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My friend is reading a text on hydrology for its school subject methodology, but we cannot understand what bend migration and bends as dynamic mean in this context?

Focus on bank erosion has in turn demanded greater emphasis on bends as dynamic, migrating forms, and a wide range of modes of bend migration has been identified.

Actually, I do not understand this sentence.

10

There is no mention of a concept of bends as dynamic; you're parsing the sentence in too short parts:

Bends have to be seen as [dynamic, migrating forms]. A bend is a curve in the river.

Those bends are not static: they move! So they are dynamic, and they move from one place to another: they migrate.

6

Here, bend is a noun, meaning:

something that is bent: as

  • a curved part of a path (as of a stream or road)

The passage here probably talks about bends in a river or stream. (The phrase "bank erosion" probably refers to the banks of a river.)

When we treat bend as a noun, we see the meaning:

bends as dynamic, migrating forms => bends as forms that are dynamic and migrating

and

bend migration => migration of a river bend (i.e. over a period of time)

The sentence talks about how scientists and ecologists should think about the turns in rivers. (e.g., River bends are not still; they move ("migrate") over time.)

5

...emphasis on bends as dynamic, migrating forms

River bends (bent places in rivers) are both dynamic and migratory. For hydraulic engineers, in this context, dynamic means that change is taking place. Migratory describes the type of change - movement.

Bend migration is a noun related to the observation that river bends are not fixed in location over time - the movement of bent places in rivers. Here is an example of one mode of bend migration (my words):

  • When the river forms a bend, there will be different velocities on the inside and outside of the turn, as the water is accelerated around the corner. The faster moving water at the outside will cause increased erosion, while the slower water on the inside will deposit material. Over time, this causes the thalweg (deepest part of the river) to move to the outside.
1

Over time the bends of rivers change. Erosion and sediment close off some watercourses and open up others. This leaves behind 'oxbow lakes'. Here's a picture from the Yamal Peninsula, Russia (image from Wikimedia):

oxbow lakes in the Yamal Penisula

You can see the large sandy riverbanks in the photo, and at the very right what looks to be some changes in the river banks (due to the colour differences and sharp angle of the river.)

I would guess that the text you are reading is saying that rather than considering erosion to ultimately do little to change the shape or position of a river bend, we should understand that sometimes there can be big changes, and that other factors can be at play sometimes too.

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    It would be useful to relate this photograph to the terms used in the question. – David Richerby Nov 19 '14 at 9:57

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