Rains
Each time there is more rain than the drainage system can take, there can be floods. Sometimes, there is heavy rain for a short period which results in floods. At other times, there may be light rain for many days and weeks which can also result in floods.

 River overflow
Rivers can overflow their banks to cause flooding. This happens when there is more water upstream than usual, and as it flows downstream to the adjacent low-lying areas (also called a floodplain), there is a burst and water gets into the land.

Hurricanes, Strong winds in coastal areas
Seawater can be carried by massive winds and hurricanes onto dry coastal lands and cause flooding. Sometimes this is made worse if the winds carry rains themselves. Sometimes water from the sea resulting from a tsunami can flow inland to cause damage.

Example: Wrightsville Beach in North Carolina USA, September 14, 2018.

A massive North Atlantic Hurricane that covered a distance of about 6,500km made landfall. The hurricane was an unusually slow-moving one, and stayed over that location for a long time, with non-stop rains. This led to catastrophic flooding, as the land was already saturated from the wet summer. Communities such as Wilmington were cut off. More than 40 people died from the hurricane and massive destruction of homes and infrastructure occurred.

Dam breaking
 (Ruptured dam or levee) (Embankments, known as levees, are built along the side of a river and are used to prevent high water from flooding bordering land)
Dams are man-made blocks mounted to hold water flowing down from a highland. The power in the water is used to turn propellers to generate electricity. Sometimes, too much water held up in the dam can cause it to break and overflow the area. Excess water can also be intentionally released from the dam to prevent it from breaking and that can also cause floods.

February 26, 1972 – Buffalo Creek Valley, West Virginia 

The failure of a coal waste impoundment at the valley’s head took 125 lives and caused more than $400 million in damages, including the destruction of over 500 homes.

Ice and snow-melts 

In many cold regions, heavy snow over the winter usually stays unmelted for some time. Some mountains have ice on top of them. Sometimes the ice suddenly melts when the temperature rises, resulting in the massive movement of water into places that are usually dry. This is usually called a snowmelt flood