Wave Attenuation in Vegetation Based Breakwater

The relationship between time period, wavelength and depth of water is given by
The density of seawater considered in the design of marine structures is
In REEF3D, as per relaxation beach method the typical wavelength of the dissipating relaxation zone should be
The mesh is described by
In the REEF3D execution command “mpiexec -n 4 reef3d”, what does “4” signify?