APSC AE WRD 2019 Questions: Force Exerted by Flowing Water on Sediment Particles

Force Exerted by Flowing Water on Sediment Particles

Question: The force exerted by flowing water on the sediment particles to cause their motion is called:
(A) Buoyant force
(B) Tractive force
(C) Kinematic force
(D) Eddy force

Answer: (B) Tractive force

Tractive force is the component of water weight acting in the direction of flow, responsible for moving sediment particles along the riverbed. It acts tangentially along the bed, causing sediment particles to move. Tractive force is also known as shear force or drag force acting on the riverbed.

Understanding the Forces

Below is a summary of the forces mentioned in the options, clarifying their roles in the context of sediment transport:

Force	Description	Role in Sediment Transport
Buoyant Force	Upward force exerted by a fluid on a submerged object.	Does not directly cause sediment movement along the riverbed.
Tractive Force	Component of water weight acting tangentially along the bed, moving sediment particles.	Primary force responsible for sediment transport.
Kinematic Force	Force related to the motion of the fluid itself.	Not directly responsible for sediment particle movement.
Eddy Force	Forces from small, swirling currents causing mixing and turbulence.	Contributes to turbulence but not the primary driver of sediment transport.

Key Notes

• Tractive force, also called bed shear stress, is critical in determining the initiation of sediment motion in rivers and streams.
• It depends on factors like water velocity, flow depth, and bed slope.
• Buoyant force may reduce the effective weight of sediment particles, making them easier to move, but it is not the primary driver of motion.
• Eddy forces contribute to turbulence, which can indirectly influence sediment suspension but not direct bed movement.

Note: Understanding tractive force is essential for analyzing sediment transport and designing stable riverbeds and channels.

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