MOHID Water Modelling System 

MOHID Water

Particle tracking transport

Lagrangian transport models are very useful to simulate localized processes with sharp gradients:

  • Submarine outfalls;
  • Phytoplankton blooms;
  • Sediment erosion due to dredging works;
  • Hydrodynamic calibration;
  • Oil dispersion;
  • Hazardous and Noxious Substances (HNS);
  • Fish larvae dispersion;
  • Exchanges between different areas in a estuary;
  • Exchanges between the deep ocean and the continental shelf;
  • Etc...

MOHID Lagrangian module uses the concept of tracer. The most important property of a tracer is its position (x,y,z). For a physicist a tracer can be a water mass, for a geologist it can be a sediment particle or a group of sediment particles and for a chemist it can be a molecule or a group of molecules.

A biologist can spot phytoplankton cells in a tracer (at the bottom of the food chain) as well as a shark (at the top of the food chain), which means that a model of this kind can simulate a wide spectrum of processes.

The movement of the tracers can be influenced by the velocity field from the hydrodynamic module, by the wind from the surface module, by the spreading velocity from oil dispersion module and by random velocity.

At the current stage the model is able to simulate oil dispersion, water quality evolution and sediment transport.




To simulate oil dispersion the Lagrangian module interacts with the oil dispersion module, to simulate the water quality evolution the Lagrangian module uses the feature of the water quality module. Sediment transport can be associated directly to the tracers using the concept of settling velocity.

Another feature of the Lagrangian transport model is the ability to calculate residence times. This can be very useful when studying the exchange of water masses in bays or estuaries.

Numerical Characteristics


Main characteristics

Spatial InterpolationBilinear horizontally and linear vertically
Particle movementParticle movement can the sum of the follow effects:
  • Flow velocity;
  • Flow turbulence;
  • Fraction of the wind velocity (floating particles);
  • Stokes drift (wave effect);
  • Oil spreading;
  • Density gradient between the particle and the water (positive gradient – sediments, negative gradient – oil droplets).
Flow turbulence Random walk based in the turbulent mixing length (constant or turbulence model) and turbulent velocity standard deviation (Allen, 1982a)
Land mapping
  • Grid or set of nested grids with land cells;
  • Set of polygons.
Flow and water properties input
  • One grid – default;
  • Set of nesting grids – default in nesting models;
  • Set of met-ocean solutions for each grid interpolated for each particle in rum time.
Particles emission

Set of origins

In each origin particles can be emitted:

  • Point;
  • Polygon;
  • Accident (oil spill – point where an initial spatial distribution is assumed);
  • Along track;
  • Cloud of points.
Particles monitoring
  • Time spent inside a set of polygons by particles emitted in a specific origin;
  • Time spent to reach a polygon boundary;
  • Beached or not;
  • Deposited or not.
Output

  • Lagrangian – X,Y,Z and n properties;
  • Eulerian – integration for the default grid (or grids when a nested approach is followed).

0D coupling

Faecal indicator mortality
  • Imposed;
  • Function of salinity, temperature and radiation in the water column.
Sediments

Sedimentation velocity imposed or function of D50

Bottom erosion and deposition critical stress

Water qualityWater quality modules of MOHID
Oil spill
  • Oil transport in water column, sedimentation and beaching;
  • Oil processes as evaporation, dispersion, entrainment, sedimentation, dissolution, emulsification and dispersion;
  • Eulerian concentration output.
Hazardous and Noxious Substances (HNS)
  • HNS transport in air of the evaporated part and in water including sedimentation processes;
  • HNS processes as evaporation, volatilization, entrainment, dissolution and degradation;
  • Eulerian concentration output.
Buoyancy jet model or MOHID Jet
  • Integral model aims focus in the near field dispersion of outfalls jets;
  • Near field trajectory assuming a volume variation of a volume in a Lagrangian referential with a cylindrical geometry;
  • Far field simulated by the particle tracking model;
  • Velocity: buoyancy vs drag;
  • Volume: shear + drag entrainment;
  • More details in the MOHIDJET Technical Manualb.
Renewal and residence time
  • Computation of particle number and times inside a polygon (i.e. estuary) from several origins;
  • Renewal times obtained when only a small percentage of particles remain in the polygon (i.e. estuary);
  • Seasonal renewal times with different conditions (e.g. tide, river inflow).
Fish larvae
  • Vertical migration implemented with positive and negative relation to light;
  • User defined radiation limit;
  • Minimum and maximum larvae depth;
  • Larvae velocity defined by the used or using the migration time;
  • More details in the MOHID Fish larvae Manualc.

References

aAllen C.M. Numerical simulation of contaminant dispersion in estuary flows. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 1982; 381(1780): 179-194. doi: 10.1098/rspa.1982.0064

bMOHIDJET Technical Manual (2003). Available for download here

cMOHID Fish larvae manual (2012). Available for download here

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