments, air ground water surface and marine water. Through contaminant precipitation, absorption or direct influent from point and diffuse source into the media contaminant are retained or transported directly into surface and marine water (Jain & Ram, 1997; Moss et al., 1996). This can be followed by bio-accumulation in food web communities triggered by the disturbance of sediment included from dredging activities (De Nobili et al., 2002, Moss et al., 1996)

Environmental impacts of dredging can take place during extraction, followed by transport and disposal of dredged sediment. Sediment extraction causes the water change in seabed surface, formation of dredging plumes and exposure of benthos to fishes to and contamination. The dredged sediments are then transported to designate disposal sites. The impacts of these two stages can include bio-accumulation, contamination exposure, change of sediment type and rise in turbidity level (De Nobili et al., 2002, Moss et al., 1996).

Historical background of dredging activities

Dredging activities started modesty in the late 1930s in a company known as “De Groot Nijkerk” working as a dredging construction of the Ijsselmeer polders. Gradually the focus shifted from contracting jobs to repairs of dredgers both own dredgers and fellow contractors. The next logically step was new building, many dredgers have been built.

De Groot Nijkerk was acquired by the Damen group in 1988. It was a perfect match and both companies combined both their specific knowledge of the dredging and trailing suctions pipe systems under its own name. Eventually in 2004 it was decided it was time to change the company’s name to “Damen Dredging Equipment”.

Damen Dredging Equipment(DDE) provides as it has done for the past 70 years, high innovative solutions to the challenges dredging contractors face. Modularity built dredgers and the variety of dredging components available to meet all the tough conditions faced by the dredging activities.

The Dop dredge pump concept is not new. It has been designed in the early 1990’s by Damen dredging equipment together with dredging contractor Ballast Nedam. The combination of the Damen dredging pumps together with day to day experience in the dredging field of Ballast Nedam has resulted in a compact dredging tool that fits in every corner.

The Dop design has evolved the past decades in a heavy duty, fit for purpose dredging tool. Recently, the latest generation of Dop pumps was launched, integrating new practical features. More types were added to the range for more versatility.

Global health concern on dredging activities

Environmental Risk Assessment (ERA) and its advantages and limitations for environmental scientist, engineers and regulators making decisions about the environmental effects of dredging projects. The literature includes detailed explanations of risk assessment methods e.g. DGE in preparation, (Den Besten, 2003; EPA, 1997;Suteret al., 1993 and the supporting science e.g. Lano, 2003). The working group reviewed such resources and applied them within the context of dredging projects. Guidance for dredged materials program in the USA provides a useful starting point (Cura et al., 1999). The European Commission Joint Research Centre (European Chemicals Bureau, 2005) provides robust technical guidance to support the human health risks assessment and ecological risk assessment for exposure to chemicals in the environment. The ecological chapter on the aquatic environment and specific to risk assessment in the marine environment.

Various capital maintenance and cleanup projects worldwide require annually dredging hundreds of millions of cubits meters of sediment. There are many ways that these projects may affect the environment during dredging, transport and placements are in the longer term at the disposal site. Evaluation of all impacts and their importance is normally the function of an Environmental Impact Assessment or similar study. The environmental risk assessment described in this guideline will address specific ecological or human health effect relating to a particular contaminant and a particular receptor (e.g. fish species, human group).

As part of the commitments made by the government of Canada and the United states in the Great Lakes Quality Agreement Remedial Action Plans (RAPs) are being developed and implemented at great lakes of around the Great Lakes of concerns. The areas of concerns are specific places around the great lakes basin ecosystem. Where environmental quality is degraded to the point that certain beneficial uses (ability of fish, wildlife and humans) to thrive are impaired. Many communities are struggling to determine what point in time, the ecological conditions such that the geographic area of concern can be considered no longer degraded. Due to semantics and lack of clarity, one example is called restrictions on dredging.

Many communities continue to debate approaches in resolving dredging activities, the paper outlines how and when this particular Great Lakes Water Agreement Impairment should be considered to no longer be impaired.

The 1909 Boundary Waters Treaty between Canada and the United States established the International Joint Commission (IJC) 1989. The commission purpose under the treaty to prevent or resolve disputes surrounding quality between the two countries and act impartially in terms of dredging activities and disposals. The National Environmental Policy Act (NEPA) of 1969), it is through the NEPA process that the dredged materials disposal alternatives including actions open water disposal or confined disposal of dredged materials are evaluated, documented and publicly disclosed. Section 404 of the Clean Water Act designates the United States army corps of engineers as they lead federal agency in the regulation and enforcement of dredge and fill discharge activities in all navigable water of the United States.

The corps is also responsible for the maintenance of federal navigation channels potential water column contaminant effects of open water disposed sediment are evaluated by comparing contaminant release in an elutriate of the material to be disposed with applicable water quality criteria or standards as appropriate. A chemical comparison of the material to be disposed with that of a reference sediment is conducted. If contaminant concentrations in the dredged materials and concentrations at an adjacent disposals site are substantially similar and contaminants will not leave the adjacent disposal site or if controls are available to reduce contamination to acceptable levels within the disposal site no further evaluation may be required if this is not the case, bio assays and bio accumulation tests are required to complete the evaluation (EPA, 1992) and determine whether active intervention in sediment remediation is warranted.

Environment windows are periods in which regulators have determined that the adverse impacts associated with dredging and disposal can be reduced below critical threshold and dredging and disposal activities are prohibited when the perceived increase in potential harm to aquatic resources is above critical thresholds. They are intuitively simple means of reducing risk to biological resources from stressors generated  during dredging and disposal activities including entrainment of fish eggs and larvae resuspension of buried contaminant sediments, habitat loss and collisions with marine mammals (Committee for Environmental Window for Dredging Projects)(CEWDP, 2001). Impairment of beneficial use is a change in the chemical, physical or biological integrity of the great lakes system sufficient to cause any impairment or other related uses such as the microbial objectives for water’s used for body contact recreational activities (Canada and United State, 1987).

Reasons for dredging activities

According to Bertha (2009), the following factors contribute to dredging activities:

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 waterway 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.

• Construction materials: Dredging sand and gravel from off-shore which are used in the construction of houses and industries principally used for concrete.

1. 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.
2. Harvesting material: Dredging is used to harvest sediment for elements, life gold, diamond or other valuable trace substance.
3. Maintenance: Dredging is used to maintain navigable waterways 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.

• 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.
• 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 activities

The potential environmental effects of dredging activities are generally two folds;

Firstly as a result of the dredging process itself and secondly as a result of the disposal of the dredged materials. During the dredging activities effects may arise due to the excavation of sediments at the bed loss material during transport to the surface overflow from the dredger whilst loading and loss of materials from the dredger and pipelines during transport (Wishart et al., 2008); Mrmom et al., 2012).

Depending on where these activities takes place a marine SAC may be affected by either dredging or disposal alone, by both of these activities. In considering the environmental effects of dredging activities and disposal is highly varied and site specific depending upon a number of factors shown below;

• Magnitude and frequency of dredging activity method of dredging and disposal.
• Method of dredging and disposal.
• Channel size and depth.
• The size, density and quality of the material.
• Intertidal area.
• Background levels of water and sediment quality.
• Suspended sediment and turbidity.
• Tidal range.
• Current direction and speed.
• Rate of mixing.
• Seasonal variability and meteorological conditions affecting wave conditions and freshwater discharges.
• Proximity of the marine features to the dredging or disposal activity.
• Presence and sensitivity of animals and plant communities (including birds, sensitive benthic communities, fish and shellfish).

Prediction of the potential effects might be caused by dredging activities and/ or disposal in a marine SAC cannot be made with any degree of confidence if these parameters are not known on a site-by-site basis. Generally the potential impacts of dredging and disposal can be summarized as follows; (IADC/CEDA, 1988; ICE, 1995; PIANC, 1996).

Removal of subtidal benthic species and communities short-term increases in the level of suspended sediment can give rise to changes in water quality which can effect marine flora and fauna, both favorably and unfavorably such as increased turbidity and the possible release of organic matter, nutrients or contaminants depending upon the nature of the material in the dredging area.

Settlement of these suspended sediments can result in the smothering or blanketing of subtidal communities and or adjacent intertidal communities, although this can also be used beneficially to raise the level of selected areas to offset sea level rise or erosion (short term impact (v) long term gain). The impact of dredged materials disposed, largely depends on the nature of the material (inorganic, organically enriched, contaminated) and the characteristics of the disposal area (accumulative or dispersive areas) (SOAEFD, 1996). The potential impacts of the disposal and dredging on the marine environment, such as restricting the disposal of heavily contaminated sediments, is to some extent minimized through the FEPA licensing process by conditions imposed by the licensing authority.

The evaluation of the environmental effects of dredging and disposal must take account of both the short term and long-term effects that may occur both at the site of dredging or disposal (near field) and the surrounding area (far field). The IADC and CEDA, (1988) guide provides a useful table that illustrates the temporal and spatial scales in which various environmental effects of dredging might be realized. Near field effects are simply defined as phenomena occurring within the geographic bounds of the activity and far field effects as occurring more than approximately 1km from the activity. However, other sources suggests that caution should be used when adopting an arbitrary distance to distinguish between near and far field effects due to the site specific nature of the potential effects that arise from dredging.

In addition to the environmental effects that may occur as a direct result of dredging and disposal activities, we must also consider the environmental effects that may occur as a result of the physical changes to bathymetry and hydrodynamic processes that dredging makes. Although such changes may occur as a result of maintenance dredging, they are more commonly associated with capital dredging activities. These changes can be summarized as follows (IADC/CEDA, 1998);

• Alterations to coastal or estuary morphology, for example alteration of sediment pathways and changes to siltations patterns which may affect coastal habitats and species in addition to marine ones;
• Alteration to water currents and waves, climates which might affect navigation and conservation interests and
• Reduction of water quality.

Each of the potential effects from dredging and disposal are discussed in the following sections. It should be stressed that there will be few dredging and disposal operations in the marine SACS where all of these potential effects will be realized;

• Dredging: Removal of benthic animals
• Dredging and disposal: Suspended sediments and turbidity
• Dredging and disposal: Organic matter and nutrients
• Dredging disposal: Contaminated sediments
• Dredging and disposal: Settlement or suspended sediments
• Dredging and disposal: Changes to hydrodynamic regime and geomorphology
• Disposal: Discharge of dredged materials at the disposal site.
• Disposal: Intertidal recharge

Probable remedies for dredging activities

According to the Environmental Impact Assessment (EIA) the potential impacts would normally be identified and quantified through the environment assessment, which also include a degree of project optimization to reduce impacts through dredge methodology, dredge schedule, sediment spill sources and climatic conditions encountered during dredging (Auld, 1978).

Environmental Impact Assessment: (EIA, 2008) states that before works are carried out it is a common practice that an EIA is undertaken to determine the potential impacts and define mitigation measures that usually includes detailed hydraulic modeling.

Permanent Impacts: Induced by the proposed structures/works on currents, water levels, waves, sediments, transport, water quality, store line, evolution in the area and nearby etc. These impacts will last as long as the structures/works are in place.

Temporary Impacts: Occurs during construction (dredging) works. The extent and potential impacts of sediment plumes generated during the dredging works are determined by the type of dredger, dredging methodology, type of sediments and flow conditions during dredging works. These impacts are usually limited to the duration of the dredging works, however if not managed properly could lead to permanent impacts.

To account for these uncertainties a level of “conservatism” is usually spilled in the modeling works, however, it is not possible to assure that impacts will not take place at sensitive receptors due to the nature of the uncertainties. It is a good practice to manage the dredging works based on actual observation during dredging to ensure that no unforeseen impacts are realized and that dredging works are carried out with minimal disruptions.

Environmental Feedback Monitoring and Management of Dredging Works;

Ensuring that no minimal adverse impacts are caused by dredging works requires a careful assessment of the dredging work and stresses induced on the sensitive receptors to guide the works. In order to address the limitations of statics monitoring, adaptive management strategies have been developed specifically aimed at addressing the environmental management by;

1. Implementation and collection of baseline information.
2. Monitoring including measurements and modeling works.
3. Evaluation of data.
4. Adaption: the adaptation includes not only the reassessment of the implemented dredging strategy but also the evaluation of the objective target values Berry, W. Rubinstein, (2003).

The adaptive monitoring is targeted to evaluate at the environment sensitive receptors and provides a response to relevant triggers values. This process can be proactive or reactive. The static manner based monitoring works and compliance to single trigger values with threshold values defined as the best practice approach to managing and minimizing impacts from dredging and post construction around sensitive areas where corals are present. While the proactive approach is usually defined in an environmental feedback monitoring and management plan based on;

• Spill budget which is used to form a first level control of potential impacts.
• Results from online instrumentation (at recurrent areas) are used as an indirect indicator of potential health of the sensitive receptors (e.g. corals based on tolerance limits).
• Predictive numerical models are used extensively to hindcast/forecast the location of the plumes from the construction operations and for providing a detailed temporal and spatial picture of potential impacts filling the gaps between monitoring stations and allowing a segregation of the impacts arising from the dredging activities.
• The tolerance limits are updated based on monitoring data of sensitive receptor areas. This is the so called feedback loop. This is carried out only if the project duration is long enough to allow this evaluation as receptors reaction to impacts may require time to become noticeable and if the dredging period is short, it will allow for re-assessment of these values.

The spill budget is the first level of control set as the limit on sediment spill to ensure that set tolerance limits are not exceeded. It is typically expressed through a set of numbers at different dredge locations and potentially for different climatic conditions.

This usually goes hand in hand with other mitigation measures that are included in the feedback monitoring such as.

• Reducing as much as practical and amount of sediment introduced into the water column at a passive plume for a given dredge operation. This can be achieved by using green valve technology and ensuring that, well maintained equipment is used in avoiding unintentional leaks.
• Careful management of the dredging plume to direct it away from sensitive receptors. This is done through planning and working carefully with the current conditions to ensure that dredging with overflow in critical overflow in critical areas is only carried out when currents will carry the dredge plume away from sensitive receptors.

Physical measures to control sediment spill such as silt contains can be used, however, the efficiency is largely dependent on the environmental conditions at the site (mainly current, speed, water depth and wave heights).Environmental conditions of the site, which has to do with the effectiveness of different mitigation measures disposal that deals very much on the hydrodynamic conditions of the site. It is critical to adopt mitigation options that are practicable and effective for the local conditions to ensure that they will get effectively implemented and achieve the objectives in terms of minimizing any impacts (Denhred & Mauck, 2006).

Feedback monitoring program

A feedback monitoring program was implemented to minimize impacts during dredging work with the following environmental management objectives (EIA, 2008)

• No reversible impacts to primary benthic producer habitats or other environmental receptors e.g. no mortalities to corals and destruction of coral reefs.
• Minimize impacts during dredging works
• Minimize risks of real or perceived impacts that could lead to stoppages after dredging.
• Key environmental receptors were identified in the area during the EIA
• Coral reefs north of the dredging areas.
• Tourist resorts
• Aquaculture installations
• Fishing grounds

Monitoring approach

A considered approach to carefully manage the spill during dredging to ensure that the set tolerances for environmental receptors were not exceeded. This was achieved through a spill control and feedback monitoring of the various processes (Apitz, 2006)

• Apply a spill budget approach
• Continuous monitoring and modeling based on (mud transport model) through the dredge period to ensure that;

1. The spill budget is adhered to
2. The sediment transport model to describe the transport of the sediment spill is applied to evaluate the location of the sediment plume both in space and time. The model is revalidated against measured data to ensure that the predictions are accurate as possible.

• Adoption of mitigation measures if required to achieve the environmental objectives.

Dredging spill limit

The spill limit was re-assessed before the start of dredging works. This was done based on more detailed information provided by the dredging contractor and a value was defined as a starting spill limit the dredging period that occurred during the North-East (NE) monsoon. Adjustment to the spill limit the value would be re-assessed during the dredging period as part of the adaptive management program (Aubry, 2006)

Plume monitoring and management:

A comprehensive monitoring campaign was implemented that includes:

• Monitoring of overflow to calculate that spill
• Hindcast modeling of all dredging operations based on actual dredging records.

This provides a detailed image of the sediment plume both in space and time.

• Daily water sampling at fixed stations
• Online

(Acoustic Doppler Current Profiler)(ADCP) measurement at two locations to derive Training Stress Score (TSS) levels and current flow conditions.

Current and TSS transects and three stages of the project to produce details of the spatial extent of the sediment plume for model calibration (EIA, 2008).

Trigger level

According to Aubry and Ethhist (2006), three trigger levels were defined for the project. A first level was when a daily “spike” exceedance occurs. The second level was based on the analysis of (3) three days running average values. The level 1 is unlikely to cause any impacts and no immediate actions is required, however, these events are analyzed to avoid any further issues. For level 2 cases the exceedance has to be investigated based on results from the monitoring and modeling works and mitigation measure have to be implemented to ensure that levels are brought back under the limit. Level 3 indicates a long term violation of the trigger values and immediate action required. The trigger levels were defined at the start of the feedback monitoring program for;

• Sediment spill:- The three (3) levels were defined based on the duration as a daily spike 3-days running average exceeds the spill limit and 14 running average exceeds the spill limit.
• Modeling:- Three (3) levels were also defined based on duration e.g. excess of TSS>5mg 11 for more than 10% of the time for daily 3-day and 14-days running periods
• Monitoring data:- Measured data does not distinguish between background and dredged derived concentrations but they are important to verify the models and effects that are not resolved by the model are not missed out at the sensitive receptors. The trigger levels defined based on the type of receptor and the conditions at the site “clear” and “turbid” water based on the baseline data with different trigger valves. These valves are assessed on a daily 3-day and 7-day running period.

Analysis:

The dredging works were monitored continuously and based on daily records of the dredging works. This information was provided by the dredging contractor and included location of the dredger in time and operational status (dredging, travel time, disposal etc.) that together with overflow sampling was used to carry out modeling works. This provides a conjunction with the daily spill records, daily monitoring data at sensitive receptors and the environmental team evaluated the conditions on daily basis to determine, if any violation occurred and if necessary defined mitigation measures. A close interaction between the dredging contractors and environmental team which allows the best approaches in order to minimize impacts especially on the northern area where the most sensitive receptors are placed. One of the mitigation actions implemented was that of dredging with overflow when currents were southward, in this way the generated sediment plume would be directed southward, away from the most sensitive receptors. A comparison of dredging works with controlled and no controlled spill is the control operation at the TSS levels are reduce in the sensitive areas. (Adams, 2005)

Another implemented mitigation measure was to concentrate the overflow along the outer off shore dredging areas so that the plume moves away from the sensitive receptors, this was only practical at the initial stages of the project when the dredging area included the overall channels, however later stages when the dredging had to focus on particular sectors of the channel this option was not viable.

During the dredging works the communication between the dredging contractors and the environmental team was extremely important as mitigations measures had to take into consideration operational conditions of the dredging works. In particular conditions increase in short term dredging was necessary and this was closely followed, mainly at the sensitive receptor areas based both on monitoring data and modeling results (Aubry & Ethist, 2006)

The modeling works were extremely important in the analysis as it provided a link between dredging and monitoring data, in one case an exceedance was observed at one particular station and the hindcast modeling confirmed that this was caused by the dredging due to a combination of high spill rates and overflowing for one of the trips at the eastern end of the channel just during flow reversal. This discussed with the dredging contracts or and corrective measure were taken to reduce TSS levels at the sensitive receptive receptor areas.

Governance, compliance monitoring and reporting

Compliance monitoring and reporting for feedback monitoring was carried out to confirm that the works were meeting the quality objectives. Berry, Melzian and Hill (2003) an environmental monitoring and management team was established to follow the works, this team produced a daily report that was submitted to the contractor and other parties and a summary report was produced every 14 days setting out details of the monitoring program, this report was issued to authorities meetings which were organized regularly to discuss the evolution of the project and also visits to the dredgers, these visit includes, different stakeholders as the Department of Environment, the Department of Irrigation and Drainage and others. The main ideas of the report and meetings was to present a clear status of the situation and the approach applied in the study (Berry et al., 2003).

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