Floods are categorized into natural and artificial floods in terms of their specific causes.
Flood is basically a natural hydrological phenomenon. Its occurrence is usually the aftermath of meteorological events. These included:
- an intense and prolonged rainfall spells;
- unusually high coastal and estuarine waters due to storm surges, seiches, etc.
Floods are also caused, indirectly, by seismic activities. Coastal areas are particularly susceptible to flooding due to tsunamis (seismic sea waves). Sinking of land due to earthquakes reduces the elevation of land areas. In the vicinity of lakes and rivers, these areas become flood-prone. Likewise, the uplifting of lake and river beds from seismic causes sometime results in the overflowing of these bodies of water. The water then inundates the surrounding and adjacent areas.
To a certain extent, astronomically influenced phenomena such as high tides coinciding with the occurrence of heavy rainfall frequently cause flooding.
Occasionally, floods occur unnaturally. These are usually the result of human activities. Such activities include:
- Blasting - this causes landslides in the slopes of hills and mountains which may result in the unintentional damming of rivers and streams.
- Construction of temporary dams - this produces an impediment to the flow of a river or stream which then results in an overflow;
- Failure of hydraulic and other control structures - accidents like the breaking of a dike result in the entry of an enormous quantity of water in a protected area; and
- Mismanagement of hydraulic structures - control structures like dams which are utilized for various purposes are usually operated according to what is known as an "operation rule" and mismanagement which results in the violation of the rule may necessitate an untimely and sudden release of large amounts of excess water.
While not quite so obvious, human activities that tend to alter the ecological system in a river basin will have an impact on the hydrology of the catchment. This could, in the future, result in frequent floods. Foremost among such activities is the denudation of forest and watershed areas.
Floods vary in degree of severity in terms of areas extent or magnitude and in depth. They are, thus, classified as minor or major flooding.
In a minor flooding, inundation may or may not be due to overbanking. When there is no bank overflow, flooding is simply due to the accumulation of excessive surface run-off in low lying flat areas. Floodwaters are usually confined to the flood plain of the river along the channel, on random low-lying areas and depressions in the terrain. Floodwater is usually shallow and there may not be a perceptible flow.
During a major flood, flooding is caused by the overflowing of rivers and lakes; by serious breaks in dikes, levees, dams and other protective structures; by uncontrollable releases of impounded water in reservoirs and by the accumulation of excessive runoff. Floodwaters cover a wide contiguous area and spread rapidly to adjoining areas of relatively lower elevation. Flooding is relatively deep in most parts of the stricken areas. There is a highly perceptible current as the flood spreads to other areas.
While floods take some time, usually from 12 to 24 hours or even longer, to develop after the occurrence of intense rainfall, there is a particular type which develops after no more than six hours and, frequently, after an even less time. These are what are known as "flash floods".
Flash floods develop in hilly and mountainous terrains where the slope of the river is rather steep. The rapid development of the flood is due to the extremely short concentration time of the drainage catchment. This means that precipitation falling on a point in the catchment farthest from the river takes only a short time to reach the river channel and become part of streamflow. Thus, the amount of streamflow rapidly increases and, consequently, the rise in water level. When the flow capacity of the stream is exceeded, the channel overflows and the result is a flash flood.