Flooding occurs when water cannot drain away fast enough in the rivers. In area of non-porous rocks, water runs off the land very quickly and streams and rivers soon overflow. Flooding also happens when winter snows thaw in spring. Huge floods cover parts of Siberia every spring, when snow melts while the rivers are still iced up. Low-lying coastal lands are vulnerable to flooding, especially when gales and high tides cause water to flow inland. Low–lying Bangladesh is particularly liable to this kind of flooding. In addition, melting snow in the Himalayan Mountains adds huge amounts of water to Bangladesh’s rivers, increasing the flood risk.
A flood is an overflow of water onto land that is normally dry. Floods can happen almost anywhere. They can cover an area with just a few inches of water or they can bring enough water to cover the roof of a house. Floods can be dangerous for communities, lasting days, weeks or sometimes even longer.