Sediment in a Curved River

Let's take a look!

Watch the video on YouTube.

What type of experiment is this?

Experimental procedure and explanation:

• When it rains heavily, rivers become muddy because of earth and sand (sediment) in the water. Where will this sediment be deposited?
• As explained in “Speed of Flow in a Curved River” and “Flow in a River at a Corner,” the pressure is greater towards the outside of a river bend.
• Close to the bottom of the river, the flow is slower because of viscosity. The centrifugal force decreases by this amount. (In fluid dynamics, when considering flow at a curve or a rotating flow, we often observe the phenomena of rotation from the perspective of the fluid particles. We will explain the operating principle in this experiment in this way, using apparent force and centrifugal force.)
• In the top part of the river, away from the bottom, the action of viscosity is weak, and therefore the top part of the river flows quickly. Therefore, the centrifugal force acting here is greater than that acting on the flow at the bottom.
• Because the centrifugal force is weaker near the bottom, the flow near the bottom is pushed by the high pressure on the outside, creating an inward flow. Conversely, near the top, the centrifugal force is large, and therefore an outward flow is generated, resulting in a circulating flow as shown by the red line in the figure above.
• The overall flow (main current) is going downstream along the river; however, some flow components are created that are perpendicular to the main current. This is called secondary flow. Particles of earth and sand that sink in the water are deposited at the sides by this secondary flow. The same is true in an actual river.
• In the video, the particles that have sunk to the bottom look streaked, but we can also observe them moving to the inner side over time.
• If we look at the particles floating on the surface of the water in “Speed of Flow in a Curved River,” we can also observe the particles moving outwards due to secondary flow.

[Keywords]	Secondary flow
[Related items]	Speed of Flow in a Curved River, Flow in a River at a Corner, Collecting Tea Leaves, Centrifugal Force
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P60・・1.