Most flooding occurs when the volume of water in a river or stream exceeds the capacity of the channel. Flooding also takes place along lake and coastal shorelines, when higher than normal water levels inundate low-lying areas. |
Numerous factors affect streamflow, and therefore the potential for flooding. Most important are the amount and type of precipitation, the nature and condition of the drainage basin, and climate. Precipitation
All rivers are subject to fluctuations in flow. During a rainstorm, the amount, intensity, duration, area of storm, and path of the storm all influence the runoff reaching the stream. The amount, intensity, and duration of storms affect the ability of the land to absorb the precipitation and therefore affect the rate of runoff. The area and path of the storm relate to the area of the basin receiving rainfall, which in turn, represents the area contributing runoff. The area and the runoff rate determine the volume of water that will pass a given point downstream. Drainage basins
The shape, size, soil type, and topography of the drainage basin are other factors affecting the quantity of water reaching the stream. These factors are usually constant. However, the absorptive or shedding properties of the soil vary with vegetation cover, season, and previous rainfall.
In the case of reforestation and revegetation, the rate at which surface water flows to the main channel may be slowed, and hence the runoff is spread over a longer period. In addition, the passage of water tends to be retarded in basins with many natural storage areas, such as lakes and wetlands, and even those with artificially created storage. Consequently, smaller peak flows are produced than in basins without these modifying influences. The best North American example of a basin with large natural storage is that of the St. Lawrence River, which has the Great Lakes in its headwaters. Climate
Climate has an important influence on the relationship between precipitation and runoff. Frost makes most soil impenetrable if the soil contains moisture. In northern latitudes, the winter season, during which a large part of the year's precipitation is stored in the form of snow, is often followed by sudden melting, with much of the runoff flowing quickly over frozen ground to reach streams. Also, the heavy ice formation on rivers influences flooding, particularly on the northward flowing rivers. Climate controls the strength of an ice cover in winter, and its manner of breakup, which governs the severity of ice jamming. Rainfall
Flash floods can be extremely dangerous. Unanticipated, they usually happen on small watersheds as a result of a torrential downpour, often caused by heavy thunderstorm activity.
A flash flood is characterized by the occurrence of the peak of the flood within six hours of the onset of rainfall. The flood conditions develop rapidly because the rainfall is so heavy the ground is incapable of absorbing the water quickly enough, resulting in a very high runoff rate. The events are generally locally intense and damage is usually restricted to a limited area. Large rivers remain unaffected, while smaller streams can overtop their banks, even in a drought year.
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