In order to collect run off from streets, parking lots, and buildings, all of which block the infiltration of water into the soil, storm sewers are installed to provide underground drainage of the surface. While this may prevent local flooding of streets, it moves water more rapidly to the major stream systems and thus decreases the lag time and increases the peak discharge of the streams collecting the runoff from the storm sewers. 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.
An engineering approach, to control flooding, and a regulatory approach designed to decrease vulnerability to flooding. As discussed above, channelization. In other words by enlarging the cross-sectional area, higher discharge can be held within the channel. Channelization also increases water velocity, and thus reduces drainage time. Dams - Dams can be used to hold water back so that discharge downstream can be regulated at a desired rate.