Floods are usually caused by excessive volume of water finding its way onto dry land that leaves destruction in their wake. Nature has its drainage system just like we humans do. Some of these floods are as a result of the natural drainages- the rivers, streams, creeks, tributaries and the ocean- being overwhelmed by excessive volume of water than they could normally handle. When natural drainages are overwhelmed and overflow their banks to cause flooding, it is usually because of natural occurrences such as heavy and torrential rainfall and tropical storms; and hurricanes which are usually above the normal rainfall indices that these drainages can handle.
Floods that occur as a result of manmade drainages being overwhelmed are usually caused by both natural factors or agents as mentioned above and manmade factors. Flooding could occur when rivers and oceans overflow their banks and spill on dry land usually inhabited by humans and other animals. The most common reasons for rivers and oceans overflowing their banks are as a result of excessive rainfall. Manmade drainages also get overwhelmed under heavy downpour and also overflow, causing devastation floods. In some cases, floods are not as a result of a natural occurrence as addressed above. They may be caused by humans independent of any natural agents. A very vivid example is the bursting of a manmade dam. Other humanly induced types of floods can be as a result of deforestation. Forests and plants protect the soil from direct impact from rain and also give soil higher water retention capacity. When trees and forests are hacked down by humans, the soil is exposed and is vulnerable to an otherwise normal or mild weather conditions.