Measuring River Velocity
The velocity of a river is the speed at which water flows along it. The velocity will change along the course of any river, and is determined by factors such as the gradient ( how steeply the river is losing height), the volume of water, the shape of the river channel and the amount of friction created by the bed, rocks and plants.
Velocity can be measured using very simple equipment. A watch capable of timing in seconds, something to float on the water and a tape measure are all that is required to find the velocity of the water surface. If you wish to find the velocity of the water below the surface, a velocity meter of some kind will also be required.
When measuring the velocity on the surface of the water it is common to time how long a float takes to travel a set distance. If you know how far it travelled and how long it took, you can find it’s velocity.
There are certain things to look for in a float, apart from the obvious fact that it has to be able to float!
Firstly , you must be able to see it. Bright colours are much better than dull ones; transparent floats are not a good idea.
Secondly, it must be able to withstand some rough treatment, especially in fast flowing and turbulent water.
Thirdly, it must not catch the wind. Only the water should be able to move it. A paper boat or any other float that sticks well out of the water can be blown by the wind and won’t give reliable readings.
It is possible to buy special floats that are bright, strong and which float almost submerged, but they are expensive and you are likely to lose a few every time you do some fieldwork. The good news, however, is that an Orange makes an excellent float, it’s a bright colour, quite strong and it floats almost submerged so the wind doesn’t blow it along. If you lose it you’ve only lost an orange; if you don’t lose it you can eat it on the way home!
At the more sophisticated end of the market are special flow vanes which are designed to record velocity both at the surface and below the surface. These are not cheap, but they do allow you to make measurements throughout the river and in specific places, rather than over a stretch of water.
The cheaper versions usually consist of a pole with a screw device at the lower end and some type of counter at the top end. When the pole is placed in the water the screw is turned by the flow of the water and every complete turn of the screw is recorded as the counter clicks up by one. More expensive versions tend to offer greater accuracy, longer poles and a display of actual velocity rather than number of ‘clicks’.
Method and Calculations: Float
1. Measure out a length of the river area you are investigating. Try to measure right along the section which interests you. If you are wanting the velocity of a riffle, try to use the whole riffle, not just a part of it. A distance of 10m is usually quite long enough and also keeps the maths quite easy.
2. Get somebody to wade out into the river, or carefully hang over it if it’s too dangerous or deep, at the upstream end of your measured section. It will be their job to release the float. It is important that they are able to just release it without throwing or pushing it, because it should start from rest. Throwing it in from the bank will give it extra speed and ruin your results. If you want to re-use the float you will need someone else at the downstream end to recover it too. An even better way to do this is to release the float just a little way upstream of the starting point . This allows the float to get up speed and be moving at the speed of the river when it reaches the start point, rather than starting from stationary.
3. A person with a watch which can record in seconds (or tenths of seconds) tells the upstream person to release the float and begins timing it. They stop timing when it reaches the end of the measured section.
4. Write down the result NOW – Don’t rely on your memory however good you think it is.
5. Repeat the experiment three or more times if possible. The float will get caught in different currents and, perhaps, behind different obstacles every time. By averaging several different reading a better result can by obtained.
Now for the maths…
Let’s assume that we measured the float four times over a distance of 10m. Our results were :
First time 28 seconds Second time 34 seconds Third time 36 seconds Fourth time 30 seconds
To find the average time we need to add the times together and divide by the number of readings:
Average = ( 28+34+36+30) / 4 = 32 seconds
Now that we have an average time of 32 seconds we can work out the velocity of the river in meters per second. The float travelled 10m in 32 seconds, so to find the time to travel 1m, we divide the distance travelled by the time taken:
Distance / Time = 10/32 = 0.3125
So we have a surface velocity of 0.3125 meters per secon.
Method and Calculations: Flow Vane
Flow vanes from different manufacturers will all have different instructions, but there are some things which apply regardless of the type you use.
Always stand downstream or to the side of the flow vane. If you stand upstream of it you will disturb the water before it flows over the screw.
Hold the vane vertically and with the screw pointing upstream. It is important that the vane is vertical so that the screw faces directly into the flow, not at an angle to it. If you put it in with the screw facing the wrong way, some flow vanes will count backwards!
Vanes that count the number of revolutions of the screw sometimes have a ‘reset’ switch to return the counter to zero before taking a reading. For those without this facility you need to always write down the reading shown before you start using the vane, and the reading shown when you have finished each measurement. Subtracting the first reading from the second will leave the number of clicks actually recorded. The number of clicks per minute can then be converted into a velocity reading using a formula or table supplied with the flow vane.
It is worth noting that the majority of flow vanes are designed to work best at the middle of their velocity range. They lose accuracy in very slow or fast water.