How do changes in the velocity of the River Lyn compare with the Bradshaw model?

Consolide your thinking – fieldwork to calculate changes in the velocity of the River Lyn

According to the Bradshaw Model the average speed or mean velocity of a river (measured in metres per second) will always increase with distance along its channel.  This is because a river tends to become deeper, wider and have a higher discharge the further downstream it moves.  As a result relatively less water is in contact with the wetted perimeter, so friction on the river water from the bed and banks of the channel is reduced downstream and it consequently flows faster.   In addition in the upper course the wetted perimeter of the river is higher in proportion to the area of the river than it is downstream.  This increases friction on the water and reduces its speed.  As well as this channel bed roughness decreases from source to mouth and therefore so will its frictional effect on water flowing through the channel from source to mouth.


To test this assumption against the real world example of the River Lyn measurements can be taken of its speed or velocity at sample points along the course of the river and then tested statistically to ascertain whether the changes identified are significant.   Measuring the speed of a river requires only simple equipment such as a stop watch, a tape measure and an orange.  More precise and accurate results can be obtained by using more sophisticated equipment such as a flow vane of one kind or another. 

 The film and narration on this page shows clearly how to measure the speed of a river and further guidance is available at and

The speed of a river needs to be measured over a fixed distance of 10 m and the following sites along the river are suggested as possible locations to take the sets of measurements required:

Assessing risk and identifying control measures to manage risk

Please refer back to guidance on Pages 7 – 8 and use the template with regard to fieldwork above.

Secondary data sets

If access to these locations is not possible for primary data collection then the following tables provide sets of secondary data obtained from the sites for the students to test statistically:

Dist. From Source (m)100011501375287033603725664071907630839010330
Average Velocity (m/s)3.311.671.120.850.680.570.500.440.390.360.33

Now go to Investigation 9 - How do changes in the mean occupied channel width and channel depth of the River Lyn compare with the Bradshaw model?