Fluvial processes• As with most geomorphic processes, Rivers

operate as a function of a dynamic equilibriumbetween

- 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(/)

- where V = mean velocity

- R = hydraulic radius = A x P> A= cross-sectional area > P= wetted perimeter

- = density of fluid

- = molecular viscosity

• often used as prediction tool

- determines at what velocity and depth flow changes fromlaminar 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 Turbulentflow

- tranquil flow (Fr <1)

- critical flow (Fr = 1)

- rapid flow (Fr > 1)

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

- V = 1.49/n (R S )> where n = manning roughness coefficient

- assumed as a constant for a range of channelcharacteristics> sample n values have been calculated for a bunch of different

channel types

2/3 1/2

• one of many channels

depicted in the Barnesreference for determiningManning n

What Purpose

Manning n values associated with bedforms

Components of sediment transport• suspended load

- held aloft by turbulent flow and in some cases colloidalelectrostatic 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 thebottom 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 canaccommodate 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 theyactually do

- Bed material load is much more closely related to dischargefluctuations

sediment entrainment• most bed load materials travel infrequently

- do so in bursts of motion associated with dramaticincreases in energy> i.e., velocity (and indirectly discharge)

- maximum size of the particles capable of beingtransported is called competence

- total amount of material the stream carries is calledcapacity

• should be an easy thing to determine, but oftenisn't

Competence• critical bed velocity

- weight or volume of largest particle varies as a functionof the sixth power of the velocity > involves ascertaining depth and flow velocity during extreme

events

• critical shear stress (tractive force)

- DuBoys equation

- c = RS

> where c = 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

• = QS

- where = stream power

= specific weight of water

Q= discharge

S= slope

• divided by width yields stream power per unitarea--> or a function of velocity and shear

= QS/width=dSV = V

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 whichthey 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 cancause 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, andfloodplain 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%) sedimentmovement 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 geometricdata 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 numberof streams

b = 0.26

f = 0.40

m = 0.34

• These variables represent what proportion oftotal discharge is affected by each dimension at

specific locations

• These 3 variables w, d, v, increase in thedownstream 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 downstreamfining of sediment load

- however the slope may in fact be a function of particlesize and not vice versa

mean particle size vs slope

Adding in area

Channel patterns and shape• shape is related to particle size (Schumm, 1971)

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

-1.08

Channel terminology• thalweg = the area of maximum velocity in thechannel

• pool = an area of deeper water; may or may not beslower flowing

• riffles = areas of shallower water;

• point bar = that area on the inside of the channelmeander bend

• cut bank = that area where the bank is steepend byerosion on the outside of the meander bend

Characteristics of flow and Channelpatterns

• flow is generally turbulent, but has areas ofconvergence and divergence

- convergent -flow lines come together, increases energy

- divergent- flow lines spread apart, decreases in energy

• occurs in downstream direction (horizontally) and inthe 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 anddivergence creates bedforms and areas of erosion

• when coupled with helicoidal flow it begins to triggermeanders, even in straight channels