Residence time on semi-closed lagoons - Forum - Delft3D Flexible Mesh

Hi,

I am developing a model to study the residence time inside a semi-closed lagoon. I have done the model using the fractions data and set 1 to the area of interest and 0 to the domain. I know we can check the concentration changing within the internal area over time, but I was trying to generate an output to verify the age of the tracer over time. So, I would have both the concentration map and also another map showing age variability.

I actually tried to set up a conservative tracer and the corresponding decayable tracer with the same initial and concentrations and a decay rate of 0.01. However, as I am only interested in the lagoon area, this method requires setting the concentration to either 1 or 100 for initial and bnd. The simulation gives the age for the entire domain, which is not what I want. I also tried drawing a polygon over the lagoons and set the value to 100 and 0 outside, keeping the bnd to 100 (it doesn't run when set to 0), but it mixes both tracers. 

I am not sure if it can be done or for residence time studies, we have to use the fraction methods.
   
 I really appreciate all suggestions and clarifications.
 
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