The duration of floods can last from several hours to months at a time. The period of a flood is dependent on factors including rainfall rate time span, soil and ground conditions, and topography.
There are four general types of flooding: river, coastal, urban and flash floods. Rivers often flood during spring melting and rain events that cause a waterway to overflow its banks. Cities in general are more susceptible to floods, as the majority of the ground in an urban environment is covered in concrete and asphalt. This kind of ground cover does not absorb excessive rainfall, and sewer systems are typically unable to keep up with an excessive deluge.