twodee - 2 : A Shallow Layer Model for Dense 1 Gas Dispersion on Complex Topography ⋆

6 twodee-2 is a Fortran 90 code based on a previous code (twodee). It is de7 signed to solve the shallow water equations for fluid depth, depth-averaged horizon8 tal velocities and depth-averaged fluid density. The shallow layer approach used by 9 twodee-2 is a compromise between the complexity of CFD models and the simpler 10 integral models. It can be used for forecasting gas dispersion near the ground and/or 11 for hazard assessment over complex terrains. The inputs to the model are topog12 raphy, terrain roughness, wind measurements from meteorological stations and gas 13 flow rate from the ground sources. Optionally the model can be coupled with the 14 output of a meteorological processor which generates a zero-divergence wind field 15 incorporating terrain effects. Model outputs are gas concentration, depth-averaged 16 velocity, averaged cloud thickness and dose. The model can be a useful tool for gas 17 hazard assessment by evaluating where and when lethal concentrations for humans 18 and animals can be reached. 19