Water on the land

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The River Valley

-The path of a river as it flows downhill is called its course

-Rivers have an upper, middle and lower course

-Rivers form channels and valleys as they flow downhill 

-They erode the landscape (wear it down), the transport the material to somewhere else and it is deposited 

-The shape of the valley and channel changes along the river depending on whether erosion or deposition is having the most impact (the dominant process)

-The long profile of a river shows how the gradient (steepness) changes over the different courses 

-The cross profile of a river shows you what a cross-section of the river looks like '

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Long profile of a river

-UPPER COURSE;

  • steep gradient
  • v-shaped valley, steep sides, narrow, shallow channel 

-MIDDLE COURSE;

  • meduim gradient 
  • gently sloping valley sides, wider, deeper channel 

-LOWER COURSE;

  • gentle gradient 
  • very wide, almost flat valley, wery wide, deep channel 
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Vertical and Lateral Erosion

-VERTICAL EROSION;

This deepns the river valley and channel, making it v-shaped, it is dominant in the upper course of the river 

-LATERAL EROSION;

This widens the river valley and channel, it is dominant in the middle and lower courses

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Four processes of erosion

-Hydraulic action-> the force of the water breaks rock particles away from the river channel 

-Abrasion-> eroded rocks picked up by the river scrape and rub against the channel, wearing it away 

-Attrition->eroded rocks picked up by the river smash into each other and break into smaller fragments, and their edges get rounded off 

-Solution-> river water dissolves some type of rock e.g chalk and limestone 

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Transportation

-Transportation is the movement of eroded material 

-Traction-> large particles like boulders are pushed along the river bed by the force of water 

-Saltation-> pebble sized particles are bounced along the river bed by the force of water 

-Supsention-> small particles like silt and clay are carried along by the water 

-Solution-> soluble materials dissolve in the water and are carried along 

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Deposition

-Deposition is when a river drops the eroded material it's transporting 

-It happens when a river slows down (loses velocity)

-Why rivers slow down and deposit material;

  • the volume of water in the river falls 
  • the amount of eroded material in the water increases 
  • the water is shallower 
  • the river reaches its mouth 
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Meanders

-Occur in the middle and lower course

-The current (flow of water) is faster on the outside of the bend because the river channel is deeper,there is less friction to slow the water down

-So more erosion takes place on the outsides of the bend, forming river cliffs

-The current on the inside of the bend is slower because the river channel is shallower, there is more friction to slow the water down 

-So eroded material is deposited on the inside of the bend forming slip-off slopes 

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Ox-bow lakes

-Meanders get larger over time and turn into ox-bow lakes;

-Erosion causes the outside of the bends to get closer 

-Until there is only a small bit of land left between the bends called the neck

-The river breaks through this usually in times of a flood

-And the river flows along the shortest course

-Deposition takes place eventually cutting off the meander 

-This forms a ox-bow lake 

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Waterfalls

-Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock

-The softer rock is eroded more than the hard rock, creating a 'step' in the river 

-As water goes over the step it erodes more and more of the softer rock 

-A steep drop is eventually created called a waterfall 

-The hard rock is eventally undercut by erosion. It becomes unsupported and collapses

-The collapse rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion, this creates a deep plunge pool

-Overtime, more undercutting causes more collapses, and the waterfall will retreat leaving behind a steep-sided gorge  

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Flood plains

-This is the wide valley floor on either side of a river which occasionally gets flooded 

-When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting

-This builds up the flood plain making it higher 

-Meanders migrate (move) across the flood plain, making it wider 

-The deposition that happens on the slip-off slopes of meanders also build up the flood plain

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Levees

-These are natural embankments (raised bits) along the edges of a river channel 

-During a flood, eroded material is deposited over the whole flood plain 

-The heaviest material os deposited closest to the river channel, because it gets dropped first when the river slows down 

-Over time, the deposited material builds up, creating levees along the edges of the channel 

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Contour Lines

-Contour lines are the orange lines drawn all over maps

-They tell you about the height of the land by the numbers marked on them 

-They also tell you about the steepness of the land by how close the lines are (the closer the lines, the steeper the slope)

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River discharge

-River discharge is the volume of water that flows in a river per second 

-It is measured in cumecs-> cubic metres per second (m /s)

-Hydrographs show how the discharge at a certain point in a river changes over time 

-Storm hydrographs show the changes in a river discharge around the time of a storm 

-Peak discharge-> the highest discharge in the period of time your looking at 

-Lag time-> the delay between peak rainfall and peak discharge #

-Rising limb-> the increase in river discharge as the rainwater flows into the river 

-Falling limb-> the decrease in river discharge as the river returns to its normal level 

-Lag time happens because most rainwater doesn't land directly in the river channel, there is a delay. It gets there quickly overland (surface runoff), or by soaking into the ground (infiltration) and flowing slowly underground 

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Factors affecting river discharge

-Amount and type of rainfall-> lots of rain and short, heavy periods of rainfall means there's more runoff, lag time is decreased so discharge increases 

-Temperature-> hot, dry conditions and cold, freezing conditions both result in hard ground, which increases runoff, lag time is decreased so discharge is increased 

-Previous weather conditions-> after lots of rain, soil cna become saturated (it can't absorb any more water), more rainwater won't be able to infiltrate into the soil so runoff will increase, lag time is decreased, discharge is increased 

-Rock type-> water infiltrates through pore spaces in permable rock and flows along cracks in previous rocks, this means there isn't much runoff. Lag time is increased, so discharge is decreased. Water can't infiltrate into impermable rock which means there is lots of runoff, lag time is decreased, discharge is increased 

-Land use-> urban areas have drainage systems and covered with impermable materials like concrete increasing runoff, lag time is decreased, discharge is increased 

-Relief (how the height of the land changes)-lots of runoff occurs on steep slopes,lag time decreases, discharge increases 

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Physical factors causing flooding

-Prolonged rainfall-> after a long period of rain, the soil becomes saturated, any further rainfall cant infiltrate, which increases runoff into rivers, this increases discharge quickly which can cause a flood 

-Heavy rain-> heavy rain means there is lots of runoff, this increases the discharge quickly, which can cause a flood

-Snowmelt-> when a lot of snow or ice melts it means that a lot of water goes into a river in a short space of time, increasing discharge, which can cause a flood 

-Relief-> if a river is in a steep-sided valley, water will reach the river channel much faster because water flows more quickly on steeper slopes, this increases discharge quickly, which can cause a flood 

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Human factors causing rivers to flood

-Deforestation-> trees intercept rainwater on their leaes, which then evaporates. Trees also take up water from the ground and store it. This means cutting down trees increase the volume of water that reaches the river channel, which increases discharge and makes flooding more likely 

-Building constructuion-> buildings are often made from impermable materials e.g concrete and they're surrounded by roads made from tarmac. Impermable surfaces increase runoff and drains quickly take runoff to rivers. This increases discharge quickly, which could cause a flood 

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Hard vs Soft Engineering

-Hard engineering-> man-made structures built to control the flow of rivers and reduce flooding 

-Soft engineering-> schemes set up using knowledge of a river and its processes to reduce the effects of flooding 

-HARD ENGINEERING;

  • Dams and reservoirs-> (What is it?) Dams are huge walls that are built across the river, usually in the upper course, a reservoir (artificial lake) is formed behind the dam. (Benefits) Reservoirs store water, during periods of prolonged rain or heavy rain, which reduces the risk of flooding. The water in the reservoir is used as drinking water and can be used to generate hydroelectric power (HEP). (Disadvantages) very expensive to build. Creating a reservoir can flood exising settlements. Eroded material is depsoited in the reservoir not along the river's natural course so farmand downstream can be less fertile 
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Hard vs Soft Engineering 2

-Channel straightening-> (What is it?) the river's course is straigtened, meandes are cut out by building artifical straight channels. (Benefits) water moves out of the area more quickly because it doesn't travel as far (Disadvantages) flooding may happen downstream as water is carried there faster. There is more erosion downstream because the water is flowing faster. 

SOFT ENGINEERING;

-Flood warnings-> (What is it?) the Environmental Agency warns people about possible flooding through TV, radio, newspapers and the internet (Benefits) impact of flooding is reduced, gives people time to move possessions upstairs, put sandbags in position and evacuate                        (Disadvantages) don't actually stop the flood, living in places that get lots of warnings makes it difficult to get insurance, people may not hear, or have access to warnings 

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Hard vs Soft Engineering 3

-Preparation-> (How it works?) Building modified to reduce damage, people make plans for what to do in a flood, keep important items like torches, blankets in a handy place (Benefits) impact of flooding is reduced, less damage to buildings, people know what to do, people less likely to worry (Didadvantages) doesn't guarantee safety, could give people a fasle sense of security, expensive to modify homes and buildings 

-Flood plain zoning-> (How it works?) restrictions prevent building on parts of flood plain that's most likely to get affected by flood (Benefits) risk of flooding is reduced, impermable surfaces aren't created, impact of flooding is reduced (Disadvantages) expansion of urba area is limited, no help for areas already been built on 

-Do nothing-> (How it works?) no money spent on new engineering methods or maintaining existing ones, natural process (Benefits) the river floods, eroded material is deposited on the flood plain, making farmland more fertile (Disadvantages) risk of flood and impacts aren't reduced, could cause a lot of damage 

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