Fluvial Process

Fluvial Process

1. Hjulstrøm diagram

   • Establish relationships between a stream’s flow velocity and its ability to erode and transport grains of a particular size. It cover a wide range of grain sizes and flow velocities.
     • the Hjulstrøm diagram applies only to erosion, transport, and deposition in alluvial channels.
       • In bedrock channels, the bed load abrades the rock floor and causes vertical erosion.

2. Erosion and transport

   • Clay and silt particles stay in suspension at velocities of 1–2 cm/s, which explains why suspended load deposits are not dumped on streambeds.
     • the flow velocity at which erosion starts for grains larger than 0.5 mm is roughly pro - portional to the square root of the grain size.
   • Channel initiation
     • Stream channels can be created on a newly exposed surface or develop by the expansion of an existing channel network
       • Arid
         • For arid to semiarid conditions critical hillslope length was required to generate a channel
           • Hortonian overland-flow mode for channel initiation
             • It says that, before overland flow is able to erode the soil, it has to reach a critical depth at which the eroding stress of the flow exceeds the shear resistance of the soil surface
       • Humid regions
         • In humid regions, channel initiation is more related to the location of surface and subsurface flow convergence, usually in slope concavities and adjacent to existing drainage lines
           • Pipe networks and slope wash and similar mass movements (that dominate soil creep and creep-like processes). can help initiate channel development
             • Rills can develop as a result of a sudden outburst of subsurface flow at the surface close to the base of a slope
   • FLUVIAL EROSIONAL LANDFORMS
     • Rills
       • Rills are tiny hillside channels a few centimetres wide and deep that are cut by ephemeral rivulets. They grade into gullies
         • Gullies are intermediate between rills and arroyos, which are larger incised stream beds.
     • Types of channels
       • Bedrock channels
         • Bedrock channels are eroded into rock. They are resistant to erosion and a river can cut into it in any reach
           • Incised meanders do form in horizontally bedded strata.
             • Intrenched meanders
               • Are symmetrical forms and evolve where downcutting is fast enough to curtail lateral meander migration (fall of base level)
             • Ingrown meanders
               • Are asymmetrical and result from meanders moving sideways at the same time as they slowly incise owing to regional warping
       • Alluvial channels

3. Deposition

   • Ocurrs (mainly) in valley bottoms where gradients are low, at places where gradients change suddenly, or where channelled flow diverges, with a reduction in depth and velocity.
     • Types of fluvial deposit
       • channel deposits
       • channel margin deposits
       • overbank floodplain deposits
       • valley margin deposits
       • Terms/process
         • Alluviation
           • Large-scale deposition affecting much of a stream system. It results from fill > scour, for long periods.
         • Aggradation of channels or floodplains
           • Sediment gains exceed losses, and storage increases
             • Common patterns in valley storage and fluxes
               • a) A quasi-steadystate typical of humid regions
               • b) vertical accretion of channels and aggradation of floodplains
               • c) valley trenching (arroyo cutting)
               • d) episodic gains and losses in mountain and arid streams
     • Stream erosion and deposition take place during flood events

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