Theme 2- Rivers and River Management

River processes
1. Rivers have gravitational energy so they transport load (boulders, gravel, sand, silt) downstream
  1. When rivers have high energy (eg after heavy rainfall or if it flows quickly), they erode. When rivers have low energy, they deposit
2. Erosion
  1. Abrasion- Scraping, scouring and smashing action of the load carried onto the river banks
    1. Attrition- When pebbles and rocks carried smash against one another so they get smaller and more rounded
      1. Hydraulic action- When friction and pressure cause water to be forced into cracks of the river bank and bed, causing the particles to break apart
        Corrosion- When weak acidic water disscolve rocks such as limestone and chalk
3. Transportation
  1. Solution- Dissolved minerals carried within water
    1. Suspension- When small particles such as sand and silt are carried by the flow
      1. Saltation- Small stones are bounced along the river bed
        Traction- Large stones and boulders are rolled along the river bed
  2. Factors affecting transportation
    1. Size of particles
      1. Type of rock as sofer rocks like limestone are more like to be dissolved as they are soluble
        The velocity of the river
4. Deposition
  1. Caused by a decrease in the gradient or velocity of the river
    1. In floooding, rivers overflow onto the floodplain so they deposit material after floding
      1. An increase in the size of sediment casues deposition
        When there is shallow water (eg in the inside of a meander bend) rivers deposit its load
River landforms
1. V-shaped valleys
  1. Formed in the upper course
    1. The water erodes sharply downwards(vertical erosion) and cuts the bed like a knife, forming a steep valley
      1. Rain washes stones ans soil from the sides so the valley becomes V-shaped
        Steep gradient+high energy= lots of erosion
        The river winds and bends and avoids areas of hard rock, creating interlocking spurs
2. Meanders and oxbow lakes
  1. Water flows faster on the outer curve of its bend and slower on the inner curve
    1. The outer bend is eroded whilst the lower bed has depsoited materials
      1. As the outer bend gets eroded (forms a river cliff), the inner bedn grows (forms a point bar)
        Over time, the process continues so the meander becomes more loopy
        An oxbow lake is a cut-off meander
        As erosion continues, the neck of the meander gets narrower
        During flooding the river takes a shortcut stright across the neck
        The loop of the meander gets sealed off
        In time, the lake gets covered by weeds and soil and disappears
        Happens in the middle course
3. Waterfalls and gorges
  1. Water tumbles over a ledge of hard rock and erodes the sofer rock underneath it faster
    1. This leaves a ledge of hard rockand a plunge pool
      1. Overhang makes the ledge unstable so it falls into the plunge pool
        This is repeated so the water reatrests upstream and carves out a gorge
        Happens upstream
4. Floodplains
  1. In the lower course, the river has a high voulme and a large discharge and deposition happens
    1. A floodpalin is an area around a river that is covered in times of flood
      1. Very fertile as it has alluvium deposits
        More deposition= build-up of alluvium, which creates levees, which raise the river bank
5. Estuaries
  1. it is where the river meets the sea.
    1. The river here is tidal so during high tides, large areas of valey floor will be flooded.
      1. As the tide falls, materila will be deposited, creating mudflats, which are important habitats fro wildlife
The Drainage Basin
1. Factors affecting the drainage basin
  1. Geology- Impermeable rocks and clay soils do not allow precipiation to inflitarte, increasing surface runoff, which can increase the discharge and thus increase flooding risk
    1. Deforestion+Afforestation- Trees reduce surface runoff due to interception as well as soaking up groundwater. More trees= less discharge and vice versa
      1. Urbanisation- Buildings and roads are impermeable, increasing surface runoff sue to less inflitartion. Also, drains remove rainwater quickly from urban areas towrds rivers, increasing discharge
        Amount and duration of rainfall- Haevy rain can cause floods as there is too mich water to infiltarte into the ground so more surface runoff. Long rain times causes the soil to get oversatuared so more surface runoff so higher discharge
2. Definitions
  1. Drainage basin- an area of land drained by a river and its tribuateries
    1. Watershed- The boundary between 2 drainage basins
      1. River channel- Where the water flows
        Confluence- Where two rivers meet
        Source- the start of a river
        Tribuateries- A small river joining the mian river
        Mouth- Where the river meets the sea
        Interception storage- Water stopped by vegetation
        Surface storage- Water stored on the surafce of the eartg
        Infiltartion- Water that soaks from the surface to soil
        Percolation- Water travelling from soil to rock
        Soil storage- Water stored in the soil
        Groundwater storage- Water stored in rocks
        Through flow- Water that travels through soil
        Groundwater flow- Water that flows through the soil
3. Storm hydrographs
  1. A hydrogarph is a way of displaying hwo the discharge of a river can chnage over a ranfall event
    1. Discharge is the voulme of water flowing in a river and is measured in cumecs. Peak discharge is when a river is at its mots full
      1. Lag time is the time taken for a channel to reach peak discharge after peak rainfall
        Rising limb can be steeper due to less inflitartion
        Peak discharge can be higher due to less infliltartion
        Lag time also depends on the factors affecting a draingae basin eg longer lagtime due to more infiltration and groundwater flow
Case Study- The Cumbrian Floods- 2015
1. Cause
  1. The warm air from the mid-atlantic caused relief rainfall over the Cumbrain mountains. The warmer the air, the more moisture it has
2. Event
  1. In December 2015, Storm Desmond set a new record with 341.4mm of rain faling over 24 hours
    1. Cockermouth was mainly affected despite its flood defences in response to previous floods
3. Previous flood defences
  1. River dredging took place after 2009 to increase the capacity of the river
    1. A self-closing flood barrier was installed in 2013 to increase the height of the river banks
      1. The Environmental Agency moniters the area for potential flooding and issues flood warnings (RAG)
        The government has peldged to spend £2.3 billion on flood defences between 2016 and 2022
4. Impacts
  1. Social
    1. Over 43,000 homes suffered from power cuts
      1. 5200 homes were affected by flooding
        One person died
  2. Economic
    1. Many businesses suffered from property damage
      1. Estimated cost was £500 million across Cumbria. Government provided £50 million for flood victims and businesses
  3. Environmental
    1. Vegetation was flooded, causing habitat loss
      1. Heavy rain washed nutrients fro soil
        Rivers were largely altered sue to more erosion
Flood management strategies
1. Hard Engineering
  1. Embankments- Raising the banks of a river so it can hold more water.
    1. Pros- Allows for floodwater to be contained within a river. Cons- Water speeds up, increasing flood risk downstream+ looks unnatural
      1. Levees- Embankments built along rivers so river can hold more water
        Pros- Allows the floodplain to be built upon. Cons- If levees are breached, there is a risk of severe flooding+ expnsive
        Dams- Huge walls built across rivers and resevoirs are built behind dams. Floodwater is caught by dams and is realesed by a steady flow throughout the year
        Pros- Resevoirs provide a reliable water source and turbines in dams generate electricity. Also allows for irrigation. Cons- Wildlife affected as fish migrate upstream to breeding groungs+ very expensive
        Straightening- River seppeds up the water so high volumes of water can pass quickly
        Pros- Reduces flood risk in built-up areas. Cons- Increases flood risk downstream due to speeding of river
        Dredging- Makes the river deeper to hold more water
        Pros- More water held into the channel. Cons- Needs to be done frequently
        Flood relief channels- Water is diverted from areas being protected
        Pros- Removes excess water from the river channel to reduce flooding. Cons- Expensive+ relief channel can also flood if water levels continue to rise
2. Soft engineering
  1. Flood plain zoning- Allowing only ceratin land uses on the flodplain to reduce flooding risk on buildings
    1. Pros- Costlier buildings moved further from river so lower flood risk.+ less damage= less insurance claims Cons- Not always possible to change current land uses+ restricts development of an area
      1. Wetalnd and river banck conservation- Wetland store water ad slow it down so planting vegetaion increases interception, increasing lagtime and reducing discharge
        Pros- Creates wildlife habitats+ vegetation protects against soil erosion. Cons- Less alnd available for farming
        Flood warnings and prepartion- Environmentqal agencies monitor rivers and issue warning via th emedia so people can prepare for floods
        Pros- People have time to protect theor properties and evacuate+ reduces insurance claims. Cons- Some people can't access warnings and flash floods happen to fast for a warning to be effecive
        River restoration- Making the river more natural though man-made levees and allowing natural flooding
        Pros- Less maintainacne needed as river is left in its natural state+ river provides a better habitat to wildlife. Cons- Local flood risk increases
Views on flood defences
1. For
  1. Planners advice that permeable materials such as gravel are used for more infiltartion
    1. Soft engineering moves flooding away from urban areas
      1. Expensive defences guarenatee home protection (home owner)
        Peat bogs allow for more water to soak into marshland and protects habitats (envoronmnetalist)
2. Against
  1. Motorists want impermeable materials such as tarmac on roads
    1. Hard engineering increases discharge downstream and affects other towns (River scientist)
      1. Floodplains must not be built upon as there is a higher flooding risk (housing minister)
        Drains carry water downstream, increasing flooding risk (environmentalist)
The River Tees Case Study
1. Upper Course
  1. Rises from the Penines and is located in the moorlands of Cumbria in soggy land
    1. Annual rainfall is 1200mm and altitude is 800m
      1. Bedload is large and angular and land is mainly made of impermeable rocks
        There is a V shaped valley
        A resevoir is built to store water for urban areas downstream
2. Middle Course
  1. Bedload is medium and rounded and altitude is 350m
    1. V-shaped valley becomes flatter and meanders and floodpalins forms
      1. Highforce Waterfall is made of Whin sill (hard rock) and is 21m high and forms a gorge
        Lateral erosion happens so more depsoition happens and river is wided and meanders erode the valley floor, creating a U shaped valley
        More land is used for farming and pasture
        Flood defences-Yarm
        Cost of defences was £2.1 million
        Embankmenst installed to increase the level of river banks
        Gabions reduce erosion along river banks
        Street lighting and flower bed placed to visually improve the area
3. Lower Course
  1. Channel becomes wider and deeper
    1. Bedload made of fine silt
      1. Land use is mainly industrial (steel and iron factories) and there is a nuclear power plant
        Levees formed sue to flooding
        Estuary important for wildlife such as birds
        River has been straightened for navigation+ to reduce flooding
        Flood defence- Tees Barrage
        Barrage stops tidal flows upstream and has opened 25km clean navigable waterway for leisure
        Regeneration of derelict land and attracted new jobs and businesses. Also led to increased tourism
        Barrage cost £54 million
        Around barrage there is a caranvan site, restaurants, woodlands, encouraging tourism

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	Criado por Meghna Ranjit Nair aproximadamente 2 anos atrás