Fluvial Processes Flashcards
Some applications of fluvial geomorphology
- Conduits for water and sed movement
- Conduits for nutrients and contaminants and support biological systems
- Critical for river ecology, population dynamics, envy chemistry
- Flood prediction and mitigation
- Fish habitat
- Dams
- Bridges and infrastructure to withstand bankfull (but should consider unusual strong events w/ low recurrence intervals)
How do landscape materials get from valley floors to their ultimate sink (oceans and lakes)?
- Streamflow accounts for 85-90% of total sed transport
- Glaciers 7%
When discharge increases, what also increases? or Decreases?
- Increase: Width, Depth, Velocity
- Decrease: Gradient
What does Q = ?
v x A
Hydraulic driving variables
- Discharge
- Shear velocity
- Shear stress
- Stream power
- Slope
- Base level
- Sediment load
Hydraulic Response variables
- Channel width, depth
- Channel bedrooms
- Channel patterns (geomorphology)
What are flow rates controlled by?
- Slope
- Velocity
- Sediment Size
- Channel Form
- Roughness
Streamflow velocity
- Vector quantity with both magnitude (speed) and direction
- Varies in 4 dimensions (distance from bed, across stream, downstream, time)
- Highly variable in time and space
- Effects processes of erosion, transport, and deposition of sed
What is the shape of the velocity profile influenced by?
- Size of roughness elements on streambed
- Depth of flow
- Logarithmic velocity profile due to friction of the bed
Where does velocity generally increase?
- Toward stream center
- but more complex
- Degree of symmetry can be highly variable, changing with shape of channel
What happens to velocity when Discharge increases?
- Depth increases
- Reduces influence of roughness elements on the bed
- So Velocity increases
- Seasonal freshet and diurnal fluctuations strongly dependent on discharge
Laminar
- Water travels along parallel paths with no significant mixing
- Relatively rare in low viscosity Newtonian fluids
Turbulent
- Chaotic movement of water
- Fluctuations in velocity, considerable mixing
- Irregular paths of fluid flow
Reynolds Number, Re
- Defines the transition between laminar and turbulent flow
- Ratio between the driving (inertial) forces and resisting (viscous) forces
- As driving forces (numerator) increases, flow becomes more turbulent
Re <500
= Laminar
Re >2000
= Turbulent
Froude Number, Fr
- Streamflow, compares inertial and gravitational forces
- = mean velocity/ (square root of gravitational acceleration x channel depth)
Fr < 1
- Subcritical
- Deep, slow flow (tranquil)
- Ripples can travel upstream
Fr > 1
- Supercritical
- Shallow, fast flow
- Flow velocity is larger than wave velocity
- Ripples cannot travel upstream
Flow states and energy regime: Uniform
Velocity is constant with position
Flow states and energy regime: Non-uniform
Velocity is variable with position
Flow states and energy regime: Steady
Velocity is constant with time
Flow states and energy regime: Unsteady
Velocity is variable with time
Flow states and energy regime: Tranquil
Fr < 1
Flow states and energy regime: Rapid (rough)
Fr > 1
Where is velocity usually the greatest?
Near the surface where channel is deepest
Flow measuring tools
- Ott meter
- Price-Gurley meter
- Fish-mounted Price meter
- Laser Doppler Velocimeter
- Electromagnetic Current Meter
- Acoustic Doppler Profiler
Velocity-Area method
- Q, Discharge measurement
- Volume of water passing through channel cross-section per unit time
- Q = W x D x V =A x V
Q Rating curves
- Use stage to find Q
- Indirect measurements made using plots of observed Q vs. Stage (height) at a given cross section
Hydraulic geometry
- Relationships btwn the mean stream channel form/dimensions and Q both at-a-station and downstream along a stream network
At-a-station
- Tells us how the channel dimensions change as flow changes at one cross section over time
Hydraulic geometry of rivers: Downstream
- Tells how the channel dimensions change along the channel
Basic definition of flooding
- flow that exceeds channel banks onto the floodplain
-
What are 2 critical stages (water depths)
- Bankfull discharge
- Mean annual flood
Bankfull Discharge
- Flow to bank tops (just before overflow)
- Effective or dominant discharge as largely responsible for altering channel forms (return period of 1-2 years)
Mean annual flood
- Flow breaches bank
- Spill onto floodplain
- Return period of 2.33 years
Recurrence interval
- Return period
- Average interval of time within which a given flood will be equaled or exceeded once
- R = (n plus 1)/m
- n = number of years of record
- m = rank
Probability
P = 1/R x 100
- R = recurrence interval
Discharge hydrograph
- Q measured over time
Event storm hydrographs
- Usually right skewed
- Steep rising limb reflecting rapid runoff
- Prolonged decline from gradual depletion of soil water and gw
- Deforestation and urbanization enhance peak Q
Discharge hydrograph
- shape depends on many factors including:
- Drainage basin shape and size, storm intensity, land use
- Deforestation and urbanization enhance peak Q
Hortonian overland flow
Rainfall>infiltration
If rainfall < Infiltration
- water moves via interflow (vadose) zone and in gw
Rills
- Series of small, parallel channels
- Sheet flow concentrates into rivulets
- Cuts small parallel channels
- Typically several cm wide and deep
- Best developed where vegetation is lacking
Gullies
- Series of large, parallel, v-shaped channels
- Carved by concentrated runoff
- Not easily undone
- Large scale form of rills that have joined
- Typically several meters deep/wide
- Prominent in arid areas (badlands), cleared land (cut-blocks, grazing areas, poorly contoured summer fallow fields)
Sheet flow
- Overland flow as a shallow layer
- Removes particles in thin layers
- Very important process effecting agricultural soil erosion
What is the resisting force for channel initiation?
- Given by the weight of the sediment on the slope and cohesive forces (clay, veg, roots)
Driving (erosive) force (tractive) is controlled by what?
- Weight of the water (depth and density) and the slope
- Stress = density x gravity x depth x slope
LOF
Limit of Overland Flow
- Where local depth increases due to various random perturbations
Hjulstrom Diagram
- Entrainment vs. grain size
- Grain-size increases with current velocity for material in transport
- But erosion of fine sediments requires disproportionally high velocities
Drainage Basins
- Includes both streams and the land surface
- Separated with drainage divide
- Basins are nested
- Geomorph and Geology exert strong controls on important effects such as flooding and stream sediment geochem
What is Canada’s largest drainage basin
- Mackenzie River
- 1.8 million km^2
- 4200km long
What are the 3 general sub-regions of drainage basins?
- Colluvial
- Alluvial
- Depositional Zones
Colluvial zones
- Mass wasting and bedrock channels dominate
- Headwater channels drain mountainous environments, Typically very steep in bedrock
- Transport capacity > sed supply (sed input < output)
- Channels erosive
- River channels actively eroding
- Typical V-shape in x-section unless other geomorphic agents active
- Channels relatively straight, steep, with pools and riffles
Alluvial Zone
- Rivers flow through own deposits
- Lowland river channels flowing in relatively wide valleys (compared to colluvial zone)
- Wider, flatter valley floors
- Channel slopes less steep than colluvial streams
- Low gradient and higher sinuosity
- Transport Capacity < sed supply (input > output)
- Rivers erode and deposit at different places and times
- Point bars
Terraces
- Abandoned floodplains formed when river flowed at a higher level than now
Strath Terraces
- Bedrock terraces
Depositional Zone
- Floodplains adjacent to stream channel, overlap with alluvial zone
- Dominated by wide, low gradient floodplains, deltas and fans
- Aggrading and laterally accreting floodplains, well-developed levees, distributary channels
- Make the depositional zone flood prone
Delta
- Classic triangular shape like Nile after Greek letter, or fan-shaped, or birds-foot (series of branching channels)
- Depositional landform where river enters bodies of water and drop sed load
- No longer confined so channels expand, velocity drops, sed depositied
- Geomorph reflects process (tidal, wave, river dominated)
Alluvial Fans
- Deposited on land where river leaves confines of narrow constriction (mountains) and flows into broad valley
- Vary from debris flow dominated colluvial fans to river-dominated fluvial fans
- Geomorph strongly controlled by depositional processes and events in the basins
- Steeper and coarser than deltas, larger magnitude flows/floods
Temporal changes of alluvial fans
- Past different than present
- Past: lots of glacial melt
- Less melt in last 6000 yrs
- Determined to be only 2-3 m in last 6000 b/c of Mount Mazama ash layer
Controls on drainage basin structure
- Bedrock (lithology, structure, tectonics)
- Vegetation type, density, fire, logging
- Soil thickness, grain size, composition weathering
- Slope (tectonics, lanscape development stage)
- Climate (Precip amount, duration, intensity)
- Base level: controlled by sea/lake level, tectonics, damming
Organization of drainage basin
- 3 systems (Horton, Strahler, Shreve)
- Minor tributaries low order and main trunk highest
Strahler Organization
- When 2 first order streams join, forms second order
- Third order only formed when 2 second order join
- Downstream order only increases if joined to a branch of the same order
- Low order streams not always counted, so under represented (Shreve’s accounts for this)
Three main categories of drainage basin morphometry
- Linear (avg stream length, bifurcation ration, LOF)
- Areal (Length-area, drainage density, constant of channel maintenance, mainly dimensionless numbers)
- Relief (Hypsometric analysis, max basin relief)
Bifurcation Ratio
- Ratio of number of streams of a given order to the number in the next highest order (Strahler)
- Remarkably consistent from one basin to another, except where powerful geological controls dominate
- Typically 3 -5 and approximates number of second order streams
- Rb = No/(No plus 1)
- No = number of streams in each order
Drainage density
- Empirical relationship btwn drainage basin area and the total length of streams w/in basin
- Largely reflects interactions btwn geology and climate
Why is Drainage density important
- Real measure of drainage basin structure, not dependent on ordering schemes
- Drainage effects the time frame btwn concentrated rainfall and river discharge
- Drainage can be used to define other useful geomorphic properties of the landscape
- Constant of channel maintenance and length of overland flow
- High discharge, low time = High drainage density
- Low discharge, large time = small drainage density
How might geology (erosion resistance) and climate effect the constant of channel maintenance and LOF?
- Erosion resistant surfaces exhibit widely spaced streams = low Drainage Density
- Arid areas lack vegetation = higher drainage densities
- Humid areas have a good veg cover = Lower drainage density
What leads to widely spaced streams (lower drainage density)?
- Resistant bedrock surfaces
- High infiltration capacity
- Thick vegetation cover
Hack’s Law
- Relationship btwn length of streams and area of their basins
Dendritic drainage
- Common
- Homogeneous materials
- Governed mostly by slope
- Confluences of tributaries at acute angles
Parallel Drainage
- System develops where slope is uniform and pronounced
- May be fault controlled
- Sometimes indicated presence of major faults
- Confluences of tributaries at very small angles
Trellis Drainage
- Systems develop in folded topography (synforms)
- Geologically controlled
- Short tributary streams enter main channel at sharp right angles
- Flow in 2 directions
Rectangular Drainage
- Systems develop in regions of faulting or have 2 main directions of jointing but are of otherwise similar resistance
- Confluences at right angles
Radial Drainage
- Systems develop around a central elevated point (volcanoes, domes)
- Can also develop in closed basins (centripetal drainage)
Deranged Drainage
- No coherent pattern
- Result from disruptions to existing drainage patterns (glaciated, karst, permafrost terrains)
- Short streams enter trunk streams at all angles
- Hollows in quasi-random locations get filled w/ water, commonly become wetlands
Straight Channel
- Uncommon
- Distinguished from meandering when sinuosity < 1.5
- Structurally controlled or artificially straightened
- Few straight for entire length
- Steep headwater streams
- Pool-riffle and step-pool sequences
Sinuosity
Channel L/Valley L
Pools
- Areas of active erosion
Riffles
- Regions of shallower flow over gravel
Length of pool-riffle sequence
- Generally 5-7 stream widths
- Only in gravel-bed rivers
Step-pools
- Often replace pool-riffles in steep bedrock channels
- Also common in bouldery creeks
- Steps comprised of pebbles, cobbles, or boulders (in alluvial) or resistant bedrock outcrops in bedrock channels
Meandering Streams
- Series of curves of similar shape and amplitude
- Much higher suspended load than bedload
- Thalweg alternates from side to side creating the typical cut-bank/point-bar geomorphology
Braided Channels
- Multiple small channels and islands (braid bars)
- Steep gradients, broad valleys
- High and variable Q, qsed, and stream power
- Erodible banks (non-cohesivue sediment)
- Unstable bars
- High bedload transport, with rapid and frequent Q changes
- Glaciofluvial systems (huge sed and variable dirurnal discharge or seasonal)
Anastamosing/ Anabranching rivers
- Multiple, deep, narrow, stable channels with vegetated islands
- Shallow gradients, low stream power
- Most sed transported as suspended lot (clays, silts)
- Vertically aggressing systems
Graded river
- Water and sed pass w/o net change in form
- Channel characteristics adjust until slope and discharge provide just enough velocity to transport sediment supplied from the basin and no more
