Rainfall over an extended period and an extended area can cause major rivers to overflow their banks. The water can cover enormous areas. Downstream areas may be affected, even when they didn’t receive much rain themselves.
With large rivers the process is relatively slow. The rain water enters the river in many ways. Some rain will fall into the river directly, but that alone doesn’t make the river rise high. A lot of rain water will run off the surface when the soil is saturated or hard. It will flow to small rivers that flow to larger rivers and these rivers flow into even larger rivers. In this way all the rain that fell in a large area (catchment area) comes together in this one very large river. When there is a lot of rain over a long period, you see the river rise gradually as it is fed with water form smaller rivers. It takes time for all the rainwater to reach the river, but once it is in the river it has to flow downstream to sea.
While the water level slowly rises, officials can decide to evacuate people before the river overflows. The area that is flooded can be huge. Villages surrounded by large stretches of water were cattle would normally graze. Whole communities can become isolated from the rest of the world as roads are blocked and communications are down. Here you see a simulation of the water level over time after a river dike breach in a low lying part of the Netherlands.
When a dike or a dam breaks and a lot of water is released suddenly, the speed of the water at the breach can be compared with the speed of a flash flood. As a larger area gets covered the speed will be reduced. The water spreads out as much as possible flowing to the lower lying areas before slowly rising. A breach is very dangerous for the people living close to it. The strength of the water may carry cars, trees and even houses away and cause loss of live. If you want to study the details of breach growth, go here.
In the virtual tour a few examples of river flooding are shown but there are many more.
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