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The Hydro Risk is represented in the territory by natural and human factor resultant , in the area of interest made up by surface and natural drainage networks and hydrological dynamics, that characterize the relations between the inflow and outflow of an hydro geographical basin; this can be calculated with:
R = H x V x E = H x D
R = The risk, Extent of damage expected for a certain area in a certain interval of time, as consequence of a disaster.
H = The danger, i.e. the probability of a disaster within a given period of time in a certain area, considered as an addition to the safety factor, rigidly related to recurrence interval. (time in which the intensity I is exceeded, on average once only – I being the maximum capacity):
H = (1-1/T)2
V = Vulnerability, i.e the degree value loss caused to a given element exposed to disaster event
E = The risk element value; the monetary value of properties and economic activities at risk in a certain area;
D = The total damage.
With the factors above, having established the specific risk value (of high flood), measures should be designed that reduce it e.g. by structural intervention.
In alluvial context, the extent of danger "H" in flood state. is evaluated through statistical and probabilistic analysis of available data.
These are capacity, rainfall that determine the extent of vulnerable areas and flooding maps with associated recurrence interval.
The extent of danger may be of two classes:
medium : alluvial areas with flood over spill recurrence interval from 10 to 100 years;
low : alluvial areas with flood over spill recurrence interval from 100 to 300 years;