To assess whether the modified channel of the river ash is effective in reducing the risk of flooding to local homes.

GEOGRAPHY COURSEWORK INTRODUCTION

Aim: To assess whether the modified channel of the river ash is effective in reducing the risk of flooding to local homes.

To achieve this aim I will conduct a number of experiments within both the natural and modified channel to find out if the modified channel is effective in reducing the risk of flooding to local homes.

A satellite image of the area studied in which the experiments on the River Ash took place. Both the modified and natural channel can be clearly seen.

The location of the River Ash in relation to the UK.

A map showing Staines at the source of the River Ash- the River Colne. The area in which the River Ash starts and flows out of the River Colne is very close to the A30 and the George VI Reservoir.

A map showing the River Ash in Shepperton near Upper Halliford, Littleton and the Queen Mary Reservoir.

A map showing Sunbury at the mouth of the River Ash where the Ash flows into the River Thames

Location

The River Ash is located in the south east of the UK. Although the river flows through Shepperton- in Surrey- the river is confined within the Spelthorne borough. The river’s source is the River Colne in Staines. It flows from Staines in an easterly direction before flowing southeast through Laleham and bypasses Queen Mary’s reservoir. It then turns east through Littleton and Shepperton before joining the River Thames in Sunbury.

Causes of river flooding

There are several factors that increase the risk of flooding in rivers. There are generally four main factors that are:

Heavy rainfall is the most frequent cause of flooding. After several days of heavy rainfall the soil that once soaked up the rain becomes saturated so infiltration becomes difficult. This causes a large amount of surface run-off when water simply lies on the surface of the ground. This surface run-off can initiate flooding.

In Britain flooding often occurs in summer. After large periods of dry hot weather the ground becomes hard and dry and hard to infiltrate. A heavy rainstorm can often follow this where the surface run-offs caused by poor infiltration results in a rapid rise in water level, or a flash flood.

Heavy snowfall means that large amounts of water are held in storage. If there is a slight rise in temperature the water melts and is released. The risk of flooding increases if there is: a greater rise in temperature; there is a large period of rain during the rise in temperature; if the ground is frozen preventing infiltration. The large amount of water released results in surface run-off, ending in a flood.

Some rocks and soils allow water to pass through easily e.g. sandy soils, limestone, and sandstone. These are called permeable rocks and soils. Clay soils and granite do not allow water to infiltrate so easily. They are called impermeable rocks and soils. If the rocks and soils in the river basin are impermeable there is an increase in the risk of flooding as there is less infiltration and more surface run-off.

Vegetation can decrease the risk of flooding. Rivers with forested river basins are at less risk of flooding. The leaves intercept the rainfall delaying the run-off to the river that could reach potentially reach the river and raise the water level. Evapotranspiration means that the water caught by the leaves evaporates of them. Although only a small amount of water is prevented from getting to the river it can make a difference in water level. Roots can prevent the water from getting to the river by obstructing it. Also the roots take up large amounts of water to help the plants in photosynthesis.

If a river basin is in an urban area the human activity like building houses and roads can largely increase the risk of flooding. The impermeable materials used for making roads and houses can lead to a large amount of surface run-off which can flow quickly into the rivers so therefore prompting a flood. Drains and sewers take water quickly and directly to the river channel so can encourage flooding instead of preventing them. Houses with sloping roofs further increase the amount of run-off.

The History of Flooding in the River Ash

The River Ash was at risk of flooding because the river was not large enough to hold the amount of water it was supposed to. Alos the sewage works and the fact that the River Ash is backed up by the River Thames and River Colne make it vulnerable if those rivers had excess water. There is an extensive history of flooding on the River Ash; the most recent and most serious flood was in 1947. During this flood many roads and houses were flooded in Staines, Ashford and the local towns nearby. This was because of excessive floodwater entering the River Ash via the River Colne and the River Ash not being able to flow into the Thames because of high water levels.

How flooding can be controlled

There are many ways in which either the flooding of a river can be reduced or the amount of damage or cost can be minimised. There are both disadvantages and advantages to these options.

One way the risk of flooding can be reduced is raising the levées on either side of the river. This means that the water level has to rise also, to the height of the levées, before it can flood. This method is frequently used, especially in America e.g. the Mississippi River in the U.S.A. A disadvantage of this method is that if the water level does manage to overflow the levées it can have catastrophic effects. This is because the steep gradient results in a high velocity of the river so it takes less time to reach settlements.

Dredging river beds involves taking out some sediment to increase the depth of the river so it can hold a greater amount of water. This means the water level has to rise by a larger amount for the river to flood the river.

A Dam is a piece of infustructure used to hold back by river water and release it in a controlled way. This controls flooding. This means that if there is a large amount of rain and the water level is too high a small amount of water can be passed through and vice versa. The water held can then be used to generate hydroelectric power when it is released. However dams are an eye sore and can flood valleys up stream of the dam.

A reservoir can store excess flood water so it is not flowing through the river at risk and making the water level rise so it could flood. A disadvantage to both dams and reservoirs is that they can be very expensive to build.

Although flood warnings do not prevent or reduce the risk of flooding they can help the public prepare for them. Television and radio can be used to spread the information, often through the news or the weather. People can then take the necessary precautions to protect themselves and their homes e.g. moving valuables upstairs, putting sandbags out and in the most severe cases- evacuation. This can reduce the cost of repairs and in the worst floods- save lives.

Land use zonation means restricting development on the flood plain, or the land that would be affected by flooding. When the river floods the damage is largely decreased and in turn, so does the cost of repairs when the river does flood. Instead of trying to prevent flooding, it is simply preventing the damage that comes with it.

Straightening rivers can be a huge job but a very effective way of reducing flood risk. Meanders in rivers slow down the velocity and so the speed at which the river can flow away from the settlement areas. Straightening rivers means rivers can flow with a higher velocity and so efficiently away from houses as it is not slowed by meanders.

Planting more trees and plants in the flood plain can reduce the amount of water. The leaves intercept the rainfall delaying the run-off to the river that could reach potentially reach the river and raise the water level. Evapotranspiration means that the water caught by the leaves evaporates of them. Although only a small amount of water is prevented from getting to the river it can make a difference in water level. Roots can prevent the water from getting to the river by obstructing it. Also the roots take up large amounts of water to help the plants in ...