River Characteristics Investigation
Cross-section - Velocity - Discharge - Gradient - Bedload
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Measuring & Calculating the Cross-Section
Measuring
• Measure the width of the channel, across the surface
of the water.
• Measure the wetted perimeter (the distance down
the banks & across the bed – where water touches
land).
• Take a metre ruler or ranging pole, stand it upright
in the water and measure the depth across the
stream at equal intervals (50cm).
Quick Calculation
A quick method to give an approximate cross-sectional area is to the use the following formular:
Average width x Average depth
Graphing the cross-section
1. Using graph paper & your field data sheets, decide on a suitable scale. Both axes should be identical in
scale.
2. Plot the distance across the stream against depth, just like an ordinary graph.
3. Join the depth points with a flowing line, and shade or colour the land and water.
Cross-section of River Site 1
0
50
100
150
200
250
300
350 cm
50
100
depth cm
How to calculate the cross-sectional area
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2.
3.
4.
Using the cross section you have drawn:
Add up all the squares above the river bed.
What if a square is divided by the bed-line? If it is more than half above the bed-line, include it. If it is more than half below the bed-line, ignore it.
Work out what each square is worth in area and add together.
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Measuring Flow Velocity
1.
2.
3.
4.
5.
6.
Velocity is the speed at which the water is flowing.
Measure out a distance of 10 metres, and mark it at either end with a ranging rod.
Throw in a float upstream of your first marker.
When the float passes the first marker, begin timing its passage downstream
When the float passes the second marker, stop timing.
Write the time down, and repeat 9 times:
-3 times near the left bank
-3 times in the centre
-3 times near the right bank
Calculating Discharge
Discharge is the volume of water flowing in a stream at a certain point in time.
It’s unit of measurement is cumecs (cubic meters/second)
To calculate it use the formular below:
Discharge = Cross-sectional Area x Average Velocity
Bedload Sampling
Take 5 random samples from each site. Collect data on particle size, shape.
1. What is the main type of particle in your sample?
• Clay and silt: < 0.075 mm
• Sand (Fine, medium & coarse grained) : 0.075 to 4.75 mm
• Gravel: 4.75 to 75 mm
• Cobbles: 75mm to 300mm
• Boulders: > 300mm
2. The measurement of the particle’s long axis.
3. What sort of material is your sample made up of:
• is it sedimentary rock,
• igneous rock,
• organic material etc?
4. Angularity – is your material:
• angular,
• sub-angular,
• sub-rounded or
• rounded?
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Data Recording Sheet
Date: Location:
Site No:
Cross-section Data
Width: Bankfull Width: Distance
Depth
1
2
3
4
Intervals:
5
6
7
8
9
Extra depth at bankfull:
10 11 12 13 14
Velocity Data
1
2
3
Average
Left Bank
Mid-stream
Right Bank
Gradient:
Bedload Sampling
Sample
Details (long axis, angularity, particle type)
1
2
3
4
5
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