The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water, the water table is the surface where the water pressure head is equal to the atmospheric pressure. It may be visualized as the surface of the materials that are saturated with groundwater in a given vicinity. The groundwater may be from precipitation or from groundwater flowing into the aquifer, in areas with sufficient precipitation, water infiltrates through pore spaces in the soil, passing through the unsaturated zone. At increasing depths water fills in more of the spaces in the soils. Below the water table, in the zone, layers of permeable rock that yield groundwater are called aquifers. In less permeable soils, such as tight bedrock formations and historic lakebed deposits, the water table should not be confused with the water level in a deeper well. The elevation of the water in this well is dependent upon the pressure in the deeper aquifer and is referred to as the potentiometric surface.
The water table may vary due to changes such as precipitation and evapotranspiration. Springs, rivers and oases occur when the table reaches the surface. Springs commonly form on hillsides, where the Earths slanting surface may intersect with the water table, Groundwater entering rivers and lakes accounts for the base-flow water levels in water bodies. Within an aquifer, the table is rarely horizontal, but reflects the surface relief due to the capillary effect in soils, sediments. In the aquifer, groundwater flows from points of pressure to points of lower pressure. The slope of the table is known as the hydraulic gradient, which depends on the rate at which water is added to and removed from the aquifer. This occurs when there is a layer of rock or sediment or relatively impermeable layer above the main water table/aquifer. If a perched aquifers flow intersects the Earths dry surface, at a wall for example. On low-lying oceanic islands with porous soil, freshwater tends to collect in lenticular pools on top of the denser seawater intruding from the sides of the islands, such an islands freshwater lens, and thus the water table and falls with the tides.
In some regions, for example, Great Britain or California, winter precipitation is higher than summer precipitation
Impervious surfaces are mainly artificial structures—such as pavements that are covered by impenetrable materials such as asphalt, brick, stone—and rooftops. Soils compacted by urban development are highly impervious.4 percent of the nation as having an estimated impervious cover of 21 to 40 percent, in some cities, the flood waters get into combined sewers, causing them to overflow, flushing their raw sewage into streams. Polluted runoff can have negative effects on fish, plants. 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, 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, 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 areas, it is over 90 percent. In the contiguous 48 states of the USA, urban impervious cover adds up to 43,000 square miles — an area nearly the size of the State of Ohio, continuing development adds another quarter of a million acres each year. Typically two-thirds of the cover is pavements and one-third is building roofs, Impervious surface coverage can be limited by restricting land use density, but this approach causes land elsewhere to be developed, to accommodate growing population. Rainwater from impervious surfaces can be collected in tanks and used in place of main water. The percentage imperviousness, often referred to as PIMP in calculations, is an important factor when considering drainage of water and it is calculated by measuring the percentage of a catchment area which is made up of impervious surfaces such as roads and other paved surfaces.
An estimation of PIMP is given by PIMP =6. 4J^0.5 where J is the number of dwellings per hectare, for example, woodland has a PIMP value of 10%, whereas dense commercial areas have a PIMP value of 100%. This variable is used in the Flood Estimation Handbook, the total impervious area can be expressed as a fraction or a percentage. The U. S. NLCD impervious surface data set may provide a high-quality nationally-consistent land cover data set in a GIS-ready format that can be used to estimate TIA value, the NLCD consistently quantifies the percent anthropogenic TIA for the NLCD at a 30-meter pixel resolution throughout the Nation. Within the data set, each pixel is quantified as having a TIA value that ranges from 0 to 100 percent, TIA estimates made with the NLCD impervious surface data set represent an aggregated TIA value for each pixel rather than a TIA value for an individual impervious feature. For example, a two lane road in a field has a TIA value of 100 percent, but the pixel containing the road would have a TIA value of 26 percent
Pervious concrete is made using large aggregates with little to no fine aggregates. The concrete paste coats the aggregates and allows water to pass through the concrete slab, Pervious concrete is traditionally used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and greenhouses. It is an important application for construction and is one of many low impact development techniques used by builders to protect water quality. Pervious concrete was first used in the 1800s in Europe as pavement surfacing, cost efficiency was the main motive due to a decreased amount of cement. It became popular again in the 1920s for two storey homes in Scotland and England and it became increasingly viable in Europe after WWII due to the scarcity of cement. It did not become as popular in the US until the 1970s, in India it became popular in 2000. The proper utilization of pervious concrete is a recognized Best Management Practice by the U. S. Environmental Protection Agency for providing first flush pollution control and stormwater management.
As regulations further limit stormwater runoff, it is becoming expensive for property owners to develop real estate, due to the size. Pervious concrete lowers the sites SCS Curve Number by retaining stormwater on site and this allows the planner/designer to achieve pre-development stormwater goals for pavement intense projects. Pervious concrete reduces the runoff from paved areas, which reduces the need for stormwater retention ponds. This allows property owners to develop an area of available property at a lower cost. Pervious concrete naturally filters storm water and can reduce pollutant loads entering into streams, all of these benefits lead to more effective land use. Pervious concrete can reduce the impact of development on trees. A pervious concrete pavement allows the transfer of water and air to root systems allowing trees to flourish even in highly developed areas. Pervious concrete consists of cement, coarse aggregate and water with little to no fine aggregates, the addition of a small amount of sand will increase the strength.
The mixture has a water-to-cement ratio of 0.28 to 0.40 with a content of 15 to 25 percent. The correct quantity of water in the concrete is critical, a low water to cement ratio will increase the strength of the concrete, but too little water may cause surface failure. A proper water content gives the mixture a wet-metallic appearance, as this concrete is sensitive to water content, the mixture should be field checked
A wetland is a land area that is saturated with water, either permanently or seasonally, such that it takes on the characteristics of a distinct ecosystem. The primary factor that distinguishes wetlands from other forms or water bodies is the characteristic vegetation of aquatic plants. Wetlands play a number of roles in the environment, principally water purification, flood control, carbon sink, Wetlands are considered the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal life. Wetlands occur naturally on every continent except Antarctica, the largest including the Amazon River basin, the West Siberian Plain, the water found in wetlands can be freshwater, brackish, or saltwater. The main wetland types include swamps, marshes and fens, and sub-types include mangrove, pocosin, the UN Millennium Ecosystem Assessment determined that environmental degradation is more prominent within wetland systems than any other ecosystem on Earth. International conservation efforts are being used in conjunction with the development of rapid assessment tools to people about wetland issues.
Constructed wetlands can be used to treat municipal and industrial wastewater as well as stormwater runoff and they may play a role in water-sensitive urban design. A patch of land that develops pools of water after a storm would not be considered a wetland. Wetlands have unique characteristics, they are distinguished from other water bodies or landforms based on their water level. Specifically, wetlands are characterized as having a table that stands at or near the land surface for a long enough period each year to support aquatic plants. A more concise definition is a community composed of hydric soil, Wetlands have been described as ecotones, providing a transition between dry land and water bodies. In environmental decision-making, there are subsets of definitions that are agreed upon to make regulatory and policy decisions. A wetland is an ecosystem that arises when inundation by water produces soils dominated by anaerobic processes, There are four main kinds of wetlands – marsh, swamp and fen.
Some experts recognize wet meadows and aquatic ecosystems as additional wetland types, the largest wetlands in the world include the swamp forests of the Amazon and the peatlands of Siberia. Under the Ramsar international wetland conservation treaty, wetlands are defined as follows, Article 2.1, may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands. Although the general definition given above applies around the world, each county, Wetlands generally include swamps, marshes and similar areas. This definition has been used in the enforcement of the Clean Water Act, some US states, such as Massachusetts and New York, have separate definitions that may differ from the federal governments. It is not uncommon for a wetland to be dry for long portions of the growing season, the most important factor producing wetlands is flooding
In addition to describing a particular form of urbanization, the term relates to the social and environmental consequences associated with this development. In Continental Europe the term peri-urbanisation is often used to denote similar dynamics and phenomena, There is widespread disagreement about what constitutes sprawl and how to quantify it. For example, some commentators measure sprawl only with the number of residential units per acre in a given area. But others associate it with decentralization, segregation of uses, the term urban sprawl is highly politicized, and almost always has negative connotations. It is criticized for causing environmental degradation, and intensifying segregation and undermining the vitality of existing urban areas, due to the pejorative meaning of the term, few openly support urban sprawl as such. The term has become a cry for managing urban growth. Definitions of sprawl vary, researchers in the field acknowledge that the term lacks precision and he argued that a better way to identify sprawl was to use indicators rather than characteristics because this was a more flexible and less arbitrary method.
He proposed using accessibility and functional space as indicators. Ewings approach has been criticized for assuming that sprawl is defined by negative characteristics, what constitutes sprawl may be considered a matter of degree and will always be somewhat subjective under many definitions of the term. The following characteristics are associated with sprawl, This refers to a situation where commercial, residential and industrial areas are separated from one another. Consequently, large tracts of land are devoted to a use and are segregated from one another by open space, infrastructure. The degree to which different land uses are mixed together is used as an indicator of sprawl in studies of the subject. Job sprawl is another land use symptom of urban sprawl and car-dependent communities, spatial mismatch is related to job sprawl and economic environmental justice. Job sprawl has been documented and measured in various ways and it has been shown to be a growing trend in Americas metropolitan areas.
The Brookings Institution has published articles on the topic. In 2005, author Michael Stoll defined job sprawl simply as jobs located more than 5-mile radius from the CBD, and measured the concept based on year 2000 U. S. Census data. These two authors used three geographic rings limited to a 35-mile radius around the CBD,3 miles or less,3 to 10 miles and this compares to the year 1998 -23. 3%,34. 2%, and 42. 5% in those respective rings. The study shows CBD employment share shrinking, and job growth focused in the suburban and exurban outer metropolitan rings, Sprawl is often characterized as consisting of low-density development
A flash flood is a rapid flooding of geomorphic low-lying areas, rivers, dry lakes and basins. It may be caused by heavy rain associated with a thunderstorm, tropical storm. Flash floods may occur after the collapse of an ice or debris dam, or a human structure such as a man-made dam. Flash floods are distinguished from regular floods by a timescale of less than six hours, the water that is temporarily available is often used by foliage with rapid germination and short growth cycles, and by specially adapted animal life. Flash floods can occur under several types of conditions, Flash flooding occurs when it rains rapidly on saturated soil or dry soil that has poor absorption ability. The runoff collects in gullies and streams and, as they join to form larger volumes, often forms a fast flowing front of water, in areas on or near volcanoes, flash floods have occurred after eruptions, when glaciers have been melted by the intense heat. Flash floods are known to occur in the highest mountain ranges of the United States and are common in the arid plains of the Southwestern United States.
Flash flooding can be caused by extensive rainfall released by hurricanes and other tropical storms, human activities can cause flash floods to occur. When dams fail, a quantity of water can be released. Many people tend to underestimate the dangers of flash floods, what makes flash floods most dangerous is their sudden nature and fast-moving water. A vehicle provides little to no protection against being swept away, it may make people overconfident, more than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections. As little as 2 feet of water is enough to carry away most SUV-sized vehicles. The U. S. National Weather Service reported in 2005 that, using a national 30-year average, more people die yearly in floods,127 on average, than by lightning, tornadoes, in deserts, flash floods can be particularly deadly for several reasons. First, storms in arid regions are infrequent, but they can deliver an enormous amount of water in a short time.
Second, these rains often fall on poorly absorbent and often clay-like soil, in fact, in some areas, desert roads frequently cross dry river and creek beds without bridges. From the drivers perspective, there may be clear weather, when a river unexpectedly forms ahead of or around the vehicle in a matter of seconds. Finally, the lack of rain to clear water channels may cause flash floods in deserts to be headed by large amounts of debris, such as rocks, branches. 1889, Johnstown Flood, Pennsylvania, U. S. more than 2,200 people dead 1903, Heppner Flood of 1903, United States,247 dead, 25% of the city 1938, Los Angeles Flood of 1938, California, U. S. S
The Minoan civilization was an Aegean Bronze Age civilization on the island of Crete and other Aegean islands which flourished from about 2600 to 1100 BC. It preceded the Mycenaean civilization of Ancient Greece, the civilization was rediscovered at the beginning of the 20th century through the work of British archaeologist Arthur Evans. It has been described as the earliest of its kind in Europe, the term Minoan, which refers to the mythical King Minos, originally described the pottery of the period. Minos was associated in Greek mythology with the labyrinth and the Minotaur, according to Homer, Crete once had 90 cities. The Minoan period saw trade between Crete and Aegean and Mediterranean settlements, particularly the Near East and artists, the Minoan cultural influence reached beyond Crete to the Cyclades, Egypts Old Kingdom, copper-bearing Cyprus and the Levantine coast, and Anatolia. Some of its best art is preserved in the city of Akrotiri on the island of Santorini, although the Minoan language and writing systems remain undecipherable and are subjects of academic dispute, they apparently conveyed a language entirely different from the Greek.
The reason for the end of the Minoan period is unclear, theories include Mycenaean invasions from mainland Greece, the term Minoan refers to the mythical King Minos of Knossos. Its origin is debated, but it is attributed to archeologist Arthur Evans. Minos was associated in Greek mythology with the labyrinth, which Evans identified with the site at Knossos. However, Karl Hoeck had already used the title Das Minoische Kreta in 1825 for volume two of his Kreta, this appears to be the first known use of the word Minoan to mean ancient Cretan, Evans said that applied it, not invented it. Hoeck, with no idea that the archaeological Crete had existed, had in mind the Crete of mythology, although Evans 1931 claim that the term was unminted before he used it was called a brazen suggestion by Karadimas and Momigliano, he coined its archaeological meaning. Instead of dating the Minoan period, archaeologists use two systems of relative chronology, the first, created by Evans and modified by archaeologists, is based on pottery styles and imported Egyptian artifacts.
Evans system divides the Minoan period into three eras, early and late. These eras are subdivided—for example, Early Minoan I, II and III, another dating system, proposed by Greek archaeologist Nicolas Platon, is based on the development of architectural complexes known as palaces at Knossos, Phaistos and Kato Zakros. Platon divides the Minoan period into pre-, proto-, neo-, the relationship between the systems in the table includes approximate calendar dates from Warren and Hankey. The Thera eruption occurred during a phase of the LM IA period. Efforts to establish the volcanic eruptions date have been controversial, the eruption is identified as a natural event catastrophic for the culture, leading to its rapid collapse. Although stone-tool evidence exists that hominins may have reached Crete as early as 130,000 years ago, evidence for the first anatomically-modern human presence dates to 10, the oldest evidence of modern human habitation on Crete are pre-ceramic Neolithic farming-community remains which date to about 7000 BC
A swale is a low tract of land, especially one that is moist or marshy. The term can refer to a landscape feature or a human-created one. Artificial swales are often infiltration basins designed to manage water runoff, filter pollutants, the swale concept has been popularized as a rainwater harvesting and soil conservation strategy by Bill Mollison, Geoff Lawton and other advocates of permaculture. In this context it refers to a water-harvesting ditch on contour. Another term used is contour bund, swales as used in permaculture are designed to slow and capture runoff by spreading it horizontally across the landscape, facilitating runoff infiltration into the soil. This type of swale is created by digging a ditch on contour, in arid climates, vegetation along the swale can benefit from the concentration of runoff. The term swale or beach swale is used to long, narrow
An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials from which groundwater can be extracted using a water well. The study of flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude, if the impermeable area overlies the aquifer, pressure could cause it to become a confined aquifer. Aquifers may occur at various depths and those closer to the surface are not only more likely to be used for water supply and irrigation, but are more likely to be topped up by the local rainfall. Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources, overexploitation can lead to the exceeding of the practical sustained yield, i. e. more water is taken out than can be replenished. The beach provides a model to help visualize an aquifer, if a hole is dug into the sand, very wet or saturated sand will be located at a shallow depth.
This hole is a well, the wet sand represents an aquifer. In 2013 large freshwater aquifers were discovered under continental shelves off Australia, North America and they contain an estimated half a million cubic kilometers of “low salinity” water that could be economically processed into potable water. The reserves formed when ocean levels were lower and rainwater made its way into the ground in areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100x the amount of extracted from other aquifers since 1900. The above diagram indicates typical flow directions in a view of a simple confined or unconfined aquifer system. The system shows two aquifers with one aquitard between them, surrounded by the bedrock aquiclude, which is in contact with a gaining stream, the water table and unsaturated zone are illustrated. An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another, an aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge.
Aquitards are composed of layers of clay or non-porous rock with low hydraulic conductivity. Groundwater can be found at every point in the Earths shallow subsurface to some degree. Saturated means the head of the water is greater than atmospheric pressure. The definition of the table is the surface where the pressure head is equal to atmospheric pressure. Unsaturated conditions occur above the table where the pressure head is negative
A French drain or weeping tile is a trench filled with gravel or rock or containing a perforated pipe that redirects surface water and groundwater away from an area. A French drain can have perforated hollow pipes along the bottom to quickly vent water that seeps down through the gravel or rock. French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations, French drains may be used to distribute water, such as a septic drain field at the outlet of a typical septic tank sewage treatment system. French drains are used behind retaining walls to relieve water pressure. The earliest forms of French drains were simple ditches, pitched from an area to a lower one. Frenchs own drains were made of sections of ordinary roofing tile laid with a 1⁄8 in gap left in between the sections to admit water, specialised drain tiles were designed with perforations. To prevent clogging, the size varied from coarse at the center to fine at the outside and was designed based on the gradation of the soil surrounding the drain.
The particle sizing was critical to keep the soil from washing into the voids in the gravel. The development of greatly simplified this procedure. Ditches may be dug by hand or with a trencher, an inclination of 1 in 100 to 1 in 200 is typical. Lining the bottom of the ditch with clay or plastic pipe increases the volume of water that can flow through the drain, modern French drain systems can be made with perforated pipe surrounded by sand or gravel and geotextile or landscaping textile. Landscaping textiles are used to prevent migration of the material as well as preventing dirt. The direction of percolation will depend on the conditions inside and outside the pipe. Subsurface drainage systems have been in use for centuries. They take many forms, but are all similar in design, variations on the French drain model include, French drains can lead to a downhill slope or to dry wells or rain gardens where the extra water is held and absorbed by plants. This is useful when city water systems or other wastewater areas cannot be used, depending on the expected level and volume of rain water or runoff, French drains can be widened or founded on 2 or 3 underground drain pipes.
Multiple pipes provide for redundancy, in case one pipe becomes overfilled or clogged by a rupture or defect in the piping, 2) Envelopes – Gravel, rock or surrounding pipe. These are permeable materials placed around pipe or drainage products to improve conditions in the area immediately around the drain and for improving bedding
Urbanization refers to the population shift from rural to urban areas, the gradual increase in the proportion of people living in urban areas, and the ways in which each society adapts to the change. It is predominantly the process by which towns and cities are formed, the United Nations projected that half of the worlds population would live in urban areas at the end of 2008. It is predicted that by 2050 about 64% of the developing world and that is equivalent to approximately 3 billion urbanites by 2050, much of which will occur in Africa and Asia. Notably, the United Nations has projected that nearly all global population growth from 2017 to 2030 will be absorbed by cities. Urbanization is relevant to a range of disciplines, including geography, economics, urban planning, the phenomenon has been closely linked to modernization, industrialization, and the sociological process of rationalization. Urbanization can be seen as a condition at a set time or as an increase in that condition over time.
The first major change in settlement patterns was the accumulation of hunter-gatherers into villages many years ago. This unprecedented movement of people is forecast to continue and intensify during the few decades. Outside Asia, Mexico City, São Paulo, New York City, Lagos, in England the proportion of the population living in cities jumped from 17% in 1801 to 72% in 1891. Growing trade around the world allowed cereals to be imported from North America and refrigerated meat from Australasia, cities expanded due to the development of public transport systems, which facilitated commutes of longer distances to the city centre for the working class. Urbanization rapidly spread across the Western world and, since the 1950s, at the turn of the 20th century, just 15% of the world population lived in cities. According to the UN the year 2007 witnessed the turning point when more than 50% of the population were living in cities. Living in a city can provide opportunities of proximity, diversity, as against this, there may be alienation issues, increased cost of living, and negative social aspects that result from mass marginalization.
In cities, services and opportunities are centralized, many rural inhabitants come to the city to seek their fortune and alter their social position. Businesses, which provide jobs and exchange capital, are concentrated in urban areas. Whether the source is trade or tourism, it is through the ports or banking systems, commonly located in cities, many people move into cities for the economic opportunities, but this does not fully explain the very high recent urbanization rates in places like China and India. Rural flight is a factor to urbanization. Farm living has always been susceptible to environmental conditions, and in times of drought, flood or pestilence