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Measurments of a river

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Methodology

Equipment

  1. 50m Measuring tape
  2. 2 Ranging poles
  3. Meter ruler
  4. Ping Pong Balls
  5. Stopwatch
  6. Clinometer
  7. Pebbleometer
  8. Data Sheet

Width

I measured the width by giving one person one of the four measuring poles. That person carefully crossed the river and stopped at the bank the other side. The next person would then hold one of the other four poles at the nearest bank and by passing the end of the 50m tape measure across the river, measured the width of the river. We measured the river in cm and we had to keep the measuring tape as tight as possible to be accurate. We divided the length of the river by 10, and at each interval we put the meter ruler down and that is how we went on to measured the depth.

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Depth

I measured the depth of the river using a metre ruler. We divided the length of the river by 10, and at each interval we placed the meter ruler down and measured the depth. We took the measurement from the bed of the river to the surface of the water, and we also took the measurement from the bed of the river to the top of the tape measure.

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Flow

I measured the flow of the river by firstly getting one member of my team to carefully stand in the river at the left side near the bank. That person then dropped one of the plastic balls into the water. When the ball had travelled the 10meter length I had measured for it to travel, I stopped the stopwatch and record time on my data sheet. I repeated this procedure three times for the left, middle and right side of the river of get an average to make it a fair result. I noticed that the middle of the river flows faster than the sides.image03.jpg

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Gradient

The gradient of a river is a measure of how steeply it loses height. A river with a high gradient loses height quickly and is typically fast flowing and youthful. A river with a very gentle gradient loses very little height and is typically a slow flowing mature river. In fact, a lowland stream may have a gradient which is so gentle that it is impossible to measure without specialised equipment.

In mountain areas the Gradient of the river bed is an important factor to measure. In lowland, the gradient is less significant as it is very gentle (between 1-3 degrees) along most of the rivers length. Only close to the source does it rise to between 3-10 degrees. In order to measure gradient, you need two ranging poles and a clinometer. The two ranging poles are held vertically, about 10 metres apart but further if possible). The person with the clinometer holds it level with the bottom of the red stripe and looks through the sight to the same position on the other pole. On a gun clinometer, the angle is recorded when the lever is pressed and then released, then the measurement is read.

A possible problem is that if the river bed is uneven and the distance between the ranging poles is small, it is not unusual to find that the river is going uphill! The accuracy of the clinometers is not good and differences may show up even where they don't exist.

A graph to show how the gradient of a river changes along its length is called a long profile. For lengthy sections of rivers this can be constructed using the contours on a good map; for shorter sections, especially where the gradient is noticeable, it is possible to measure the gradient directly with very simple equipment.

Firstly you need to break up the river into short lengths which you can easily measure. Then you need to stand at one end of the stretch with a pole whilst getting a helper to stand at the other end with the other pole. Make sure that both poles are vertical and the right way up. Place the clinometer against the height mark on your pole and look through the sight, adjusting the angle of the clinometer until you are looking directly at the height mark on the other pole. Taking care not to move the clinometer, look to see what angle is shown by realising the handle. It is a good idea to record the value at this point, and not to rely on memory. Your helper now moves the same distance again along the river and you take another reading. Keep repeating this until you have surveyed the whole of your study area.

Short sections of a couple of meters will give a very accurate profile, but you will need to take many readings to cover a long stretch of river. Long sections of 10meters or more can generate difficulties around meanders.

Anything from 5m to 15m is a good size stretch of river to survey per measurement. You can choose different distances every time you take a measurement, but this can make record keeping very difficult and confusing. Unless you have to alter your measuring length, you should keep it the same all the time. image06.jpg

This is a diagram of how to measure the gradient.image06.jpgimage06.jpgimage06.jpg

Velocity

As we move from the source to the mouth of the river, the velocity of the river will increase. I think this because the gradient will increase as you move from the source to the mouth of the river because of the location of the source and mouth of the river. The source of the river is located in the hill tops, in high, mountainous areas, the centre or middle part of the river, will obviously be located further down the mountain, as the water works its way down hill by the force of gravity, the erotional process that will occur for this to take place are corrosion, because acid or chemicals in the water could erode softer soil and even help to erode small rocks that may be in the path of the starting river.

Water in streams can flow in one of two ways, Laminar flow in which the water particles flow in straight line paths that are parallel to the channel. The water particles move steadily downstream without mixing. Turbulent flow in which the water particles move in a confused and irregular fashion that is characterized by swirling whirl-pool like eddies.

Laminar flow is only possible when the water is moving very slowly through a smooth channel.  If the velocity increases or the bed becomes rough, the water flow changes to turbulent.  The multidirectional movement of water in turbulent is very effective both in keeping sediment suspended in the water and eroding the stream's channel.image07.jpg

Water velocity is the most important parameter in controlling the extent of erosion due to a stream.  When a stream is straight, the fastest flow occurs at the centre of the channel, as there is less friction there.  Minimum velocities occur at the banks and bed where friction is greatest.  Thus, a river with a semi-circular cross section will give rise to the fastest flow, as this cross section provides for the least surface in contact with the banks.  When a river is curved, however, the fastest velocities occur towards the outside of each bend, eroding the bank more effectively.  In addition, fastest flow occurs in rivers with smooth beds, as the zone of turbulence (and, hence, friction) is smaller.

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The most important and obvious factor controlling the velocity of flow is the gradient or slope of a stream channel. Parts of the Mississippi have gradients of only 10 cm/km. Mountain streams may have gradients of more than 40 m/km

Sean Williams.

This student written piece of work is one of many that can be found in our AS and A Level Hydrology & Fluvial Geomorphology section.

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