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The UK's evolving physical landscape - Rivers Topic 4, GCSE Geography

- - - - Heliocoidal flow sends river's energy laterally - thalweg is forced ono the outside, creating a river cliff; sediment is moved inwards forming a point bar and consequently a flood plain
      - This creates a bend known as a meander
      - River bends in the middle course as water has a heliocoidal flow
    - - Continued erosion can create a narrow neck between two meanders
      - Overtime, the energy of the river cuts through the narrow neck of the meanders, cutting of the meander
      - Now that the meander is cut off the river, it becomes an ox-bow lake
  - - - An area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding
    - - When a river floods, the sediment is also moved with the water, and builds up on the banks of the river
    - - When sediment is deposited past the coastline
    - - Two directions of flow - outwards and inwards due to tides
      - Where the river meets the sea
  - - - Ridges of land which jut into the river valley, looking like they are interlocked; are steeper on one side where the river cuts into the side
    - - When a waterfall retreats, it leaves behind steep valleys on either side called gorges
    - - Erosion of less resistant rock underneath continues undercutting the hard rock above; river's energy hollows the bottom of waterfall known as a plunge pool
      - Less resistant beneath is eroded more reapidly via erosion and hydraulic action, creating a ledge which overhangs and collapses
      - Waterfalls occur when a river crosses a bed of more resistant rock
      - Waterfall takes a new position, leaving a gorge
- - - - River sediment (items 5-7)
      - Gradient (Item 8)
      - Channel (Items 1-4)
      - Cross Profile / Valley Shape
- - - - Occurs when there's a lot of rain and the ground becomes saturated; soil/rock slides down the slope
    - - Occurs on a curved slip plane - large chunks of land break off
    - - Weathering (physical/biological) weakens rock causing it to break and fall
    - - Lots of rain causes the land to be saturated, the land slides
  - - - Such as landslides and mudflows
    - - Such as soil creep
- - - - When the soil become saturated, extra rain flows overground as surface runoff
      - Little precipiation falls directly into river; mostly trees and plants catch this (interception zone) - amount depends on vegetation and season (e.g. deciduous plants intercept more in summer when in leaf)
      - Some intercepted is evaporated, rest is dripped into soil and soaks in (infiltration)
      - How quickly surface runoff occurs...
        
        Permeability of geology and the soil type - e.g. sandstone/chalk are permeable and absorb water quickly whereas clay soils are impermeable
        
        Heaviness of rainfall - heavy storms cause low infiltration and rapid runoff
        
        Antecedent rainfall - the soil may already be saturated
        
        River basine shape - circular increases flood risk whilst longer/ thinner basins have a lower flood risk
    - - Once water enters the soil...
        
        Some seeps through soil air spaces (throughflow)
        
        Some continues into solid rock - upper limit is water table; from here water seeps to river as groundwater flow, which keeps a river flowing when there's no rain
        
        Some is transpired through leaves
  - - - A
        
        Reacts to quick rainfall - happens during heavy rainfall in urban regions with impermeable rock, allowing for surface runoff; drains force water into rivers rapidly rising the levels quickly
      - B
        
        How a river behaves in rural regions - either permeable soils or summer; each increases lag time
    - - Land Use
        
        Replacing fields with infrastructure OR pasture with arable (crops) - both increase runoff, reducing lag time
      - Deforestation
        
        Changes hydrograph shape from 'B' to 'A', reducing interception and infiltration - afforestation has opposite affect and is used in upland areas to reduce flooding
    - - Peak Rainfall
        
        Highest rainfall in the time period you're looking for
      - Peak discharge
        
        Highest discharge of time period you're looking at
      - Base Flow
        
        Normal flow of a river when water level is sustained by groundwater flow
      - Rising Limb
        
        Increase in river discharge as precipitation flows into river
      - Failing Limb
        
        Decrease in river discharge as precipitation returns to its normal level
      - Lag time
        
        Delay between peak rainfall and peak discharge
- - - - Prolonged Rain
        
        On 15th June, 90mm precipitation fell over Sheffield - more than 1 month's normal rainfall - which saturated the soil
        
        25th June led to a larger rainfall (100mm in 24hrs) (record in Sheffield); ground was saturated so drains couldn't cope
        
        Antecedent rainfall fell across South Yorkshire
        
        190mm in 2 days made June the wettest Yorkshire month since 1882, creating a hydrograph like 'Shape A'
      - Rivers & Landscape
        
        Sheffield AKA 'City of Seven Hills' - steep so quicker surface run-off
        
        City lies at foot of Pennines, where 3 rivers (Rivelin, Loxley, Don) meet; water reservoirs for Sheffield in upper course help regulate river flow in rainy periods, but this time they filled and overflowed
        
        River Rivelin joins River Loxley at Confluence X on map; water volume increased hugely increasing flood risk
        
        Confluence Y on map occurs w/ River Don, which was already almost up to its bank so a backflow occured down the other rivers
    - - Most of Sheffield is built up with impermeable surfaces
      - Extensive drain network was overwhelmed by extreme flows
      - Difficult to predict flooding
      - Fallen trees blocked river channels
      - Drainage system was designed to withstand precipitation which could occur once every 30 years - this rainfall was a 1 in 35 years event and the floods were a 1 in 400 year event!
      - The Shopping Centre flooded despite having defences to protect it in a 1 in 100 yr flood
- - - - One of Uk's lowest lying lands
      - During last Ice Age, was covered by sea
      - Artificially drained in 17th century
  - - - Both hard and soft engineering i.e. dredging/ beach nourishment/ levées
    - - Rainfall
        
        Jet streams from Atlantic drive low pressure weather systems w/ wind & rain
        
        Normally end up in N. but during winter 2014, moved 800km south to S.England
        
        Winter 2014 bought:
        
        12 major storms - stormiest UK winter in 20 years
        
        Most severe gales of any winter since 1871
        
        Most rainfall since 1871 - 235% of average winter amounts
        
        More days when high winds combined w/ very high tides / tidal surges
        
        More rainy days since 1961
- - - - Costs up to £1m depending on materials
      - Effectiveness
        
        :check: / :red_cross: Expensive but allow to live close to farm/river w/ reduced risks
        
        :red_cross: Increases flood risk downstream
        
        :red_cross: Can fail by overtopping, slumping, erosion
      - Increases capacity of river like flood walls
    - - Costs £50,000
      - Effectiveness
        
        :red_cross: Needs to be done every year
        
        :red_cross: Concrete lining is expensive to build
        
        :red_cross: Increases flood risk downstream
        
        :check: Cheap to maintain
      - Dredge river to increase channel capacity OR line w/ concrete to speed up river
    - - Cost depends on type of wall and material
      - Effectiveness
        
        :check: Fairly cheap & are 'one-off' costs
        
        :check: Useful for city centres w/ limited space
        
        :red_cross: Disperses water quickly which increases flood risk downstream
      - Build a high wall alongside a river to increase capacity to prevent flooding
    - - 1km in Rotherham cost £14m in 2008; 7km in Exeter cost £30m in 2015
      - Effectiveness
        
        :check: Protects built up areas
        
        :red_cross: Can cause flooding elsewhere
      - Create extra channels to divert excess water from city centre
  - - - £1.2m for 2km
      - Effectiveness
        
        :check: Increased ability to store floodwater
        
        :check: Flooding reduced in 2007 despite heavy rains
        
        :check: Only flooding in Darlington that year was due to backlog in drains
      - Flood plain level is lowered, surfaces restored to grassland, allowing gradual water retainment
    - - £1.2m for 2km
      - Effectiveness
        
        :check: Improved ecology w/ 30% increase in birds & insects in one year
        
        :check: Improvement in natural look
      - Some meanders are rebuilt, banks are lowered, hard engineering methods stripped away, replaced w/ more natural methods