| Product(s): | OpenFlows FLOOD |
| Version(s): | CONNECT Edition |
| Area: | Modeling |
How are Lagrangian tracer results presented in OpenFlows FLOOD. For instance, when viewing sediment results.
Lagrangian tracer results can be written in Lagrangian or Eularian format. In the Lagrangian format, result files contain the tracers ID, origin, location, their properties, etc. In the Eularian format, only the properties are outputted, and in the form of gridded data using the grid data file used to run the model. This can include the bathymetry or an HDF file containing the gridded results for velocity, temperature, etc.
The model automatically writes the results in these two formats. However, there are two options to compute the values obtained for each grid cell in the Eularian format.
The keyword to specify which option is used is “OUTPUT_CONC” and has two possible integer values:
1 – maximum value of:
1.a - The total mass of tracers inside each grid cell divided by their correspondent grid cell volume.
1.b - The total mass of tracers inside each grid cell divided by the total volume of tracers.
2 – An average approach where:
2.a - If grid cell volume > total volume of tracers in a grid cell, result is:
[((Volume of grid cell - Volume of tracers) * ambient concentration) + (tracers mass / tracers volume)] / Volume of grid cell. This means the result is the total concentration of the water parameter(such as sediment) inside a grid cell
2.b - If volume of tracers > volume of grid cell, the result is:
mass of tracers / volume of grid cell
This is added to the output section:
!Output Options
OUTPUT_TIME : 0 3600.
OUTPUT_CONC : 1
While option 2 is just an average concentration inside delimited area (grid cell), option 1 can be used more in risk assessment situations where a localized maximum concentration is important for decision making processes (example: fecal coliforms).
For example, if the Lagrangian results are processed into an Eularian grid cell with a volume of 100 m^3 and only one tracer (with cohesive sediment mass equals 10 g and volume equals 10 m^3) is present inside a grid cell, then the concentration value using option 1 would be 10 g / 100 m^3 or 0.1 g/m^3 and using option 2 would be 10 g / 10 m^3 or 1 g/m3.
As such, looking at these outputs one should bear in mind how these calculations are made and in what context they are best used. And if necessary, adjust the number of tracers used in the simulation to achieve a better statistical significance of the Eularian results.
Also for consideration is whether the type of tracers used is subject to dilution, degradation, deposition or any process that changes the mass or the volume of the tracers – therefore changing the concentration over time.
Eularian results include other type of results. Following the sediments example the output hdf will contain the property under group “Results”:
"Group_1/Data_2D/WaterColumn/sediment”. This property is computed by dividing the total mass of the tracers in each grid cell by the grid cell area (UNITS are mg/m^2). However, if the objective is to see the concentration of sediment, then property to display in the map should be under group “Results”: “sediment".