---
title: "Bed forms: ripples, dunes in alluvial streams"
subject: "Hydraulics"
chapter: 10
key: "semester_4/hydraulics:ripples-dunes-river-bed-formation"
source: https://www.bctengineer.com/q/semester_4/hydraulics/ripples-dunes-river-bed-formation
site: BCT Engineering
---

# Bed forms: ripples, dunes in alluvial streams

Hydraulics · Chapter 10: Flow in mobile boundary channel

- Asked in: 2080 Chaitra, 2 marks; 2076 Baisakh, 3 marks (asked 2 times)
- Video: [3.3 Movable bed and bed forms development of bedload layer](https://www.youtube.com/watch?v=Iw9TxjobqSE) — UTAR Public Courses
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/hydraulics/ripples-dunes-river-bed-formation
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-10

## Question

Briefly explain ripples and dunes (river bed formation in alluvial streams).

## Short answer

Ripples are small bed forms (wavelength <0.6 m) in fine sand at low shear stress, with gentle upstream and steep lee slopes, migrating downstream. Dunes are larger, depth-dependent, occur at higher flow strength with subcritical flow, and have a water surface out of phase with the bed, causing high form resistance.

## Answer

Flow over an alluvial (sand) bed forms regular bed features called **bed forms**. As the flow strength increases the sequence is: plane bed (no motion), ripples, dunes, transition (washed-out dunes), plane bed with motion, then antidunes.

#### Ripples

- Small, triangular bed forms with a **gentle upstream slope and a steep downstream (lee) slope**.
- Wavelength is small (typically less than 0.6 m) and height is a few centimetres (less than about 0.06 m); the size depends on the grain size and not on the depth of flow.
- They occur in **fine sand** (grain size below about 0.6 mm) at low shear stress and low Froude number, just above incipient motion.
- Grains are rolled up the gentle slope, fall into the separation zone behind the crest and are deposited on the lee face, so the ripples **migrate downstream**.
- The water surface is almost undisturbed. They add form roughness.

#### Dunes

- Larger bed forms of similar triangular shape with a long gentle upstream slope and steep lee face, but with **wavelength and height related to the depth of flow** (wavelength about 5 to 7 times the depth, height up to about one third of the depth).
- They occur at **higher flow strength** than ripples, in sand of any size, with subcritical flow ($Fr < 1$).
- Flow separates at the crest and a strong eddy forms in the lee. The water surface is **out of phase** with the bed (the water surface is lowest above the dune crest).
- They move downstream at a slow rate and give a large form resistance (high roughness and a large increase in flow depth).

```text
 Ripples:   /\/\/\/\/\/\    small, regular
 Dunes:    __/\___/\___/\   large, depth-related; flow separation
 flow ---->
```

## Check yourself

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

1. What controls the size of ripples compared with dunes?
   A. Both depend only on grain size
   B. Ripples depend on depth, dunes on grain size
   C. Ripple size depends on grain size, dune size depends on flow depth
   D. Neither depends on anything
   Answer: C. Ripples are small with size set by grain size, while dune wavelength and height scale with flow depth. (Key point: Size control)

2. How do ripples and dunes move and what is their shape?
   A. Symmetric humps that stay fixed
   B. Triangular with steep upstream face that move upstream
   C. Flat beds with no movement
   D. Triangular with gentle upstream slope and steep lee face; grains move up the back and deposit on the lee, so they migrate downstream
   Answer: D. Grains roll up the gentle slope and settle on the steep lee face, so the forms migrate downstream. (Key point: Bed form movement)

3. What is true of dunes and the water surface?
   A. The water surface is out of phase with the bed and flow separates at the crest, giving large form resistance
   B. The water surface is in phase with the bed
   C. The water surface is undisturbed
   D. Flow is supercritical
   Answer: A. Dunes form in subcritical flow with strong lee eddies and an out-of-phase surface, greatly increasing resistance. (Key point: Dunes and water surface)

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