fluvial processes

• Fluvial processes


  Control local deposition and erosion of sediment, allowing rivers to migrate across valley bottoms and form flood plains. They also govern the way that rivers incise into bedrock and generate the topography of upland valleys
• Graded stream


  A stream profile that is adjusted to carry its sediment load. It is concave up, with steeper channels in headwaters declining progressively downstream toward the outlet of the river network
• Factors controlling the shape of channels

  • Flows and sediment loads they carry
  • Cohesive strength of material in their banks
  • The slopes they flow down
• Factors controlling the shape of channels

  • Velocity
  • Slope
  • Discharge
• Climate influence on channels

  Establishes the amount, type, and seasonal pattern of precipitation and runoff, and controls the dominant runoff- generating mechanisms and thereby streamflow magnitude and variability
• Discharge in humid vs arid regions

  In humid regions, discharge systematically increases downstream within channel networks. In arid regions, discharge often decreases downstream as water infiltrates through the streambed
• Ways sediment reaches river channels

  • Erosion
  • Transport
  • Upstream channel reaches
  • Bed & bank erosion during channel incision and migration
  • Neighbouring hillsides
• Dissolved load


  Composed of ions in solution travelling at speed of the flow
• Suspended load


  Composed of material suspended by turbulence of the flow and moving at the speed of the flow
• Bedload


  Moves by rolling or sliding. Saltating sediment is swept from the bed and travels some distance before settling onto channel bottom
• Bed and bank material

  Control the rate at which sediment is mobilised, the banks are eroded, and the bed is incised
• Alluvial channels


  Most common in lower gradient stream valleys. No valley walls, thick sediment that prevents incising, allows channels to migrate along valley bottoms
• Bedrock channels


  Found in narrow uplands or hilly mountainous terrain. Narrow valleys, rocky walls, over bedrock
• Riparian vegetation influence on channels

  Holds material together & slows erosion. Influences scour, deposition, and sediment transport. Can divert streams. Can cause massive erosive and transport episodes
• Physical constraints determining flow velocity

  • Fluid driving force – controlled by depth and slope
  • Bed roughness – frictional resistance of bed and banks
• Manning's equation


  U= [R^2/3 S^1/2]/n, where N = channel roughness, S = surface slope, R = (width x depth) / wetted perimeter
• Discharge formula


  Q = W (width) x D (depth) x V (velocity)
• Hydraulic radius formula


  Acs/ Pw
• Manning roughness coefficient


  A commonly used empirical assessment of frictional resistance to flow
• Physical properties determining channel roughness

  • Flow depth
  • Slope
  • Velocity
  • Obstacles in the stream/ on the stream bed
• Froude number


  The ratio of flow velocity to the speed at which a surface wave will propagate (flow depth and gravitational acceleration). Indicates whether a stream is moving faster or slower than its wake, differing ratios all transport and deposit sediment differently
• Effective discharge


  The discharge that transported the most sediment over a period of years to decades. Corresponds to major precipitation events like monsoons
• Velocity distribution in channel cross section

  High flow will occur at the top of the water and lowest flow at bottom. Good place for average is 0.6 of the way
• Bankfull flow


  Flows that fill a channel to the point of overflowing. Important because it is the point to be exceeded for floods that inundate valley bottoms and deposit sediment
• At-a-station hydraulic geometry


  Relationships between wetted width, flow depth, and velocity when compared to discharge
• Fluid forces moving sediment

  Lifting, dragging & rolling of sediment particles. Force of moving water exerts shear stress directly on the sediment, overcoming forces of gravity and cohesion holding sediment in place
• Types of sediment load

  • Dissolved Load
  • Suspended Load
  • Bedload
• Critical shear stress


  The flow velocity needed to move a piece of sediment. Entrainment begins here. Below this, there is no motion, above this transport rates increase with increasing shear stress
• Stream power


  The ability of a stream to transport sediment and carve into bedrock. Deeper, faster flowing water can carry more sediment and incise more bedrock
• Discharges for sediment transport vs bedrock incision

  Discharges with high area & low velocity are good for sediment transport. Discharge with small areas but high velocity down cut. Incision occurs only after sediment is in motion already
• Ways rivers incise into rock

  • Abrasion
  • Plucking
  • Dissolution
• Alluvial channel migration


  Erosion on the outside of bends (faster water) makes cutbanks and depositions on the inside of banks (slower water) makes point bars
• Pools and riffles at high vs low flows

  Low = riffles have steep water surface slopes and rapid flow, and pools water surface flow and slower flow. High = pools have greater velocity than riffles
• Channel meandering


  Natural flow dynamics and erosion and deposition of sediment along river bends. Water flow velocity is higher on outer edge of bend due to centrifugal force leading to greater erosion. On the inner edge of the bend where water flow is slower, sediment is deposited
• River sinuosity


  The ratio of the channel length measured along the centre of the channel to the straight line distance measured down the valley axis
• Channel patterns


  • Straight
  • Meandering
  • Braided
  • Anastomosing
• Channel reach vs channel unit

  Reach = stretch of channel that exhibits similar characteristics: bed and bank material, position in landscape. Unit = within reaches, groups of morphologically distinct forms: bars, steps, pools, riffles
• Downstream change in channel reach types

  Gets less steep towards lowland valleys. Cascade, steep-pool, plane-bed, pool-riffle, dune-ripple
• Large woody debris in channels

  Creates stable obstructions to flow and forces flow convergence, divergence, and sediment impoundment by stream channels causing pools, bars, and steps
• Ways rivers build floodplains

  • Overbank deposition: sediment laden discharge spills out over natural levees and deposits on plains
  • Meander migration: bedload material is deposited on the inside of meander bends
  • Avulsion: integration of deposits from stable log jams