2D Decay

Many processes can be approximated by a simple first-order decay, such as die-off of E.Coli due to exposure to light, decay of the activity of radioactive substances or estimating the age of water bodies.

First order decay of a component is generally described by

(6.1)   

or

(6.2)   

where c is the specific concentration, c₀ is the initial concentration and k is the decay factor. In the model the decay term is added to the general transport equation.

For each WQ component, the decay is specified individually for each component.

The format of the 2D decay factor can be specified as:

• None
• Constant (in time)
• Varying in time.

For the case with time varying decay factors, a time series file (.dfs0) containing the decay factors must be supplied. The data must cover the complete simulation period. The time step of the input data file does not, however, have to be the same as the time step of the hydrodynamic simulation. A linear interpolation will be applied if the time steps differ.

Note on simulation stability

The decay may affect the stability of the numerical solution, in a way similar to the advection or diffusion terms. If the decay represents a very rapid process such that the product k\(\Delta\)t>1 the decay term may be the source of instability or at least occurrence of negative concentrations. A solution is then to reduce the time step.

Note on MIKE ECO Lab decay

The decay is only relevant for Advection-Dispersion components. For state variables from a MIKE ECO Lab model, decay is simulated as part of the biological processes. As a consequence, no decay information has to be specified for WQ components associated to a MIKE ECO Lab state variable.