1. Flooding can happen when torrential rains occur. The amount of water, which falls in the form of rain, falls at a rate which exceeds the drainage rate (of man-made displacement systems, like sewers). Sewers become filled to capacity and can no longer drain the water. The water then is forced to stand and build up. This is one way flooding can occur.
2. Another way flooding can happen is if there has been a constant rain. As it rains, the ground absorbs the water. Like the sewers, the ground can only hold so much water. Once the ground has absorbed as much water as it can, it becomes supersaturated. After supersaturation occurs, and the water has no where to go, it continues to build up and flooding occurs.