UNESCO International Glossary of Hydrology defines “Flood” as follows;
The Glossary also defines flooding as an overflowing by water of the normal confines of a stream or other body of water, or accumulation of water by drainage over areas that are not normally submerged.
Types and Causes of floods
CAUSES OF FLOODING
These different types of flooding can be caused by the following factors;
EFFECTS OF FLOODING
PROBABILITY OF FLOOD OCCURRENCE
A sound understanding of the likelihood of occurrence of a flood hazard is a fundamental step in dealing with flood risk. Risk from flooding can be conceptualized into four stages as in Figure below:
To fully evaluate risk, the degree of exposure and the nature of exposed receptors and their potential
to sustain or resist damage also need to be considered.
.Flood forecasts for a natural drainage area or a city are usually obtained by analyzing the past occurrence of flooding events, determining their recurrence intervals, and then using this information to extrapolate to future probabilities.
This common approach is described below in simplified form for fluvial flooding. The probability of occurrence for pluvial, groundwater, flash, and semi-permanent floods is much more difficult to estimate, even if historical data is available. This is due to the fact that the causes of these types of floods are, as seen above, a combination of a meteorological event such as heavy rainfall and other factors such as insufficient drainage capacity, mismanagement of key infrastructure and other human factors. In the case of coastal floods caused by seismic activities, predicting their probability is as difficult as predicting the occurrence of an earthquake. For coastal floods caused by storms or hurricanes, their probability of occurrence can, in principle, be computed using historical data or numerical simulations of key variables such as wind speed, sea level, river flow and rainfall.
FREQUENCY OF OCCURRENCE
The recurrence interval or return period is defined as the average time between events of a given magnitude assuming that different events are random. The recurrence interval or return period of floods of different heights varies from catchment to catchment, depending on various factors such as the climate of the region, the width of the floodplain and the size of the channel. In a dry climate the recurrence interval of a three meter height flood might be much longer than in a region that gets regular heavy rainfall. Therefore the recurrence interval is specific to a particular river catchment. Once the recurrence intervals are determined based on the historical record, some assumption about the flood frequency distribution has to be made in order to extrapolate or interpolate to events that have not been recorded historically. To achieve this, an assumption about the distribution of flood frequency has to be made. In this way the recurrence interval for any discharge (and not just those
present in the observational record) can be inferred.
For any discharge, or alternatively, any recurrence period, the probability of occurrence is the inverse of the return period p=1/T
The figure below shows the trend of flood events around the world since 1950.
Figure: Number of reported flood events between 1950 and 2011 (Source: EM-DAT/CRED, v. 12.07)
September 23, 2019