The frequency with which a river overflows can predict how quickly a river delta changes shape. That is what three researchers from the University of Texas conclude in a scientific journal PNAS. Using satellite images they studied 48 river deltas worldwide, and came to the conclusion that three processes influence the migration pattern. In addition to the floods, these are the amount of sediment and the ‘steering power’ of the river. Although other factors also play a role, such as human interference.
Deltas – the regions where rivers with all their branches flow into the sea – are fertile areas and therefore heavily inhabited. Hundreds of millions of people around the world live in a delta: from the Dutch Rhine delta to the Ganges in Bangladesh, and from the Yangtze in China to the American Mississippi.
In practice, most deltas deviate considerably from the Δ-original form
They owe their name to the historian Herodotus, who once noted that the mouth of the Nile resembled the Greek letter delta (Δ). In practice, most deltas deviate considerably from this archetype. Some are mainly formed by wave action from the sea, with heavily eroded straight coastlines. In other deltas, the tide or rivers are the dominant force.
In addition, deltas change shape over time. For example, river branches will be added, or the existing waterways will become wider or change location. Such ‘migrating’ deltas are particularly risky in heavily populated areas, and it is important to predict the speed at which they change shape as accurately as possible, so that this can be taken into account in urban development plans.
Although much research has already been done into individual deltas, a large-scale overview is lacking, according to the Texan engineers. For that reason, they decided to have the computer analyze satellite images of 48 deltas worldwide, including all of the above examples. Through deep learning The computers ‘learned’ to recognize changes in flow patterns, and with a special technique the flow speed of the water and sediment could also be calculated.
In doing so, the scientists discovered three processes that can predict the rate at which watercourses change course. First of all, the fastest changes – with individual branches moving at least three meters per year – always take place in deltas in which the river has a dominant guiding role, and not the wave action or the tide. Examples of such fast-migrating, river-driven deltas are those of the Yangtze and Yellow Rivers in China.
Natural sandbanks
Secondly, the sediment quantity is important. The Dnieper Delta in Ukraine, for example, although river-driven, has a slow migration rate, at only 0.66 meters per year. According to the authors, this is because there is little sand and gravel flowing through the river. Precisely when a lot of sediment is supplied, natural sandbanks can form that influence the course of a watercourse.
Finally, the flood frequency plays a role. For example, relatively little water flows through the Dnieper year-round. A river such as the Ganges, on the other hand, which regularly overflows its banks, will cause erosion more quickly, and thus widen and change waterways.
Although the system provides an interesting insight into the delta dynamics, it does not work flawlessly. Too many other variables play a role for that, the authors themselves emphasize in their article. For example, a river flowing in a rocky bed will not change course so easily. River deltas in the polar region, such as those of the Russian Lena, are also above average inhabited, because permafrost has a stabilizing effect there. And in the Mississippi Delta, many waterways are already held in place by artifice.
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