Velocity

Lesson Aims

• To understand the main factors which influence a river’s velocity.

1. Channel shape in cross-section2. Roughness of the channel’s bed and banks3. Channel slope

How will you do this?You will achieve this by calculating the hydraulic radii of rivers and considering what happens as we move down a river from its source to its mouth.

Channel Shape

• This is best described by its hydraulic radius; that is, the ratio between the cross sectional area and the length of the wetted perimeter.

Hydraulic Radius = Cross-sectional area

Wetted Perimeter

Cross-sectional area

This is measured by multiplying the width and the mean depth of the channel.

Wetted perimeter

The portion of the perimeter of a stream channel cross section that is in contact with the water.

Which river is the most efficient?

These two channels have the same cross sectional area but are different shapes and as such different hydraulic radii.

Answer

• Stream A – has a larger hydraulic radius with less water in contact with the bed or banks, so it has less frictional drag and more of the water can move at a higher velocity.

The reality• Streams are often asymmetrical and as

such we get varying velocity from one river bank to the other.

The reality• A river’s velocity will change throughout the year.

Consider the two diagrams below.

Bankfull stage

Complete the activities

• Summarise the text from the paragraph at the bottom of page 10 into 4 bullet points.

• From page 11 of the book.

Demonstrate your understanding

• Your understanding of this part of the work will be tested in Stansfeld when you investigate the following hypothesis.

AIM – A river’s channel characteristics change significantly as it moves downstream.

Objectives

1) The further you move from the source of the river the greater the discharge and hydraulic radius.

2) The further you move from the source of the river the smaller and smoother the bedload will become.

Review your thinking

Explain to your table why river velocities increase as you move downstream.