Paper 4 Measurements Flashcards
What is the aim of measuring surface velocity?
To investigate the changes in the discharge of the river along its length
To investigate how the human management of rivers can affect discharge and velocity
What are the limitations of the ‘float’ method?
It must be visible, durable and not affected by wind
User error means that the start and finish line of the object may not be exact
Throwing or pushing the object can affect results
How can the accuracy of the ‘float’ method be improved?
The float can be placed upstream and start and finish lines can help to minimise errors
How can the speed of flow be measured
Flow meter
OR
1. The impeller of a hydro prop is placed at 1/2 the depth of the river and then wound to the end of its shaft
2. With the impeller facing upstream, it is placed in the water and timed using a stopwatch to see how long it takes to reach the end of its shaft
3. The distance travelled is measured and the speed=distance/time equation is used to measured speed
What are the limitations using a hydro prop?
The hydro prop may not be completely covered in shallow water
In turbulent flow it may be difficult to see the impeller
How can the impeller be made more visible?
It could be brightly coloured
How is channel width measured?
- Stretch a tape measure taut across a river from the point the water meets the bank on one side to the point the water meets the bank on the other side
- The tape should be kept parallel just above the surface of the water
- Measure the distance in m
What are the limitations and solutions to measure channel width?
Water at high flow could come into contact with the tape measure which would influence width measurement
The tape measure can be raised further above the water to avoid this problem
Measuring depth
- Divide the width of the river by 4
- Use a meter ruler/ranging pole to measure the depth at these intervals
Do not use unnecessary force and push the ruler into the sediment
How is finding cross sections useful?
Allows you to compare straight and meandering sections of the same river
How do you work out the cross sectional area of a river?
Multiply width by average depth
Wetted perimeter
- Use a chain to measure the distance that the water is in contact with the bed
- Use a tape measure to measure the length of the chain
The chain should be held tightly to the shape of the bed
Gradient
- Measure a 10m stretch of the river
- Carefully hold 2 ranging poles in the river, 10km apart vertically
- Line the clinometer at the top of one pole and aim it at the other
- Measure the angle
More than one person may be needed to achieve a steady reading
How do you measure the size of a pebble?
- Hold a ruler against each rock
- Measure the longest axis and read off in cm/mm
Callipers would be a more accurate way
How do you measure the weight of a rock?
- Weigh an empty plastic bag
- Put each rock in the plastic bag and attach to a spring balance
- Subtract weight of plastic bag
How do you measure the roundness of a rock?
- Put the rock next to a powers roundness index
- Choose the category that best fits the appearance of the rock
This is subjective so isn’t very reliable
Why are measurements generally taken?
To measure how each feature changes downstream
Surface velocity
- Record time taken for an object to travel over set distance
- The floats may be caught up behind stones or held in turbulence so it is a good idea to do the measurements at a minimum of 3 places across the stream many times
- Use speed=distance/time to work out average speed for all measurements taken
- Surface water flow is reduced by friction with the air so you should multiply your results by 0.85 to gain a more accurate idea of the speed for an overall stream
List some safety precautions that should be taken when taking measurements
Protection from weather (rain jacket/hat/sunscreen)
Carry out experiments in groups
Carry out in daylight and wear reflective clothes
Check the safety of a study area
Carry a mobile phone
What can be done to make measurements more reliable?
Away from human impact
More investigations along a site
Pilot study to make sure equipment is working
More rocks may be measured
More than one student can measure subjective data
What are the two different techniques that can be used?
Systematic: samples at regular intervals and take a number of samples at each point across a river
Random: use random numbers or pick up stones which the pole touches
