# Fluvial processes - PowerPoint PPT Presentation

Fluvial processes

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## Fluvial processes

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##### Presentation Transcript

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