Urban runoff

Overwatering during landscape irrigation 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 melt, urban flooding is characterized by its repetitive, costly and systemic impacts on communities, regardless of whether or not these communities are located within formally designated 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 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.

Flood flows in urban environments have been investigated relatively recently despite many centuries of flood events.[14] Some researchers mentioned the storage effect in urban areas. Several studies looked into the flow patterns and redistribution in streets during storm events and the implication in terms of flood modelling.[15] Some recent research considered the criteria for safe evacuation of individuals in flooded areas.[16] But some recent field measurements during the 2010–2011 Queensland floods showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by the velocity and water depth fluctuations.[14] These considerations ignore further the risks associated with large debris entrained by the flow motion.[16]

Urban flooding has significant economic implications. In the US, industry experts estimate that wet basements can lower property values by 10%-25% and are cited among the top reasons for not purchasing a home.[17] According to the U.S Federal Emergency Management Agency (FEMA), almost 40% of small businesses never reopen their doors following a flooding disaster.[18] In the UK, urban flooding is estimated to cost £270 million a year in England and Wales; 80,000 homes are at risk.[19]

A study of Cook County, Illinois, identified 177,000 property damage insurance claims made across 96% of the county's ZIP codes over a five-year period from 2007-2011. This is the equivalent of one in six properties in the County making a claim. Average payouts per claim were $3,733 across all types of claims, with total claims amounting to $660 million over the five years examined.[17]

Despite concerted efforts, many communities lack the funds to fully address these issues, and often seek funds elsewhere. Numerous watersheds within Los Angeles County, California do not meet state water quality standards, despite spending $100 million a year on clean water programs to combat issues such as urban runoff. To combat this problem, officials have introduced a measure that would assess a fee to homeowners and local businesses in attempt to raise $290 million for effective urban runoff management.[20]

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

Urban runoff contributes to water quality problems.[21]

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.[22]

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 incising 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.

An open runoff system in Africa

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. 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.

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.[23] 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.[24] See River engineering.

There are many different ways  that the polluted urban runoff can come to harm humans: through contaminating drinking water, disrupting food sources and even causing parts of beaches to be closed off due to risk of illness, it has a high harm-causing potential. It is often the case that post heavy rainfall there are warnings given for people to avoid beaches or water based activities. 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 then all the chemicals that the crops have been treated with are at least partially washed away and introduced into the water table. The new environments that these chemicals are introduced to suffer due to their presence as they kill native vegetation and bugs.

Effects of Urban Runoff on rural areas.

Stormwater management is hard to contain due to the lack of drainage systems facilitated to take in runoff that tends to overflow. There are possible changes that could be made that would mitigate the effect that urban and stormwater runoff has on salmon and the ecosystem at large.(18) This comes in many forms; from not dumping pollutants down the storm drain; to taking your car to the car wash and not washing it in the street. one of the most impactful ways is swales. Swales also known as Rain Gardens are relatively shallow patches of vegetation that lay on the side of many roads and busy streets. These gardens act as a form of filtration for the polluted water before it joins the water table, or seeps back out in the streams.These rain gardens are only necessary due to the fact that urbanization has disrupted the ecosystems natural filtration systems. The plants that are kept in these rain gardens are durable and referred to as ‘hyperaccumulators’ essentially meaning that they can absorb mass amounts of toxins and pollutants without being poisoned themselves. This means that there are ways to protect the ecosystem on a greater scale (oceans, beaches, and streams) but it does not mean that it will be widely implemented due to the fact that it would take mass amounts of funding and infrastructure changes in cities where swales became a norm. Some advantages of maintaining swales are: low capital costs, easy to incorporate into landscaping, and pollution blockage is easily dealt with. The positives of swales is that they are compatible with most environments they are in, utilizing native plants in an often aesthetically pleasing way to remove the pollutants. They have a low cost to implement and are beneficial in more ways than simply removing pollutants, they can help improve air quality. The positives of swales is that they are compatible with most environments they are in, utilizing native plants in an often aesthetically pleasing way to remove the pollutants.(5) They have a low cost to implement and are beneficial in more ways than simply removing pollutants, they can help improve air quality.

The effect on Salmon.

As water moves across the cement, it picks up all the particulates and oils that have been left on the surface from cars and urban living. The toxins that this water gathers include: heavy metals such as copper and nickel, and many different products of oil based origins, the microscopic bits of rubber that have shed off the tires of cars and the gasoline that leaked from the gas tank, these are both derived directly from oil, and are both detrimentally impacting our ecosystem. This water then runs down a storm drain, into the vegetation, or directly into a watershed. These places then lead into the oceans, streams, rivers, or lakes, in the area. In areas where rivers are home to the breeding grounds of Salmon the effect is quite shocking and visible.   The impact that the runoff has on our water table is disproportionate, affecting everything in the water, the soils around it, and the plants on the banks. What scientists have been focusing most on recently is why all the salmon are dying before breeding. The answer to that can be largely attributed to the effects that water runoff has on their brains. In some extreme cases the road runoff results in ‘acute mortality’ or the rapid death of the fish. It deteriorates the protection of the brain and the blood that surrounds it, corrupting the cerebrovascular system of the fish and rendering them either dead or severely sick and dying. The fish that do not die immediately; the ones that are left very sick are also left with neurological damage due to the lack of protection of the brain. In the period of development from being fry to being sea ready fish, most salmon feed on the small microorganisms that inhabit the streams: the plankton and the midge larva.(1)The issue here being that the little creatures that the Salmon feed on are also being impacted greatly by the runoff of from the roadways. This means that not only are these Salmon facing the detriment that is the poisonous pollution tainting their waterways, but they are also facing depleted food sources.  The increasing numbers of dying salmon is not an isolated negative impact, the lack of salmon then goes on to remove the food sources of Orca whales in the pacific northwest and other predators that rely on their existence to stay alive. On a less environmental scale the desolation of salmon would cause a drop in jobs and work opportunities for people working in the fishing industry, it would cause hatcheries that grow and release fish to become moot because the environment that the fish would be released into would not be sustainable or healthy.(3,4,)

The canopy of trees has a significant impact on the amount of stormwater or rain that actually reaches the ground during a storm. The water lands on the higher levels of trees or the foliage and when it lands at that level it has a higher likelihood of being evaporated back into the atmosphere without it ever entering the water table on the ground, it doesn’t get the opportunity to get become polluted by the chemicals because it never comes into contact with them. There is a way to calculate the total loss of water in the water table to the previously described process of evapotranspiration and it is the process of subtracting the amount of water that flows down the trunk or center of the tree, also called stemflow, along with throughflow, which is the water that makes its way through the canopy of the tree from the gross amount of water falling on the tree. The implementation of planting trees throughout cities that have issues with polluted runoff would largely reduce the amount of runoff created.(1,18)