How to evaluate plan previews for a safe and sustainable town

Human activities on the territory often produces some modifications in the natural environment that can be particularly dangerous. Transformations sometimes improve the physical situation but in other cases they are the cause of the decay of the environment. Thus, risk management policies and the consequent planning choices call for a coherent determination of the acceptable level of risk, both when compared to benefits coming from human activities and to the available resources to mitigate hazards. The paper tries to show how it is possible to combine a transformation policy to a risk mitigation approach, in order to get a result rather than imposing a constraint. In fact land use transformations have as consequence in terms of permeability degree of the soil and so on the risk level. In the paper a study case will be described, into which it has been observed that a transformation that interests a small portion of the territory causes a limited increase of the average permeability degree for the whole drainage area but it produces a considerable increase of the flow at a local level. So it is important to evaluate the effects of plan previews before their application and, if it is necessary, to introduce mitigation measures.