Rivers

• Drainage Basin


  An area of land where all flowing surface water converges to a single point
• Tributary

  A river or stream flowing into a larger river or lake
• Levee

  A levee is a natural or artificial wall that blocks water from going where we dont want it to go
• Sections of a river

  Upper - starts as many tributaries which are narrow and V-shaped. Very shallow amounts of water. A lot of vertical erosion creating a valley

  Middle - tributaries form a channel wider and deeper than the upper course. The water has more energy as there is more water meaning more lateral erosion. Flat land around the river is called the floodplain which gets covered in water when the river floods

  Lower - at this point, the river is carrying the largest volume. Very wide and very deep. There are bridges on either side called levees which blocks water from flooding
• Why does the velocity of a river change?

  The speed of the water depends on the course its travelling on. Friction occurs between the water and the riverbed which slows the water down. If the water has more contact with the riverbed it slows as there's more friction.
• The changes in velocity of a river

  In the upper, water is shallow meaning that there is more friction between the water and riverbed as the water is closer in contact with the riverbed which slows the speed. As the river gets deeper the water is in less contact with the riverbed which means velocity increases as there isn't much friction. In the lower, water travels faster as there is less friction from the riverbed. The river is wider and deeper due to the erosion that occurred from the fast water
• What are the 4 erosional processes in a river?

  Abrasion

  Attrition

  Solution

  Hydraulic Action
• Abrasion


  Rocks scrape and bang against the sides of the river wearing away the channel gradually (like sandpaper on wood)
• Attrition


  Rocks and pebbles hit each other wearing each other down becoming smaller eventually. This doesn't change the shape of the channel
• Hydraulic Action


  The sheer power of the waves as they smash against the banks of the river. This cracks in the rock and eventually the rock breaks apart and collapses, expanding the channel
• Solution (erosion)


  When the acidity of the water dissolves certain types of rocks e.g. limestone and chalk
• What are the 4 transportation processes?

  Solution

  Traction

  Suspension

  Saltation
• Solution (transport)


  Dissolved rocks e.g. limestone and chalk are carried in a solution
• Traction


  Big rocks that roll across the sea bed
• Suspension


  Small rocks light enough to float within the water
• Saltation


  Rocks too heavy to be suspended that bounce across the sea bed
• Deposition


  The dropping of a river's load when the velocity of the water decreases. If the river is slower, it has less energy and can carry less material
• River Landforms

  Upper - Erosional landforms are most prominent.

  Waterfalls, Interlocking Spurs, V-shaped Valley

  Middle - Both erosional and depositional landforms

  Gorges, Meanders

  Lower - Depositional landforms are most prominent

  Floodplains, Ox-Bow Lake, Estuary
• Interlocking Spurs


  In the upper course, the river doesn't have much energy meaning it isn't strong enough to erode resistant rocks. Instead, the river re-routes and curls around them

  
• Waterfall

  • When a river flows over an area of hard and soft rocks, soft rock is eroded first
  • The soft rock erodes away under the hard rock, creating a step
  • As the soft rock continues eroding, the hard rock is left suspended in the air as an overhang. Underneath the hard rock where the soft rock has eroded a deep plunge pool is created
  • The overhang falls into the plunge pool due to the force of gravity
  • Erosion continues to erode underneath the hard rock creating another overhang
  • The continuation of overhangs collapsing causes the waterfall to retreat and the plunge pool deepens

  
• Gorges


  Form from waterfalls. As the waterfall retreats it leaves behind a steep valley on the side of the waterfall

  
• Meanders


  • In the middle course, water flows around obstacles creating bends
  • Faster water causes lateral erosion to happen on the outside of the bends (through hydraulic action and abrasion) which forms a river cliff
  • Slower water deposits material on the inside of the bends forming a slip-off slope
  • The bends now become meanders The thalweg is the path of the fastest water, when drawing a meander we must draw a thalweg with arrows

  
• Ox-Bow Lake


  • Since in a meander, the sides are eroded due to fast water and deposition occurs on the inside of the bend due to slow water, gradually erosion bends the river so that the meanders travel towards each other
  • This narrows the neck of the meander. Often during a flood, the river takes the shortest course which is through the neck leaving a new straighter course
  • The meander is abandoned forming one body of water.
  • New deposition seals off the river and the cut-off becomes an oxbow lake and a straight river course

  
• Floodplain


  When a river floods the water spills to the surrounding land which is the floodplain. The water loses its velocity and deposits its load. Floodplains are often agricultural land, as the area is very fertile because it's made up of alluvium. The floodplain is often a wide, flat area caused by meanders shifting along the valley.
• Levee


  Forms on the banks of the river. Is higher than the height of the water. During a flood when the water rises above the banks, the water deposits to the banks. This increases the height of the river once again increasing the possible river capacity preventing the river from flooding in the future as now the channel can hold more water
• Estuary


  Where the river meets the sea, the river water is affected by the tides. Since the river flow becomes disrupted and slowed greatly by the tides and there is a large amount of deposition. This can create mudflats and saltmarshes that can build into permanent habitats on the river/coastline.
• River Tees

  Memorise its major landforms of deposition and erosion

  
• Human factors that increase flood risk

  Deforestation - Trees intercept the rain so it takes longer for the rain to travel to the river. Cutting trees will speed up the time taken for water to flow into the river

  River Capacity - If the river becomes filled with rubbish and debris, the channel size decreases meaning the river can carry less water increasing its likelihood of flooding

  Urban land use - when an area around a river is built on, there is an increase in the amount of tarmac and concrete which are impermeable surfaces. This decreases the time taken for water to flow to the river
• Physical factors that increase flood risk

  Amount of rain - If there is a storm and a large amount of rain falls in a short amount of time, the amount of water running to the river increases

  Geology - If an area has a lot of impermeable surfaces the rain cant go into the soil and runs straight into the river instead

  Topography (shape of the land) - Steeper hills have higher gradients meaning rainwater travels quicker down it
• Storm Hydrographs

  Represents the variation in the river's discharge (the volume of water flowing through the river at a specific point). Shows the effect of precipitation on a drainage basin
• Features of a storm hydrograph

  Peak rainfall - The maximum rainfall that occurs

  Rising limb - The increase of river discharge

  Peak flow - The maximum discharge

  Lag time - The time delay between peak rainfall and peak discharge

  Falling limb - The decrease of river discharge

  Base flow - The discharge returns to its normal level

  
• Flash flooding

  A flood with little warning where a large volume of water suddenly overwhelms the river. Can happen if a large amount of rainfall happens in a short amount of time.
• Subdued flooding

  Could take a week of consistent rainfall, isn't a large quantity of water falling per day. Because the ground is saturated with water any rain runs straight to the river. Would cause long term flooding but wont sweep away people or buildings
• Storm hydrographs on flash flooding and subdued flooding


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• Why do we manage drainage basins?

  They are important to the people and animals that live in a drainage basin. If drainage basins aren't managed then there is a risk that it might flood
• Soft engineering strategies (rivers)

  Floodplain zoning - constructing houses and buildings furthest from the river reducing their risk of flooding

  • Reduces damage caused to houses in the future
  • Can only be done for new developments as you cannot reconstruct an existing village

  Flood warnings - issuing warnings when areas are likely to flood so people can prepare.

  • Possessions can be saved and people have more time to protect their properties
  • Some may not be able to access the warnings and they do not stop land from flooding - they just warn people that a flood is likely.
• Hard engineering (rivers)

  Dams - a barrier preventing the flow of water or of loose solid materials, storing it

  • Can be used to produce electricity and reservoirs can attract tourists
  • Very expensive and dams trap sediment which means the reservoir can hold less water.

  Embankments - The banks of the river are built up in brick and concrete to increase channel capacity

  • Reduces lateral erosion so the river doesn't grow wider and are cheap with a one-off cost
  • Looks unnatural and destroys habitats along the river bank
• Hard engineering (rivers) pt2

  Dredging - Rubbish and sediment are dug up from the bottom of the river making it deeper and can hold more water

  • More water can be held in the channel
  • Dredging needs to be done frequently

  Channel Straightening - The river is straightened, speeding up the water so high volumes of water can pass through an area quickly.

  • Can be used to reduce flood risk in built-up areas
  • Speeding up the river increases flood risk further down the river
• Hard engineering (rivers) pt3

  Flood relief channels - Floodwater flows into the relief channel and is taken either to an area where it can be absorbed or re-enters the river further down its course.

  • Removes excess water from the river channel to reduce flooding.
  • Expensive to build and if water levels continue to rise, the relief channel may also flood.