When floods develop, the places in which they do so are called water sheds. These water sheds can be either man made or they can be natural. A watershed is defined as an area that is between a land mass and a water source. A watershed can be a lake or a river. If this particular watershed is working appropriately, then the water should be drained into the ground naturally. However, if there is too much snowfall or rainfall, then the watershed cannot hold so much water and this results in overflowing of the water.
However, when we are talking about the water sheds it is just not a small quantity of water but we are talking about huge volumes. So, a lot of water can actually drown an entire city. A combination of rains and excessive water can form the perfect recipe for a flood.
When it rains the water levels constantly rise up and the rivers or lakes that act as water sheds cannot hold anymore water and so they also start overflowing and this results in a major flood. Even humans are some of the contributing factors to a flood actually. With so much development in the form of roads, infrastructure and buildings, there is very left ground left. So, there is no mud left for absorbing the water that comes in the form of rainfalls. If there are drains available, then the waters naturally flow into the drains and then led to the sea. However, when the drains are full due to continuous rainfall, then the cities get flooded.