Canal


Canals or artificial waterways are waterways or engineered channels built for drainage management or for conveyancing water transport vehicles. They carry free, calm surface flow under atmospheric pressure, and can be thought of as artificial rivers.
In most cases, a canal has a series of dams and locks that create reservoirs of low speed current flow. These reservoirs are referred to as slack water levels, often just called levels. A canal can be called a navigation canal when it parallels a natural river and shares part of the latter's discharges and drainage basin, and leverages its resources by building dams and locks to increase and lengthen its stretches of slack water levels while staying in its valley.
A canal can cut across a drainage divide atop a ridge, generally requiring an external water source above the highest elevation. The best-known example of such a canal is the Panama Canal.
Many canals have been built at elevations, above valleys and other waterways. Canals with sources of water at a higher level can deliver water to a destination such as a city where water is needed. The Roman Empire's aqueducts were such water supply canals.
The term was once used to describe certain linear features on the surface of Mars, though modern imaging has shown this to be a misnomer.

Types of artificial waterways

A navigation is a series of channels that run roughly parallel to the valley and stream bed of an unimproved river. A navigation always shares the drainage basin of the river. A vessel uses the calm parts of the river itself as well as improvements, traversing the same changes in height.
A true canal is a channel that cuts across a drainage divide, making a navigable channel connecting two different drainage basins.
File:Canal, Westbury Court Garden - geograph.org.uk - 1416966.jpg|thumb|Westbury Court Garden: the garden "Canal"

Structures used in artificial waterways

Both navigations and canals use engineered structures to improve navigation:
  • weirs and dams to raise river water levels to usable depths;
  • looping descents to create a longer and gentler channel around a stretch of rapids or falls;
  • locks to allow ships and barges to ascend and descend.
Since they cut across drainage divides, rather than following the natural water level, true canals are more difficult to construct and often need additional improvements, such as viaducts and aqueducts.

Types of canals

There are two broad types of canal:
  • Waterways: canals and navigations used for carrying vessels transporting goods and people. These can be subdivided into two kinds:
  • Aqueducts: water supply canals that are used for the conveyance and delivery of potable water, municipal uses, hydro power canals and agriculture irrigation.
File:Chutes Loading the Canal Boats on the Lehigh Canal.jpg|thumb|Loading Anthracite on the Lehigh Canal to feed the early United States industries in the pioneer-era
File:MurtfaltalrDanubeChannel.JPG|thumb|The Danube-Black Sea Canal in Romania
File:AmsterdamRijnkanaal.air.jpg|thumb|The Amsterdam-Rhine Canal near Rijswijk, Netherlands

Importance

Historically, canals were of immense importance to the commerce, development, growth and vitality of a civilization. The movement of bulk raw materials such as coal and ores—practically a prerequisite for further urbanization and industrialization—were difficult and only marginally affordable to move without water transport. The movement of bulk raw materials, facilitated by canals, fueled the Industrial Revolution, leading to new research disciplines, new industries and economies of scale, raising the standard of living for industrialized societies.
The few canals still in operation in the 21st century are a fraction of the number that were once maintained during the earlier part of the Industrial Revolution. Their replacement was gradual, beginning first in the United Kingdom in the 1840s, where canal shipping was first augmented and later superseded by the much faster, less geographically constrained, and generally cheaper to maintain railway system.
By the early 1880s, many canals could not compete with rail transport and were abandoned. In the 20th century, oil was increasingly the heating fuel of choice, and the volume of coal shipments began to decrease. After the First World War, technological advances in motor trucks as well as expanding road networks saw increasing amounts of freight being transported by road, and the last small U.S. barge canals saw a steady decline in cargo ton-miles.
The once critical smaller inland waterways conceived and engineered as boat and barge canals have largely been supplanted and filled in, abandoned and left to deteriorate, or kept in service under a park service and staffed by government employees, where dams and locks are maintained for flood control or pleasure boating. Today, most ship canals primarily service bulk cargo and large ship transportation industries.
The longest extant canal today, the Grand Canal in northern China, still remains in heavy use, especially the portion south of the Yellow River. It stretches from Beijing to Hangzhou at 1,794 kilometres.

Construction

Canals are built in one of three ways, or a combination of the three, depending on available water and available path:
;Human made streams
  • A canal can be created where no stream presently exists. Either the body of the canal is dug or the sides of the canal are created by making dykes or levees by piling dirt, stone, concrete or other building materials. The finished shape of the canal as seen in cross section is known as the canal prism. The water for the canal must be provided from an external source, like streams or reservoirs. Where the new waterway must change elevation engineering works like locks, lifts or elevators are constructed to raise and lower vessels. Examples include canals that connect valleys over a higher body of land, like Canal du Midi, Canal de Briare and the Panama Canal.
  • A canal can be constructed by dredging a channel in the bottom of an existing lake. When the channel is complete, the lake is drained and the channel becomes a new canal, serving both drainage of the surrounding polder and providing transport there. Examples include the. One can also build two parallel dikes in an existing lake, forming the new canal in between, and then drain the remaining parts of the lake. The eastern and central parts of the North Sea Canal were constructed in this way. In both cases pumping stations are required to keep the land surrounding the canal dry, either pumping water from the canal into surrounding waters, or pumping it from the land into the canal.
;Canalization and navigations
  • A stream can be canalized to make its navigable path more predictable and easier to maneuver. Canalization modifies the stream to carry traffic more safely by controlling the flow of the stream by dredging, damming and modifying its path. This frequently includes the incorporation of locks and spillways, that make the river a navigation. Examples include the Lehigh Canal in Northeastern Pennsylvania's coal Region, Basse Saône, Canal de Mines de Fer de la Moselle, and canal Aisne. Riparian zone restoration may be required.
;Lateral canals
Smaller transportation canals can carry barges or narrowboats, while ship canals allow seagoing ships to travel to an inland port, or from one sea or ocean to another.

Features

At their simplest, canals consist of a trench filled with water. Depending on the stratum the canal passes through, it may be necessary to line the cut with some form of watertight material such as clay or concrete. When this is done with clay, it is known as puddling.
Canals need to be level, and while small irregularities in the lie of the land can be dealt with through cuttings and embankments, for larger deviations other approaches have been adopted. The most common is the pound lock, which consists of a chamber within which the water level can be raised or lowered connecting either two pieces of canal at a different level or the canal with a river or the sea. When there is a hill to be climbed, flights of many locks in short succession may be used.
Prior to the development of the pound lock in 10th-century China and 15th-century Europe, vessels had to ascend and descend either with flash locks, consisting of a single gate, or with ramps, sometimes equipped with rollers. Flash locks were cumbersome and only usable where there was plenty of water available.
Cycling a lock uses a large amount of water, so builders have adopted other approaches for situations where little water is available. These include boat lifts, such as the Falkirk Wheel, which use a caisson of water in which boats float while being moved between two levels; and inclined planes where a caisson is hauled up a steep railway.
The crossing of a stream, road or valley, where the delay caused by a flight of locks at either side would be unacceptable, may be achieved by the construction of a navigable aqueduct, such as the Pontcysyllte Aqueduct across the valley of the River Dee in Wales.
Another option for dealing with hills is to tunnel through them. An example of this approach is the Harecastle Tunnel on the Trent and Mersey Canal. Tunnels are only practical for smaller canals.
Some canals attempted to keep changes in level down to a minimum. These canals known as contour canals would take longer, winding routes, along which the land was a uniform altitude. Other, generally later, canals took more direct routes requiring the use of various methods to deal with the change in level.
Canals have various features to tackle the problem of water supply. In cases, like the Suez Canal, the canal is open to the sea. Where the canal is not at sea level, a number of approaches have been adopted. Taking water from existing rivers or springs was an option in some cases, sometimes supplemented by other methods to deal with seasonal variations in flow. Where such sources were unavailable, reservoirs – either separate from the canal or built into its course – and back pumping were used to provide the required water. In other cases, water pumped from mines was used to feed the canal. In certain cases, extensive "feeder canals" were built to bring water from sources located far from the canal.
Where large amounts of goods are loaded or unloaded such as at the end of a canal, a canal basin may be built. This would normally be a section of water wider than the general canal. In some cases, the canal basins contain wharfs and cranes to assist with movement of goods.
When a section of the canal needs to be sealed off so it can be drained for maintenance stop planks are frequently used. These consist of planks of wood placed across the canal to form a dam. They are generally placed in pre-existing grooves in the canal bank. On more modern canals, "guard locks" or gates were sometimes placed to allow a section of the canal to be quickly closed off, either for maintenance, or to prevent a major loss of water due to a canal breach.