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
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
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
Reynolds Number (Re)
• Re = VR( r/m )
- where V = mean velocity
R = hydraulic radius = A x P
>
>
-
A= cross-sectional area
P= wetted perimeter
r = density of fluid
m = molecular viscosity
• 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
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)
Flow and Resistance
• Chezy equation
- V=C RS
> where R = hydraulic radius
>
S = slope of channel
>
C= constant of proportionality (a fudge factor!)
• Manning equation
- V = 1.49/n (R 2/3 S 1/2 )
> 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
• one of many channels
depicted in the Barnes
reference for determining
Manning n
What Purpose
Manning n values associated with bedforms
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
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
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
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
-
tc = gRS
> where
tc = critical shear
>
g = specific weight of water
>
R= hydraulic radius
>
S = slope
Hjulstrom Diagrams
Stream Power
• defined by Bagnold to relate the processes, the
velocity, and the particle sizes
• w = g QS
- where w = stream power
g = specific weight of water
Q= discharge
S= slope
• divided by width yields stream power per unit
area--> or a function of velocity and shear
w = g QS/width=gdSV = tV
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
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)
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
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
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
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
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
mean particle size vs slope
Adding in area
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
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
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
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