Human Intervention
Humans can modify the landscape in many ways. Sometimes humans attempt to modify drainage systems to prevent flooding, but sometimes these efforts have adverse effects and actually help to cause flooding in other areas. Any modification of the landscape has the potential to cause changes in the drainage system, and such changes can have severe consequences.
Channel Modifications
Humans often decide that a stream should flow along a specified path for such reasons as flood control, enhancement of drainage, control of erosion, increasing access to the floodplain for development, or improvement of the appearance of the channel. Such channel modifications involve measures such as the straightening the channel, deepening or widening the channel, clearing vegetation from the banks, or lining the channel with concrete. These modifications are referred to as channelization.
In order to control floods, channel modification should involve increasing the channel cross-sectional area, so that higher discharge will not increase the stage of the river. Straighter channels also allow higher velocity flow and, enable the stream to drain faster when discharge increases. Lining the channel with concrete provides a smoother surface over which the water can flow, thereby reducing friction and also increasing the velocity of the stream.
While channelization for flood control may reduce the incidence of flooding in the channelized area, it often results in more severe flooding both upstream and downstream from the channelized area.
Channelization can also interfere with the natural habitat of the stream system and decrease the aesthetic value of the stream.
Channelization, or any other modification of a stream system, changes the validity of all historic data collected over the years on that stream. During flooding of the Mississippi River in 1973 and 1993 water levels rose to higher levels than expected from the statistical data, because modification of the stream had made the data invalid. The 1973 flood caused damage and rose to levels that might be expected from a 200-year flood, even though discharge was only at a level predicted for a 30-year flood. Thus, it appears that modifications of the drainage system had an adverse effect.
Effects of Development on Flood Hazard
Whenever humans modify the landscape in any way changes are to be expected in the way water drains from the land. Unless careful consideration is given to the possible drainage consequences, such landscape modifications can result in higher incidence of flooding. Development on floodplains should therefore be undertaken only with great care. Existing developments that have enhanced flooding problems are often costly to fix. Among the factors that enhance the flood potential are: