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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 IDPT makes use of rapid assessment dredge plume modelling, a database with computed hydrodynamic background conditions and a database with ecological information, i.e. locations, species and species tolerance information.

    General Tool Description

    Context, purpose and results

    Dredging and other human-induced increases in turbidity levels and sedimentation rates may impact coastal ecosystems such as coral reefs and seagrass meadows. The effect of this increased turbidity and sedimentation on the ecosystems is site and species specific, but has been addressed for certain species in Singapore in different projects within the Building with Nature program.

    Dredging operations can be optimised in terms of environmental impact by assessing these effects before commencement of the work. In particular during the tender phase of a project, however, time hardly allows for a detailed (modelling) study to assess the environmental effects of different work methods. The Interactive Dredge Planning Tool (IDPT) enables the user to make a rapid quantitative assessment including the newly developed knowledge on ecological response to (dredging-related) sediment stresses. The IDPT is developed based on the MapTable concept.

    The Interactive Dredge Planning Tool enables (initial) assessments of the spreading of dredging-induced turbidity plumes and their environmental effects. The results of this assessment are clearly visualised, along with other project-relevant data, on an interactive map which be used for communication with stakeholders and other interested parties. The tool in its present form has been developed for Singapore, but can serve as an example for applications elsewhere.

    In addition, the IDPT can create so-called dredging suitability maps, which indicate the advised maximum turbidity production given a specific maximum allowable ecological effect. Thus it facilitates the transition from the current emission-based practice to a more impact-based approach.

    Usage skills

    This tool is easy to use for rapid assessments of the expected direct environmental effects of dredging operations. The user interface is easy to use and enables defining and evaluating different dredging scenarios. Clearly, the proper interpretation of results requires a certain level of knowledge on dredging operations, ecology and the models and databases underlying the tool.

    BwN interest

    The IDPT consolidates knowledge and tools related to dredging-induced turbidity and ecological effects into a (prototype) rapid-assessment tool for the effects of dredging on ecology. By making BwN knowledge available to practice, it contributes to spreading the Building with Nature philosophy on ecosystem-based norms and rules.

    The tool is especially useful for projects in ecologically sensitive areas where hydraulic engineering works are planned or executed and where different stakeholders are involved in the project. It can serve as a means to communicate with stakeholders about the effects to be expected and to involve them in the project process.

    How to Use

     

    Requirements

    The IDPT is available in several forms:

    1. as a stand-alone tool locally installed on the user's PC, in order to be used for the planning of dredging operations, for instance;
    2. as a web-based tool, to be used for demonstration and communication purposes.

    For the first option, installation of Python, Pylons, the Matlab wrapper and Matlab is required. A tutorial of installing these components can be found here. The open-source code of the IDPT is included in the OpenEarth tools repository and can be downloaded from there.

    The second option is platform-independent and can be used on any computer with the latest version of Google Earth and either Mozilla or Google Chrome as browser. The Microsoft Internet Explorer browser experiences compatibility issues with regard to the Javascript parts on the web interface and therefore the tool cannot be used in this browser. The user can define a scenario and see the results on the OpenEarth Viewer. The tool itself resides and runs on a remote computer that is in contact with the web page.

    Working principles

    The working principles of the IDPT can be described with the steps of the workflow. The technical workflow diagram in figure 2 shows the relations between the different modules.

    1. User interface
      Users can specify the input in the user interface. The user interface contains a Google Earth plug-in in which the dredging tracks can be drawn and in which the results are visualised.
    2. Matlab backbone
      The Matlab backbone is the basis of the communication between the different modules and between the user interface and the modules.
    3. Dredge input
      The dredge input module translates the user input to input for the dredge plume model. In future, this module can be replaced by Turbidity ASsessment Software (TASS).
    4. Hydrodynamic and suspended sediment background conditions
      The background conditions corresponding to the selected ambient conditions will be obtained from a location-specific database with hydrodynamic and suspended sediment background conditions. This database is used by the dredge plume dispersion module, which should be able to read the database. Because the present used dredge plume dispersion model is Delft3D part, the database should be developed with the software package Delft3D and be able to couple with Delft3D part. More information about the present requirements of the hydrodynamic database can be found in the set-up guideline for this tool. (Tool set-up report) In case a different dredge plume dispersion model is used, different requirements for the database will apply.
    5. Dredge plume dispersion
      The dredge plume dispersion module simulates the sediment plume dispersion under the influence of the prevailing currents. Detailed output is obtained for the locations with a vulnerable ecosystem.
    6. Ecological input
      The ecological input module contains a database with the locations of vulnerable ecosystems and the associated species response curves or other available criteria. The set-up of this database can be found in the set-up guideline of this tool. (Tool set-up report)
    7. Translation into ecological stresses
      This module translates the increased turbidity and sedimentation into ecological stresses, such as light attenuation.
    8. Ecological assessment
      The ecological stresses due to the dredging activities can be compared with the background conditions, if relevant. The expected stresses are translated to effect regimes via the species response curves (SRCs). The result is a map of the ecological effects of the dredging activities.
    9. Presentation module
      The presentation module visualises the results of the dredge plume modelling and the ecological assessment in the Google Earth interface.

    Note that different types of ecological criteria can be part of the IDPT, such as the mentioned SRCs. Also other ecological can be used, or non-ecological ones such as criteria related to territorial waters between countries. Criteria can simply be added to the database and assessment modules described above.

    Functioning of the IDPT prototype version

    This section describes how in general the user can work with the tool, for the stand-alone as well as the web-based version.

    1. The user specifies the required input parameters of the dredging operation (location(s), schedule, sediment characteristics, spill rate, etc.) and selects the pre-computed hydrodynamic ambient conditions scenario (e.g. normal conditions, storm, a particular season, anomalies), which is stored in the hydrodynamic database. The hydrodynamic database in the prototype IDPT consists of the validated Delft3D-FLOW Singapore modelling results. This means that the hydrodynamic model computations do not need to be carried out in the rapid assessment (since the dredge plume does not substantially affect the overall hydrodynamics), but only the sediment dispersion with the Delft3D-PART particle tracking model. Within the hydrodynamic database also other site-specific information such as the bathymetry is included as part of the hydrodynamic modelling input and results. The location(s) of the dredging operation can be defined interactively, e.g. by drawing points or a polygon on the map (white line in the Example screen shot of the IPDT). In the prototype version of the tool, the sediment spill rate and characteristics of the dredging operation are entered manually, see for more information the set-up guideline. (Tool set-up report) For the prototype, the source terms are specified directly, i.e. in S(x,y,z,t) (kg/s). A more detailed coupling with a dredging operations model (e.g. (TASS model)) can be developed at a later stage to derive these source terms more comprehensively.
    2. When the required input has been given, the sediment plume model embedded in the tool rapidly assesses the sediment plume dispersion based on the ambient hydrodynamic conditions. In the present version, the dredge plume dispersion module is based on the Delft3D-PART code, which is made available freely for use in this tool, but it can be replaced relatively easily by another dispersion model. Based on the sediment plume dispersion the expected stresses (suspended sediment concentration, resulting in light attenuation and sedimentation) are determined at the locations with sensitive ecosystems in the form of ‘time series’ at these locations. Furthermore, the ‘footprint’ of the dredge plume is presented for further reference (see figure 1).
    3. The locations of the ecologically sensitive areas and species and the associated Species Response Curves for Seagrass and coral (Tool set-up report) are included in a database in the tool (XML format, so it can easily be extended and edited). To be able to use the SRC in practical applications with the tool, it is mapped onto a discrete number of effect regimes, distinguishing different time scales and intensity levels. In the current version of the tool the ecological assessment of Van Doorn-Groen (2007) is used (see table below). This assessment method classifies effects on the basis of the excess stress above background conditions in (mg/l) and the duration of this stress in percentage of time. In the ecological assessment module of the tool, this assessment method indicates the ecologically sensitive areas and species. They are linked to the effect regimes to enable assessment in terms of ‘no effect’, a ‘sub-lethal’ or a ‘mortal effect’ for the different species, given the excess stress and its duration (also see figure 3 for a schematic presentation of the mapping of model results and species response curves to effect regimes).
    4. The expected effects on the ecologically sensitive areas and species will show up in green (ok), orange (sub-lethal) or red (mortal) on the map (also see the IDPT screen shot). For each of the timescales at which an effect is expected, more information (and possibly monitoring advice) is provided by clicking on the highlighted areas (markers) in the map.

    Severity

    Definition (excess concentration) (according to Van Doorn - Groen 2007)

    Effect regimes IDPT

    No Impact

    Excess Suspended Sediment Concentration > 5 mg/l for less than 5% of the time

    level 1 - no effect (green)

    Slight Impact

    Excess Suspended Sediment Concentration > 5 mg/l for less than 20% of the time
    Excess Suspended Sediment Concentration > 10 mg/l for less than 5% of the time

    level 1 - no effect (green)

    Minor Impact

    Excess Suspended Sediment Concentration > 5 mg/l for more than 20% of the time
    Excess Suspended Sediment Concentration > 10 mg/l for less than 20% of the time

    level 2 - sub-lethal effect (orange)

    Moderate Impact

    Excess Suspended Sediment Concentration > 10 mg/l for more than 20% of the time
    Excess Suspended Sediment Concentration > 25 mg/l for more than 5% of the time

    level 2 - sub-lethal effect (orange)

    Major Impact

    Excess Suspended Sediment Concentration > 25 mg/l for more than 20% of the time
    Excess Suspended Sediment Concentration > 100 mg/l for more than 1% of the time

    level 3 - mortality (red)

    Developing a case with the IDPT

    The set-up of the IDPT is generic, such that this software can be used for any site or location. In the current research program of Building with Nature the tool has been applied to the Singapore coastal waters. Application at another location requires site-specific data, like:

    • adaptation of the tool settings file, which includes settings regarding the geographical location of the tool application, references to the background conditions, model parameters, such as the dispersion coefficient, etc.
    • an ecological database with species response curves and locations of vulnerable ecosystems or species;
    • a database with ambient hydrodynamic conditions. These hydrodynamic data are in the present implementation based on Delft3D-FLOW modelling results. These hydrodynamic conditions can be prepared by running a Delft3D-FLOW model with the on-line WAQ coupling activated. For convenience, a switch to activate this option has been implemented in the Delft Dashboard tool, which could be used to set up an initial Delft3D-FLOW hydrodynamic model. For other use and more details, reference is made to the Delft3D-FLOW User Manual;
    • a database with background sediment conditions (optional, if ecological criteria are used which include this).

    The IDPT-code is easily adaptable by experienced Matlab users, because of its modular set-up. If desired, modules can be replaced or added to the workflow and the behaviour and appearance of the tool can be changed. This flexibility is used, for instance, when making dredging suitability maps (based on a maximum specified effect) instead of assessing the effects of a specific dredging operation

    Advice and recommendations

    Limitations

    The IDPT is intended for rapid assessment of the effects of dredging activities on vulnerable coastal ecosystems. The first-order estimate of ecological effects it produces cannot replace more advanced analyses or detailed modelling. The usefulness of the results depends, among others, on the accuracy of the ecological database, the hydrodynamic and sediment background conditions and the dredge plume dispersion model. This means that the computed ecological effects need to be considered carefully, taking into account the possible uncertainty ranges associated with the complex nature of the chain of operations leading to the estimated ecological effects. Expert interpretation and explanation are essential for proper use.

    Tips and Tricks

    • The stand-alone version of the IDPT also contains scenario management. This means that different scenarios (e.g. dredging operations/specifications, background conditions, etc.) can be prepared and run. The IDPT allows for easy switching between (prepared) cases so that the impact of certain measures (e.g. different dredging strategy) can easily be demonstrated. The cases can also be stored and reloaded for later use, e.g. in a meeting with a client or stakeholders to discuss the work method and its implications.
    • The stand-alone version of the IDPT allows the user to change the current settings (XML) file to adapt e.g. the simulation settings from the File menu. To activate these adapted settings, 'Reload settings' need to be selected also from the File menu. The settings used for a certain case are stored in the case file for consistency and reference, but only after (re-)running the case. Reloading the settings does not affect the completed cases.
    • The dredging input pop-up screen contains a table for the different points in space of the dredging operation. The lines in this table can be moved up and down, as well as copied and deleted, to customise the envisaged dredging operation to the user's needs without having to click each point in the Google Map. The dredging schedule can also be saved and loaded, so that even an off-line specification of the dredge track (e.g. in a text editor) can be used (possibly convenient if an actual dredging log file is used).

    SWOT analysis

    These lessons are derived from a workshop session on 22 December 2011 involving Ecoshape Partners. The overview has been updated on 1 March 2012 and 21 June 2012.

    Strengths

    Weaknesses

    • Applicable to full cycle from source to effect
    • Tool usable as Operation-based AND Impact-based (Reverse cycle)
    • Rapid-assessment
    • Easy visualization for communication purposes
    • Generic method
    • Predictions of impact
    • Modular and flexible set-up
    • The graphical output is understandable to a lay audience
    • Tailor-made (site-specific databases)
    • Insights into the ecosystem and its response
    • Limited accuracy (rapid-assessment tool)
    • Difficult to validate full cycle
    • SRC not completely generic (location & species specific)
    • Time consuming to collect ecological data
    • For each ‘new’ area a new database with ambient conditions need to be set-up using modelling

    Opportunities

    Threats

    • Standard method/approach (generic)
    • Optimisation of the monitoring program (for adaptive management)
    • Helps to create awareness
    • Stimulates sharing knowledge
    • Interactive
    • Can be used to make the process operational and transparent
    • Enables active stakeholder participation
    • Provides information about ecosystems
    • Inclusion of different types of criteria, e.g. ecological, institutional, operational, industrial, etc.
    • Garbage in = garbage out (use any model with care)
    • Uncertainties in the far-field spreading of plumes
    • Uncertainties in physical parameters, such as the sediment characteristics
    • Uncertainty propagation in the computational process
    • Other stressors (natural or human-induced)
    • A tool this comprehensive may create false expectations
    • Difficult to determine source term during dredging operation

    Lessons learned

    Lessons Learned development process
    • Keep stakeholders pro-actively informed, also if these stakeholders are not pro-actively involved to keep monitoring their requirements and if the product (i.e. IDPT tool) still meets these requirements.
    • The use of a modular software design in a programming language that many development team members are familiar with, hugely expedites the development process, since modules can be developed in parallel.
    • Making individual team members responsible for their own module development stimulates the involvement and motivation of the team members
    Lessons Learned user experience
    • Based on the comments gained during the Building with Nature conference, the attendees commented that they were impressed by the appearance and the possibilities of the prototype tool and found this development very interesting. Also the suitability map mode was found to be useful.
    • In particular from a communication perspective, the tool is considered to be potentially very useful and powerful, specifically for communication with the community (e.g. living close to the project area, public). It was mentioned that the local community can have a very large influence on the progress of a construction project and a tool such as this could improve the communication with them substantially.
    • It was also requested if such tool could be developed in a similar manner for other aspects, such as swimmer safety at the Sand Engine; and this is obviously possible in an efficient way based on the flexible framework that is developed in this project.
    • The attendees quickly realised that the ‘weakest link’ in the tool would be the ecological criteria and assessment of the ecological effects, mainly fed by the ecological response data available (or actually not available).
    • Some attendees had doubts about the applicability of rapid-assessment tools in general. The results of such tools generally provide an indication only, while in many cases more than just an indication is required. These cases require detailed modelling which is not rapid-assessment. Nevertheless, a rapid-assessment tool could also provide insight in the necessity of detailed modelling or other approaches in an early stage of the project design. It is noted however, that this prototype tool development should be regarded as a proof-of-concept and technique which could be used in many ways and for many applications in different dredging project phases and that it could be useful to support these phases in addition to more traditional approaches (and not so much replace other assessments or detailed modelling). In particular, during project execution and monitoring, this tool set-up, and the possibility to easily combine results with measurements in a Google Earth map environment, can be very useful.
    Technical Lessons Learned

    Lessons learned setting up a MapTable tool

    • Setting up a database with unknown data requires a generic approach. Ecological data was not yet available at the start of the project. However, this did not impose any problems, due to the generic approach of the setup of the database.
    • Using separate modules makes the backbone of the MapTable tool and the data transfer more generic. For instance, modules can be replaced in the future without adapting the backbone of the tool.
    • Using separate modules makes it easier for people to work independently on the tool.
    • Using settings files makes the MapTable tool easier adaptable.
    Lessons learned Delft3D-PART - Dredge Plume Dispersion Modelling
    • By using a closed boundary the particles will be reflected. In case of an open boundary the particles will leave the domain and never return. Therefore it is necessary to have a sufficiently large model domain to capture the full relevant extent of the plume;
    • In the area of interest the grid size on which the Delft3D-PART results are projected should be small enough (e.g. <100-200 meter) to obtain a realistic value for the suspended sediment concentrations. It is noted that Delft3D-PART is a langrangian sub-grid model, but in order to derive the sediment concentrations, control volumes (i.e. a grid) is required. This grid can be the Delft3D-FLOW grid (by default) or a separately defined Delft3D-PART ‘zoom grid’;
    • The concentration in an observation point is given as cell averaged of the grid cell it is in. If an observation point is located within the zoom grid, the output concentrations provided are the zoom grid cell averaged concentrations;
    • The time scale of simulations is usually limited to a few weeks, in order to simulate accurately using a large number of particles;
    • The number of particles needed for an accurate solution would be ideally 100 particles in one grid cell, implying a deviation of 1% (99% accuracy);
    • If the bed shear stress at any location is greater than the critical shear stress for erosion, all deposited particles at that location will be returned in the water column instantaneously. This gives a 'sudden' plume which is not desired. Therefore it is advised to exclude erosion by choosing a high threshold for the critical shear stress for erosion;
    • When using a large fall velocity there is an artefact in the module that the particles will 'bounce' at the bed and return in the water column, while they should stay at the bed. To reduce this difficulty all settings should be chosen realistically;
    • Vertical dispersion is around 10^-3 m^2/s for low current velocities and a non-stratified model. In case of high current velocities the vertical dispersion can be increased to 10^-2 m^2/s. This value can be specified in the IDT settings file;
    • Horizontal diffusion increases in about a day from 0.01 m^2/s to 1 m^2/s, depending of the size of the grid. At first the plume is concentrated, later on it will become larger with as consequence more horizontal diffusion.

    Practical Applications

    Introduction IDPT Showcase Singapore

    The objective of the IDPT Showcase is to demonstrate how the Interactive Dredge Planning Tool can be used to assess ecological responses to dredging operations (sediment plumes and sedimentation) in a specific dredging project. A fictitious dredging project in Singapore is described, including dredging scenarios and source term modeling. This showcase yields lessons learned on the tool and shows users how to apply this tool. The ecological results and their reliability are not further assessed and interpreted.

    The showcase presents an initial assessment of the response of sensitive species to turbidity and sedimentation, starting from a basic background hydrodynamic model. This configuration improves with time, analogous to the situation in a real dredging project, where information is initially limited but progressively improves as the project develops.

    Within the Singapore case, the East Coast Park (ECP) has been selected as a location for the fictitious dredging project. Sediment plumes cause by a Trailing Suction Hopper Dredger (TSHD) operation are assessed by using the Turbidity ASsessment Software (TASS). TThe resulting sediment source terms are subsequently used as inpust to IDPT. The results of IDPT give an indication of the response of coral and seagrass ecosystems in the vicinity to dredging-induced suspended sediment and sedimentation.

    The following aspects are considered within this showcase:

    1. Selection of the area of interest.
    2. Selection of dredging and reclamation work method.
    3. Schematisation of dredging operations and site conditions.
    4. Schematisation of the sediment source term with the TASS model.
    5. Hydrodynamic model set up. Study of available background information and data sets (e.g. bathymetry and tidal datasets); extension and upgrading (resolution) of models, etc.
    6. Evaluation of results and lessons learned

    Dredging and reclamation works: Southern Islands region and ECP

    The showcase concerns fictitious land reclamation in East Coast Park (ECP). For this area the BwN innovation program has developed the EDD design pilot ECP. This design was used to define the reclamation project. For the sake of argument, the sand for this reclamation is assumed to be found in the Southern Islands region in Singapore, because this is the area with the most sensitive receptors.

    Reclamation project

    East Coast Park (ECP) is the result of various reclamation projects since the 1970’s on the coast of Katong (Southern Singapore) and extending from Changi to Tanjong Rhu. Today, East Coast Park is the largest park in Singapore, fully built on reclaimed area. Due to wave action, lack of sediment supply, and presumably sea level rise, ECP has been reported to experience coastal erosion, causing beachfront areas to collapse.

    The BwN-design selected for the showcase is the "Lagoon, unprotected corals [1]" variant, the. The purpose of the reclamation project is to create additional land for recreational purposes while favouring the development of local habitats.

    The design involves a seaward extension of the existing land and the construction of a hard substrate to enables coral reef formation (see figure). The hard substrate in shallow water should favour coral colonization; the ridge allows reducing the amount of landfill (perched beach).

    Dredging

    Although in reality landfill material is scarce in the Singapore area, we assume for the sake of argument that the borrow area for landfill material for the ECP is located in the Southern Island region. This area is rich in coral and seagrass and therefore suitable to investigate the effect of dredging plumes on this ecosystem.

    The dredge parameters used are fictitious and not necessarily valid in practice.

    The dredging and reclamation works for this showcase include:

    • Dredging of soil by means of a TSHD;
    • Bunding the reclamation area;
    • Pumping of dredged material into the bunded reclamation area (by means of a discharge pipeline);
    • Control of dewatering discharges using bunded settling basins.

    The required volume of dredged material is 8.5 million cubic meters. It is hereby assumed that all the dredged material is suitable for reclamation purposes. Therefore, the approximate output per dredger of 5.400 m3 in a dredging cycle of 6.2 hours will be 16.200 m3/day, assuming 3 dredging cycles daily. The total time for completing the dredging works will be 8-9 months

    _____________________

    [1]This design has been prepared as part of the multidisciplinary work “Developing an eco-dynamic design for additional land in front of East Coast Park, Singapore” as part of the Building with Nature Singapore project (Innovative coastal protection).

    Source term schematisation

    The source term which is determined here is the source for the far field plume, which may serve as input for dispersion modelling, see for more information Assessment of dredging-induced turbidity. As mentioned before, the borrow area for the ECP reclamation works is fictitious. Correspondingly, the sediment properties are specific for this study and should not be considered as a reference for sediment properties in other studies in this area.

    TASS software was used to model the source term for overflow and dynamic plumes from the TSHD. Initial TSHD dimensioning and operating characteristics, site conditions and in-situ sediment characteristics are required as inputs.
    The TASS Overflow Model computes the overflow sediment loss from the hopper in each time step of the model run and for each sediment fraction. The output is used to compute the dynamic plume, which is used as input (source term) to the IDPT. Parameter settings can be found in the tables alongside, the source term is given in the figure.

    The percentage of fines in the overflow amounts to 10%. In the computation the fine and the course sediment fractions were lumped, respectively, yielding to binary grain-size distribution, with a fine fraction (particle size up to 50 µm) and coarse fraction (particle size from 50 µm to 100 µm).

    The TASS Dynamic Model computes the fate of the (near-field) plume and the rate of fine sediment release from the plume. The model calculates separately the sediment flux from the dynamic plume into suspension near the bed (density current) and in the water column (dilute suspension) and the sedimentation on the bed.

    For the sediment source computation of the IDPT-showcase the flux into the near-bed density current and into the dilute suspension are used. The sediment source term resulting from the TASS model is shown in the diagram.

    Other IDPT input - Dredging scenario

    The figure below schematically shows the dredging operations, between sand sourcing in the area of Southern Islands and the reclamation area near East Coast Park. The dredging scenario consists of five different phases, each with its own input into the IDPT. These five phases are:

    1. Loading of the TSHD until the overflow level is reached.
    2. Loading of the TSDH until maximum permissible load is attained while discharging sediment-laden overflow.
    3. Loaded journey of the TSHD from dredging area to reclamation area.
    4. Discharge operation at the reclamation area.
    5. Sailing back empty to dredging site (Southern Island).

    1 Loading of TSHD until the overflow level is reached

    The overflow level is reached after 27 minutes of dredging. The table shows the input for the IDPT as long as there is no overflow discharge.

    2 Loading of TSHD until maximum permissible load is attained while discharging sediment-laden overflow
    From the moment that the overflow level is exceeded until the maximum TSHD’s tonnage is reached, sediment-laden overflow will be released into the water column. The maximum load is reached 180 minutes later after the overflow level is exceeded (27 minutes from start). The sediment fluxes into the near-bed density current and the dilute suspension in the water column are used to calculate the depth-averaged discharge rate (see table for the IDPT input in this phase).

    3 Laden journey of TSHD to reclamation site (ECP)

    The travelling time from the dredging site to ECP is assumed to take approximately 72 minutes, assuming a laden vessel speed of 13 knots and a vessel speed approaching the coastline of ECP of 3 knots. During this period no overflow is assumed.

    4 Discharge operation

    Discharging the sediment from the TSHD in the reclamation area will take approximately 30 minutes, assuming a discharge rate of 3.18 m3/s.

    5 Sailing back to dredging site

    The travelling time from ECP to the Southern Islands borrowing area is approximately 65 minutes, assuming an average sailing speed of 14 knots.

    Description of Models and Tests

    When incorporating the hydrodynamic forcing and the species response indicators into the model, several factors need to be taken into consideration, such as:

    • the sediment characteristics in the borrow and reclamation areas;
    • the use of a certain bathymetry dataset;
    • the definition of the model domain and resolution (e.g. by selecting an specific grid size and/or grid resolution), and
    • the definition of indicators and criteria of the ecological response.
      These factors are crucial to the accuracy of the IDPT results.

    Three nested hydrodynamic models (different in model domain and resolution, bathymetry, background suspended sediment concentration) were implemented and tested. The largest-scale model was obtained from Deltares; it uses the Singapore Regional Model SRM with detail in the Johor Estuary. The second model and third model were built using Delft Dashboard, which enables generating a model from available on-line data such as bathymetry and tidal datasets. It also facilitates a quick schematisation of the model by using tools to create and modify the model grid, to nest different models or to create domain decompositions. Tidal stations from on-line data bases can also be used to create Time-series of model results can be obtained in preset stations, in order to compare them with on-line tidal data, for instance. This sequence of models is used as a background hydrodynamic model for IDPT to predict the dispersion of the dredging plume.

    Critical factors for the hydrodynamic model configuration are:

    Model domain: important to accurately include the relevant large scale phenomena in the region, such as tidal forcing. For the Singapore region, the model domain needs to include the transition between the dominantly diurnal tidal regime from the South China Sea and the semi-diurnal regime from the Indian Ocean.

    Grid resolution: determined according to the required detail of output data; nesting of models can be used for efficiency reasons.

    Bathymetry: implementation of detailed bathymetry data may increase the accuracy of the simulations, but may also induce additional computational time.

    Tidal forcing: tidal models are available to be incorporated via Delft Dashboard; the tidal stations in these models are used as observation stations.

    Model 1: Singapore Regional Model SRM Grid Johor Estuary

    Model 2. A 2D hydrodynamic model configured in Delft Dashboard. Large grid covering the area of the Malacca and Karimata Straits

    Model 3: A 2D hydrodynamic model, inner fine grid size within prior large model configured in Delft Dashboard.

    Grid size: Outer (black): Curvilinear grid. Average grid size: 3.7 km by 1.8 km Middle (red): Curvilinear grid. Average grid size: 470 m by 450 m Islands (blue): Curvilinear grid. Average grid size: 420 m by 150 m Bathymetry database: Deltares source. Tidal forcing database: Deltares source.

    Grid size: Rectangular grid. 2 km Bathymetry database: GEBCO_08 Tidal forcing database: TPXO 7.2

    Grid size: Outer (blue): Rectangular grid size. 2 km Inner (red):  Rectangular grid. 500 m Bathymetry database: GEBCO_08 Tidal forcing database: TPXO 7.2

    IDPT results

    The three different models presented in 'Models & Tests' are run with the dredging scenario described before. The results of these simulations on the ecology are shown in the figures. Within the IDPT three different ambient hydrodynamic conditions are available: summer period including an East residual current, winter period including a West residual current and the intermediate period where there is no residual current.

    Model 1: Singapore Regional Model SRM. Grid Johor Estuary (Source: Deltares)
    This model computes the large-scale hydrodynamics in three different periods (summer, winter and intermediate). The first scenario, summer, is presented in the top figure. For the three periods, neither of the stressors, light attenuation and sedimentation, has any significant effect on either the coral or the seagrass. For every station the Code green indicates that dredging activities under the given conditions do not increase the concentration of suspended sediments in the water to a dangerous level. At most a slight impact is expected as a result of the dredging-induced turbidity. A maximum value of 0.022 g/l occurs in winter in the southern part near dredge track 2.

    Model 2: Configured in Delft Dashboard; large grid covering the area of the Malacca and Karimata Straits
    This model was setup using Delft Dashboard to simulate a month of TSHD discharges during the summer period. The results shown in the figure are maximum values of suspended sediment concentrations in each grid cell (2 km2). Note that the scale of sediment concentrations was adjusted in order to visualise differences between the values in the area of interest. The red-coloured grids show a maximum concentration of 0.026 mg/l (scale is x10^-5 g/l). These suspended sediment concentrations are too low to have a significant effect on the Southern Islands ecosystem. There is a possibility, however, that this is a model artefact associated with the configuration of the particle tracking component (PART) of Delft3D.

    Model 3: Nested model (Delft Dashboard); inner fine grid covering the Singapore area

    In order to improve the results of the previous models, a nested model was built with detail in the area of the Southern Islands. The grid size was refined by a factor four. At the time of writing, however, this model required further examination of the hydrodynamic boundary conditions derived from Model 2 and the nesting process.

    Analysis of results

    The results of Model 1 (SRM Grid Johor Estuary) show that there is a well-defined control volume in Delft3D-PART for each computational grid. This may be the result of the relatively small grid size of the model at the Southern Islands location, when compared with the large grid of Model 2. Results of IDPT for Model 2 could be improved modifying the amount of available particles to be eroded for each computational grid in Delft3D PART.

    The bathymetry dataset used in Model 1 is unknown, whereas for Model 2 and Model 3 GEBCO_08 was used (with a resolution of 30 arc-seconds). When preparing a hydrodynamic model for IDPT different bathymetric datasets can be used. Depending on model domain and resolution, the results may differ.

    Lessons learned

    • Creating a model grid with Delft Dashboard may seem relatively easy, but it requires a basic understanding of the relevant large-scale processes. In Singapore Strait flow velocities are strongly influenced by non-tidal processes such as local winds and they are sensitive to changes in depth and the presence of embayment or islands. This means that a large model is needed.
    • A model with many grid points produces large result-files which take a long time to be read into and processed in IDPT and PART. It is therefore recommendable to use the nesting function within Delft Dashboard or Delft3D to define a smaller-scale model that includes the hydrodynamics of the larger-scale model. This will reduce the time required to obtain results from IDPT.
    • When computing the sediment source terms with TASS, the model can be used in a dynamic (depth-resolving) or a passive (depth-averaged) mode. The dynamic mode produces produces more conservative estimates of the sediment fluxes into the near-bed density current and the dilute suspension than the passive mode.
    • The passive plume source term is difficult to estimate, because TASS-results are given in therms of depth-averaged SSC and not terms of sediment fluxes. Hence they include the effects of the ambient conditions and the operability of the TSHD.

    References

    Literature

    • Deltares. 2011. User Manual Delft3D-Part, version 2.14.
    • Van Doorn - Groen, Stéphanie M., 2007. 'Environmental monitoring and management of reclamations works close to sensitive habitats'. In: Terra et Aqua, number 108, September 2007.
    • Van Maren, Bas, 2011. ‘Tides and residual flows around Singapore’.
    • Van Rijn, Leo C., 1993. ‘Principles of sediment transport in rivers, estuaries and coastal seas’. Aqua Publications, ISBN: 90-800356-2-9

    Suggestions for further reading about the Interactive Dredging Tool:

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