Reduction of infiltration - Any time the surface materials of the Earth are covered with impermeable materials like concrete, asphalt, or buildings, the infiltration of water into the soil is prevented. Urbanization tends to reduce infiltration, and thus water must collect in storm sewers and eventually in the main drainage systems. Thus, extensive urbanization also decreases the lag time and increases the peak discharge even further. Urbanization can therefore lead to a higher incidence of flash floods.