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SUPPLEMENTARY READING

Text 1

Urbanization

Before an area is urbanized, rain would have infiltrated the soil and recharged groundwater or slowly runoff into streams and coastal environments. Water quality would only be influenced by natural processes.

How does urbanization change things? Urban areas have large areas of hard surfaces such as concrete, bitumen and roofs, which don't allow rain to percolate into the soil. Instead, rainwater runs off these hard surfaces very quickly, becoming what is called stormwater. As stormwater travels across roads, carparks, gardens, etc, it collects many pollutants that are produced in cities. These pollutants include litter (e.g. plastic bags and drink bottles), nutrients (e.g. phosphate and nitrate in fertilizers and detergents), heavy metals (e.g. copper from motor cars and zinc from metal roofs), suspended solids (e.g. through soil erosion on building sites) and disease-causing bacteria (often from overflowing sewage pipes).

How does urbanization affect water quality? As urbanization has reduced water infiltration and groundwater recharge, urban streams now don't flow for long periods. Now, during rain events, urban areas quickly generate stormwater, inducing fast stream flows of short duration. This urban stormwater carries many pollutants that are quickly transported to downstream water bodies such as the creeks which run through an urban area. The effects of these polluted stormwater discharges can include significant ecological changes to the plant and animal communities in both urban streams and coastal waters, as well as rendering water unsafe for recreational activities.

Text 2

Silt.

Silt is soil or rock derived granular material of a specific grain size. Silt may occur as a soil or alternatively as suspended sediment in a water column of any surface water body. It may also exist as deposition soil at the bottom of a water body.

Source. Silt is generated by a variety of mode capable of breaking and splitting up generally sand-sized quartz crystals of primary rocks by exploiting deficiencies in their lattice. These involve chemical weathering of rock and a number of physical weathering processes. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. Sedimentary rock composed mainly of silt is known as siltstone. Silt, deposited by annual floods along the Nile River, created the rich and fertile soil that sustained the ancient Egyptian civilization. This silt was depended on for this purpose. A decrease in silt deposited by the Mississippi River throughout the 20th century has contributed to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans.

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Environmental impacts. Silt can occur as a deposit or as material transported by a stream or by a current in the ocean. Silt is easily transported in water and is fine enough to be carried long distances by air as 'dust'. Thick deposits of silty material resulting from aeolian deposition are often called loess (a German term) or limon (French). Silt and clay contribute to turbidity in water. One of the main causes of river siltation in the year 2006 is the result of slash and burn treatment of tropical forests. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive; for example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. The resulting sediment load in rivers flowing to the west is ongoing, with most rivers a dark red brown colour. The resulting fish kills in most of these rivers have resulted in the process of extinction of a variety of Madagascar's fish species.

Text 3

Pesticides.

Pesticides can be defined as chemicals to kill pests. Pesticides used in agricultural regions can be washed into rivers and streams after rain. Some pesticides are persistent and can be detected in water long after use. In South Australia pesticides have been found in waterways and groundwater, and historical contamination of soil is common around cattle and sheep dip sites. Pesticides have caused fish and aquatic invertebrate kills in inland and estuarine waters. Bird deaths have been attributed to pesticides, and spraying to kill locusts has been shown to affect other organisms.

Pest species. Pests or invasive species are usually introduced by humans. They threaten the survival of native plants and animals, and can also damage valuable agricultural and personal resources. Terrestrial and aquatic pests affect the health of our waterways as well as native animals and plants. For example, the mosquito fish (Gambusia holbrooki) was introduced from the USA to control mosquitoes. However, it now outnumbers native fish in many parts of south eastern Australia, as they out compete indigenous species for food. Exotic trees such as willows (Salix sp.) are another problem. Willows produce dense shade, suppressing under storey growth, resulting in bare banks that are susceptible to erosion. The trees are a poor habitat for land animals, and the population and diversity of aquatic invertebrates and native fish is greatly reduced under their canopy.

Text 4

Alteration of natural flows (hydroenergy)

Water is pumped from rivers and underground water supplies for use by rural towns, farms, industries and cities. Many rivers also feed dams and reservoirs for public water supplies and hydro-power, and are used as transport routes for boats.

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While these activities provide economic and social benefits, there are many adverse environmental effects caused by altering the natural flow of rivers (river regulation). These include the decline and loss of native species of plants and animals, encouragement of habitats favourable to pest species (carp, gambusia and redfin), declining water quality, and loss of amenity.

It is now widely recognised that changes to the flow regime have severely degraded most, if not all, regulated rivers in some way. The River Murray highlights this. So much water is removed from the river that less than 20% of the normal annual volume reaches the river mouth at Goolwa.

River regulation in the Murray-Darling Basin is so severe that giant river redgums which rely on frequent flooding are dying and the Murray Cod is threatened.

Major efforts are now under way to understand the impact of river regulation, and to develop strategies to restore and/or protect the natural flow regime of rivers and creeks to improve the environmental condition of our waterways.

To permit automobile traffic to travel along a road at any time of the year at high speeds and with economic fuel consumption, the road pavement must be of an adequate rigidity, uniformity and resistance to wear. These requirements can be satisfied by means of various combinations of pavement structural layers consisting of different road-building materials. The pavement service qualities, i.e., permissible speed and traffic comfort, are determined mainly by the nature of the surfacings, which can be divided into the following basic structural types, given in consecutive order of their development.