A dam is a bank or construction of stone, earth, or wood etc across a stream or river for the purpose of keeping back the current to give it increased head, for holding back supplies of water, for floodinglands, for rendering the stream above the dam navigable by increased depth, and for generating electricity. Its material and construction will depend on its situation and the amount of pressure it has to bear. For streams which are broad and deep strong materials are required, usually stone masonry bound in hydraulic cement and a strong framework of metal or timber. The common forms of a dam are either a straight line crossing the stream transversely, or one or two straight lines traversing it diagonally, or an arc with its convex side towards the current. Research Dam
A levee is an embankment along the course of a river. Natural levees are low banks that are produced by the river during floods, when the overflowing of the river decreases the speed of the water and permits the deposit of silt. Artificial levees are considerably higher than natural ones and are built in order to protect the surrounding countryside from floods, and as such levees are similar to the protective dikes in the Netherlands that prevent flooding by the sea. On a large river such as the Thames in England or the Mississippi in the USA, floods cannot be controlled by levees alone because the waters rise to heights that would overwhelm any embankment. Research Levee
A valley is a long narrow depression in the earth's crust, flanked by well defined ridges and usually due to the erosive action of rivers or glaciers but sometimes due to trough-faulting.
Longitudinal valleys are the hollows between the up-folded mountain ranges, parallel to the mountains, and they usually contain a largee river. Similar valleys occur between upfoldod mountains and the crustal plateau which has resisted upheaval. The Indo-Gangetic valley between the upfolded Himalayas and the Deccanplateau is the largest example of this type.
The valley cut by vertical erosion is usually V-shaped in cross-section and irregular in its course, its gradient being punctuated by sudden drops and long shelves. These irregularities represent local base levels which are gradually removed by denudation, so that as the falls are worn back and lakes infilled the breaks in the profile are reduced. In southern England the valleys of the Severn and the Thames show the results of denudation, which has carved away the softer rocks, and left the more resistant ridges of the Cotswolds, Downs, and Chilterns, which confine the drainage system.
With lateral erosion and mass movement, the valley broadens. Deposition occurs as the gradient slackens, and floodplains fill the valley floor. Rejuvenation leaves remnants of old floodplains above the new ones in the form of terraces, the highest of which are the oldest. A lowering of the water-table may leave dry valleys, and sudden uplift may leave hanging valleys, while the flooding of valleys by the sea gives rias or 'drowned valleys' which are existing estuaries where the sea has encroached upon the lower courses of rivers, such as the Gulf of St Lawrence. Research Valley
The beaver (Castor) is the only genus of the family Castoridae. The family contains a single genus, Castor, with two species: Castor canadensis, found in the New World, and Castor fibre, found in the Old World. Both species are semi-aquatic rodents noted for the building of dams. The two species differ chiefly in the shape of the nasal bones and are so much alike that some authorities consider them to be varieties of the same species. They are large rodents; the average adult beaver weighs about 16 kg, but specimens as heavy as 40 kg have been found, and some extinct beavers were almost bear- like in size. The beaver is usually about 76 centimetres long and stands less than 30 centimetres high, with a broad, flat, scaly tail about 25 centimetres long.
The body is plump, the back arched, the neck thick, the hind feet webbed, and all the digits clawed. The fur is usually reddish- brown above and lighter or greyish below. The eyes are small and the nostrils closable. The skull is massive, with marked ridges for fixing the muscles that work the jaws. The two front teeth on either jaw are like those of other rodents, wearing away more rapidly behind so as to leave a sharp, enamelled chisel edge. With these the beaver can cut down large trees. It usually selects trees five to 20 centimetres in diameter, but it can fell trees with diameters as large as 76 cm.
Beavers have a pair of anal scent glands, called castors, that secrete a musk-like substance called castoreum, probably for marking territories. The animals tend to be monogamous and have a life span of 20 years or more. The female has one litter a year, usually of two to four young.
Beavers are social animals and in areas where food is abundant and the locality secluded, the number of families in a beaver community can be rather large. The so-called beaver lodge is a unique structure. Three distinct kinds exist, their differences depending on whether they are built on islands, on the banks of ponds, or on the shores of lakes. The island lodge consists of a central chamber, with its floor a little above the level of the water, and with two entrances. One of these, the 'wood entrance', is a straight incline rising from the water, opening into the floor of the hut. The other approach, the 'beaver entrance', is more abrupt in its descent to the water.
The lodge itself is an oven-shaped house of sticks, grass, and moss, woven together and plastered with mud. The room inside is carpeted with bark, grass, and wood chips, sometimes with special store rooms adjoining. The pond lodge is built either a short way back from the edge of the bank, or partly hanging over it, with the front wall built up from the bottom of the pond. The lakelodge is built on the shelving shores of lakes. Dams are used by beavers to widen the area and increase the depth of water around their homes and are constructed either of sticks and poles or more firmly and solidly of mud, brushwood, and stones. As time goes by the beaver repairs and adds to the dam. Floating material lodges there, and vegetation growing on the top adds its roots to the strength of the dam. Frequently the beaver builds a smaller dam downstream in order to back up some water against the original dam and thus decrease the pressure of water on it from the other side. The dams are about 1.5 metres high, usually more than three metres wide at the base, and narrow at the top. A beaver dam more than 300 metres long was found in Rocky Mountain National Park, Colorado.
Beaver ponds eventually fill with sediment, and the animals move to a new location. The abandoned area becomes good meadowland. Beaver dams also help control runoff. Although the beaver is a powerful swimmer, it has difficulty dragging over the ground the logs and branches it needs for building and for food. Colonies of beavers therefore often dig canals from the pond to a grove of trees. Such canals are up to one metre wide and deep and often a few hundred metres long. The timber is then readily floated down the canal toward the pond.
Beavers have long been exploited for their fur, and for many years during the 18th and 19th centuries hundreds of thousands of beaver skins were exported to Europe from North America annually. The animals were also sometimes destroyed because of the damage they did to forests and the flooding occasionally caused by dams. Ceaseless slaughter led to near extinction of beavers in both Europe and North America. The beaver is almost extinct in Europe, but is becoming re-established in Canada and in protected areas of the USA. Research Beaver
In Sumerian mythology and in Babylonian mythology, Adad is a storm god, son of Anu. He holds a lightning bolt in his right hand and an axe in his left. He is partially responsible for the helpful annual flooding of the rivers which brought fertility to the land. He relates to the Canaanite god Hadad. Research Adad
In Chinese mythology, the dragon or long is a beneficial spirit of the moist yang principle. The dwell in the clouds or waters. There were five sorts: heavenly, which guarded the mansions of the gods; spiritual, which controlled winds and rains, and only accidentally caused flooding; earthly, which cleared rivers and deepened seas; those of hidden treasure; and imperial, marked by five claws, the others having four.
In Christian tradition, during the Middle Ages the dragon was symbolic of paganism, rather than a winged crocodile-like creature that breathes fire. Research Dragon
Channel efficiency is a measure of the ability of a river channel to discharge water. Channel efficiency can be assessed by calculating the channel's hydraulic radius . The most efficient channels are generally semicircular in cross-section, and it is this shape that water engineers try to create when altering a river channel to reduce the risk of flooding . Research Channel Efficiency
The hydraulic radius is a measure of a river's channel efficiency, and is used by water engineers to assess the likelihood of flooding. The hydraulic radius of a channel is defined as the ratio of its cross-sectional area to its wetted perimeter. The greater the hydraulic radius, the greater the efficiency of the channel and the less likely the river is to flood. The highest values occur when channels are deep, narrow, and semi-circular in shape. Research Hydraulic Radius
Denial of Service (DoS) is a form of malicious attack conducted against a computer server with the intention of denying the server the ability to conduct its rightful service. Typical denial of service attacks are launched against http (web) servers to prevent access to the web pages published by them, and against email servers to prevent their functioning. Other Denial of service atacks may target firewalls and routers on networks, thereby preventing tarffic in and out of a network. The most popular and easiest forms of denial of service attacks are caused by flooding (such as with the Pepsi UDP Flooder), the attacker simply sends a constant stream of data to a victim machine. In a typical attack (known as SYN Flooding), a malicious attacker sends a TCPSYN signal with a non-existant origination IP address to the victim machine. The victim machine then sends a SYN-ACK signal to the non-existant IP address and awaits a response, which of course it never receives, and so the victim machine resends and rewaits. All this sending and waiting takes up resources on the victim machine, until eventually it may be so busy sending and awaiting replies from a non-existant IP address that it cannot perform it's legitimate duties. Research Denial Of Service
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Dam
