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Fluvial processes

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  1. Fluvial processes As with most geomorphic processes, Rivers • operate as a function of a dynamic equilibrium between - Driving forces and Resisting forces Driving Forces include • - Gravity Resisting Forces include • - Geology > rock type, topography - Friction > channel shape, particle size of channel > molecular

  2. Types of Flow Laminar Flow • - flow lines are parallel - water molecules don't disrupt flow paths of one another - Not a common type of flow in natural settings > channel is usually irregular which contributes to non-laminar flow Turbulent flow • - flow lines are not parallel - flow lines are semi-choatic - flow velocity varies in all directions > shear stresses are transmitted across layers

  3. Flow flow in turbulent conditions • - varies with depth > related to viscosity and channel conditions max flow velocity in the channel • - occurs up from the bottom of the channel - occurs away from the edge of the channel > due to friction with the channel surface

  4. Reynolds Number (R ) e Re = VR / ) ( r • m - where V = mean velocity - R = hydraulic radius = A x P > A= cross-sectional area > P= wetted perimeter - = density of fluid r - = molecular viscosity m often used as prediction tool • - determines at what velocity and depth flow changes from laminar to turbulent > values less than 500 = laminar flow > values more than 750 = turbulent flow > values between 500 to 750 = situational


  5. Froude Number (Fr) Fr = V / (dg) • - where V = mean velocity - d = depth - g = gravity used to differentiate between types of Turbulent • flow - tranquil flow (Fr <1) - critical flow (Fr = 1) - rapid flow (Fr > 1)

  6. Flow and Resistance Chezy equation • - V = C R S > where R = hydraulic radius > S = slope of channel > C= constant of proportionality (a fudge factor!) Manning equation • 1/2 2/3 - V = 1.49/n (R S ) > where n = manning roughness coefficient - assumed as a constant for a range of channel characteristics > sample n values have been calculated for a bunch of different channel types

  7. one of many channels • depicted in the Barnes reference for determining Manning n

  8. What Purpose

  9. Manning n values associated with bedforms

  10. Components of sediment transport suspended load • - held aloft by turbulent flow and in some cases colloidal electrostatic forces > the more turbulent the flow, the higher the likelihood that material will be transported in suspension - usually restricted to fine grained particles > coarse grains can travel in suspension, infrequently and for short distances and times Bedload • - sediment rolled, bounced, and scooted along the bottom of the channel > usually associated with coarser particle size fractions

  11. Other means of categorizing the load Wash Load • - particles so small that they are absent from the stream bed Bed material load • - particle sizes found in abundance on the stream bed this categorization scheme is dynamic and can • accommodate the natural variability in stream flow discharge only partly controls wash load (fines) • - sediment supply is a much more limiting factor - most streams can naturally carry much more than they actually do - Bed material load is much more closely related to discharge fluctuations

  12. sediment entrainment most bed load materials travel infrequently • - do so in bursts of motion associated with dramatic increases in energy > i.e., velocity (and indirectly discharge) - maximum size of the particles capable of being transported is called competence - total amount of material the stream carries is called capacity should be an easy thing to determine, but often • isn't

  13. Competence critical bed velocity • - weight or volume of largest particle varies as a function of the sixth power of the velocity > involves ascertaining depth and flow velocity during extreme events critical shear stress (tractive force) • - DuBoys equation t - c = RS g t > where c = critical shear > g = specific weight of water > R= hydraulic radius > S = slope

  14. Hjulstrom Diagrams

  15. Stream Power defined by Bagnold to relate the processes, the • velocity, and the particle sizes = QS • w g - where = stream power w = specific weight of water g Q= discharge S= slope divided by width yields stream power per unit • area--> or a function of velocity and shear = QS/width= dSV = V t w g g

  16. Bank erosion generated by two processes • - corrasion > removal of materials by flowing water that exerts a critical shear - this then contributes to a second process > slope failure due to undercutting of the bank > slab failure > often observed when trees drop into the river as banks on which they grow collapse - failure may also result from tension cracks, shrink swell, sapping, or some combination of the above

  17. deposition related to energy as well • - decreases in energy or changes in particle shape can cause sediments to be deposited > coarse stuff first, then finer particles as velocity and or depth changes. - long term deposition is termed aggradation > creates episodes of fill punctuated by episodes of incision > responsible for point bars, gravel bars, terraces, and floodplain formation - vertical aggradation vs lateral migration (point bars)

  18. Geomorphic work when do streams move materials? • - low frequency, high magnitude? or - high frequency, moderate magnitude events? what is the definition of geomorphic work? • - movement of material? - maintenance or modification of channel form? some data indicate most (90%) sediment • movement occurs during normal flow events - sediment is moved during frequent (1-5 year) events > the dominant discharge = approximated by bankfull discharge or the 1.0 to 2.33 yr flood event

  19. other factors include vegetation cover along the channel • recovery time • - has the stream had time to recover > accumulate sediments or re-establish the original channel form environmental conditions • - geologic and topographic setting - climatic variations as well

  20. Hydraulic Geometry streams are in constant state of flux • - discharge and sediment loads vary all the time stream is in equilibrium with these conditions • - Quasi-equilibrium compilation of all kinds of discharge and geometric • data provided statistical relationships for the variable involved - w = aQˆb - d = cQˆf - v = kQˆm > since Q =wdv > Q= (aQˆb) x (cQˆf) x (kQˆm) = ackQˆ(b+f+m) - ackbfm are constants, whereas discharge is the variable

  21. values for b, f, and m avg. values for a statistically significant number • of streams b = 0.26 f = 0.40 m = 0.34 These variables represent what proportion of • total discharge is affected by each dimension at specific locations These 3 variables w, d, v, increase in the • downstream direction - also climate and vegetative cover affect the value of Q

  22. Channel slope concave up longitudinal profile represents a • stream in equilibrium - e.g., the gradient decreases in the downstream direction this helps to explain the general downstream • fining of sediment load - however the slope may in fact be a function of particle size and not vice versa

  23. mean particle size vs slope

  24. Adding in area

  25. Channel patterns and shape shape is related to particle size (Schumm, 1971) • -1.08 F = 255 M > where F is depth to width ratio > M is percent clay and silt (fines) - those with more fines have deep narrow channels - those with coarse-grained banks have wider than deeper Channel Shape • - sinuosity= stream length/valley width > straight channels = sinuosity < 1.5 > meandering = sinuosity ≥ 1.5 > braided = any value-not related to sinuosity

  26. Channel terminology thalweg = the area of maximum velocity in the • channel pool = an area of deeper water; may or may not be • slower flowing riffles = areas of shallower water; • point bar = that area on the inside of the channel • meander bend cut bank = that area where the bank is steepend by • erosion on the outside of the meander bend

  27. Characteristics of flow and Channel patterns flow is generally turbulent, but has areas of • convergence and divergence - convergent -flow lines come together, increases energy - divergent- flow lines spread apart, decreases in energy occurs in downstream direction (horizontally) and in • the vertical direction (up and down) - erosion occurs where lines come together - deposition where lines move apart

  28. Origins of meanders hypothesized as a result of helicoidal flow • - spiral in the downstream direction meander size and shape are shown to be related to • - bankfull discharge and sediment size once flow initiates, random convergence and • divergence creates bedforms and areas of erosion when coupled with helicoidal flow it begins to trigger • meanders, even in straight channels