The River Arrow in Herefordshire
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The overall aim of this investigation is to find which factor has the greatest effect on deciding the planform of the River Arrow in Herefordshire, by studying slope angle and discharge amongst other varying elements.
Rivers are a primary feature of the world’s surface area. Hundreds of networks of river channels together drain almost 70 percent of the earth’s land surface and their characteristics differ from location to location and also at the same location over a time period, known as a river regime. In order to fully investigate the main question of this investigation, a number of subsidiary questions have been devised. Each of these in turn will help to provide answers in investigating the reasons for variation in planform.
* 1. How does the planform vary along the river?
* 2. Is the slope angle steeper on meandering bends than on straight
* 3. How do discharge and velocity compare on different planforms?
* 4. Does Channel roughness effect planform?
* 5. Does Landuse on the banks influence the planform shape?
Turbulent flow is the major flow type in most natural river channels. It is this which justifies their efficiency in both the eroding and transporting of materials and sediment. The velocity of a river is affected by three main things, channel roughness, channel shape and gradient. Channel shape can be described by the hydraulic radius of the water cross section, and is defined as the area of water per unit of channel water contact. The discharge of a river is dependant on the velocity of the river and the size of the channel at that point. The river’s velocity should increase as the discharge rises. The higher the velocity of a river, the more energy it has, and is therefore able to carry larger particles. A pattern should become apparent when comparing the velocity and discharge readings.
The velocity of a river when at a gentler gradient should be greater than when it is at a steeper one. This is because the channel roughness and shape are more efficient in carrying water downstream and these two factors outweigh the effect that gradient will have on the river. If the river is surrounded by a vegetated area there may be variations in the discharge. Rainfall that reaches the ground surface does not flow immediately into the streams and rivers, but infiltrates into the soil and is transported to the rivers and streams later. Water can also be held in the vegetation above a river or stream if the precipitation is intercepted in the canopy layer. However, if the river or stream is in a built up, urbanised area, then the rates of run off are greater, and more water is likely to make it to the rivers at a quicker pace. River systems often provide us with a multivariate problem, as all variables affect eachother.
A river’s shape, or planform, is established in undertaking out its two chief purposes: transporting water and sediment from source to mouth. When a river is in flow it will seek out a course which has less resistance, which will mean that it can flow more effectively. An estimated “95% of a river’s energy is expended in order to overcome friction” (Geography, An Integrated Approach, David Waugh).
A river’s channel will change due to interaction between the water flowing through the channel and the channel materials. Meandering streams with a high sinuosity and generally linked with areas of a reasonable gradient with fine bank sediment whilst the rivers themselves are identified to have steep overhanging banks. Streams with a low sinuosity, which are comparatively straight, are associated with areas of a gentle gradient.
“Their low sinuosity is a reflection of their aimless plan where sluggish downstream water velocity has not imposed a regular meander pattern on the system.” (Challenge of the Natural Environment, Knapp, Ross and McCrae)
Study Area: The River Arrow, Herefordshire
The river Arrow, is a tributary of the river Lugg which in turn is a tributary of Herefordshire’s largest river, the Wye. The Arrow is a relatively small river and so this meant that it would be safe to take readings from. I chose to focus on a small area of the Arrow near to Kington, one of Herefordshire’s market towns. It is situated between the river Arrow and its tributary the Gilwern Brook. The landuse around the river in the area around the Kington area is very similar, most of it being fields for the grazing o livestock, although there are small areas of woodland either side of the town. The town centre is set away from the river’s banks and so there has been very little development along the river barring a few houses and a small campsite.
When deciding how to collect the field data required to carry out this investigation, it is important that the process is both practical and efficient, and therefore needs to be planned well. The results should be collected over a period of three days, and it is crucial that the data collection of this investigation is not attempted alone due to safety risks involving the river such as drowning or other minor accidents hat may occur while readings are being taken. A mobile phone was also kept nearby whilst carrying out the fieldwork so that if something were to happen it would be fairly easy to notify others.
It was decided that the stretch of river on which the readings would be taken should have areas of both low and high sinuosity. It was also essential that the surrounding area is easily accessible in order to reach the river with all the equipment, and be able to move up and down the course of the river without difficulty.
Primary Data Collection:
Width (in m)
The width of the river should be measured simply by stretching a tape measure from one bank to the other, recording the result of the given site.
Depth (in m)
To find the average depth of the water a metre rule should be placed at ten regular intervals across the river, at each recording the depth of the river at that point; a systematic sampling. This method of obtaining results is appropriate as it should give an average measurement across the width of the river. After recording the results of each depth they will need to be added up, and then the answer divided by ten.
Cross-sectional Area (in m sq)
After finding the width and the average depth readings it will be possible to find the cross sectional area of each site, which is calculated by multiplying the width and average depth. It is also possible to transfer the depth and width readings onto a piece of graph paper and count the number of squares covered after drawing the river profile. This process however is much more lengthy and often inaccurate due to human error.
Wetted Perimeter (in m)
In finding the wetted perimeter it is possible to make use of the tape measure once again, yet this time placing it along the bed of the river across it’s breadth and taking a reading for the distance from water edge to water edge, also taking into account the distance the water rises up the banks.
Velocity (in m/sec)
In finding velocity a flow meter should be used, placing approximately two thirds of the depth beneath the waters surface for one minute and recording the result, which will show you how many revolutions the propeller has turned. By doing this three times across the width of the river at regular intervals you will get a reading across the whole of the river. These count figures can then be added together and divided by three to get an average. This is then used alongside a mathematical equation which will give the velocity reading: Velocity = (0.144 * count) +5. To obtain the answer in m/s as apposed to cm/s you would then divide your answer by 100.
Pebble Size (in cm)
Pebble size should also be recorded when depth readings are taken, at each given depth, ten regular intervals across the river, a systematic sample the pebble on the river bed should be measured by its B axis. This sampling method is appropriate as it should give fair results, with pebble sizes across the whole width of the river
Secondary Data Collection
Gradient (in degrees)
The gradient will be measured through taking readings from contour lines on a map. By taking the measurement this way an exact reading will be given, providing the sites are marked definitely on the map. It is sometimes not possible to take gradient readings when in the field as the banks can be inaccessible. The use of an OS map subtracts any chance of unsuccessful readings on the day due to physical obstacles.