Building with Nature BwN Guideline Environments Project phases Governance BwN Knowledge base
BwN Building Blocks BwN Toolbox Pilots and cases BwN Knowledge

Log in

The TASS-model (Turbidity ASsessment Software) aims to model the development of dredging-induced turbidity plumes arising from Trailing Suction Hopper Dredgers (TSHD). The model enables the user to predict sediment release rates during dredging activities and thereby helps determine specific effects of dredging projects. The tool can be used specifically for development of turbidity caused by the discharge of sediment-laden water through the overflow of the TSHD. A first order assessment can be made by in-experienced users, but more detailed model usage requires expert knowledge on dredging methods,TSHDs and soil characteristics.

    General Tool Description

    TASS aims to provide a tool to calculate the overflow losses from hoppers and the residual impact of dredging operations on turbidity levels.

    History

    The project to develop a software package on turbidity generation from dredging was initiated in 1998 by Rijkswaterstaat, VBKO and the Dutch dredging contractors, after substantial field data was collected since 1984. Since then, a number of measurement campaigns have been carried out in order to gain more insight into the development of dredging-induced turbidity plumes and to collect measuremed data to calibrate the model. Trial campaigns were performed with a grab dredger (2001) and with a trailing suction hopper dredger (2002). After evaluation of these campaigns, it was decided to focus only on the modelling of dredging-induced turbidity generated by trailing suction hopper dredgers (TSHDs). Measuring campaigns to collect calibration data were carried out at several location under various conditions in the North Sea (Bremerhafen 2006, Hook of Holland 2007 and Den Helder 2007). The validation report based on these measurements can be found here.

    A first version of TASS has been delivered in 2010. Since then it has been validated with measurements in clear tropical water in North-West Australia in July 2011, (see validation report). The latest version of the TASS model (v4.0) was delivered by the end of April 2013.

    Model description

    The TASS model is a tool to compute turbidity plumes arising from dredging by Trailer Suction Hopper Dredgers (TSHDs). Such plumes are caused by the discharge of sediment-laden water from the hopper (usually through the hull or overflow of the dredger) which descends towards the bed in a so-called dynamic plume. A certain part of the dynamic plume goes into suspension and is transported with the flow: the passive plume. It is this passive plume which is responsible for increased turbidity levels in the far field. Other sources which add to the passive plume are disturbance by the drag-head and suspension by propeller wash (see Figure 1). Further information about the generation of turbidity in dredging operations can be found in the building block: assessment of dredging induced turbidity

    The TASS model consists of three sub-modules:

    1. Overflow / trailer process module represents the hopper as a horizontally averaged 1dv model. It predicts the settling behaviour in the hopper and the overflow losses. The overflow losses are a flux and the particle size distribution of sediment in the overflow from a TSHD, as well as the sediment release rate resulting from the propeller jet.
    2. Dynamic plume module uses a Lagrangian integral modelling technique whereby a thin disc of released dyanmic plume is tracked as it moves downward under the forces of momentum and negative bouyancy. It predicts the spatial distribution, particle size distribution and mixing of sediment in the dynamic plume, as well as the source concentration for the passive plume. At present TASS does not predict the % of re-entraiment of sediment from the density current caused by the dynamic plume (the so-called surface plume, see figure 1), but leaves this to be defined by the user.
    3. The passive plume module is a relative simple model bassed on the application of the 2D analytical solution for the dispersion of slug of material released into an environment of homogenous and time invariant current velocity and depth. The simplified model calculates the expected dredging-induced suspended sediment concentrations at one user-defined location. It using the contributions from the dynamic plume, surface plume, and propeller scour as inputs to the simulation for the far-field plume dispersion. At present the contribution of the disturbance by the draghead is not included in the TASS model, as measurements showed this contribution to be an order of magnitude smaller than other sources.

    The results of one module as used as input to the next as shown in Figure 2.

    The underlying formulas and assumptions can be found in the TASS user guide.

    Usage skills

    The model requires extensive input, hence information and expertise on TSHD characteristics, soil characteristics and dredging production. To make the model easier to use for less specialized users, several sets of default values are available in the user interface. A user can choose, for instance, between a small, medium, large or jumbo vessel and the software automatically fills in the vessel dependent input (see figure 3). Clearly, the user may overwrite these values.

    BwN interest

    This tool helps to determine the fate of overflow losses from a TSHD in detail. This is valuable information when operating in environmentally sensitive areas where dredging-induced turbidity needs to be predicted as accuratly as possible. Also, operational scenarios may be analysed with this tool, for effects on turbidity. The tool is applicable in different phases of a dredging project with TSHDs:

    • In the Initiation phase a first order assessment can be made to assess release rates.
    • In the Planning and Design phase this tool can be used to assess the overflow losses in more detail.
    • In the Construction phase, the model can be used to support dredging operations, so as to minimise thier environmental impact.
    • After realisation of the project, the tool can be used for post-project analysis of environmental changes, if necessary in court cases.

    How to Use

    Requirements

    The software is suitable for installation and operation on a PC having the following minimum specification:

    • An IBM or compatible Pentium based PC with a VGA monitor;
    • Windows VISTA, Windows XP, Windows 7 or Windows 8 operating systems;
    • 64 Mb RAM.

    A first-order assessment can quite easily be made using the available scenarios and default values in the model. For more specific assessments, including specific dredging vessels and soil characteristics, dredging and soil expertise is necessary to get the right input.

    Phased plan process

    The Tass model involves running the three modules (overflow, dynamic plume and passive plume module) consecutively. For each module a seperate input file should be created. After running of each module output is generated and visualised. Therefore one can also use only one of the modules.

    Required input

    The input file for the overflow module requires the most extensive input, concerning the vessel characteristics, the site characteristics and operational input parameters.

    • TSHD characteristics:
      • Size (length, width, depth) of hopper;
      • Tonnage of TSHD;
      • Volume of hopper;
      • Suction pipe diameter;
      • Draft;
      • Weir / overflow diameter; and
      • In case propeller scour is included: main engine and bow thrusters properties.
    • Site characteristics:
      • Hydrodynamic conditions such as water depth and current speed. The latter is only required if propeller scour is included; and
      • Soil conditions, such as the grain size distribution.
    • Operational characteristics:
      • Vessel speed;
      • Length of dredging track;
      • Cycle time;
      • Production rate;
      • Density and velocity of slurry upon dredging; and
      • Overflow (time) restrictions, if any.

    The dynamic plume module requires the results file from the overflow module (.in file) and additional information on the current speed in the direction of (U current) and prependiculat to (V current) the dredging path.

    Finally the passive plume requires the results file from the dynamic plume module and input regarding the point of interest (e.g. the location of a sensitive receptor) relative to the dredge track. The passive plume module will calculate the depth-averaged suspended sediment concentration at this point of interest as a function of time. The user needs to define the distance of this point relative to the dredge track and the offset with respect to the dredge track. Another value that needs to be specified in the passive plume module is the initial width of the surface plume, as a first guess the vessels width may be used for this parameter.

    The current and vessel speeds and directions are both assumed to be constant during a simulation. If a simulation with a time-dependent current speed is needed, one may use the TASS outcomes as input to a more sophisticated hydrodynamic and sediment transport model to describe the spreading of the passive plume.

    For each module the input is saved in a steering file (.str file).

    Output

    Each TASS module generates a set of output files, of which one can be used as input file for the subsequent module. In addition to these output files per module, all modules generate a CSV-file (ASCII file), which is a standardized data file allowing for subsequent use for modelling (e.g. MIKE21, Delft3D, Telemac, etc.) or visualisation (Excel, OpenEarth, etc.).

    • The Overflow module generates:
      • .DRG-file – user-friendly format allowing for data analysis and verification of the simulation.
      • .IN-file - gives the results of the simulation and can be used as input for the dynamic plume module.
    • The Dynamic plume module generates:
      • .RES-file – results file and input file for subsequent TASS module;
      • .LOG-file - log of the simulation, giving the dimensions and the location of the dynamic plume for each timestep.
    • The Passive plume module generates:
      • .OUT-file - results file of the passive plume module.

    Upon completion of a module run a standard plot of the output appears. For the overflow module this includes the overflow loss rates (kg/s) as a function of time for each user-defined particle size, see Figure 4. The dynamic plume module generates two plots. One is the release rate for each type of release (Figure 5), such as the rate going into the surface plume, the propeller scour etc. The other plot from the dynamic plume shows the total mass (kg) for each item (Figure 6). The passive plume module generates two plots: the concentration (mg/l) in time at the point of interest (Figure 7) and an overview of the dredge track, the location of the point of interest relative to the dredge track and the direction of the flow velocity.

    More information on how to use the TASS model and the underlying methods and assumptions can be found in the User guide TASS v4.0 TASS 4.0 User Guide R_2-0.pdf

    Practical Applications

    General application of TASS

    The TASS model can be used for different purposes, examples of TASS applications are:

    • Rapid worst case assessments by non-dredging experts;
    • Describing specific TSHD processes in more detail for research purposes;
    • Assessment of specific working methods;
    • Ex-Ante analysis of specific dredging processes (use of single module);
    • Online support of dredging operations in order to minimise environmental impacts;
    • Ex-post analysis of ecological degradation in area where dredging works have been carried out.

    Example files can be found in the TASS manual.

    The table below suggests when TASS can be used (based on the BwN guideline's phases and steps):

    Phases
    Steps

    Initiation

    Planning & design

    Construction

    Operation & maintenance

    type of application

    Understand system

     

     

     

     

     

    Identify Alternatives

    TASS

    TASS

    TASS

    TASS

    estimation of effect of specific work method, e.g. size of TSHD

    Valuate and pre-select

     

    TASS

    TASS

    TASS

    valuation of environmental effect of work method

    Embed in project

     

     

    TASS

     

    in case of adaptive monitoring:
    tool to predict effect of specific work methods in order to adjust planning of operations

    Prepare next phase

     

     

     

     

     

    Interactive Dredge Planning Tool Singapore

    TASS can also be used in combination with other, more complex models, such as Delft3D, Mike21 or Telemac in order to determine the spreading of the passive plume. This has been applied for the showcase of the Interactive Dredge Planning Tool Singapore (IDPT), which is based on Delft3D-Part. Here the TASS model has been applied to determine the dredging related sediment release rate. Based on the outcomes of the TASS model (overflow module and dynamic plume module) the sediment release rates which add to the passive plume are determined and used as input for the IDPT. In the present experimental case, these values were manually inserted in the interface of the IDPT, but this can of course be automated if this combination will be used more often.

    References

    Back to Top

    PDF