Anthropogenic avulsion in the Huang He (Yellow River) delta
The term ‘avulsion’ describes the process of natural channels abruptly changing course. This process is typical in sedimentary systems in which the dispersal pattern is distributive, or spreading out — as in deltas, alluvial fans, and submarine fans. To put it another way, avulsion is one of the processes that is responsible for creating these morphologies — channels switch back and forth over time distributing the sediment into a ‘fan’ shape. There is, of course, a lot more going on (e.g., bifurcation of channels), but I’m just going to keep it simple for this post.
What causes avulsion? This is a fundamental question — I have many colleagues and peers conducting research that address this question. In a simplified sense, if the downstream segment of a channel begins to back-fill with sediment (and, thus, reduce gradient along the reach), the upstream segment will respond by ‘seeking’ a new course to a lower site. Now, exactly where along the channel that happens, and when it occurs is what researchers would like to figure out. Another idea is that during a particularly large flood, or particularly large turbidity current, the flow simply breaches its natural levee, thus, creating a new course.
The example of avulsion I’m showing in this post, however, is the product of human engineering. The two images below are from a recent post on Earth Observatory that shows a series of images of the Huang He (Yellow River) delta in China. I’m really loving NASA’s Earth Observatory site lately — it seems to be getting better and better.
Here’s an excerpt from the explanation of the series of images that show recent change in the delta morphology:
Between 1989 and 1995, the delta became longer and narrower along a southeast-bending arc. In 1996, however, Chinese engineers blocked the main channel and forced the river to veer northeast. By 1999, erosion and settling along the old channel caused the tip of the delta to retreat, while a new peninsula had formed to the north.
The two images below are from 1995 and 1999, respectively, and show this anthropogenic avulsion nicely.
Compare those two images and note the changes in the abandoned part of the delta after the avulsion. As the excerpt above mentions, the coastline of the abandoned part of the delta is receding. The supply of sediment, which is what constructs and builds the delta seaward, has been cut off. A combination of natural subsidence — a slow sinking as the underlying sediment compacts — with rising sea level is now eating away at that part of the delta. Also notice how the new channel is contributing to net-construction of the delta to the north.
About a year ago I posted similar images of the Huange He, also from Earth Observatory, that showed changes in the delta over a bit longer time scale and, more importantly, comparing before major human influence to after.
Note how significant the delta morphology is in the 1979 image — showing a much more rounded delta front, whereas the more recent morphology protrudes into Bohai Bay creating what delta researchers call a “bird’s foot” morphology. While the Chinese have been constructing levees along this river for a very long time I wonder if natural avulsions were able to occur at the lowermost reaches — the river mouth — until more recently. [note: this paragraph edited from original version; see comments]
Management of delta regions is an increasingly important area of research, especially as we deal with rising sea levels. The example of the anthropogenic avulsion shown here will certainly be something to watch. A significant proportion of the Earth’s population lives on delta and coastal plain environments, so we will undoubtedly continue to geo-engineer these sedimentary environments. The more effort we put into researching their dynamics and processes, the better our strategies will be for doing smart engineering that takes advantage of the natural processes that shape and maintain the health of a delta.