Canal

• 1. Introduction

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  A single horse, walking a towpath beside a canal, could draw a barge carrying up to thirty tons. The same horse on a soft road might manage five eighths of a ton, and a packhorse only an eighth. That gap in numbers explains why artificial waterways once shaped the wealth of civilizations. A canal is a waterway or engineered channel built for drainage, irrigation, or carrying transport, and it can be thought of as an artificial river. It carries free, calm surface flow under atmospheric pressure. The oldest known canals were irrigation channels dug in Mesopotamia around 4000 BC, in what is now Iraq. From those first ditches to the locks of the Panama Canal, the story runs through emperors, dukes, and speculators who bet fortunes on water. How does a channel climb a hill? Why did the canals that fueled an industrial age fall silent? And why are so many cities still nicknamed "the Venice of" somewhere? The answers begin where the water meets the rock.

• 2. Navigations Versus True Canals

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  A navigation runs roughly parallel to the valley and stream bed of an unimproved river, always sharing that river's drainage basin. A vessel uses the calm parts of the river itself along with engineered improvements, climbing the same changes in height the river already makes. A true canal does something harder. It cuts across a drainage divide, joining two different drainage basins into one navigable channel. The best known example of a canal that crosses a divide atop a ridge is the Panama Canal, which generally requires an external water source above its highest point. Because they ignore the natural water level, true canals are more difficult to build than navigations. They often demand extra works like viaducts and aqueducts. Both kinds lean on the same toolkit of structures: weirs and dams to raise river levels to usable depths, looping descents to ease a path around rapids or falls, and locks to let ships and barges climb and descend. The term once described certain linear features on the surface of Mars, though modern imaging has shown that label to be a misnomer. The distinction between following a valley and breaking out of it would shape every difficult choice an engineer faced next.

• 3. Climbing The Hills

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  At its simplest, a canal is a trench filled with water, and the shape seen in cross section is called the canal prism. Depending on the stratum the cut passes through, the channel may need a watertight lining of clay or concrete. When clay is used, the technique is known as puddling. The pound lock solved the central problem of changing levels. It is a chamber in which the water can be raised or lowered, connecting two stretches of canal at different heights, or linking a canal to a river or the sea. Where a hill must be climbed, flights of many locks in short succession are used. Before the double gated pound lock appeared in 10th century China and 15th century Europe, vessels moved with flash locks of a single gate, or with ramps sometimes fitted with rollers. Flash locks were cumbersome and only worked where water was plentiful. Cycling a lock spends a great deal of water, so builders found other ways when supply was scarce. Boat lifts such as the Falkirk Wheel carry boats in a caisson of water between two levels, and inclined planes haul a caisson up a steep railway. To cross a stream, road, or valley without a slow flight of locks, engineers built navigable aqueducts. One is the Pontcysyllte Aqueduct, now a UNESCO World Heritage Site, carrying a canal across the valley of the River Dee in Wales. Hills could also be pierced, as with the Harecastle Tunnel on the Trent and Mersey Canal, though tunnels suit only smaller canals. Some routes avoided the problem entirely. Contour canals took longer winding paths along land of uniform altitude, trading distance for level ground.

• 4. Feeding The Channel

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  Water supply is a constant problem, and canals carry many features built to solve it. The Suez Canal sidesteps the issue by lying open to the sea. Where a canal sits above sea level, builders drew water from rivers or springs, sometimes supplementing those sources to cope with seasonal swings in flow. When no such source existed, reservoirs separate from the canal or built into its course supplied the water, often aided by back pumping. In some places, water pumped out of mines fed the channel, and extensive feeder canals were dug to bring water from far away. A canal fall, or canal drop, is a vertical drop in the canal bed, built where the natural ground slopes more steeply than the desired canal gradient. The structure dissipates the falling water's kinetic energy to keep it from scouring the bed and sides. The ogee fall follows an s shaped curve for a smooth transition, but that very smoothness fails to dissipate the energy and causes heavy scouring, so the channel must be reinforced with concrete or masonry. The vertical fall, called simple and economical, uses a depressed cistern just downstream to cushion the water in a deep pool. Vertical falls suit drops of up to 1.5 m and discharge of up to 15 cubic metres per second. To drain a stretch for maintenance, stop planks are dropped into pre existing grooves in the bank to form a dam. On more modern canals, guard locks or gates can close off a section quickly, both for repairs and to prevent a major loss of water from a breach.

• 5. Ancient And Medieval Waterways

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  The Indus Valley civilization, in what is now Pakistan and northwestern India around 3000 BC, developed sophisticated irrigation and storage, including reservoirs built at Girnar. In Egypt, Pepi I Meryre, who reigned from 2332 to 2283 BC, ordered a canal built to bypass the cataract on the Nile near Aswan. In ancient China, large canals appeared as far back as the Spring and Autumn period of the 8th to 5th centuries BC. The longest of that era was the Hong Gou, or Canal of the Wild Geese, which the historian Sima Qian recorded as linking the old states of Song, Zhang, Chen, Cai, Cao, and Wei. The Grand Canal of China remains the longest and oldest extant canal in the world, stretching 1,794 km. It was built to carry the Emperor Yang Guang between Zhuodu, now Beijing, and Yuhang, now Hangzhou. Work began in 605 and finished in 609, though it joined older channels, the oldest dating to at least 486 BC. Even in its narrowest urban stretches it is rarely less than 30 m wide. In the 5th century BC, the Achaemenid king Xerxes I of Persia ordered a canal cut through the base of the Mount Athos peninsula in Chalkidiki, northern Greece, as he prepared for the second Persian invasion. Greek engineers were among the first to use canal locks, regulating flow in the Ancient Suez Canal as early as the 3rd century BC. Across the Atlantic, the Hohokam of the North American Southwest built the most complex irrigation network in ancient North America, in what is now the Phoenix area, supporting the largest population in the Southwest by 1300 AD. Part of those ancient canals has been renovated for the Salt River Project. In the Middle Ages, the first artificial canal in Western Europe was the Fossa Carolina, built at the end of the 8th century under the personal supervision of Charlemagne. In Britain, the Glastonbury Canal, thought to be the first post Roman canal, was built in the middle of the 10th century to link the River Brue at Northover with Glastonbury Abbey, a distance of about 1.75 km.

• 6. The Golden Age

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  In the mid 18th century the 3rd Duke of Bridgewater, who owned coal mines in northern England, wanted a reliable way to move his coal to the industrializing city of Manchester. He commissioned the engineer James Brindley, whose design carried the canal over the River Irwell on an aqueduct that immediately drew tourists. The Bridgewater Canal opened in 1761, funded entirely by the Duke, and became the first major British canal. The results were dramatic. Boats were drawn by a single horse along a towpath, and one canal boat could carry thirty tons, more than ten times the cargo per horse of a cart. Within a year of opening, the Bridgewater cut the price of coal in Manchester by nearly two thirds, and it earned back its construction cost within a few years. Industrialists everywhere took notice. In Staffordshire the potter Josiah Wedgwood saw a chance to bring clay to his factory and ship fragile finished goods to market by water, minimizing breakages. Canals like the Oxford Canal and the Trent and Mersey Canal followed, and the period from the 1770s to the 1830s became the Golden Age of British canals. Each canal required an act of Parliament to authorize construction. As investors saw the high incomes from tolls, proposals multiplied for the sake of dividends, and during this canal mania speculators bought shares simply to sell them on for quick profit. Many schemes came to nothing, yet the network grew to nearly 4000 mi. Competition turned fierce. At Worcester Bar in Birmingham, two canals ran only seven feet apart, and a dispute over tolls forced goods to be carried by hand from boats in one canal to boats in the other.

• 7. Crossing A Continent

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  The Erie Canal, opened in 1825, was chartered and owned by the state of New York and financed by bonds bought by private investors. It runs about 363 mi from Albany on the Hudson River to Buffalo at Lake Erie, contains 36 locks, and spans a total elevation difference of around 565 ft. Through the Hudson it completed a navigable route from the Atlantic Ocean to the Great Lakes. The canal repaid its invested capital of seven million US dollars and turned a profit, halving transportation costs and speeding settlement of the American mid west. Its success set off a canal building boom in the United States that lasted until about 1850, when railroads grew seriously competitive. Earlier American canals had been built by private joint stock companies. Four were finished before the War of 1812: the South Hadley Canal of 1795 and the Middlesex Canal of 1802 in Massachusetts, the Santee Canal of 1800 in South Carolina, and the Dismal Swamp Canal of 1805 in Virginia. To the north, three Canadian canals served very different ends. The first Welland Canal, opened in 1829, bypassed Niagara Falls between Lake Ontario and Lake Erie. The Lachine Canal of 1825 let ships skirt the rapids on the St. Lawrence at Montreal. The Rideau Canal, completed in 1832, linked Ottawa to Kingston as a military route after the War of 1812, offering an alternative to a St. Lawrence stretch vulnerable to American blockade. The great sea canals came later, beginning with the Suez Canal in 1869, which carries tonnage many times that of most other canals, followed by the Kiel Canal in 1897. A De Lesseps company that had run the Suez Canal first tried to build a Panama Canal in the 1880s, but difficult terrain, heavy rain, and high worker mortality from disease drove it bankrupt. Its abandoned excavating equipment still sits as decaying machines and tourist attractions.

• 8. Cities And Second Lives

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  Canals are so deeply tied to Venice that many canal cities have been nicknamed "the Venice of" somewhere. The city is built on marshy islands, with wooden piles supporting the buildings, so the land itself is man made rather than the waterways. By the 12th century Venice was a powerful city state. Amsterdam was built in a similar way on wooden piles, becoming a city around 1300. Many of its canals began as fortifications and became grachten when the city expanded and houses rose along the water. Its nickname as the Venice of the North is shared with Hamburg, St. Petersburg, and Bruges. Marco Polo dubbed Suzhou the Venice of the East during his travels there in the 13th century, and its canalside Pingjiang Road and Shantang Street are now major tourist attractions. The narrow early industrial canals tell a different story. Made obsolete for most commercial transport, many were abandoned to navigation, though some still carry untreated water and others gave their routes to railways, as with the Croydon Canal. A movement that began in Britain and France turned these historic channels over to pleasure boats and hotel barges, and abandoned canals such as the Kennet and Avon Canal have been restored for boaters. The 21st century found fresh uses. The Seine Nord Europe Canal is being developed into a major waterway linking France with Belgium, Germany, and the Netherlands. Canals now serve as easements for fibre optic cabling, avoiding roadways while easing access and reducing the hazard of damage from digging. On the 26th of June 2016 the expanded Panama Canal began commercial operation, its new locks letting larger New Panamax ships pass through a route first imagined as a way to get from one hemisphere to the other.