Floods can be classified into fluvial, coastal and those that result from deficiencies in urban drainage. Fluvial floods occur when river discharge exceeds its bankfull capacity. The return period of out-of-bank flood flow is generally 2.3 years (Newson 1989), and the magnitude of floods and their probability of occurrence are strongly connected, although these relationships are regionally specific and depend on climatic conditions and river catchment character (ibid.).

Coastal flooding occurs where tide levels exceed land levels, exacerbated by extreme wave The historic city of Venice in northern Italy is one of the most famous cases of increasing flood threat. A combination of subsidence in the city and rising sea levels means that the frequency of flooding in St Marks Square in the city centre has risen from seven per year in 1900 to about fifty per year today. A major flood occurred in 1966, causing widespread damage. Projections of sea level rise indicate that this frequency could rise to over 300 per year by the year 2050. Many solutions have been suggested, including a system of gates between the lagoon in which Venice is located and the Adriatic Sea (Bandarin 1994). However, these proposals are controversial in that they appear to tackle only the symptoms of Venice's many problems rather than their causes (Penning-Rowsell et al. 1998). They therefore may not solve the flooding problem in the long term - i.e. 50+years - and do not tackle the associated problems of pollution in the city and the Lagoon, the city's declining population, and the decay of the ancient Venetian buildings. Even the local government Comune in Venice has voted against the proposals, which remained mired within the labyrinthine and corrupting Italian political system for years, until vetoed by the Italian Environment Minister in 1998: the problem remains unsolved.

Flooding in urban areas away from the coast or major rivers occurs when summer thunderstorm conditions (or intense cyclonic rain) occur on urban catchments, where infiltration rates are reduced by paved surfaces. The result is rapid and almost complete runoff far exceeding the capacity of drainage and sewer systems. Without deliberately designed storage ponds or other control systems for this runoff, it is liable to cause damage, especially in the basements of buildings or where underground railways or telecoms systems are at risk.