Provisions in design and maintenance to protect water quality from roof catchments

An analytical model has been developed to study time‐dependent atmospheric diffusion in the protected zone a ≥ z ≥ h, considering heavy admixture's concentration, where h is the height of the surface layer, a is the height of the mixing layer and z is the the vertical coordinate. This model takes care of the variation of surface and mixing layer heights on thermal stratification, geostrophic drag by terrain and several other meteorological parameters. Pollutants are emitted from two sources. One is continuous in the surface layer 0 ≤ z ≤ h and the other is instantaneous above the surface layer h ≤ z ≤ a. We have used an appropriate boundary condition at the surface layer height to model the effect of surface layer source, effect of terrain and settling of larger particles. Results have been analysed for the case of stable and neutral atmospheric conditions. It is found that the effect of settling of larger particles is to reduce concentration throughout the protected zone for large settling velocities. For smaller values of settling velocity there will be an increase in concentration in the lower part and decrease in the upper part. It is also found that the concentration in the lower part of the protected zone is higher in the stable case compared to the neutral case at large travel time. The effect of settling of larger particles is to reduce the concentration significantly in a stable atmospheric layer compared to the neutral case. The effect of roughness of terrain, ground absorption and geostrophic wind have also been discussed.