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
         • Alluvial channels form in sediment that has been, and is being, transported by flowing water
         • Basic forms
           • straight
             • structural control exerted by faults or joints
               • Even in a straight channel, the thalweg (the trace of the deepest points along the channel) usually winds from side to side, and the long profile usually displays a series of deeper and shallower sections (pools and riffles, p. 223)
           • Meandering
             • Meandering is favoured where banks resist erosion, but meandering is caused by the intrinsic instabilities of turbulent water against a movable channel bank.
           • Braided
             • Depositional form that occur where the flow divides into a series of braids separated by islands or bars of accumulated sediment
               • Braided channels tend to form where (1) stream energy is high; (2) the channel gradient is steep; (3) sediment supply from hillslopes, tributaries, or glaciers is high and a big portion of coarse material is transported as bed load; and (4) bank material is erodible, allowing the channel to shift sideways with relative ease
           • Anastomosing
             • Are a set of interconnected channels separated by bedrock or by stable alluvium. Its formation is favoured by an aggradational regime involving a high suspended-sediment load in sites where lateral expansion is constrained
           • Anabranching channels
             • Consist of multiple channels separated by vegetated and semipermanent alluvial islands or alluvial ridges. The islands are cut out of the floodplain or are constructed in channels by the accretion of sediments.
               • Developed by frequent floods, channel banks that resist erosion, and mechanisms that block or restrict channels and trigger avulsions.

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
         • Avulsion
           • Avulsions are the sudden change in the course of a river leading to a section of abandoned channel, a section of new channel, and a segment of higher land (part of the floodplain) between them
     • Stream erosion and deposition take place during flood events

4. Taken from Huggett, 2011

5. Hydraulic geometry, profiles, baselevel, and grade

   • Hydraulic geometry
     • Hydraulic geometry considers the relationships between the average channel form and discharge. Q = w (width) d (depth) v (velocity)
       • the mean velocity and width–depth ratio (w/d) both increase down stream, as discharge increases
         • If stream width reduces, then water depth increases as well as velocity, this cause bank erosion, widening the stream again and returning the system to balance
           • Channel variables respond at different rates to changing discharge
   • Longitudinal profile
     • Is the gradient of its water-surface line from source to mouth.
       • Depth increases with increasing discharge and so, in moving downstream, a progressively lower gradient is sufficient to transport the bed load
   • Baselevel
     • Is the lowest elevation to which downcutting by a stream is possible and is the water body into which it flows (ex. Sea, lake)
   • Grade
     • Is a state of a river system in which controlling variables and baselevel are constant. Is a system in equilibrium

6. Main geometrical properties of stream networks and drainage basins

7. Structural control

   • Drenaige development depends upon structural control. Most stream networks are adapted to regional slope and geological structures, picking out the main fractures in the underlying rocks.
     • major types of drainage pattern and their relationship to structural controls
     • Anomalous drainage patterns
       • Captured rivers
         • Abrupt changes in stream course
       • Diverted rivers
         • Glacial ice, uplifted fault blocks, gentle folding, and lava flows may all cause major river diversions
       • Antecedent rivers
       • Superimposed rivers
       • Persistent rivers
         • Streams adjusted to a particular structure-geology may, on downcutting, meet a different structure

Images and files in the map