Surface runoff flowing down cobblestone streets in a Brazilian city

Urbanization typically results in alterations to the natural ground surfaces and stream channels of a basin. Surface permeability, basin size, stream density, roughness, and channel length and slope can all be affected in a way that results in greater magnitude and speed of runoff.

Rainwater flowing down a street and through a storm drainage grate.  Nearby, water is flowing up and out of a manhole cover and onto the street again

In urban areas the greater coverage of pavement, buildings, and compacted soils prevents infiltration of rainfall and snowmelt compared to the natural ground surface. This can greatly increase the magnitude of runoff.

Depiction of sub-basins formed by road across a basin.

Urban features such as road embankments and berms can act to break down natural basins into smaller sub-basins. Smaller drainages react much more quickly to localized rainfall than larger basins.

Illustration of the effect of urban concrete grid on runoff contribution to stream in a basin.

Road grids, ditches, and storm sewer systems act as a network of tributaries and effectively increase stream density. Higher stream density results in more rapid runoff to the stream channels.

Compared to a natural stream bed, road surfaces, culverts, and storm sewers have smooth surfaces. This decrease in surface roughness allows runoff to move much more quickly to the main stream channels than it would in a more natural setting.

Brick culvert in West Yorkshire, United Kingdom

Streams in urban areas often have vegetation removed and are sometimes lined with concrete in a process called "channelization". This also decreases roughness and causes water velocity to increase.

Illustration of the effect of straightening and channelizing a stream in an urban setting.

Sometimes as part of channelization urban streams are straightened by having meanders removed. This decreases the distance that water travels from the top to the bottom of the drainage basin. It also effectively increases the slope because the stream now experiences the same elevation drop but in a shorter distance. Decreasing the distance traveled and increasing slope will cause a more rapid flood response from the runoff.

Overall, an urban environment will result in faster runoff with more runoff reaching the streams than in a rural setting.