Urban runoff

Overwatering through irrigation by sprinkler may produce runoff reaching receiving waters during low flow conditions.[10] Runoff carries accumulated pollutants to streams with unusually low dilution ratios causing higher pollutant concentrations than would be found during regional precipitation events.[11]

Flooded streets in New Orleans

Relationship between impervious surfaces and surface runoff

Urban runoff is a major cause of urban flooding, the inundation of land or property in a built-up environment caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers.[12] Triggered by events such as flash flooding, storm surges, overbank flooding, or snow melts, urban flooding is characterized by its repetitive, costly, and systemic impacts on communities, even when not within floodplains or near any body of water.[13]

There are several ways in which stormwater enters properties: backup through sewer pipes, toilets and sinks into buildings; seepage through building walls and floors; the accumulation of water on the property and in public rights-of-way; and the overflow of water from water bodies such as rivers and lakes. Where properties are built with basements, urban flooding is the primary cause of basement flooding.

Weasel Brook in Passaic, New Jersey has been channelized with concrete walls to control localized flooding.

Urban runoff contributes to water quality problems. In 2009 the US National Research Council published a comprehensive report on the effects of urban stormwater and stated that it continues to be a major contamination source in many watersheds throughout the United States.[14]^: vii The report explained that "...further declines in water quality remain likely if the land-use changes that typify more diffuse sources of pollution are not addressed... These include land-disturbing agricultural, silvicultural, urban, industrial, and construction activities from which hard-to-monitor pollutants emerge during wet-weather events. Pollution from these landscapes has been almost universally acknowledged as the most pressing challenge to the restoration of waterbodies and aquatic ecosystems nationwide."[14]^: 24

An open runoff system in Africa

The runoff also increases temperatures in streams, harming fish and other organisms. (A sudden burst of runoff from a rainstorm can cause a fish-killing shock of hot water.) Also, road salt used to melt snow on sidewalks and roadways can contaminate streams and groundwater aquifers.[15]

One of the most pronounced effects of urban runoff is on watercourses that historically contained little or no water during dry weather periods (often called ephemeral streams). When an area around such a stream is urbanized, the resultant runoff creates an unnatural year-round streamflow that hurts the vegetation, wildlife and stream bed of the waterway. Containing little or no sediment relative to the historic ratio of sediment to water, urban runoff rushes down the stream channel, ruining natural features such as meanders and sandbars, and creates severe erosion—increasing sediment loads at the mouth while severely carving the stream bed upstream. As an example, on many Southern California beaches at the mouth of a waterway, urban runoff carries trash, pollutants, excessive silt, and other wastes, and can pose moderate to severe health hazards.

Because of fertilizer and organic waste that urban runoff often carries, eutrophication often occurs in waterways affected by this type of runoff. After heavy rains, organic matter in the waterway is relatively high compared with natural levels, spurring growth of algae blooms that soon consume most of the oxygen. Once the naturally occurring oxygen in the water is depleted, the algae blooms die, and their decomposition causes further eutrophication. These algae blooms mostly occur in areas with still water, such as stream pools and the pools behind dams, weirs, and some drop structures. Eutrophication usually comes with deadly consequences for fish and other aquatic organisms.

Oil slick created by runoff

A percolation trench allows infiltration of stormwater through permeable soils into the groundwater aquifer.

An oil-grit separator is designed to capture settleable solids, oil and grease, debris and floatables in runoff from roads and parking lots

Excessive stream bank erosion may cause flooding and property damage. For many years governments have often responded to urban stream erosion problems by modifying the streams through construction of hardened embankments and similar control structures using concrete and masonry materials. Use of these hard materials destroys habitat for fish and other animals.[16] Such a project may stabilize the immediate area where flood damage occurred, but often it simply shifts the problem to an upstream or downstream segment of the stream.[17] See River engineering.

There are many different ways that polluted urban runoff could harm humans, such as by contaminating drinking water, disrupting food sources and even causing parts of beaches to be closed off due to a risk of illness. After heavy rainfall events that cause stormwater overflows, contaminated water can impact waterways in which people recreate or fish, causing the beaches or water-based activities to be closed. This is because the runoff has likely caused a spike in harmful bacterial growth or inorganic chemical pollution in the water. The contaminants that we often think of as the most damaging are gasoline and oil spillage, but we often overlook the impact that fertilizers and insecticides have. When plants are watered and fields irrigated, the chemicals that lawns and crops have been treated with can be washed into the water table. The new environments that these chemicals are introduced to suffer due to their presence as they kill native vegetation, invertebrates, and vertebrates.