Soil sealing

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Introduction
A soil of good quality is of paramount importance to human living and well-being. Soil provides food to humans and animals. Soil is an important factor to human societies providing to humans with raw materials to create building materials. Decomposition of organic materials releases carbon which is stored in the soil. Portions of carbon not incorporated into soil are released to the atmosphere as carbon dioxide (CO2) thus contributing to greenhouse effect. Furthermore, soil has an important role to the hydrologic cycle by infiltrating, storing and replenishing groundwater (where potable water is mainly supplied from).


In the past soil quality deterioration was attributed to excessive fertilization and pollution with pesticides. In recent years though, a new factor was added: rapid urban population growth results in a large-scale residential expansion. Today, approximately half the world's population lives in cities and towns, while back in 1900 the percentage of population living in urban areas was almost 15%. City expansion is usually consuming nearby areas of precious agricultural or forest land. Even in coastal areas uncontrolled tourism development leads to the same result: the soil is covered with impermeable materials such as block paving, concrete or asphalt. It is this permanent covering of the soil surface with impermeable materials for the construction of infrastructure that is called “soil sealing”. In general, soil sealing can occur as side effect of any activity that causes changes in soil structure rendering it impermeable (e.g. compaction of soil from heavy agricultural machinery).

As a consequence of soil sealing, Due to sealing soil cannot fulfill its important ecological functions any longer. Soil can be regarded as a non-renewable resource in that its degradation rates can be rapid whereas the formation of few centimeters requires hundreds of years. The consequences of this degradation have immediate impact on human health, on the number and variety of habitats, on climate change, as well as on the economic development and quality of human life.

Soil sealing and water
Among the consequences of soil sealing, converting the soil into a virtually impermeable medium poses a threat on the cycle of water in nature. Rapid residential expansion, lack of “green” building design standards plus the lack of a strategy on soil protection result in decreased infiltration and increased runoff. Therefore, percolation of the water down through the soil to the aquifer is reduced, thus not replenishing the main source of potable water: the groundwater. Furthermore, in sealed areas during and after a storm event, runoff is both rapidly observed and increased, resulting in catastrophic flooding of areas characterized by uncontrolled residential development. Finally, the increase of mean air temperature observed in densely-built urban areas (Urban Heat Island phenomenon – UHI), is widely accepted by meteorologists to contribute to extreme storm events.

In 1994 the term Water Sensitive Urban Design (WSUD) was introduced by australian scientists to describe the new philosophy behind urban development that takes into account the adverse impact of human activities on the hydrologic cycle. Since then more and more countries recognize the threats posed by soil sealing and adopt low-cost methodologies of urban design to mitigate its effects.

In Hydrology the degree of soil sealing is estimated using the so called «runoff coefficient». A surface’s runoff coefficient represents the ratio of direct runoff volume to the volume of rainfall received by the surface. It is widely accepted that the main factors affecting runoff coefficient are land use and slope. Soil infiltration rate is considered as a secondary factor especially in urban areas. It is easily understood that a surface of high slope which is covered by impermeable materials (e.g. concrete, asphalt etc) will have higher value of runoff coefficient (and thus will be more vulnerable to floods) comparing to a level surface covered with vegetation. Therefore, the three aforementioned parameters must be accounted for in the determination of the type of urban development. The primary aim of WSUD should be the minimization of a region’s runoff coefficient change. In the opposite case, the quality of everyday life is deteriorating and economic growth is hampered.

In every region under urban development and in order to avoid the effects of soil sealing, studies to estimate the potential danger should be carried out. The scale of these studies can range from city level to the level of a single residential lot. To mitigate the sealing effects a series of measures should be adopted. Possible measures are:

  • Use of permeable construction materials. Permeable materials exist in the market for almost 30 years now. They can mitigate the sealing effects since they allow water to percolation through their structure down to the underlying soil.
  • Construction of vegetated swales, to allow runoff volumes to percolate to deeper soil layers before they are added to runoff volumes of neighboring lots
  • Expanding the urban forest through tree planting. Apart from improving air quality and improving human health, trees contribute also to a) the reduction of runoff volumes and to b) the reduction Urban Heat Island effects
  • Green roof construction. Green roofs (roofs with a vegetated surface and substrate) absorb rainwater, thus decreasing the total amount of runoff and slowing down the rate of runoff from the roof. This way a reduction of surface runoff volumes can be achieved