Why Flooding Occurs

Flooding refers to the overflow or accumulation of water on land, which has the effect of submerging it. There are several known causes of floods – that are either natural or man-made. These include heavy or frequent precipitation, snowmelt, and land use. However, the causes alone do not explain why one area might flood with 30 millimetres of rainfall while another would not. To do this, one has to understand flooding as a process. Science in the sphere of geography has provided some insight into how flooding occurs; causes are just one factor or catalyst in the process.

• Causative factors and catalysts

For flooding to occur, there must be a stimulus to precipitate the phenomenon. Natural weather events such as storms and rainfall are a common cause. According to environmentlaw.org, snow melt, overflowing rivers, and a combination of tides, low air pressure and storms contribute to flooding as well. In addition, man-made factors contributing to flooding include poor drainage systems, irregular dredging, human use of flood plains, and run-off from developed roads and areas. In some cases, animal activity (beaver dams) or accumulation of natural debris can cause channel constrictions that affect the carrying capacity of these water channels.

• How floods occur

Although human activity significantly contributes to flooding, it can occur in natural watersheds or man-made ones. The following elements make some areas more flood-prone than others:

* Infiltration rates

* Degree of surface run-off

These factors subsequently affect the capacity of river, streams, drains, and other water channels, to transport water to outlets. If the runoff surpasses the carrying capacity of the water channels, this causes the water to overflow the banks of the channel and flow onto a flood plain – typically a flat, low-lying area.

• The flood model

Recall that infiltrations rates and runoff rates are critical to the degree and extent of flooding. Generally, the presence of vegetation and forests maximize the infiltration of precipitation and retard the movement of runoff. With rapid urbanization in several areas, floods are becoming more severe and frequent. According to science.jrank, the flow of water through channels can be modelled in an equation:

Q = A x V, where Q is the amount of water, A is the area of the water channel and V is the velocity of the water flowing through the channel.

An overflow (excess amount of water) increases the velocity of the water. However, if the water exceeds the capacity, two things happen. The velocity of the water increases, transporting the water to low-lying areas faster, and the land has a reduced capacity to handle subsequent flows. Therefore, there is little to impede the movement of water to susceptible areas and the land is less capable of absorbing water.

This is why urbanization has increased the frequency and severity of floods in other areas. Erecting buildings reduces the capacity of the land to absorb the water. Thus, areas with a high building density are essentially funnelling water to other areas. The reduced infiltration also leads to a higher degree of surface run-off. In some instances where drainage systems are poor, this not only affects surrounding flood plains (which flood first). In severe cases, the urban areas might be affected eventually – even if above sea level.

Analyzing flooding as a process that is based on infiltration rates and surface run-off that is precipitated by various causative elements is very useful. This is because it explains why different areas receiving similar amounts of precipitation can respond so differently to the conditions. Scientific explanations of flooding have helped to alleviate flooding issues by emphasizing the importance of drainage. In addition, it placed emphasis on catchment areas and drainage as a system.