Physical Causes of Floods
- The most basic physical cause of floods is heavy rain, often a downpour that lasts for several days without stopping. Heavy rain raises water levels in dams, creeks, rivers and oceans and often causes them to overflow past their natural banks or artificial barriers. However, heavy rain needn't pour for days to cause a flood; sometimes rain is so intense and comes down with such ferocity that 4 to 5 inches in a few hours could be enough to precipitate flooding.
- Melting snow can also cause flooding, especially in colder areas where the temperature is rising, making snowcaps melt at a faster rate. Melting snow can raise ocean levels, which can have a domino effect on river levels, making rivers overflow their banks and leading to a flood. Frozen ice that breaks apart and flows into water sources can also cause flooding by destroying bridges and other structures that are in place to prevent flooding.
- The cutting down of trees for timber and other uses, known as deforestation, contributes to flooding in that trees naturally prevent sediment runoff and absorb water from the earth, making the soil able to store more rainwater instead of overflowing. Deforestation also creates the release of sediment -- eroded soil that ends up in rivers and reduces the river's ability to hold water without flooding. This means that even normal rainfall that wouldn't have raised a river's banks before deforestation can now lead to flooding.
- The rapid development of urban communities has contributed to floods because areas that were previously covered with grass and dirt capable of soaking up rain and moisture have been replaced with asphalt, steel and concrete. In heavy rainfall, urban storm sewers and drainage ditches may not be sufficient to prevent overload, leading directly to flooding. In addition, the despoiling of America's wetlands has contributed to flooding, as the wet soil and mud of the wetlands formed a natural sponge to store extra water. Without those wetlands, rivers that overflow will flood much quicker, because an effective natural drainage source has been removed.
