Introduction Extreme floods are among the most destructive forces of nature. Flooding accounts for a significant proportion of the total number of reported natural disasters occurring in the world (Figure 7.1a) and over the last 30 years this proportion has been increasing (Figure 7.1b). Reasons for this trend may not be clear; for each hazard there is a need to quantify whether this is an increase in the hazard itself, an increase in exposure to the hazard internationally or a change in the reporting of what constitutes a natural disaster. Internationally, the costs and scale of flooding are enormous but differ depending on the types of impact that are analysed and the databases used. Globally in 2007 it was estimated that annually 520 million people are affected by floods and that the death toll is approximately 25000 people in any one year. Jonkman (2005) found for a study using data from 1974 to 2003 (from data maintained by the Centre for Research on the Epidemiology of Disasters in Brussels) that floods are the most significant natural disaster type in terms of the number of people affected – some 51% of the total of that period of approximately five billion people affected by natural disaster (droughts are second with 36%, and earthquakes third at 2%). However, in terms of overall estimated deaths flooding accounts for 10% of the approximately two million reported deaths associated with natural disasters over the 1974–2003 period (droughts 44% and earthquakes 27%). In monetary terms an assessment by Munich RE for the period 1980–2010 determined that at 2010 prices the losses totalled US$3000 billion from ~19400 events with 2.275 million fatalities. Of these, hydrological catastrophes (flooding and mass movement, i.e. landslips and debris flow in this case) accounted for 24% of these monetary losses, from 35% of the total events, and 11% of the fatalities. Other categories of natural disasters included in these totals were geophysical, meteorological and climatological (NatCatSERVICE, 2011).