Flood can be characterized by the following attributes:
- Triggering factors
- Spatial occurrence
- Duration of the event
- Time of onset
- Secondary events
These characteristics can help in categorization of different types and levels of floods as well as it enables us to compare different hazards with each other. Simply, these characteristics can be translated as what, when, where, why, how, how much and who.
1. Triggering Factors of Flooding
Floods can be triggered by different natural and anthropogenic phenomena. Sometime it is prolonged rainfall that causes floods, sometime torrential rains or storms cause flooding situation. In summer season, high average temperature can result in increased melting of the snow hence, high discharge downstream. Tidal surges due to attraction forces of sun and moon can also cause localized flooding on sea costs. Similarly, lack of permeable surface and high ground water table in a region can also trigger a flood.
2. Spatial Occurrence
Spatial occurrence refers to the area and the extent of the area affected by the flood. Floods don’t occur randomly. They occur in areas that are in geographical proximity to water bodies, where prolonged rainfall occur or in areas with poor drainage system. These characteristics enable us to categorize the flooding events.
The extent of the area covered by the flood can be small for example in the case of flash floods, it affects only a village, town or a city. While riverine floods can affect many a cities on its way.
3. Duration of the Flooding
Duration of the event means the time span between the start and end of the flooding or the event that caused the flood. Usually this is difficult to be defined for floods as the recede very slowly and does not vanish completely, rather the flood water moves from one area to another. It can also be defined as the time duration through which the flood lasted.
4. Time of Onset of the Flood
It is the time span between the start of the event causing the flooding and the time when the flood has actually occurred. For instance, the delay between the rainfall and the peak discharge.
5. Frequency of Flooding
Frequency of flood events mean how often the flooding occur in a given time period for example, a year. Flood recurrence intervals can range from multiple times a year to once in 10 years or even 30 years. It allows scientists/researchers to understand when a flood of certain magnitude and intensity will occur in a given area.
6. Seasonality of Flood
Seasonality refers to the season that has the most probability for flood. In South Asia, the probability of floods is highest in Monsoon period (July-August) as compared to the rest of the year. This is due to the seasonal winds (Monsoon) that blow over South Asia in this period. By knowing the season for the flood hazard we can take steps to prevent, mitigate or in the worst case prepare for the hazard.
7. Magnitude of Flood
Magnitude refers to the energy released during the hazardous event.
8. Intensity of Flood
Intensity of floods is the damage caused by it. It can be characterized by depth of inundation, volume of inundation, velocity of flow and rate of rise of water. The more the depth of water, more will be its volume, velocity and its damaging capacity. A high rate of rise for water also means less preparation time for people in the area.
9. Derived Events (Secondary Events)
Disasters can also trigger secondary disasters causing damages and destruction. Floods can hit down electric poles and cause widespread electrocution or even fires.