Although it is a difficult task to perform, if the storage time and amount of sediment is calculated, it is possible to work out a balance between erosion and deposition of the sediment in a catchment specific area. This is called the sediment budget. These are used by scientists, which can present the situation of the catchment area, and whether any human activity needs to take place there, for example, removing deposited sediment to reduce the risks of the river over-flowing, resulting in possible flooding of the area. Another situation could be where too much erosion is taking place in a specific area, and the area around the river needs to be protected by some sort of defence.
What the sediment budget shows is the amount of sediment which leaves a catchment area, and how much of it stays and is stored there. Most of the sediment is stored at the floodplain, though there will always be some others along the river’s course. By looking at an example we can see this more clearly.
The sediment budget for the River Start, South Devon, shows, firstly, all of the 107 tonnes/km squared/year of sediment is transported to its first catchment area, and 15% of the sediment is deposited here at Hedgerow. The remaining 85% of the sediment is carried further down the river’s course, to Lower Start Valley, where 58% of the sediment is deposited onto the floodplain. The remaining sediment is then transported to Slapton Ley. (Figure 1.40, P.20)
How does the Sediment Budget have both positive and adverse effects within drainage basin?
The positive and adverse effects can be easily spotted by the use of an example. I will use The Yellow River, China, to find the effects the sediment budget has within the drainage basins.
The Yellow River (known as the Hwang He in China) is one of the world’s greatest rivers, draining a total area of over 750,000km squared, where 84 million people live and 13 million hectors of farming land. The river bed is made up of easily eroded material, which causes great problems when the seasonal flow erodes the land, and hence increasing the chance of flooding, which is a danger to the people who live in the lower parts of the catchment.
Rather than solid rock, the river flows over yellow loess deposits in its middle stages of its course, where the loess can be up to 300m thick in some areas. The yellow loess is easily eroded, and therefore the rate of erosion and transport by the river is increased extremely, especially during the seasonal flow of the river. Loess is also picked up by the river further down towards the south, such as near Yulin, where 25000 tonnes of loess are eroded per km squared.
All this sediment which the Hwang He carries has both advantages and disadvantages. The advantages include that the fertile soil transported by the river is used for farming. The river is also used by humans as an energy source, as there is a huge potential for hydro-electric power, mainly in the upper regions of the river where sediments loads are low. This is important to the area as it is the source of their electricity, but it is also fairly cheap compared to other forms of power, and also saves the earth’s resources as it doesn’t require the burning of fossil fuels.
This also causes less pollution. Also, where the deposits have been dropped they have formed a huge delta, which goes out to sea, and this has provided a large area of land for houses to build on. 10 million people live on areas such as this and near the river, with a population density of over 500 per km squared. The formation of such deltas is vital for a country like China where the population density is so high, and also eases over-crowdness problems in urban areas.
The disadvantages are that, firstly, the river brings the risk of flooding to the area as it can easily break into its banks. The hydro-electric power plants built along the river could be in danger of becoming damaged by sediments carried by the river, which will give problems to the areas powered by that plant. Also the high rates of erosion means that homes are at risk if situated near the river, as the land can become eroded away.
Relating this to the sediment budget, sediment deposited at specific catchment areas along the river’s course could become blocked in a way due to the high amount of sediments transported by the river. This could harm farmers in particular, who rely on the river to provide fertile soil, as the flow of river could be affected. Also eutrophication could occur if the river picks up any chemicals on the way, such as from farm fertilisers.