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City Walls, Levees, and the 1642 Yellow River Flood

Editor’s note: This post about the 1642 Yellow River Flood in Kaifeng, China was written by Steve Grimmer, Artist Mechanic at Little River.

Here at Little River Research and Design, we’re committed to helping people learn about rivers and their geomorphology, especially in response to anthropogenic interventions such as mining, dredging, channelizing, and levees. My last two blog posts have specifically addressed levees (The Fate of a Small Levee is in Question, Add Anthropogenic to the List of Flood Factors), so it’s a nice coincidence to see an article in the journal, Nature, add to the discussion. I took special interest reading this article in that I visited the city of Kaifeng in Henan Province, China in 2012 as part of a research project with a university there. The night market in Kaifeng is not to be missed, in case you get the chance to go!

Kaifeng and the Yellow River

Figure 1. Kaifeng and the Yellow River. (a) Topography of the North China Plain. (b) SRTM 90 m DEM of the area around Kaifeng, note that the elevation of the Yellow River is higher than Kaifeng (Data available from the U.S. Geological Survey). (d) The city walls of Kaifeng (solid line is the Song dynasty wall, dashed line is the Ming /Qing dynasty wall) (Imagery from Google Earth Pro, Digital Globe). (d) The super elevated channel of the Yellow River hanging above Kaifeng, with the tallest building in medieval Kaifeng, the Iron Pagoda, for scale. (Image: Michael Storozum, et al, Scientific Reports volume 10, Article number: 3765 (2020))

Kaifeng was once the capital city of the Song Dynasty and was a major city as early as the 7th century. It lies on the south bank of the Yellow River, which gets its name from the heavy load of sediment that it picks up on its way through the Loess Plateau. The article, by lead author Michael Storozum, of Fudan University and the Max Planck Institute, looks at artifacts buried at several archeological sites in the old city during the catastrophic Yellow River flood of 1642. Archeological evidence in the 20 meters of sediment here indicates continuous occupation since the Bronze Age. It’s an immensely flood-prone river, with sources recording a thousand floods over the past 2000 years. (Consider for a moment that written history in China goes back some 3000 years to the Shang Dynasty!)

The particularly deadly flood that took place in 1642, killed an estimated 300,000 of the city’s 370,000 inhabitants. Though heavy rains had swollen the Yellow River to flood stage, it was held in check by levees and floodwalls. This disaster was entirely manmade. Kaifeng had been under siege by rebellious farmers for over six months and one of the city’s walls had been breached. The Ming governor of the city ordered a levee on the Yellow River to be crevassed in an attempt to wash away the rebels. The ensuing deluge entered the city through the breached wall, but had no easy way out and quickly filled the city with over 6 meters of silty water. Many of the homes in the city were built of mud bricks which quickly dissolved, and the deluge easily swept large pottery, wagon wheels and wooden doors through the streets, destroying anyone and anything in its path. When the waters receded, a 3 meter layer of silt was left behind.

Let’s look at some of the factors that came together to cause such a calamity. The first is the protective wall circling the city. Archeological evidence reveals many layers of sediment outside the city, indicating the reliable protection afforded by these walls over the centuries. However, once water entered the city in 1642, it had no way out, and the city was quickly inundated. Modern cities protected by floodwalls have extensive systems of drains, storm sewers, and pumps, along with flood gates that can be opened and closed at will. Kaifeng in 1642 did not. Still, as we learned during the flood of New Orleans after hurricane Katrina, even sophisticated protection systems are prone to failure.

Next, let’s consider elevation. The Yellow River near Kaifeng is about 14 meters above the city, though they are only 10 km apart. Most of the elevation change takes place in the form of a steep bluff, not a gradual slope.  The river here is still coming down off the Loess Plateau onto the North China Plain where Kaifeng is located, and is hanging over the head of the city, so to speak. The Yellow River, being so loaded with sediment, undergoes aggradation. That is, the supply of sediment is greater than the system can transport, causing deposition and elevation gain in the channel. Over the centuries, the Yellow River has been rising even higher over Kaifeng.

Given time, an aggrading channel bed rises sufficiently to push the river out of its banks, seeking a more direct and steeper channel in a process called avulsion. Indeed, the North China Plain carries the marks of many such events over the centuries, and paleochannels are evident in satellite imagery. These sudden and recurring changes in the river’s course claimed millions of lives and wiped out valuable agricultural land. The practice in dynastic China, as it is here in the United States, was to build ever-higher levees and floodwalls along the river in an effort to keep it within its current channel. This is only a temporary fix that leads to catastrophic flooding when the inevitable breach occurs.

90 m SRTM DEM of Kaifeng and surrounding area

Figure 2. 90 m SRTM DEM of Kaifeng and surrounding area. The DEM highlights the paleochannels around the site, specifically, the paleochannel coming out to the southeast of Kaifeng may be the relic channel of the AD 1642 flood deposit (Data available from the U.S. Geological Survey). (Image: Michael Storozum, et al, Scientific Reports volume 10, Article number: 3765 (2020))

In ancient China, as in the United States now, efforts to build flood-resilient infrastructure were successful much of the time, especially within what might be considered a typical range of conditions. But we have always lived with changing environmental, social, and structural pressures that cause rivers to behave in ways that aren’t so predictable. What has worked in the past may not work now, because today’s river isn’t the same river that existed 400 or even 40 years ago. With the input of a warming climate to an already complex system, we must build our riverside infrastructure with ever more awareness and sensitivity to all the factors that come into play. Our Emriver stream tables and accessories do an excellent job of illustrating aggrading channel beds and avulsion, along with the ways levees, floodwalls, and other manmade structures impact the fluvial environment. Whether you are an educator or researcher in any of the many fields related to river science, we are here to help you!