Fieldwork Calculations

Measuring River Cross Sections

Producing a cross-section of a river channel is a basic river fieldwork skill.  The first stage is to measure the width and depth of the river. The data gathered can then be plotted to create a scale diagram of the cross-section, or used to find the cross-sectional area and wetted perimeter of the river.

Simple measuring techniques may be developed with Year 7 pupils using a local stream and extended in Year 9 or as part of coursework at examination level.

Equipment Needed

• A long, waterproof, tape measure is needed to find the width of the river.
• A metre rule and / or a longer pole, such as a surveying pole, are needed for the actual depth readings.
• For measuring the gradient of the stream a clinometer is needed.
• A flow meter may be used to measure the velocity of the water.

WARNING: Listen to the instructions of the teacher. Water can be dangerous and the rocky stream bed can be difficult to walk steadily on. If the water is so deep that you cannot use a metre rule, think very carefully about going into the river. Check that watches are waterproof before doing this survey. You will be putting your hands under the water.

Plotting your data onto graph paper is the same as drawing any other graph, with one major difference. Instead of measuring values up from zero at the bottom of the graph, you must measure then down from zero at the top of the graph. This is because you are plotting depths that go down from ground level.
Having drawn your cross-section by 'joining the dots' on your graph you have created a scale picture of a slice through the river. This diagram can be used to calculate the cross-sectional area or wetted perimeter of the river, which are needed to find discharge and channel efficiency.

Task

Drawing a Cross Section of a River

Use the readings below to plot a cross section of the stream. Label it with the following labels:

• Shallow water
• Fast flow
• Slow flow
• Deep water
• Deposition
• Steep bank

Additional Information:

• Width of stream: 6.40 metres
• Distance from the water surface to the top of bank was 1.25 metres
• Readings taken at 30 cm intervals