AS 1: Processes that shape fluvial environments

• Precipitation entering a drainage basin

  1. Channel catch

  2. Interception

  3. Evaporation

  4. Surface run-off/overland flow

  5. Infiltration

  6. Transpiration
• Discharge


  The volume of water passing any point in a given time in the river channel
• Calculating discharge


  1. Channel cross sectional area

  2. Velocity

  3. Equals discharge in cumecs
• Cumecs


  Cubic metres per second
• Hydrograph


  A graph showing the pattern of changing discharge at any point along a river over time

  
• Types of hydrographs


  • Annual hydrograph
  • Storm hydrograph

  
• Annual hydrograph


  • Usually taken over a 30-year period to average out the mean flow pattern
  • Reflects events like spring snow melt or wet/dry seasons

  
• Annual hydrograph


  • Useful for planners examining new developments or changing land use
  • Important for water engineers to ensure adequate year-round water supply and prevent shortages/floods
• Storm hydrograph


  • Shows impact of one period of precipitation on river flow

  
• Factors impacting hydrograph shape

  1. Fast surface flow of channel catch and overland flow

  2. Slow sub-surface flows of through flow and groundwater flow
• Sequence of a typical hydrograph

  1. Slight increase in discharge due to channel catch

  2. Soil becomes saturated or rainfall exceeds infiltration, leading to overland flow and rising limb

  3. Lag time between peak rainfall and peak discharge

  4. Peak discharge, if exceeding channel capacity leads to flooding

  5. Falling or recession limb as remaining storm water drains

  6. Base flow from groundwater store

  
• Storm flow


  Discharge resulting from the period of precipitation

  
• Flat hydrograph


  • Gentle limbs
  • Low peak discharge
  • Long storm flow
  • Long lag time
  • Large base flow
  • Flooding unlikely
  • Environment is permeable or forested

  
• Flashy hydrograph


  • Steep limbs
  • High peak discharge
  • Short storm flow
  • Short lag time
  • Small base flow
  • Flooding likely
  • Environment is impermeable or urbanised

  
• Factors affecting basin discharge and the storm hydrograph


  • Nature of the storm
  • Nature of the basin
  • Previous conditions
  • Vegetation cover
  • Human influence
• Nature of the storm

  • How long the storm lasts
  • How heavy the storm is - heavy downpour or longer period of steady rainfall
  • Type of precipitation - snow, hail, rain
  • Speed of the storm - does it take a while to pass over an area or does it move quickly past
• Nature of the basin

  • Basin size - is it small or large. This would influence how quickly the precipitation would potentially reach the river. (shorter lag time)
  • Basin shape - a more circular shape will be more efficient at transferring the water to the river. (steeper rising limb) A longer elongated shape will mean the water would take longer to reach the measuring station on the river. (more gentle rising limb)
  • Relief - a steep slope will encourage greater surface run-off and the water will therefore reach the river faster. (steeper rising limb) A gentler relief will allow more infiltration, so water will take longer to reach the river by through flow. (gentle rising limb)
  • Underlying soil and geology - More permeable soil and geology (limestone) allow infiltration of surface water, reducing how quickly the water reaches the river and therefore reducing discharge. Lower peak discharge, more gentle rising limb, longer lag time. Impermeable soils and geology (clay/granite) reduce infiltration and encourage greater surface flow. Water will reach the river quicker and increase discharge, shorten lag time and create a steep rising limb.
  • Drainage density - this is a measure of the amount of streams and tributaries there are within a drainage basin. The greater the drainage density the faster water can reach the main channel and be recorded. The graph will have a steeper rising limb, higher peak discharge and shorter lag time. (flashy hydrograph)
• Previous conditions


  • This looks at the state of the drainage basin before the storm event. For example, had previous precipitation increased groundwater levels and possibly saturated the soil. This would reduce infiltration and increase surface run-off influence on the graph. (steeper rising limb, higher peak discharge, shorter lag time.) A flashy graph! A drought could cause the ground to become very hard and temporarily impermeable. When the next precipitation did fall on this hard ground infiltration rates would be reduced initially and surface run-off increased. Once the ground has softened, the infiltration rate will increase and surface run-off will reduce.
• Vegetation cover


  • Greater vegetation cover will mean greater interception and evpotranspiration. This will reduce the amount of water reaching the river. The type of vegetation will also influence the hydrograph shape. Deciduous trees in summer will intercept more moisture that coniferous trees, but in winter the deciduous trees shed their leaves reducing interception rates. Deciduous trees intercept 60% more precipitation in summer than in winter!
• Human influence


  • Human activity can alter the nature of the land surface. Things such as deforestation, changing the type of land use from crops to arable farming.
  • Urbanisation - this provides greater impermeable surfaces and reduces infiltration rates, increasing surface run-off. Urban drainage systems can also increase the speed water reaches the river, increasing discharge and the likelihood of flooding.
• Drainage basins


  Transfers of kinetic or movement energy when water starts to flow down slopes
• Types of work done by the energy in drainage basins

  • Friction
  • Erosion
  • Transportation
• Energy is lost by friction between the water and the bed and banks of the channel
• Erosion


  The wearing away of the bed and banks of the channel
• Transportation


  The carrying away of eroded material (load)
• Types of river erosion

  • Abrasion/corrosion
  • Hydraulic action
  • Solution/corrosion
  • Attrition
• Abrasion/corrosion


  Rock fragments carried by the river wear down the bed and banks of the channel (action is like sandpaper)

  
• Hydraulic action


  The erosive power of the water pushes against the river banks and removes any loose or unconsolidated material, undercutting the banks on the outer side of the channel. It can force air into cracks especially at waterfalls and rapids.
• Solution/corrosion


  Soluble rock or minerals are dissolved by the slightly acidic river water. Effective on carbonate-based rocks (limestone and chalk).
• Attrition


  Load particles strike each other and break up into smaller pieces. The load will become more rounded as it travels downstream.
• Ways erosion processes operate

  • Vertical (deepening valleys in mountains)
  • Lateral (horizontal meandering of rivers)

  
• Types of river transport

  • Solution (load dissolved in and carried by the river water)
  • Suspension (load carried by the flow without touching the river bed)
  • Traction (load rolled along the river bed)
  • Saltation (load bounced along the river bed)
• Rivers need energy to overcome friction and to erode and transport their load. When a river loses energy, the load can no longer be carried and so deposition occurs.
• Deposition


  The sediment falls to the bed, starting with the heaviest first (gravel/sand), while finer sediment (silts/clays) may be carried further.
• Point bar


  Deposition of load on the inner bank of a meander where the water flowing has a shorter distance to travel and slows down.
• Alluvial


  Material deposited by the river.
• Eyots


  Small islands that form when a river slows down and deposits sediment on its bed.
• Braiding


  The river channel becomes divided into several smaller interweaving channels.
• Eyots


  Small islands that form when a river slows down and deposits sediment load on its bed
• Eyot formation


  1. River slows down

  2. Deposits sediment load on bed

  3. Small islands form