Stormwater
Stormwater, also written storm water, is water that originates from precipitation, including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil and become groundwater, be stored on depressed land surface in ponds and puddles, evaporate back into the atmosphere, or contribute to surface runoff. Most runoff is conveyed directly as surface water to nearby streams, rivers or other large water bodies without treatment.
In natural landscapes, such as forests, soil absorbs much of the stormwater. Plants also reduce stormwater by improving infiltration, intercepting precipitation as it falls, and by taking up water through their roots. In developed environments, such as cities, unmanaged stormwater can create two major issues: one related to the volume and timing of runoff and the other related to potential contaminants the water is carrying. In addition to the pollutants carried in stormwater runoff, urban runoff is being recognized as a cause of pollution in its own right.
Stormwater is also an important resource as human population and demand for water grow, particularly in arid and drought-prone climates. Stormwater harvesting techniques and purification could potentially make some urban environments self-sustaining in terms of water.
Impacts of stormwater
Stormwater pollution
With less vegetation and more impervious surfaces, developed areas allow less rain to infiltrate into the ground, and more runoff is generated than in undeveloped conditions. Additionally, passages such as ditches and storm sewers quickly transport runoff away from commercial and residential areas into nearby water bodies. This greatly increases the volume of water in waterways and the discharge of those waterways, leading to erosion and flooding. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.A first flush is the initial runoff of a rainstorm. During this phase, polluted water entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.
Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, and other land surfaces. Such pollutants include trash, sediment, nutrients, bacteria, pesticides, metals, and petroleum byproducts. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or ocean. While there is some attenuation of these pollutants before entering receiving waters, polluted runoff results in large enough quantities of pollutants to impair receiving waters.
Stormwater runoff as a source of pollution
In addition to the pollutants carried in stormwater runoff, urban runoff is being recognized as a cause of pollution in its own right. In natural catchments surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storms. Before land development occurs in a particular area, most rainfall soaks into the ground and contributes to groundwater recharge, or is recycled into the atmosphere by vegetation through evapotranspiration.Modern drainage systems, which collect runoff from impervious surfaces, ensure that water is efficiently moved to waterways through pipe networks, meaning that even small storms result in increased waterway flows.
In addition to delivering higher pollutants from the urban catchment, increased stormwater flow can lead to stream erosion, encourage weed invasion, and alter natural flow regimes. Native species often rely on such flow regimes for spawning, juvenile development, and migration. Stormwater runoff from roadways has been observed to contain many metals including zinc, cadmium, copper, nickel, lead, chromium, manganese, iron, vanadium, cobalt, and aluminum and other constituents.
In some areas, especially along the U.S. coast, polluted runoff from roads and highways may be the largest source of water pollution. For example, about 75 percent of the toxic chemicals getting to Seattle, Washington's Puget Sound are carried by stormwater that runs off paved roads and driveways, rooftops, yards, and other developed land.Image:Natural & impervious cover diagrams EPA.jpg|thumb|Relationship between impervious surfaces and surface runoffIndustrial stormwater is runoff from precipitation that lands on industrial sites. This runoff is often polluted by materials that are handled or stored on the sites, and the facilities are subject to regulations to control the discharges.
Stormwater treatment
Stormwater management facilities are generally designed using Stokes' law to allow rudimentary treatment through the settling particulate matter larger than 40 micron in size and to impound water to reduce downstream flooding. However, regulation on the effluent from SWMFs is becoming more stringent. The effect of phosphorus, either dissolved from or bound to sediment particles from construction or agriculture runoff, causes algae and toxic cyanobacteria blooms in receiving lakes. Cyanotoxin is of particular concern as many drinking water treatment plants can not effectively remove this health hazard. In a recent municipal stormwater treatment study, an advanced sedimentation technology was used passively in large diameter stormwater mains upstream of SWMFs to remove an average of 90% of total suspended solids and phosphorus during a near 50 year rain event turning a management facility into a passive treatment facility.Passive treatment systems
Chemical treatment of stormwater to remove pollutants can be accomplished without large scale infrastructure improvements. Passive treatment technologies use the energy of water flowing by gravity through ditches, canals, culverts, pipes or other constructed conveyances to enable treatment. Self-dosing products, such as gel flocculants, are placed in the flowing water where sediment particles, colloids and flow energy combine to release the required dosage, thereby creating heavy flocs which can then be easily filtered or settled. Natural woven fibers like jute are often used in ditch bottoms to act as filtration media. Silt retention mats can also be placed in situ to capture floccules. Sedimentation in a forebay is often utilized as a deposition area to clarify the water and concentrate the material. Mining, heavy construction and other industries have used passive systems for more than twenty years. These types of systems are low carbon as no external power source is needed, they require little skill to operate, minimal maintenance and are effective at reducing TSS, some heavy metals and phosphorus.Urban flooding
Stormwater is a major cause of urban flooding, which is the inundation of land or property in a built-up environment caused by stormwater overwhelming the capacity of drainage systems, such as storm sewers. Although triggered by single events such as flash flooding or snow melt, urban flooding is a condition, characterized by its repetitive, costly and systemic impacts on communities. In areas susceptible to urban flooding, backwater valves and other infrastructure may be installed to mitigate losses.Where properties are built with basements, urban flooding is the primary cause of basement and sewer backups. Although the number of casualties from urban flooding is usually limited, the economic, social and environmental consequences can be considerable: in addition to direct damage to property and infrastructure, chronically wet houses are linked to an increase in respiratory problems and other illnesses. Sewer backups are often caused by defects in the sanitary sewer system, which takes on some storm water as a result of infiltration and inflow.
Stormwater creation of sinkhole collapses
An example of urban stormwater creating a sinkhole collapse is the February 25, 2002 Dishman Lane collapse in Bowling Green, Kentucky where a sinkhole suddenly dropped the road under four traveling vehicles. The nine-month repair of the Dishman Lane collapse cost a million dollars but there remains the potential for future problems.In undisturbed areas with natural subsurface drainage, soil and rock fragments choke karst openings, thereby being a self-limitation to the growth of openings. The undisturbed karst drainage system becomes balanced with the climate so it can drain the water produced by most storms. However, problems occur when the landscape is altered by urban development. In urban areas with natural subsurface karst drainage there are no surface streams for the increased stormwater from impervious surfaces such as roofs, parking lots, and streets to receive drainage. Instead, the stormwater enters the subsurface drainage system by moving down through the ground. When the subsurface water flow becomes great enough to transport soil and rock fragments, the karst openings grow rapidly. Where karst openings are roofed by supportive limestone, there frequently is no surface warning that an opening has grown so large it will suddenly collapse catastrophically. It is recommended that land-use planning agencies avoid karst areas when considering new development projects. Ultimately taxpayers end up paying the costs for poor land use decisions.