Pre-Lab 2: Sediment Transport

Name:______________________________

PRE-LAB 2: Sediment Transport: The Froude Number & Bedforms

For open channel flow (for instance, the flow of water in rivers and streams), gravity is the primary driving force. Consequently, the appropriate dimensionless number to study open channels (and to scale model results) is the Froude number (Fr), which expresses the ratio of inertial forces (kinetic energy) to gravity forces. It is given by:
     Where: u = average flow velocity in the channel
h = some characteristic length of the channel (typically taken as water depth)
Ögh = the speed of the fastest possible surface wave for the water depth, h

1 If the flow velocity, u, is equal to the wave speed, what will the Froude number be? (1 pt)
 

2 What will happen to surface waves traveling upstream? (1 pt)
 
 
 

3 Make a sketch of how surface waves will propagate if you drop a small stone into a glass of water. (1 pt)
 
 
 
 
 

4 Make a sketch of how surface waves will propagate if you throw a large boulder into the Mississippi River. (1 pt)
 
 
 

5 For a flow, what must you do to increase the Froude number? (1 pt)
 
 

For the following questions, use figures and graphs from lecture.
6 What flow conditions are required in order to get current ripples? (1 pt)
 
 
 
 

7 What flow conditions are required in order to get dunes? (1 pt)
 
 
 
 

8 Do you think that grain size influences the type of bedforms that can form? Why? (1 pt)
 
 
 
 

9 According to the diagram, how does the mean flow depth, h, effect the mean dune wavelength, l? (1 pt)
 
 
 
 

10 Why does the dune wavelength change when water depth changes? (1 pt)