Causes of Runoff

Causes of Urban Runoff

Urban runoff can be attributed to many things including the amount of rainfall, the soil conditions, and the degree of urbanization. The rainwater can go in many directions once it has reached the earth's surface. Rainwater can be absorbed by the soil on the land surface, it can be absorbed by surrounding vegetation, directly deposited into oceans, streams, and rivers, as well as infiltrating through the surface and subsurface soils into the groundwater. These are the natural processes that occur as the precipitation reaches the land surface. Rainwater can will also have a natural runoff percentage. This is due to the varying characteristics displayed by soils around the world. These characteristics include: soil layer thickness, permeability, and the degree of saturation in the soil before a rain event. There are many ways in which the urbanization of our land has contributed to runoff.

Since the majority of people live in cities, they are becoming larger and larger, and are sprawling into vast suburban neighborhoods. These neighborhoods have acres of hard, impermeable surfaces. Fields and forests allow for the rainwater to soak into the soil where it falls. Roofs, parking lots, and streets and other impervious surfaces of an urban environment, cause the rainwater to collect, and be forced out through a storm drain system. If the drainage system does not connect to a wastewater treatment facility, then the rainwater and everything that is carried with it will travel into local streams and rivers.

Urban runoff contributes to the many pollutants found in our groundwater and streams. Some of these pollutants include : lead, zinc, copper, chromium, and arsenic. Along with pollutants other water quality characteristics are affected because of urbanization. These characteristics include water temperature, pH, dissolved oxygen, alkalinity, hardness, and conductivity.