REVISION AS CAMBRIDGE GEOGRAPHY: RIVERS - CHANNEL EFFICIENCY.

1. AS GEOGRAPHY
HYDROLOGY
4 QUESTIONS
REVISION
CHANNEL
EFFICIENCY

2. DEFINITION
Channel efficiency is a measure of the ability of a channel shape to
transfer water downstream.

3. QUESTION 1
How is efficiency measured?

4. ANSWER
By calculating the hydraulic radius of the channel – cross-sectional area
divided by wetted perimeter.

5. QUESTION 2
How is the measure interpreted?

6. ANSWER
The higher the hydraulic radius, the more efficient the channel at
transferring water, hence the higher the velocity.

7. QUESTION 3
Why are some channel shapes more efficient at transferring water
downstream than others?

8. ANSWER
With high values of hydraulic radius, proportionally less water in the
channel is in contact with the frictional effects of bed/bank material.

9. QUESTION 4
Why does efficiency normally increase downstream?

10. ANSWER
Because the channel becomes deeper and wider due to the additional
inputs of water and the bed/bank material becomes smoothed by
erosion, resulting in less friction.

11. IMPORTANT
The key here is the proportion of water subjected to friction, not the
total amount.
Hence small, square channels may be more efficient than larger channels
that are quite wide.