---
title: "Define incipient motion condition"
subject: "Hydraulics"
chapter: 10
key: "semester_4/hydraulics:incipient-motion-define"
source: https://www.bctengineer.com/q/semester_4/hydraulics/incipient-motion-define
site: BCT Engineering
---

# Define incipient motion condition

Hydraulics · Chapter 10: Flow in mobile boundary channel

- Asked in: 2080 Chaitra, 2 marks; 2077 Chaitra, 2 marks; 2075 Bhadra, 2 marks; 2071 Bhadra, 1 marks (asked 4 times)
- Video: [Incipient Motion | Design steps of channel in alluvial soil | Hydraulic | Prashant YT | Unit:11 |](https://www.youtube.com/watch?v=ybly82vkA7k) — Prashant YT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/hydraulics/incipient-motion-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-10

## Question

What do you mean by incipient motion condition in a mobile boundary channel? (Also define mobile boundary / alluvial channel.)

## Short answer

Incipient motion is the threshold condition where hydrodynamic forces on a bed particle equal its resisting forces, just before movement begins. It is characterized by critical shear stress τc or critical velocity Vc. A mobile boundary channel has erodible boundaries of loose sediment that can be transported and reshaped by the flow.

## Answer

#### Incipient motion

**Incipient motion** is the condition at which the hydrodynamic forces (drag and lift) acting on a bed particle are just equal to the forces resisting its movement (submerged weight and friction). It is the threshold between a stable bed and a moving bed: if the flow increases even slightly, the particle starts to move.

It is described by:

- the **critical (threshold) shear stress** $\tau_c$: the bed shear stress at which motion starts; or
- the **critical velocity** $V_c$ at which motion starts.

Quantitatively, for non-cohesive grains, $\tau_c$ is obtained from the Shields diagram, $\tau_c = \tau_*(\gamma_s - \gamma)d$. Motion occurs when the actual bed shear stress $\tau_0 = \gamma R S_0$ exceeds $\tau_c$.

#### Mobile boundary (alluvial) channel

A **mobile boundary channel** is an open channel whose bed and banks consist of loose granular material (sand, gravel, silt) that can be eroded, transported and deposited by the flowing water itself. The boundary is therefore deformable: the shape, slope and bed forms of the channel adjust to the flow and sediment load. Natural rivers, and unlined earth canals in sandy soil, are examples.

An **alluvial channel** is a channel formed in alluvium, that is, in its own deposited sediment. Its geometry is partly determined by the flow, unlike a rigid boundary channel (concrete or rock lined) whose boundary does not change.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the incipient motion condition?
   A. Drag and lift on a bed particle just equal the resisting forces, so a slight increase in flow starts motion
   B. The condition when the whole bed is moving as bed load
   C. The condition when particles are suspended in the flow
   D. The condition when flow becomes supercritical
   Answer: A. It is the threshold between a stable and a moving bed, where driving and resisting forces balance. (Key point: Meaning of incipient motion)

2. Which quantities describe the threshold of motion?
   A. Froude number and depth
   B. Critical shear stress tau_c or critical velocity V_c
   C. Sediment concentration and slope
   D. Manning n only
   Answer: B. The threshold is expressed as critical shear stress or critical velocity at which motion starts. (Key point: Describing the threshold)

3. What defines a mobile boundary (alluvial) channel?
   A. It is lined with concrete
   B. Its slope is always steeper than critical
   C. Its bed and banks are loose material that the flow itself can erode, carry and deposit, so the boundary adjusts
   D. It carries clear water only
   Answer: C. A mobile boundary channel has a deformable boundary, unlike a rigid concrete or rock lined channel. (Key point: Mobile boundary channel)

## Related in Hydraulics

- [Boundary shear distribution in alluvial channel](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ashear-stress-distribution-alluvial-channel) — Hydraulics, chapter 10
- [Mobile boundary design: three approaches](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Adesign-three-methods-mobile-boundary) — Hydraulics, chapter 10
- [Rigid versus mobile boundary design principle](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Arigid-vs-mobile-boundary-design-principle) — Hydraulics, chapter 10, asked 3×
- [Boundary shear stress and compound channel flow](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ashear-stress-boundary-compound-channel-flow) — Hydraulics, chapter 6
- [Tractive force method of channel design](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Atractive-force-method-explain) — Hydraulics, chapter 10, asked 3×

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