Rivers Flashcards
What is the watershed and confluence
The boundary of a river basin and the point where two rivers meet.
Describe slumping.
This happens on river banks where part of the bank slips into the river. It is common in areas with clay and dry weather makes the clay crack and contract. When it rains, water gets into the cracks and is absorbed making the rock heavy and the soil saturated. The rock slips on a slip plain of saturated soil due to gravity.
How do the characteristics of the river change from source to mouth?
The river will be become wider as more water joins from tributaries, this will also cause the depth to increase. The velocity of the river will increase as when the river becomes wider and deeper there is less friction on the water from the bottom and bank of the river; the river is therefore slower in the hills. The discharge of the river will increase as the volume increases. The gradient will become less steep as the river moves out of the hills into flatter, floodplains.
How is a waterfall formed?
A steep hill made of soft rock in the river with a layer of hard rock on top causes the river to erode the underneath of the waterfall faster causing an overhang. The soft rock is eroded by hydraulic action and splash back and a plunge pool is created which is deepened by corrasion as sediment being carried is scraped against the bottom and sides. The overhang eventually falls into the plunge pool due to gravity and the waterfall retreats uphill to create a gorge.
How do interlocking spurs cause V-shaped valleys?
In the upper course of a river, 95% of energy is used overcoming the friction on the water and therefore the water is slower and erodes downwards. An interlocking spur is a barrier of hard, resistant rock which can’t be easily eroded and therefore the river weaves around it causing a deep valley between the two hard rocks.
Describe and explain the processes in a meander.
The outside of a meander bend has more energy and speed due to its lower friction with the bottom of the river. The outside of the river therefore is more likely to erode the banks and the bed to form a river cliff. On the inside of the meander bend the water has less energy and therefore drops its sediment and causes a slip off slope to form by deposition; this is because the water is shallower and has more friction from the river bed.
Explain how ox-bow lakes form.
In the lower course of the river, meander bends are very large. With the continual erosion of the outside of the bend, the two ends of the meander become closer and when flooding occurs the water is able to cut through the gap and in time form a new straight channel. The inside deposition of alluvium then results in the old bends being cut off as sediment blocks them.
What is a floodplain?
An area of flat land on either side of the river that floods when the river bursts its banks. The water is shallower on land than in the river and therefore drops its sediment; the heaviest materials first and the lighter ones are carried the furthest. The deposition of materials and/or the migration of meanders across a river valley adds to the floodplain.
What is a levee?
After a number of floods the larger sediment on the banks of the river builds up to form a wall of sediment that raises the height of the banks. The coarse sediment makes up a steep wall which gradually declines on the other side as the finer sediments were deposited further away from the river. This is because as the river floods, the water loses energy when moving on land due to friction and has to drop its sediment.
What are the physical causes of flooding?
If a valley has steep slopes, water will run into the valley more frequently. If there is rain for a number of days the ground becomes saturated and excess water runs into the rivers. If an area is made of impermeable rocks, all water will run into the rivers. If there has been a heavy snowfall and a sudden rise in temperature, melted ice water will run into the rivers.
What are the human causes of flooding?
If vegetation is removed from slopes, less interception will occur. A dam could burst, overflowing nearby rivers. If farmers plough up and down slopes, channels are created for water to run into the rivers. When an urban area is built, there are large areas of tarmac which is impermeable and will therefore not become saturated causing water to enter rivers at greater speed.
Effects of river flooding.
Bulgaria 2006, River Danube - 2M people were affected, 20 killed. Roads impassable; rails and bridges washed away. The economy was damaged by 346M pounds. Large areas of farmland were destroyed.
Kenya 2007, River Tana - The town of Garissa was under water. Roads were impassable and bridges destroyed.
Mexico 2007, River Grijaiva - 1M affected. 70% of the state was under water and all crops destroyed. Sandbags had to be placed around an archeological site in La Venta.
India 2008, River Kosi - 2.7M affected, 55 killed. Roads were impassable and bridges destroyed. All crops destroyed. The road linking Saharsa village to the rest of the area was washed away.
Education
Governments give advice on flood safety on the internet. The Environment Agency website gives information about the likelihood of a flood via a code system giving people information of the severity of a flood and how to act.
Forecasting
The Met Office predicts the likelihood of a flood on the TV, radio and internet. Householders are advised to ring a hotline or go onto the Environment agency website to check for a flood in their area.
Planning
A flood risk assessment much be carried out on an area before planning permission is given. A new 2010 law requires all houses in flood zones to be flood resistant. DEFRA, has the responsibility of deciding which areas are going to be defended against the risk of flooding; it also provides most of the funding. The actual construction is then carried out by the Environment Agency.
