Soils covered by impervious surfaces
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Urban Heat Islands are basically originated by the coating of natural soil surfaces with urban construction materials. This means to modify the radiatve and thermal properties of the soil surface. Thus, when vegetated land covers are changed to concrete, asphalt and others low albedo and high heat-capacity urban materials, the energy stored and released by the earth surface increases. Furthermore, most of these materials are highly impervious and hence, they prevent the soil water infiltration in a natural way, modifying the hydrologic cycle of a drainage basin.
"Hot materials" are increasingly covering the soil surface of many cities. Parking lots, roofs, roads, sidewalks, driveways, synthetic soccer fields and many others urban features block the soil water infiltration, which is the opposite way from that which most natural soil surfaces. In a drainage basin scale, impervious surfaces and waste water collectors represents lots of challenges: floods, water contamination, groundwater infiltration and water pollution. As you will see on the following paragraphs, many of these issues could be solved by changing the urbanization pattern and trend, planting trees and using well-known "cold" and porous urban construction materials.
Urban land covers are often refered as "hardscapes" in opposition to natural land covers or "softscapes". Since hardscapes are composed of impervious surfaces, they avoid the water drainage into the subjacent soil. Furthermore, subjacent natural soils can also be highly impervious because of the compactation introduced by urban development.
An example is represented by the urbanization of part of Cousino Macul vineyards in the city of Santiago, Chile. This is typical case of land cover modification as consequence of urban sprawl. The following video was made with a sequence of historic satellite's images obtained from the Google Earth free software. It shows the south-west area of the vineyard which was sold around 10 years ago, changing the land use/land cover from rural to urban.
Urban Sprawl in Santiago de Chile
Inside the yellow square you can see how an old vineyard was surrounded by suburban development and part of its area was sold to build new houses. These new houses, supermarkets and shopping centers were build in only 10 years (from 2000 to 2010). The same urbanization pattern occurs in many cities around the world. In the case showed in this video, soil and vineyards were changed to urban construction materials, thus increasing the urban heat island effect and the area of impervious surfaces inside the drainage basin of Maipo valley. This reduces the natural percolation of the water down through the soil and accelerates runoff, all of which puts additional pressure on the storm water pipelines.
Land surface temperatures of Santiago, Chile. 1998 (left) and 2005 (right). The city is warmer than the southern rural valley but colder than the northern rural valley (for further references consult Peña (2008) and Peña (2009) articles, see the bibliography section of this website).
"Urbanization increases surface runoff, by creating more impervious surfaces such as pavement and buildings, that do not allow percolation of the water down through the soil to the aquifer. It is instead forced directly into streams or storm water runoff drains, where erosion and siltation can be major problems, even when flooding is not. Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for farmers and others who depend on the water wells."(Wikipedia, 2010).
"When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to create water pollution. This pollutant load can reach various receiving waters such as streams, rivers, lakes, estuaries and oceans with resultant water chemistry changes to these water systems and their related ecosystems."(Wikipedia, 2010).
Environmental effects of impervious surfaces
"Impervious surfaces are an environmental concern because, with their construction, a chain of events is initiated that modifies urban air and water resources: The pavement materials seal the soil surface, eliminating rainwater infiltration and natural groundwater recharge. From a recent article in the Seattle Times : "While urban areas cover only 3 percent of the U.S., it is estimated that their runoff is the primary source of pollution in 13 percent of rivers, 18 percent of lakes and 32 percent of estuaries."[1]"(Wikipedia, 2010).
"Some of these pollutants include excess nutrients from fertilizers; pathogens from pet waste; gasoline, motor oil, and heavy metals from vehicles; high sediment loads from stream bed erosion and construction sites, and waste such as cigarette butts, 6-pack holders and plastic bags carried by surges of stormwater. In some cities, the flood waters get into combined sewers, causing them to overflow, flushing their raw sewage into streams. Polluted runoff can have many negative effects on fish, animals, plants and people."(Wikipedia, 2010).
"Impervious surfaces collect solar heat in their dense mass. When the heat is released, it raises air temperatures, producing urban "heat islands", and increasing energy consumption in buildings. The warm runoff from impervious surfaces reduces dissolved oxygen in stream water, making aquatic life still harder."(Wikipedia, 2010).
"Impervious pavements deprive tree roots of aeration, eliminating the "urban forest" and the canopy shade that would otherwise moderate urban climate. Because impervious surfaces displace living vegetation, they reduce ecological productivity, and interrupt atmospheric carbon cycling."(Wikipedia, 2010).
For instance, most urban rooftops are completely impervious. "The total coverage by impervious surfaces in an area, such as a municipality or a watershed is usually expressed as a percentage of the total land area. The coverage increases with rising urbanization. In rural areas, impervious cover may only be one or two percent. In residential areas, coverage increases from about 10 percent in low-density subdivisions to over 50 percent in multi-family communities. In industrial and commercial areas, coverage rises above 70 percent. In regional shopping centers and dense urban areas, it is over 90 percent. In the contiguous 48 states of the USA, urban impervious cover adds up to 43,000 square miles (110,000 km²) — an area nearly the size of the State of Ohio. Continuing development adds another quarter of a million acres (1,000 km²) each year. Typically two-thirds of the cover is pavements and one-third is building roofs.[2]"(Wikipedia, 2010).
Impacts
When rains in a natural soil with trees and plants the water do infiltrate the soil and flows slowly to the rivers, the lakes and the sea as groundwater.
When rains in an impervious surface instead, the water is concentrated in ponds flooding and running fast through the streets and flowing through the specially conceived water pipelines and sewers. But when heavy rains, all the water which can not infiltrate to the soil and goes directly to this pipelines can overload the network collapsing the load and the rain water overflow to the sewers. Therefore rainwater and wastewaters mixes.
"The water table in and around large areas of hardscape is usually very depleted because not enough rainwater is being absorbed into the soil of that area in order to help recharge the water table in that (usually urban) area. Such areas must then rely largely on "imported" freshwater from local or non-local lakes, reservoirs, dams, rivers, and streams. On the other hand, most homes in rural areas often use wells and springs as their primary source of freshwater because the local water table is being constantly recharged by the hydrologic cycle."(Wikipedia, 2010).
"Without nearby bare soil, a hardscape requires artificial methods of drainage/runoff in order to carry off the sometimes massive volumes of water that would normally be mostly absorbed into the ground as groundwater. Lack of capacity can cause major problems after severe thunderstorms, hurricanes, typhoons, etc."(Wikipedia, 2010).
"Because impervious surfaces (parking lots, roads, buildings, compacted soil) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding when the stormwater collection system is overwhelmed by the additional flow. 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.[3]"(Wikipedia, 2010).
Therefore, exist a relationship between impervious surfaces and surface runoff, the sewers and pipelines are overflowed and they run out by gravity to the deepest side of the city flooding this areas. In other cases, all this water arrives straight to the purification plant but because there is too much water to treat, they open the doors and leave the mix of wastewater and rainwater pass through the plant and finally sends all the polluted water directly to the rivers, the lakes, the sea and so on.
"Pollutants entering surface waters during precipitation events is termed polluted runoff. Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering the receiving waters, the quantity of human activity results in large enough quantities of pollutants to impair these receiving waters."(Wikipedia, 2010).
Consequences
This is a dangerous pattern not only because the danger of flood over economics and public health is high, cities are increasing his imprevoius surface every day and urban heat islands entails more violent convetive rains and hot front storms. "Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. The frequency with which this occurs is described by a return period. Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered by land use changes such as river engineering. Floods can be both beneficial to societies or cause damage. Agriculture along the Nile floodplain took advantage of the seasonal flooding that deposited nutrients beneficial for crops. However, as the number and susceptibility of settlements increase, flooding increasingly becomes a natural hazard. Adverse impacts span loss of life, property damage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness. Floods are among the most devastating of natural disasters."(Wikipedia, 2010).
Source: http://www.plataformaurbana.cl (2010)
Mitigation of environmental impacts
The most effective measure to mitigate this problem is to plant trees and urban planing. All that big surface covered by impervious materials should consider trees on its design. In some cases is just enough good to install green roofs and cover the roads and parkings with porous asphalt and cement. In that case, urban heat island will be diminished as well.
"Impervious surface coverage can be limited by restricting land use density (such as number of homes per acre in a subdivision), but this approach causes land elsewhere (outside the subdivision) to be developed, to accommodate growing population. (See urban sprawl.) Alternatively, urban structures can be built differently to make them function more like naturally pervious soils; examples of such alternative structures are porous pavements, green roofs and infiltration basins. Rainwater from impervious surfaces can be collected in rainwater tanks and used in place of mains water." (Wikipedia, 2010).
"Partly in response to recent criticism by municipalities, a number of concrete manufacturers such as CEMEX and Quikrete have begun producing permeable materials which partly mitigate the environmental impact of conventional impervious concrete. These new materials are composed of various combinations of naturally-derived solids including fine to coarse-grained rocks and minerals, organic matter (including living organisms), ice, weathered rock and precipitates, liquids primarily water solutions, and gases.[3]" (Wikipedia, 2010).
Last update: May 3th, 2010
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