The factors in the mass balance equation are not constant with time. This leads to another important concept in floods, the time lag between the precipitation and the peak discharge. The time lag is important for flood hazard mitigation. The longer the time lag the more time to prepare. The graph below show that the flood severity increases as urbanization changes the mass balance equation.
River floods are something that occurs with great regularity.
One of the best hazard mitigation procedures is to use statistical techniques to predict the frequency of floods of a given magnitude. The important parameter in this type of analysis is the annual maximum discharge - the highest level of discharge measured at a particular spot in a given year. The 100 year flood will innudate a region which is called the 100 year flood plain .