1. RIVER CHANNEL CALCULATIONS
Use this info to complete page 5 and 6 of the latest notes packet you have been given
Answer all the questions using the info in this PowerPoint

2. What is the Wetted Perimeter?
The yellow line shows the wetted perimeter
Wetted perimeter: the part of the bed and banks which is in contact
with the water in the channel – how do you think hydrologists measure this?

3. This river has a higher wetted perimeter in comparison to its volume,
The difference in the wetted perimeter of a river between the upper and
Lower course of a river
5 meters
This river has a higher wetted perimeter
in comparison to its volume,
which increases friction and reduces
Velocity (upper course)
Rocks and boulders
50 meters
This river has a smaller
wetted perimeter in comparison
with its volume, because it has
smooth banks, friction is reduced
and this allows velocity to increase
(middle/lower course)

4. Calculating the wetted perimeter
Wetted perimeter: the part of the bed and banks which is in contact
with the water in the channel – how do you think hydrologists measure this?
Calculating the wetted perimeter
This can be done using a weighted tape measure
If the stream has a rectangular shape, with no rocks and boulders in the channel like the one below
The wetted perimeter can be calculated by multiplying the depth by 2, and adding the width
Wetted perimeter=
10 x 2 = 20
= 40 meters
Av. Depth
10 meters
Width
20 meters

5. Hydraulic radius (Rh): the ratio between the area of the cross-section of a stream and the length of its wetted perimeter – it measures channel efficiency
A square-shaped stream cross-section has a larger hydraulic radius than a shallow, rectangular-shaped stream cross section
The higher the hydraulic radius, the faster the river flow (Velocity) and the more efficient the river is, because there is less contact between the water in the channel and the bed and banks, so lower friction.
(cross sectional)

6. Velocity & Discharge
Discharge is the amount of water that passes a given point on the stream every second- measured in cubic meters a second (cumecs)
Velocity is the speed of the water moving, measured as the distance traveled every second.
Speed = Distance\Time
Time how long it takes for a cork to travel 50cm (using the meter stick/ tape measure)

7. Measuring discharge in Cumecs (m³/s )
In order to calculate the discharge of a river in cumecs (cubic meters per second), you need to calculate the cross sectional area (m) (width x depth) and times this by the velocity (speed in meters a second)
Width = 10 meters
Speed of flow (velocity)
= 10 meters a second
Discharge in cumecs=
15 x 10 = 150 cumecs
Depth = 1.5 meters
10 x 1.5 = 15 (cross sectional area)

8. 3) The wetted perimeter (rectangular hypothetical streams)
Width = 12 meters
Speed of flow (velocity)
= 11.3 meters a second 1)
Av.Depth = 3 meters
Width = 32 meters
Speed of flow (velocity)
= 15.6 meters a second 2)
Av.Depth = 4.3 meters
Calculate the:
1) Cross sectional Area
2) Discharge in cumecs
3) The wetted perimeter (rectangular hypothetical streams)
4) Hydraulic radius
of these hypothetical rivers (show your working out)
Width = 56 meters
Speed of flow (velocity)
= 34 meters a second 3)
Av.Depth = 9.6 meters
Width = 2 meters
Speed of flow (velocity)
= 8 meters a second 4)
Av.Depth = 0.8 meters
Speed of flow (velocity)
= 16 meters a second
Width = 23 meters 5)
Av.Depth = 1.7 meters

9. 3) The wetted perimeter (rectangular hypothetical streams)
Width = 43.7 meters
Speed of flow (velocity)
= 2 meters a second 6)
Av.Depth = 7 meters
Width = 32.1 meters
Speed of flow (velocity)
= 0.2 meters a second 7)
Av.Depth = 5.8 meters
Calculate the:
1) Cross sectional Area
2) Discharge in cumecs
3) The wetted perimeter (rectangular hypothetical streams)
4) Hydraulic radius
of these hypothetical rivers (show your working out)
Width = 68 meters
Speed of flow (velocity)
= 2 meters a second 8)
Av.Depth = 11.3 meters
Width = 2.8 meters
Speed of flow (velocity)
= 3.0 meters a second 9)
Av.Depth = 15.9 meters
Speed of flow (velocity)
= 2.8 meters a second
Width = 23.7 meters
10)
Av.Depth = 2.7 meters

10. Data Analysis Which is the most efficient stream?
Which stream has the narrowest channel?
Which stream has the largest wetted perimeter?
Which stream has the highest discharge in cumecs?
Which stream has the lowest discharge in cumecs?
Which stream has the deepest channel?
Which stream has the widest channel?
Which stream has the fastest velocity?
Bonus: Which stream would be causing the most erosion and transporting the most load? Why?