All it takes it's an half hour storm, that's enough to put at risk people and property, nowadays. It can all be washed away by a little drain that becomes river in a second.
This is what happened recently in Sardinia, five lives drowned by the flood caused by insufficient drainage capacity of the catchment and by foolish cementing of an area at risk.
Afterwards the usual lamenting of the survivor and the damage count.
In the photos, the destructive potential of an innocent watercourse when 100mm of rain fall in half an hour.
It happened behind my house, in Cugnana, next to Olbia: the river has destroyed the IMHOFF septic tank and stormed into the house, pushing its content throughout the floor up to 50cm. About 30,000€ damage.
Fortunately no one got hurt this time.
More shots of the same happening.