A study into the bed load of the River Lemon

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A study into the bed load of the River Lemon

                

Rivers dominate the landscape all over the world, producing widespread changes and affecting human and physical environments in many ways. Rivers are hugely variable because of factors such as climate, geology and human influences, but most follow similar patterns and have similar processes. Understanding these processes is essential for humans to live in harmony with rivers, and knowing how best to use and manage them. To understand these processes we can study river variables, such as velocity, efficiency and discharge, and how they change along the river’s course. In this investigation I will try to find out how river variables change downstream in relation to the bed load. The bed load of a river is the material that sits at the bottom of the channel. The bed load is composed of material that is too heavy for the river to entrain and carry along the channel, this is mostly rocks. The bed load originates from two sources: 90% is exogenetic, coming from weathering and mass movement of slopes near the river. 10% is endogenetic, coming from the river’s own channel. Understanding the bed load is important as it affects the characteristics of the river as a whole and therefore the management of the river. For my investigation I have studied five sites along the course of the River Lemon in South Devon. I have two key questions to guide my investigation:

1. In what ways does the bed load change downstream?

2. Is there a correlation between river velocity and the size and angularity of the bed load?

The velocity of a river is a measure of the speed at which the water travels along the channel.

My predictions for my key questions are that:

1. The bed load will start off large and angular at the source (this may be due to the river vertically eroding to reach equilibrium, which leads to the degradation of slopes from weathering and mass movement causing rocks to fall into the channel) and get smaller and more rounded downstream.

2. The change in the bed load will be partly due to an increase in velocity downstream, which will increase hydraulic action on the bed load, eroding it in size and angularity. A higher velocity downstream will also increase the process of attrition, which occurs when the rocks knock into one another, which causes them to erode.

It will be interesting to compare my results to the Bradshaw model. This model shows the characteristics that rivers usually follow from source to mouth, based on data from rivers from all over the world.

The model shows what happens to different variables of a river downstream. Some increase and some decrease. I will compare my data to what the model states about velocity and load size, and see if they agree.

The River Lemon

The River Lemon is situated in South Devon. Its source is called Lemon head and is situated in Dartmoor National Park. The River Lemon is a tributary of the River Teign, and meets it at the Teign estuary in the town of Newton Abbot. The Lemon is quite a small river at 16km long. The Lemon has a stream order of 3. At Dartmoor, where the Lemon’s source is, the bedrock is made from hard, impermeable granite. Granite is very difficult to erode, and as a result, the river has not reached its graded long profile. It is still trying to erode away the granite, as this picture of the long profile shows.

Near the source, it is not a smooth curve. Further down the river the bedrock changes to softer rocks such as slate, and so the curve is a smooth one. Other factors in Dartmoor that affect the river Lemon are the high levels of precipitation and lack of trees. The consequences of this are that when there is a precipitation event, the water enters the river very quickly because water cannot infiltrate through the granite so there is high runoff, and no water is intercepted by large vegetation. So on a hydrograph, the lag time is very short.

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As a result of these factors, flooding on the River Lemon is an issue, especially at Newton Abbot, where there are two rivers. In 1979 high rainfall on Dartmoor contributed to flooding on the Lemon. The culvert in Newton Abbot was too small to cope with the amount of water and £3.6 million worth of damage was done. In 1982 a dam was completed on the Lemon, called the Holbeam dam. This dam reduces the water flow when there is danger of a flood, and an adjacent field is flooded. When there is no flood risk, the river is ...

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