Grand Gangwŏn Canal

<imgur thumb="yes" w="300" comment="Map of the Grand Gangwŏn canal (in red) and present-day cities. The Ryongdan river lies at its west end, and the Meng river at its east end.">Gyu7B9e.png</imgur>

The Grand Gangwŏn Canal (Menghean: 강원 대운하 / 江原大運河, Gangwŏn Daeunha), also known as the Twin Rivers Canal (쌍강 운하 / 雙江運河, Ssanggang Unha) and sometimes the Menghean Grand Canal, is a manmade canal in Gangwŏn province, Menghe. It connects the Meng and Ryongdan rivers, beginning at the city of Sunchŏn and terminating at the city of Yŏng'an. Its location allows barges in the Ryongdan to cross over to the Meng river, and from there sail down the Okgang branch into Lake Jijunghae and the rivers that feed it. At a time when river barges were the only affordable way to move bulk goods long distances over land, this made it a critical asset for Menghe's internal trade and tribute network.

The canal was first dug during the Meng dynasty, nearly two thousand years ago, though subsequent dynasties repaired, straightened, and improved it all the way up to the present. At the time of its completion, it was the largest artificial waterway in the world by volume, and with a length of 64.2 kilometers it remains one of the longest canals in Septentrion. Apart from its sheer size, it also faced a number of daunting geographic challenges, including unstable water levels due to monsoon rains and a stretch of elevated terrain between the two river basins. It is recognized internationally as a SLESCO World Heritage Site and domestically as one of the Seven Wonders of the Eastern World.

Background

The Meng river (멩강/孟江, Menggang) and Ryongdan river (룡단강 / 龍湍江, Ryongdangang) both originate in the Chŏnsan mountains, where they are fed by glacial meltwater and seasonal rainfall. From there, they descend to the southeast in the Twin Gorges (쌍곡/雙谷, Ssanggok). For the first 500 kilometers or so, the two rivers run parallel to one another, but are separated by a high ridge. As they descend, however, the ridge grows gradually lower, eventually leveling out to a minimum elevation of seven meters above the Black River's annual median high water level. At this point the Meng river continues eastward, flowing out to the East Menghe Sea, while the Ryongdan river bends southward, skirting along the eastern edge of the Chŏllo Plain and emptying into the South Menghe Sea.

Prior to the invention of the railroad, water transport was the only affordable way to move bulk goods such as grain, stone, and metal over long distances. Caravans of pack animals could manage shorter transportation efforts, but they also required fodder and care, and particularly large items could be hard to break down into separate loads. This was not a problem for most of north-central Menghe; the Okchŏn branch of the Meng River emptied into Lake Jijunghae, allowing ships sailing along it to access any of the other rivers in the Jijunghae Basin or sail out to sea. Yet prior to the canal's construction, goods traveling from the Ryongdan basin had to be unloaded from their ships and hauled along the road between the two rivers, a time-consuming process that was particularly demanding for military logistics.

Menghean administrators observed the need for a Meng-Ryongdan canal as early as the Jun dynasty, though they considered it unfeasible due to the geographic challenges involved. One early plan would have involved breaching the banks of the Ryongdan river at its bend near the canal's current upper link and letting the water flow naturally downhill into the Meng, but this would have wiped out the capital at Ŭngyŏng, now Yŏng'an.

Only in the Meng dynasty, when most of Menghe was unified under a single government, did serious plans to build the canal once again rise to the surface. The founding Emperor Mu of Han was said to have considered ordering construction in 178 BCE, but he declined in favor of moving the capital to Yŏng'an and relying on transport along the Meng river. In 107 CE, however, Emperor Hwa issued a decree mandating the planning and construction of a water connection between the Meng and Ryongdan rivers, on the basis that it would improve trade and allow for active flood control. Such a canal would also streamline the transport of troops and military supplies to the Chŏllo basin, a fact which cannot have escaped Emperor Hwa's mind in light of the Jang Nok Rebellion that his predecessor had only recently suppressed.

Construction

Modern-day reproductions of the dyke-bracing structures used during the Meng dynasty, based on surviving engineering treatises.

When Menghean hydraulic engineers began planning the canal in the late 100s CE, they faced a number of serious challenges. Unlike the flat terrain and soft soil of the Chŏllo and Lower Meng areas, where they had worked on small canals and irrigation systems, the ground here was uneven and rocky, trailing off the foothills of East Hemithea's highest mountain range. The rivers themselves were also subject to seasonal variation: both meltwater and rainfall increased during the summer months, creating floods that could that could overrun dykes and wash away other barriers. This was particularly true in their respective upper stretches, where they descended from the mountains in swift, narrow channels that only later panned out and slowed down. The Ryongdan river was especially notorious in this regard; while it was originally known as the Black river, small boat crews working its banks in the Jun dynasty had given it its new name, meaning "dragon rapids."

The head engineer of the project, Ryu Bing, eventually selected the current site, after determining that it was the lowest point on the ridge along the Ryongdan river's course. Rather than dig a straight passage for water, he decided to exploit the natural topography with a contour canal that snaked along the lowest course between the two rivers. This also meant that for part of the descent he could rely on the existing Ri river, a minor tributary which connected with the Meng just east of the capital at Yŏng'an.

According to surviving documents, Ryu Bing's labor team numbered close to 100,000 men, an enormous pool of labor for the time. Every bit of it was needed, given the scale of the project. The most demanding task was cutting through the elevated section of the ridge so that water from the Ryongdan could flow downhill; without gunpowder or other blasting explosives, Ryu Bing's team relied on heating and cooling the bedrock until it cracked and could be chiseled apart.

As part of the same project, Meng engineers had to tame the lower Ryongdan river, which was notorious for narrow sections with rapids and fast currents. To control the flow of water, they built weirs across the most turbulent sections, and even widened the river itself where possible. To cross the new barriers, barges would have to be pulled up twin slipways, which was still an improvement over hauling them all the way against the strong current. This project, though not technically part of the Grand Gangwŏn Canal, enabled it to be used effectively.

By the time the canal was completed twelve years later, Emperor Hwa had passed away, and his son Emperor Sŏng sat on the throne. The project had seriously depleted the state treasury, and it came at an enormous cost in human labor. Nevertheless, its effects on commerce and trade were soon felt, and it would prove immensely strategic in the long run.

Description

Upriver view of the Ryongdan river at the canal junction, with the water diversion route on the right.

In its current form, the Grand Gangwŏn Canal is 64.2 kilometers long. It descends by a vertical distance of 30.1 meters, measured from the median annual high-water levels of the two rivers, and does so over the course of 16 pound locks scattered along its course. Water in the canal flows west to east, fed by the Ryongdan river and by some natural drainage within the valley it follows.

begins at a junction in the Ryŏngdan river, near the minor city of Gŏsŏng. To overcome the problem of raising the Ryongdan's water beyond its banks, Ryu Bing built a dam across the river's main course and diverted some of its water into a new channel to the east. This dual arrangement broadened the river, reducing the speed of the water, and allowed the division of the flow to be adjusted by means of floodgates on the main dam. The east-side portion descends at a slower rate than the west-side portion and gradually peels away to the east, separated from the lower main river by a high earth-and-stone dyke in the center. Once the elevation is high enough, the narrower Grand Canal breaks away eastward and part of the diverted river section flows back into the Ryongdan over another spillway, also adjustable. Historically, both dams were designed to fail in the event of a catastrophic flood, diverting as much surplus water as possible out of the canal; today their adjustable gates are fully automated and incorporate backup power systems.

From the diversion point onward, the canal follows the natural contours of the land, tracing the center of the shallow valley between the two rivers. The exact path has been straightened over the course of the centuries to allow for easier navigation, but today's canal follows the same general route as its 2nd-century precursor. In the final two-thirds of its course, the canal follows what was once the Ri river, a tributary of the Meng which had to be widened and dredged to make way for regular traffic.

As the modern canal reaches Yŏng'an, it branches off into a number of smaller sections which flow around and through the city; most are within the city limits today, but all are outside the old city wall. The largest of these curves around the city to the south and east, and most closely follows the original exit of the canal; another large branch runs directly northward west of the city. The larger branches carry most of the through traffic, while the smaller branches are used to irrigate farmland or carry small boats around the outskirts of the city. In the Sŭng and Yi dynasties, the banks of the broad through canals were home to bustling markets where barges bought and sold their goods or took on new supplies.

Subsequent history

Repairs and improvements

Even after the canal was finished, Menghean engineers still faced the daunting task of keeping it in good condition. Natural erosion posed a continuous threat to the dykes, embankments, and weirs that kept it in operation, especially during the rainy season. Unlike the Baek river to the north, the Ryongdan does not experience any significant buildup of silt, but in several locations the canal walls are prone to landslides which can seal it off entirely during an earthquake or period of heavy rain. For the two millennia that followed, any time the state treasuries were drained, canal maintenance faltered and the region downriver was placed at risk.

Subsequent dynasties also found ways to improve on the canal while repairing it. During the Kang dynasty, engineers replaced some of the existing weirs and slipways with flash locks, allowing ships traveling downriver to traverse height distances more easily. These locks were of no use to ships traveling against the current, however, and they also sent surges of water downstream each time they were opened. Sŭng dynasty engineers corrected this in the 940s by building two-stage flash locks with water diversion channels along the sides of the gates - a description which, by dominant interpretations, would make them the first pound locks anywhere in Septentrion. Pound locks were well-documented by the time of the Yi dynasty, by which time they ran along the entire length of the canal and also crossed the weirs downstream on the Ryongdan.

The most recent reconstruction took place in 2001, as part of an effort to improve transportation infrastructure in isolated Gangwŏn province. The canal's banks were widened and reinforced and its 22 old locks, some dating to the 19th century, were replaced with 16 new ones. Some bridges over the canal were also torn down and rebuilt at a higher level to allow for increased clearance, though the oldest bridges - which were designed to accommodate wooden barges with folding masts - were preserved. When the canal opened to full traffic in 2002, it was capable of accommodating barges displacing up to 200 tonnes.

Floods and breaches

One of the inherent dangers of the canal is the elevation difference between its two ends. The entrance to the Ryŏngtan sits 32 meters higher than the exit to the Meng river. Ryu Bing considered this a desirable feature, as it ensured that the canal would be well-supplied with water and eliminated the need for an uphill portion. Yet it also meant that if the sophisticated water control measures at the entrance failed, a large portion of the Ryongdan river would be diverted directly downhill into the Meng, crashing through the capital at Yŏng'an at their junction and potentially flooding the major cities of Hwasŏng, Yŏngjŏng, Sapo, and Haeju further down its course. As the two rivers' sources are not far apart, a flood on the Ryongdan is likely to be accompanied by high base water levels on the Meng, and thus a canal failure would likely spread water on top of an existing flood.

Ryu Bing was well aware of this problem, and he put considerable effort into designing the canal's mouth so that the diversion of water between the canal and the Ryongdan itself could be controlled. The sharp bend away from the diverted section was also designed to overflow back into the Ryongdan river if water levels rose too high.

Nevertheless, the risk of catastrophe was always present, especially when periods of internal strife or state bankruptcy disrupted regular maintenance work on the canal's various weirs, dykes, and floodgates. The most notorious accidental failure came in 1235, when the gates and locks failed as a result of lack of maintenance, sending a cascade of water down the canal into the already monsoon-fed Meng. Emperor Ae of Sŭng reportedly dismissed pleas for reconstruction expenditures, instead boasting that next year's harvest would be bountiful due to the thorough flooding of farmland, a quote which would go down in history as emblematic of his disregard for the common people. Another severe accident took place in 1512, according to legend because the work teams managing the flood gates had died of plague.

There were also instances in which warring parties deliberately diverted excessive water into the canal in order to flood the Meng and slow down an attacking army. This first happened in the Five States and Seven Fiefdoms period, when Chŏllo General Wang Ryŏn's army diverted the river in 379 in order to drive out Batu forces laying siege to Yŏng'an. The ruse worked, but legend has it that when news of the devastation downriver reached the Chŏllo headquarters, General Wang's head engineer drowned himself in the still-rushing canal to absolve himself of his sins. A Namyang army opened the floodgates again in 1871 to drive back a column of Sinyi barges and troops moving up the canal's lower reaches, but were unable to restore control of the dams, and for a period of several years almost all water in the Ryongdan river was diverted into the Meng, causing devastation in the agricultural land below.

Today

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  Panorama of the upper portion of the canal, shortly after it breaks away from the Ryongdan diversion branch. Areas like these remain popular with tourists today.

More than 1,900 years after its construction began, the Grand Gangwŏn Canal remains in use. Barges carrying still use it to ferry coal and other ores from loading stations upriver to cities along the Meng, though the vast majority of bulk cargo now covers this route by railroad.

As cargo traffic has fallen and incomes have increased, the Gangwŏn provincial government has worked to rebrand the canal as an attraction for domestic tourists. While reconstruction work has altered the canal's banks along its entire length, some of this work was topped over in the old style, and several bridges and flood control buildings have been preserved from as early as the Yi dynasty. Passenger boats sailing along the canal also gain an excellent view of the natural scenery on both sides, making it an ideal link for combined Meng-Ryongdan river cruises.

See also

• Luminovian Canal