twodee-2 is a Fortran 90 code based on a previous code (twodee). It is de-8 signed to solve the shallow water equations for fluid depth, depth-averaged horizon-9 tal velocities and depth-averaged fluid density. The shallow layer approach used by10 twodee-2 is a compromise between the complexity of CFD models and the simpler11 integral models. It can be used for forecasting gas dispersion near the ground and/or12 for hazard assessment over complex terrains. The inputs to the model are topog-13 raphy, terrain roughness, wind measurements from meteorological stations and gas14 flow rate from the ground sources. Optionally the model can be coupled with the15 output of a meteorological processor which generates a zero-divergence wind field16 incorporating terrain effects. Model outputs are gas concentration, depth-averaged17 velocity, averaged cloud thickness and dose. The model can be a useful tool for gas18 hazard assessment by evaluating where and when lethal concentrations for humans19 and animals can be reached.
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