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Sediment plume modelling and monitoring has an increasingly important role in the environmental impact assessment of dredging and disposal works. Sediment plume assessments are generally based on complex, time-consuming numerical modelling. Little is known on source terms and spatial / temporal variation of the character of source terms in combination with the complex hydrodynamic conditions.

The objective of this building block is to provide background information / a guideline when assessing, modelling and monitoring sediment plumes and source terms.

    General Building Block Description

    Dredging plumes and environmental impacts

    Sediment plume modeling and monitoring has an increasingly important role in the environmental impact assessment of dredging and disposal works. Within these plumes, suspended sediments may be transported towards sensitive receptor sites, such as coral reefs, sea grass meadows and mangrove areas (Figure 1). Other locations which may be impacted by sediment plumes concern recreational areas and cooling water intakes. Potential impacts on these receptors may follow from:

    • Reduced light penetration in the water column due to enhanced turbidity levels
    • Smothering of organisms upon sedimentation when particles settle out from suspension
    • Water quality changes e.g. due to the release of contaminants from the sediment

    Although sediment plumes do occur in the direct neighbourhood of dredging operations, it is important to realize that sediment plumes generated by dredging activities not 'by definition' induce a negative environmental impact.

    Figure1 depicts a dredging-induced sediment plume (middle) from a trailing suction hopper dredger (right) in relation to a sensitive receptor site (coral reef, left).

    The background conditions in a natural system often exhibit non-zero turbidity levels and net sedimentation. Moreover, dredging activities are not the only driver of sediment-induced turbidity. Natural processes (such as river peak discharges and resuspension of fine sediments during storms) as well as other human activities (such as trawler fishing and ship-manoeuvring operations) are also associated with sediment suspension and turbidity generation (Aarninkhof et al., 2008). It is furthermore important to realize that sediment plumes may in some cases even generate a positive environmental effect. For example, organic matter resuspended by dredging activities may provide an additional food source for fish and shellfish.

    Summarizing, the severity and spatial extent of sediment plumes and the associated potential environmental impact depend on (see e.g. PIANC, 2010):

    • The presence and type of sensitive receptors relative to dredging / disposal operations, the detectable stress-response of these receptors and the timing (e.g. in relation to coral spawning periods) concerning duration and frequency of the sediment plume.
    • Existing receptor stress levels in combination with the local background conditions regarding e.g. turbidity levels resulting from natural processes and other man-induced activities.
    • Transport of suspended sediment within a plume depending on the local water depth, hydrodynamic conditions (e.g. tidal/seasonal currents) and sediment characteristics (e.g. settling behaviour).
    • The character of dredging / disposal operations (e.g. type of equipment, production rate) in combination with the properties of the dredged/disposed material (e.g. fines content, in-situ density) as both determine the type and magnitude of the source of a sediment plume.

    Figure 2 also gives a schematic overview of these important aspects determining plume characteristics and potential environmental impact on sensitive receptors.

    Depending on these factors, there can be a considerable spatial and temporal variation of environmental impacts, if any. In some cases, the impact may be confined close to the work area, whilst in others the prevailing currents may transport fine sediments over large distances, with documented cases of plumes extending to more than 70 km from the work site (PIANC, 2010).

    Problem definition

    For the assessment of turbidity near dredging activities a number of challenges can be identified:

    • Sediment plumes in particular exhibit a complex three-dimensional behaviour which varies at a wide range of spatial and temporal scale as a function of different hydrodynamical (e.g. wave and current action) and sedimentological (e.g. settling) processes. The same applies to the input of sediment plumes, which may be natural (riverine input, re-suspension by waves or currents) or anthropogenic (e.g. fishing, shipping).
    • The input of dredging plumes adds another dimension to the complex character of sediment plumes. The amount, type, rate and location at which sediments are released into the water column during dredging activities may vary with the in-situ soil conditions of the dredged material, with the applied work method (e.g. dredging sequence, planning) as well as with the type of equipment employed.
    • An important input for turbidity assessments is the release of sediments into the water column during dredging and disposal operations. However, little is known on these so-called source terms, as these depend on soil conditions, dredging work method and the complex hydrodynamic conditions around dredging equipment. Therefore, source terms are generally based on rough estimates.
    • The complex behaviour and input of dredging plumes indicate that it is difficult to accurately assess turbidity near dredging activities and to realistically predict related potential sensitive receptors impacts. Available analysis and modelling tools for sediment plume assessment range from simplified analytical tools, allowing for quick first-order assessments, to advanced numerical modelling tools, which are generally complex and time-consuming.
    • The above indicates that choosing the right approach for assessing the turbidity near dredging activities at a specific project is not straightforward. This choice is always a compromise between the required level of detail of the assessments and the available resources (manpower, time and costs).
    • The approach for assessing the turbidity near a dredging site should be in line with the character (e.g. techniques, number of stations) and accuracy of the monitoring strategy during the execution of a project. The strong spatial and temporal variation of dredging plumes and source terms indicates the challenges associated with (compliance or verification) monitoring of plumes and source terms.
    • Predicting the impact on sensitive receptors based on the outcome of a turbidity assessment near dredging activities is rather difficult. Stress-responses (e.g. mortality, decreasing health, behavioural changes) of receptors with respect to turbidity levels and sedimentation rates highly vary per receptor type, geographical location, background conditions, etc. Little information is available on these stress responses and monitoring of receptor stress is complex.

    In summary, the challenge is to effectively predict turbidity near dredging and disposal operations (process impact) as a function of project-specific conditions, in view of potential environmental impacts on receptor sites.

    A general, 2-stepped approach is proposed with increasing level of detail. The first step concerns a first-order assessment. Only when an environmental impact is anticipated, a higher-order assessment step is taken.

    Dredging plume development and impact

    This section discusses general the characteristics and behaviours of dredging-induced sediment plumes and their typical environmental impact that are deemed necessary as background knowledge for the application of the proposed assessment approach.

    Dredging and disposal methods

    A dredging plume is generated by dredging and/or disposal activities. The character of these activities in combination with the nature of the dredged/disposed material determines the type and magnitude of the source of a sediment plume. Below a list of typical dredging and/or disposal activities as well as typical soil condition parameters is presented, information that is important to dredging plume development. For a comprehensive overview of the characteristics of dredging and disposal operations (and associated potential environmental impacts) reference is made to Bray (2008).

    Dredging and/or disposal activities:

    • Type of dredging equipment:
      • Trailing Suction Hopper Dredge (TSHD)
      • Cutter Suction Dredger (CSD)
      • BackHoe Dredger (BHD)
      • Grab Dredger (GD)
    • Dredging method:
      • Hydraulic (TSHD)
      • Mechanical (BHD, GD)
      • Hydraulic-mechanical (CSD)
    • Production rate:
      • Relatively high: TSHD, CSD
      • Relatively low: BHD, GD
    • Disposal method:
      • Bottom-doors of hopper (TSHD, barge)
      • Pipeline disposal (submarine or land-based)
      • Disposal rate: continuous or intermittent

    Nature of the released material:

    • Type of material
      • Cohesive: clay, mud
      • Granular: loose sand, gravel
      • sedimentary or solid
    • In-situ conditions
      • Volume
      • Packing density
      • Grain size distribution
    • Dredged material characteristics as influenced by dredging and transport processes
      • Additional fines generation due to disaggregation
      • Bulking of material (i.e. loosening resulting in lower density)
      • Fraction of lumps of material, e.g. clay balls

    Sediment plume development

    The severity and spatial dimensions of sediment plumes depend to a large extent on the local hydrodynamic conditions (prevailing water depth, tidal/seasonal current and wave conditions), as well as on the settling behaviour of the suspended material. The stages of plume development are discussed below based on Land et al. (2004) and illustrated in Figure 3 (after Spearman et al., 2011) and Figure 4 (Land et al., 2004). 

    Dredging Zone: Dynamic plume.

    The dredging zone concerns the immediate area of the dredging activities that release sediment from the equipment (see Figure 3, Figure 4 and Figure 5). It is an area of high turbulence where the processes of plume development are dominated by ‘fall out’ of large lumps of material and by the severely hindered settling of all other particles. The latter results from the intense turbulence generated by the dredging operations.

    The sediment plume in this zone is characterized as ‘dynamic’, which means that it will sink as a whole towards the bed as a density flow, at a speed that may be orders of magnitude larger than the theoretical settling velocity the individual particles in the suspension. The size of the dredging zone is typically in the order of a few metres around the equipment and the duration of this phase of plume development is typically in the order of seconds.

    Near-field plume: Transition from a dynamic to a passive plume.

    The near-field zone concerns the area in the vicinity of dredging activities (see Figure 3, Figure 4 and Figure 5). The near-field phase of plume development is the period in which an initially dynamic plume loses its momentum as differential settling, assisted by turbulent diffusion, reduces the excess concentration. Coarse particles progressively settle out of suspension, leaving the finer particles to form a passive plume that is carried into the far-field plume by water currents.

    The duration of this near-field stage of plume development will vary considerably, depending mainly on the initial concentration and particle size distribution in the suspension, the volume of the suspension, water depth and hydrodynamics. The typical timescale of this stage is between minutes and several tens of minutes.

    Far-field plume: passive plume.

    The far-field plume (see Figure 3, Figure 4 and Figure 5) is essentially passive and comprises a dilute suspension of fine sediments, which settle at a rate proportional to the concentration and the settling velocity of the individual grains. The term ‘far field’ is most commonly used in the context of the fate of fine sediments and the limits of potential environmental effects. The path taken by the plume and the solids suspended in it are determined by the local hydrodynamics.
    Figure 5 illustrates the various stages and processes of plume development: (a) suspended sediment within the dredging zone of a TSHD at a typical scale of tens of metres (HRWallingford, 2003); (b) near-field plume at a typical scale of hundreds of metres, and (c) far-field plume at a typical scale of kilometres (PIANC, 2010)._

    Furthermore, one can distinguish primary and secondary dredging plumes. A primary dredging plume directly relates to a specific dredging activity and includes the near-field and far-field plume. The typical timescale of a primary plume is of the order of magnitude of that of the dredging activity.

    A secondary plume is generated by the re-suspension of bed material deposited from a primary dredging plume. This re-suspension of bed material is due to natural processes, for example during storm or spring-tide. Re-suspension fluxes and secondary plumes will not be further discussed in this context, mainly because they typically occur at timescales exceeding those of the dredging activities. Since secondary plumes are caused by factors that have nothing to do with dredging, however, they may just as well occur during a project.

    Source terms.

    Source terms for dredging plumes concern the flux (mass per unit time) of sediment released into the water column during dredging or disposal operations. These terms are a function of soil conditions, dredged material, work method and hydrodynamic conditions. For near-field models, or when near-field modelling is an integral part of far field modelling, the description of the ‘source’ must include:

    • the location of the source as a function of time;
    • the flux of sediment at that point;
    • the particle size distribution of thesediment.

    These three source properties are shortly discussed below, as indentified in Figure 4. This text is mainly based on Land et al. (2004).

    At dredging activities: True source.

    The ‘true source’ (see Figure 4) is the actual location where sediment becomes detached from the dredging equipment. It is in an area of high turbulence where the processes of plume development are dominated by ‘fall out’ of large lumps of highly concentrated sediment-water mixtures and by the severely hindered settling of all other particles. The lumps of material descend almost immediately to the bed within the dredging zone.

    Edge of dredging zone: Practical Source.

    As the released sediment moves out of the highly turbulent dredging zone into the near-field zone water currents or because the dredger with the true source sails away, the behaviour of the remaining sediment in suspension (i.e. excluding the large lumps) becomes more predictable and easier to model and measure. This is why this is called the ‘practical source’. The practical source is the input to the near-field plume. This includes a continuum of particle sizes, ranging from lumps that were too small to settle in the turbulent dredging zone, to sands, silt and clay.

    At dredging activities: Virtual source.

    Any definition of sediment release rate will be somewhat arbitrary. One option is to make it suitable for input into plume models. It may theoretically be possible to continuously determine settling rates from the changing particle size distribution, but within the dredging zone this is practically hardly possible due to the high level of turbulence and the occurrence of large lumps of material.

    An alternative approach is to fit a curve to measured data (if available) of the decaying flux in the near-field zone and to extrapolate this back to the location of the dredging activity (i.e. the origin of the x-axis of Figure 4). This computed release rate can then be defined as the ‘virtual source’. Although this input into the plume model may yield realistic estimates of the plume away from the dredger, it does not necessarily represent what is happening at the dredging location. Furthermore, it is noted that the reversed extrapolation of an exponentially decaying function incorporates significant inaccuracies.

    Environmental impacts

    Typical sensitive receptors potentially impacted by dredging plumes are:

    • Coral reefs
    • Sea grass meadows
    • Mangrove areas
    • Aquaculture areas
    • Cooling water intakes
    • Recreational areas

    These receptors may be impacted by dredging plumes due to:

    • Increased turbidity - reduced light penetration
    • Increased sedimentation - smothering of organisms
    • Water quality changes - reduced oxygen levels, release of contaminants, chemical reaction products.

    Although the construction phase of a dredging operation or port construction project is usually of a temporary and relatively short-term nature, sediment plume impacts may be permanent (lethal: mortality, change in species composition) or transient (sub-lethal, e.g. reduced growth rate, reduced reproduction). Severity, duration and spatial extent of sediment plume impacts depend to a large degree on (PIANC, 2010):

    • Proximity/location of the receptor relative to the work site in relation to the prevailing currents
    • Existing receptor stress levels and tolerance of receptors to elevated stress levels
    • Magnitude, duration, frequency and timing of impact
    • Prevailing water depth, current and wave conditions in the area
    • Nature of released material, release rate and plume development

    There are large differences in species’ responses to impacts. Figure 6 shows a conceptual relationship between the intensity and duration of a stress event (e.g. elevated turbidity levels) and the risk of sub-lethal and lethal effects on corals (PIANC, 2010). These species response curves (SRC) may be applied to ‘translate’ results of turbidity assessment tools to sensitive receptor impacts.

    How to Use

    A guideline to effectively assess turbidity near dredging and disposal operations is not straightforward due to the complex nature of dredging plumes. The guideline proposed herein therefore consists of four assessment steps (see below). An environmental risk assessment related to dredging-induced turbidity starts with Step i. If a Step i assessment indicates that a potential impact is anticipated, than a Step i+1 assessment is proposed. If no impact is anticipated, no further steps need to be taken. The first step concerns a rapid first-order assessment and is based on numerous assumptions and simplifications. Subsequent assessment steps go into an increasing level of detail. The final, most detailed assessment step generates rather realistic results, but requires a significant amount of effort and time.
    The proposed four assessment steps allow users to take a project-specific approach, with a level of detail in line with the project-specific requirements. This also prevents spending too much effort and/or time to a specific assessment. A preliminary environmental impact assessment as part of a project feasibility study, for instance, generally does not require a high level of detail, not only because a qualitative indication for potential impacts is usually required in this early phase, but also because detailed information (e.g. on the work method) is still lacking.
    Another reason to apply a specific assessment level in the project preparation phase has to do with the proposed monitoring method during the project execution phase: there is no point in an assessment at a level of detail that exceeds the resolution of the data from compliance / verification monitoring. Finally, it is noted that an extensive turbidity / impact assessment is only necessary if sensitive receptors occur in the vicinity of the dredging activities.

    Step 1: Situation sketch

     

     

    Step 2: Feasibility assessment

     

     

     

    Step 3: Spatial and temporal scenario assessment

     

    Step 4: Research assessment

     

    Step 1: Situation sketch: project and natural system assessment

    Objective:
    First-order qualitative assessment of potential environmental risks involved with dredging and disposal processes. The main questions to be answered by Step 1 reads: are there any sensitive receptors in the influence zone of the proposed dredging activities?

    Who:
    Non-technical / general consultant, regulator; basic understanding required.

    Duration:
    Hours – day.

    Tools:
    Qualitative description / visualization of site.

    Information sources:
    Google Earth, locals, Admiralty Charts, basic information on meteorological, tidal, wave and environmental conditions, etc.

    Required input:

    Project surroundings:

    • Qualitative description of existing physical and environmental site conditions (including variability).
    • Project location: latitude and longitude.
    • Mean water depth for the whole project site.
    • Presence of coastline, river, etc.
    • Seabed / substrate conditions: muddy, silty, sandy, gravelly, rock.

    Sensitive receptors:

    • Presence, location and type of sensitive receptor sites: coral reef, mangrove, sea grass.
    • Environment: sheltered/exposed, shallow/deep.
    • Background conditions: clear water, turbid (including variability).

    Project specifications:

    • Location of dredging area and disposal site.
    • Material to be dredged: volume, type (mud, clay, granular, rock).
    • Planning: milestones, timing relative to season, project duration.
    • Anticipated dredging (mechanical / hydraulic), transport and disposal work method.

    Plume transport:

    • Descriptive: direction (north/south/west/east), magnitude (strong/weak/ intermediate), waves (sea, swell, locally generated wind-waves / sheltered site), tidal range, river (presence, seasonal variance, effect wet season) coastal up/downwelling.

    Typical result:

    Project site map to qualitatively indicate (1) the presence of sensitive receptors relative to dredging activities and (2) the potential transport of suspended sediment, by showing:

    • Location and type of sensitive receptors.
    • Existing physical environment: geographical location (coordinates) of coastline, river mouth(s), islands, bathymetry, etc.
    • Typical hydrodynamic / metocean conditions: dominant (tidal / seasonal) flow direction and magnitude, tidal range, annual wave rose.
    • Project overview: dredging and disposal site(s), work method, planning.
    • Seabed characteristics in view of material to be dredged.

    In case of the presence of sensitive receptors a first indication of the potential environmental impact may follow from one or more of the following qualitative / quantitative assessments:

    • The typical time-scale of the planned project and that of the ambient hydrodynamics, especially the typical system-flushing time-scale.
    • The spatial and temporal adaptation scales (scales of exponential decay) of the concentration of suspended seabed sediment in unidirectional uniform flow (constant depth-averaged velocity U). For dilute suspensions these quantities are given by

    in which (lambda) c and (tau) c denote the spatial and temporal scale, respectively, U is the depth-averaged flow velocity, h the water depth and w s the settling velocity of the sediment. Results provide an indication of how far from the site sediments brought into suspension at the site can be carried by the flow (close to the work area or far away from the work site due to prevailing currents), but are independent of project-specific dredging activities.

    • Develop a site-specific mass and/or volume balance of (fine) sediment to be dredged and disposed, during dredging and transport as well as upon/after disposal. This provides insight into the potential losses of material to the environment.

    Verification:

    • Site visit.
    • Information from locals: harbour master, fishermen, university, client, local regulators.

    Points of attention:

    • If the flow is not strong enough to keep the sediment in suspension, the Suspended Sediment Concentration (SSC) decreases exponentially with the distance from dredging operations. This implies that (1) the SSC generally decreases with an order of magnitude within a few hundred metres from the operation site (i.e. the Dredging Zone, see Figure 4), and (2) reaches natural levels within a few kilometres.

    Step 2: Feasibility assessment

    Objective:
    Worst case scenario assessment based on steady state conditions to verify the (environmental) feasibility of the proposed work method. The main question to be answered by executing Step 2 is: can there be any impact (qualitatively speaking) on sensitive receptors due to the proposed dredging activities?

    Who:
    Hydraulic / environmental engineer with a consultant or the engineering department of a dredging company; specialized institute.

    Duration:
    Days – week.

    Tools:
    Numerical model: Turbidity ASSessment Tool (TASS):

    • Hopper module: simulates TSHD source term during dredging.
    • Dynamic plume module: simulates the early stages of plume development in the near-field. Hopper module provides input for the dynamic plume module. Alternatively, the source term of a dredging activity may be imposed manually, e.g. for CSD or BHD dredging operations.
    • Passive plume module: direct simulation of far-field plume

    Information sources:

    Bathymetric survey data, publically available general literature, project information document, geological / soil investigation data, measured hydrodynamic data.

    Required input (input of Step 1, but more detailed)

    Project surroundings:

    • • Bathymetry: simplified depth contour pattern.

    Sensitive receptors:

    • Characterization of receptor sites.
    • Background SSC / sedimentation levels.
    • Variability through sensitivity testing.

    Project specifications:

    • Bathymetry in the dredging area and at the disposal site(s).
    • Material to be dredged: characteristics of dredged material; average fines content, typical Atterberg limits, strength.
    • Planning per type of equipment / operation, typical work method for certain periods of the dredging program at a specific location, position of dredger in time and space.
    • Schematized source term for a single dredging operation (rate of fines release constant in time and space); see Practical Source Term in Figure 4.

    Plume transport:

    • • Constant unidirectional flow velocity (magnitude and direction), assess a few cases representative of the local conditions (residual tidal current, residual wind-induced seasonal current). No wave conditions. Simplified hydrodynamics.

    Typical result:

    TASS simulation results to indicate potential transport of suspended sediment.

    • One/two-dimensional representation of near- and far-field suspended sediment plume (see Figure 4) as a function of space (e.g. between a sensitive receptor site and a dredging operation) and/or time (e.g. at a sensitive receptor site) for a source due to a dredging activity (continuous or intermittent source).
    • Typical examples of TASS output is shown in:
      • Figure 7: 2D view of sediment plume (SSC as a function of space).Typical TASS-result with the colours indicating a sediment plume (red is a high suspended sediment concentration) generated by a dredger (red star) in a channel being dredged (black line). The arrow reflects the direction of the ambient flow and the green stars sensitive receptor sites.
      • Figure 8: Longitudinal sectional view of sediment plume (SSC as a function of space). Typical TASS-result with the suspended sediment concentration as a function of the distance from a dredging activity with a limited duration (e.g. TSHD dredging) at three different moments in time (t1, t2 and t3) clearly showing an exponential decrease (t1) of the sediment concentration when moving away from the source.
      • Figure 9: SSC as a function of time at two different locations (as shown in Figure 7) within the influence zone of the dredging activities (e.g. two sensitive receptor sites) for a dredging activity with a limited duration (e.g. TSHD dredging). The figure clearly reflects the pulse character of the source of the sediment plume.
    • Input conditions may be varied to assess the variability of the plume behaviour, e.g. by assessing various flow velocities and/or flow directions, different dredged material characteristics and time and/or space varying dredging production rates.
    • Sedimentation rates (SED) as a function of time and space can be derived from the material derivative of the suspended sediment content in the water column (hc), but it is also proportional to the product of the settling velocity (w s)and the difference between the concentration (c) and the equilibrium (i.e. background) concentration (c eq) for the prevailing ambient flow.
    • A worst case impact map showing the spatial distribution of the SSC relative to the project site and the sensitive receptors, based on (1) the project site map as discussed for Step 1 and (2) the TASS output. Predicted worst case impacts may be translated into zones of influence, low impact and high impact.

    Verification:

    • Vessel-based point-measurements using an Acoustic Doppler Current Profiler (ADCP) and a Optical Backscatter Sensor (OBS) to verify the extent and intensity of the sediment plume during the execution of a characteristic dredging activity.

    Points of attention:

    • The worst case scenario does not considers extreme conditions, hence does not yield the most probable impacts.
    • Indicate whether the predicted SSC / sedimentation is relative to background levels or absolute.
    • Incorporate existing background stress levels (SSC / sedimentation) of sensitive receptors.

    Step 3: Spatial and temporal scenario assessment

    Objective:
    Realistic (i.e. detailed in time and space) scenario assessment of the turbidity using a high-level area-model. The main questions to be answered by executing Step 3 reads: what is the (quantitative) impact of the proposed dredging activities on the sensitive receptors in the vicinity?

    Who:
    Specialized (hydrodynamics, ecology) consultants and institutes

    Duration:
    Weeks - months

    Tools:

    • Detailed, far-field area models including complex hydrodynamic and morphological processes: e.g. Delft 3D, Mike 21, Telemac, potentially in combination with TASS.
    • Interactive Dredge Planning Tool (IDPT).
    • Important input for these tools concerns an elaborate source terms assessment (e.g. using TASS). To translate the results of these models into sensitive receptor impact a species- and site-specific species response curve is required (Figure 6).

    Information sources:
    Detailed site specific hydrodynamical and morphological data for model-calibration purposes, technical department dredging contractor (input for source terms), scientific papers (empirical parameters).

    Required input (as in Steps 1 and 2, but more detailed):

    Project surroundings:

    • Realistic, complex and time-varying bathymetry

    Sensitive receptors:

    • General sensitive receptor response curve available in public literature

    Project specifications:

    • Detailed bathymetry at dredging area and disposal site
    • Space and time varying characteristics of material to be dredged
    • Space and time varying operations, dredger movements, detailed planning
    • Advanced source term assessment: varying fines release rate in time and space, see Practical Source Term and/or Virtual Release in Figure 4.

    Plume transport:

    • Time and space varying flow velocity to represent tidal (ebb-flood, spring-neap) and/or seasonal conditions, horizontal (2D) and vertical (3D). Inclusion of waves, wave-driven currents, re-suspension, time-varying river output (discharge and SSC).

    Typical result:

    • Quantitative 2- or 3-dimensional representation of the near-field and far-field sediment plume (see Figure 4) as a function of operations that vary in time and space. The assessment may either concern part of a project by investigating the effect of a typical operation (e.g. turbidity levels for a moving TSHD during a full spring-neap cycle), or a complete project by investigating the cumulative effect of multiple operations during the full project period, in order to assess, for instance, the total sedimentation at a specific receptor site.
    • Based on these plume modelling results it is possible to develop safe dredging/disposal maps. These allow to plan dredging activities depending on the hydrodynamic conditions. An example of a ‘Safe Disposal Map’ is shown in Figure 10 (Aarninkhof and Luijendijk, 2009). The dashed lines indicate the boundaries of the placement area beyond which turbidity limits apply. The red line shows the exceedance contour of the dredging plume resulting from placement at the black dot. The arrow reflects the prevailing flow direction. As the red line is to stay within the outer black dotted line, the allowable boundaries of the placement operations can be derived as a function of the tidal phase, as shown by the green-shaded areas in fig. 10.
    • The Interactive Dredge Planning Tool (IDPT); is able to perform a rapid assessment of the expected, initial ecological effects caused by interactively defined dredging operations. For this, the IDT makes use of rapid assessment dredge plume modelling, a database with hydrodynamic and background conditions and a database with ecological information, more specifically locations, species and species response information. The IDPT consists of a map platform (Google Earth), in which dredging operations can be defined in an easy way. Based on the defined dredging operations and selected background conditions, the resulting stresses at the ecological areas are determined into so-called impact zones.
      Figure 11 gives an example of the IDPT user interface with the defined dredging operation (zig-zag white line), maximum suspended sediment concentration footprint (blue coloured areas), and resulting ecological impact assessment (green and orange markers).

    Verification:

    • Continuous fixed-station ADCP/OBS monitoring to verify SSC and sedimentation levels.
    • Frequent vessel-based ADCP/OBS monitoring to verify the extent and intensity of the sediment plume during execution of the dredging activities.
    • Turbidity monitoring within overflow of TSHDs
    • Sampling of water within sediment plume and sediment bed within the influence zone of plume
    • Satellite data to verify the extent (in the upper part of the water column) of the sediment plume during execution of the dredging activities.

    Points of attention:

    • The accuracy of the model input and output should be in accordance with the accuracy of the monitoring data to be acquired during the execution phase of the project in order to verify the predictions.
    • The visualization of model results should correspond with the project / client requirements, e.g. by showing time series of SSC at receptor sites.

    Step 4: Research assessment

    Objective:
    Obtain enhanced insight and understanding of certain aspects of the 2/3 dimensional modelling as discussed for ‘Step 3’, and/or verify specific assumptions made for these simulations by carrying out detailed (in time/space) state-of-the-art model simulations. The main question to be answered by executing Step 4 is: what is the sensitivity of the dredging-induced impact on the sensitive receptors to the Step 3 modelling assumptions and limitations?

    Who:
    Universities (MSc and PhD studies), research institutes.

    Duration:
    Months – years.

    Tools:
    Computational Fluid Dynamics (CFD) models, data analysis, data-model integration.

    Information sources:
    Scientific literature.

    Required input (Input of Step 1, 2 and 3, but more detailed):

    Project surroundings:

    • Detailed and time-varying bathymetry (e.g. as a result of sedimentation, dredging, disposal, etc.).

    Sensitive receptors:

    • Complex sensitive receptor response curves based on long-term site-specific measurements (e.g. 2-5 yrs) of receptor health as function of background conditions. Identification of long term trends, effect of sea level rise, El Niño, etc.

    Project specifications:

    • Detailed bathymetry of part of the project site.
    • Detailed characterization of dredged material at a particular location.
    • Detailed planning of a specific operation: information on the dredging cycle, how long does it takes to fill a hopper, etc.
    • Detailed work method of a specific operation involving information on dredging cycle, filling of hopper, use of green valve, orientation of ship with respect to ambient current, True Source Term (see Figure 4 as well as the section ‘Source Terms’ as part of this Building Block).

    Plume transport:

    • Complex / detailed local wave, current and sediment transport processes (hindered settling, flocculation); within the Dredging Zone (see Figure 3, Figure 4 and Figure 5) this should including ship’s return flow, overflow, propeller wash, wave effects on ship movement and fines release.

    Typical result:

    • Detailed time-dependent 3D eddy-resolving quantitative model results for a specific part of the dredging activities in a particular project, incorporating complex hydrodynamic and/or morphological processes. This may e.g. concern the filling-process of the hopper of a TSHD, detailed modelling of the complex dynamics in the Dredging Zone or the near-field plume (see Figure 3, Figure 4 and Figure 5), the effect of propeller wash and/or the location of the overflow at the hull of a TSHD on the generated sediment plume, etc.
    • An example of a CFD modelling result is shown in Figure 12 (de Wit, 2010). This study focused on the assessment of the source term and near-field dredging plume for a TSHD. A 3D eddy-resolving multiphase sand-mud-air-water CFD model was developed to predict the complex near-field behaviour of the sand-mud-air-water mixture flowing through the overflow of a TSHD into the water column.

    Points of attention:

    • This Step 4 assessment requires long calculation times and significant efforts and generally exceeds the typical level of detail required for environmental impact studies. Therefore, these detailed assessments cannot be used for environmental impact assessments on a full project scale, but only on a (rather small) part of a large project, or in a hindcast study to reconstruct what happened during a project or to learn from it for a next project. The necessity for a Step 4 assessment needs to be discussed with regulators.
    • Data interpretation involved sometimes difficult decisions, e.g. on interpolation, integration and/or averaging of data.

    Verification:

    • Verification against detailed measurements using state-of-the-art monitoring tools in and around dredging activities. Note that monitoring close to a ship or within a ship’s hopper is complex (dynamic environment, presence of air bubbles, wear and tear in overflow) and may involve personnel safety issues.
    • Verification against measured data taken under controlled conditions in scaled laboratory tests.

    Practical Applications

    A systematic, well structured and in-depth assessment of dredging induced-turbidity has been and is being applied in many projects around the world. Depending on site-specific conditions differences in approach will develop. In the next sections few examples are given of projects that have documented (part of) the process.

    Cape Lambert - Australia

    (after linked documents)
    The Cape Lambert Port B Development involves the construction of new ore handling, processing and export facilities adjacent to the existing Cape Lambert operations. The Port B development includes both marine and onshore works which will increase the capacity of the existing port. Up to 16 million cubic metres (Mm3) of material will be dredged and relocated to offshore deposition areas.

    The development will result in direct and potentially indirect impacts to benthic habitats. Direct seabed disturbance and habitat removal will result from the following activities:

    • cutter suction (CSD) and trailer hopper dredging (TSHD);
    • construction of the wharf; and
    • disposal of dredged material in the deposition areas.

    The direct removal of seabed can result in the following potential impacts:

    • loss of benthic habitat and mortality of supported species and benthic assemblages;
    • subsequent indirect effects to higher species such as marine turtles and dugong through dredging-induced loss of habitat; and
    • reduction in species diversity and abundance (such as fish and sessile benthic infauna and epifauna).
      Indirect impacts to marine fauna and flora from dredging and spoil disposal relate to increases in background turbidity and increased sediment deposition from the dredging activity. Increased levels of turbidity can impact corals, seagrasses and algae by reducing the amount of light available for photosynthesis. Settling of particulate material onto the benthos and can abrade or smother sessile organisms.

    disposal management plan for the dredged material has been prepared and has been subject to a public environmental review process The project has now been completed and no serious turbidity-induced environmental impacts were observed.

    Øresund Fixed Link – Denmark/Sweden

    The Øresund Fixed Link, connecting Denmark – Copenhagen with Sweden - Malmö was designed and constructed between 1991 and 2000. The link consists of three parts; going from Denmark towards Sweden first a four km immersed tube tunnel, a four km long artificial island and an eight km double deck bridge.
    Dredging works for the tunnel trench and the island were executed within a vulnerable hydrodynamic and ecologic environment. Extensive studies were made during planning and design stage, including turbidity modeling and impact assessment, leading to a spill budget system to be adhered to during the execution of the works. Strict environmental monitoring was applied to verify compliance. This novel spill-budget approach proved instrumental in achieving the environmental targets set by the Danish and Swedish regulators.

    This Øresund project with its well founded environmental control set the tone for many projects that followed. (see also Case Øresund Fixed Link).

    Fehmarn Belt –Denmark/Germany

    The Fehmarn (or Femern) Belt is a strait between Germany and Denmark,. At its narrowest this area is approximately 18-km (10 nmi) wide, with depths of 20–30 m. 
    The Danish and German governments agreed on 29 June 2007 to build a fixed link by an immersed tube tunnel, intended to open in 2020.

    The Danish "Vurdering af Virkninger på Miljøet" (VVM) has prepared and Environmental Impact Assessment, in order to identify and assess environmental impacts and consequences that can be expected in connection with a construction project. It also considers how any negative impacts on the environment can be avoided or reduced.

    Part of this assessment is an extensive evaluation of the dredging-induced turbidity. Initially such studies were made as part of the comparison between the considered bridge- and tunnel options. Later, more detailed assessments were made for the tunnel only alternativre, in support of the permit application. Operational turbidity threshold have again been set in the form of ‘spill budget’.

    During construction, intensive monitoring will be applied, to verify compliance to the spill budget. But Adaptive Management principles are intended to be applied should monitoring demonstrate significantly different conditions than modeled and anticipated.

    Voh - New Caledonia

    (after linked document)
    In the Voh region of New Caledonia, Koniambo Nickel SAS has been constructing a new port complex to support a nickel processing plant. The port construction involved dredging of a 4,500 m long navigation channel ending in a turning basin. The marine environment around New Caledonia is renowned for its aquatic fauna and flora. This has been confirmed by enlisting the lagoons of New Caledonia as UNESCO World Heritage. Therefore, specific thresholds and required management actions have been outlined in an Environmental Management Plan (EMP).

    To check the EMP requirements, extensive monitoring campaigns have been carried out by different independent parties. During construction turbidity levels were continuously monitored at selected locations around the dredging area, both from fixed and mobile stations.

    During the entire project, set turbidity threshold values were never exceeded as a result of dredging activities in combination with different mitigation measures, which were implemented prior to and during the works to minimise the environmental impact. Also other ecological measurements and observations showed no impact by the increase of turbidity or by excessive sedimentation.

    The dredging works in New Caledonia proved again that good environmental management, based on both a pro-active and re-active approach, supported by proper pre-project assessments, can prevent undesired consequences.

    Singapore

    (after linked document)
    In Singapore, traditional methods for environmental management of marine reclamation works close to sensitive habitats have generally not provided the level of control necessary to ensure preservation of these habitats. Obtaining the level of control necessary to assure authorities and non-governmental organisations (NGOs) of compliance with environmental quality objectives requires quantifiable compliance targets covering multiple temporal and spatial scales. 
    Of equal importance to allow feedback of monitoring results into compliance targets and work methods are effective and rapid response mechanisms. Environmental Monitoring and Management Plans (EMMP), based upon such feedback principles, enable such control.

    In Singapore, methods based on refined sediment plume models are utilised to quantify compliance with daily spill budget targets and how such targets and compliances are assessed. These spill budgets take into account specific habitat tolerance limits for varying magnitudes and durations of sediment loading. Refinements were undertaken to hindcast impacts from the contractors’ complex reclamation schedules, and for segregation of impacts and assessment of cumulative impacts. The system also includes updating of tolerance limits and confirmation of spill budgets via targeted habitat monitoring.

    To date, the EMMPs have been able to document compliance of the works to all pre-project environmental quality objectives at a level of reliability that cannot be refuted by third parties. This has minimised the developers’ and contractors’ exposure to public complaints and liabilities associated with environmental impacts. The EMMPs have thus allowed the reclamation activities to proceed in an efficient manner, whilst ensuring protection of the environment.

    References

    Literature

    Books

    • Dredging and Reclamation; The Øresund Technical Publications, Øresundsbro Konsortiet. December 2001. Hardcover, 354 pp., colour illustrated. Niels J. Gimsing and Claus Iversen (editors)
    • From Salt to Peber - The Art of Dredging & Reclamation; Øresund Marine Joint Venture. December 2001. Hardcover, 80 pp. Full color. Photographs. Text: Kyle Johnson, Steen Bendtsen, Jens Borresen and Charles Knapp. Editors: Lars Carlsen, Ulla Bjerregaard and Charles Knapp

    Both books are available from:
    Øresundsbro Konsortiet
    Vesper Søgade 10
    DK-1601 Copenhagen V, Denmark
    tel. +45 33 41 60 00
    fax +45 33 41 61 02
    www.oresundsbron.dk

    Internet

    Abbreviations

    ADCP
    BHD
    CFD
    CSD
    EDD
    GD
    OBS
    SSC
    TASS
    TSHD

    Acoustic Doppler Current Profiler
    BackHoe Dredger
    Computational Fluid Dynamics
    Cutter Suction Dredger
    Ecodynamic Development & Design
    Grab Dredger
    Optical Backscatter Sensor
    Suspended Sediment Concentration
    Turbidity ASsessment Software
    Trailer Suction Hopper Dredger

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