River Cross-section

Introduction

Producing a cross-section of a river channel is a basic river fieldwork skill.
Whether you need to find the discharge or examine the profile of a feature such as a meander or riffle, it will be necessary to produce a cross-section of the river.

The first stage is to measure the width and depth of the river. The data 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.

Equipment

A long, waterproof, tape measure is needed to find the width of the river.
A rigid meter rule and / or a longer pole, such as a surveying pole, is needed for the depth readings.

Method

(Check pockets, wrists etc before you start. Watches and mobiles don’t like getting wet!)

Stream Width

First, find the width of the river. Stretch a tape measure from one bank to the other at 90° to the course of the river. The start and finishing points for the measuring are the points where the dry bank meets the water.

Don’t let the tape drag in the water – that can give you a longer reading than the actual width, so keep the tape tight and about 20cm above the water surface.

 Look straight down from above the tape to work out where it meets the edges of the river.

It is possible that the river or stream only occupies part of the whole channel, and that in flood conditions the river expands, and currently dry bank areas become part of the channel. If you wish to include a prediction of the river’s characteristics when in flood you will need to also measure the bankfull width.

Bankfull Width

A tape measure is again stretched from one bank to the other at 90° to the course of the river. This time, the start and finishing points for the measuring are the points where the vegetation and gradient of the bank suggest that the river has reached its maximum capacity. If the water were to rise above these points flooding of the surrounding area would occur. Again, keep the tape tight, don’t let it sag, and read the width from directly above the tape.

Water Depth

Having established the width of the river, find its depth at regular intervals across the width. How many readings you take depends upon the width of the river and the amount of detail you require. For most rivers, intervals of 50cm are a good compromise between excessive work and loss of detail. If in doubt, divide the width by ten, and use that value as the distance between measurements.

The tape measure which was stretched from one bank to the other can be used as a guide to ensure that you take measurements in a straight line. It is also a useful way to measure the gaps between readings.

Starting on the edge of the river, lower the measuring stick into the water and hold it vertically, with the end just touching the bottom. Read off the depth, and make sure the reading has been written down. Repeat that every 50cm as you move across the river.

Use measuring stick edge-on so it doesn’t bend under the pressure of the flowing water, and so you don’t get a bow wave pushing water up the stick. Both can make the water seem a little deeper than it is.

If the water becomes too deep for a meter rule, a surveying pole or similar item can be used, the depth marked on it and then measured with a tape.

WARNING: If the water is so deep that you cannot use a meter rule, think very carefully about going into the river. Only go into deep water if you know what you are doing.

Calculations

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.

Use the horizontal axis of the graph for distance, in 50cm units. Use the vertical axis for water depth, starting at 0 at the top and going down to the maximum depth you recorded.

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.

You can calculate the area by using graph paper or use one of many computer programs that are available.