Geomorphology 320 - UW Madison
what is the stream power erosion rule?
- E = k A^m S^n k = lithology, climate, channel width, flow hydraulics, sediment load A = stream area S = slope of river m & n = empirical constants
how does width, height , and speed of a river vary with discharge at a station?
- Q = WHV W = aQ^b H = cQ^f V = kQ^m b = 0.25, f = 0.4, m = 0.35
how does width, height, and speed of a river vary with discharge (downstream)?
- Q = WHV W = aQ^b H = cQ^f V = kQ^m b = 0.5, f = 0.4, m = 0.1
what is a braided stream?
- a stream with many channels that are shallow and wide and are separated by bars - coarse sediment moved by bed load - variable flow and sparse vegetation
what is a meandering stream?
- a stream with one primary channel that is narrow and deep with fine sediment moved by suspended load - relatively constant flow with dense vegetation
how do basins evolve?
- basins grow by headward extension - total drainage density and growth is dependent on slope
what are the controls on drainage density?
- climate: precipitation & vegetation - geology: runoff and erosion susceptibility
what is aggradation?
- continuous deposition of fluvial sediment
what is the froude number?
- dimensionless number used to distinguish subtypes of turbulent flow - Fr = V / sqrt(g*d) V = velocity g = gravity d = depth
what is chemical weathering?
- dissolved chemicals weather rocks (esp carbonates)
resistive stresses
- friction bt water molecules - water moving over/around bed or walls - channel shape changing downstream
where is shear stress highest/lowest?
- highest at bed of river - lowest at surface of river
what promotes river incision?
- increase Q - change in bed load sediment Qs - increase in rock uplift rate - drop in base level
what is abrasion?
- particles entrained in flow scour the bedrock - proportional to water velocity and debris content
why do streams meander?
- perturbation - deposition due to oscillatory sloshing - armored banks
what is plucking?
- removal of rock blocks through hydraulic action or momentum exchange with large clasts which can cause crack growth - size of removed rocks is determined by geology of the rock itself
what is deposition?
- rivers deposit based on size, shape, and density - water sorts sediments
what is mannings roughness parameter?
- roughness of a rivers surface - varies between .02 and .075
what are the properties of a drainage basin?
- streams form valleys via erosion and transport of sediment - all rivers consist of a major trunk fed by mutually adjusted branches that diminish in size away from the trunk
what is shields parameter?
- tc = pgds // (ps - pf) gDi pgds = shear stress ps = density of solid particle pf = density of liquid surrounding a particle - used to determine if a particle will be entrained or not
what is cavitation?
- water acceleration leads to vapor bubbles that can implode and cause shockwaves and erosion - can cause rapid erosion
explain how a rivers slope could increase
If river discharge Q decreases (via a decrease in depth or velocity), then the fluvial sediment flux is smaller than the sediment supply (Qs_out< Qs). The riverbed aggrades and slope increases until it compensates for decreased Q and depth, and the sediment flux returns to equilibrium (Qs=Qs_out).
explain how a rivers slope could decrease
If sediment supply Qs decreases, then the fluvial sediment flux is larger than the sediment supply (Qs_out> Qs). The riverbed erodes and slope decreases until flow velocity decreases enough to compensate, and the sediment flux returns to equilibrium (Qs=Qs_out).
what is the reynolds number?
Re = VRp / mew V = avg velocity R = hydraulic radius mew = viscosity (water) p = density (water)
what is mannings equation?
V = 1.49R^(2/3)S^(1/2) / n R = hydraulic radius V = velocity S = slope n = manning roughness parameter
How can velocity go up downstream while the slope is decreasing and the depth is increasing?
a decrease in n (manning roughness parameter) can compensate for the decrease in slope
what is a constructive delta
a delta in which fluvial action is the primary influence
what is a destructive delta
a delta in which wave action, tides, and currents are the primary influence
what is stream order?
a method of categorizing streams (1 signifies a stream that collects rain fall)
what is an alluvial fan
a semicircular area in which sediment is deposited from an escarpment or mountain
what are the four bedrock erosive processes?
abrasion, cavitation, plucking, chemical weathering
FR > 1
antidunes
where is water velocity the highest?
at the surface of the river in the middle of the stream
what is a recurrence interval
average time between floods of a given discharge
what landforms result from stream migration
chutes, cutoffs, meander scrolls
what is the hydraulic radius (R)?
cross sectional area / wetted perimeter
what is mass wasting?
debris moves downslope due to gravity; process is likely caused by river undercutting
what are the two most common drainage patterns?
dendritic and parallel
what is discharge?
discharge is the volume rate of flow transported through a given area
what is fluvial entrainment?
drag has to overcome the weight and friction between grains
Fr < 1
dunes (higher n) and then plane bed (lower n)
what is bedrock erosion?
erosion of in place bedrock that cuts vertically
why do streams migrate
erosion on cut bank and deposition on point bars
what is bank erosion
erosion that cuts outward, occurs in both bedrock and alluvial streams
what is flux?
flux is a quantity per unit area per unit time
what is a type 1 deposit
high viscosity, low water:sediment ratio, steep terrains, fine particles
what controls deposition at the base of a alluvial fan
hydraulic geometry, not slope!
~90% of GW is done on sediment loads during which events?
intermediate discharge events that occur every 5-10 years
slope is ____ with drainage area for an alluvial fan
inverse
how do laminar and turbulent flows differ?
laminant flows are straight with drag whereas turbulent flows have fluctuating velocities and water interchange between zones
re < 500
laminar flow
characteristics of a slow onset flood
large area, thick soil, low relief, elongated basin, dense vegetation
what is a type 2 deposit
low viscosity, high water:sediment ratio, flat terrain, coarse particles
what is sediment yield (drainage denundation)?
mass / area*time
t < tc (critical shear stress)
no particle movement
how do deltas differ from fans
occur in water, not as steep, deposition results from reduced velocity, can be redistributed by waves, tides, and currents
what does the equilibrium in a meandering stream look like?
one side with a cut bank (outer edge) and another point bar (inside edge)
t = tc or t > tc (critical shear stress)
particle moves
what determines if a particle can be entrained?
particles can be entrained if the stress on the particle from the water moving past is greater than the stress holding it down it will start to move
Fr << 1
plane bed , ripples
controls on sediment yield
precipitation, rock uplift rate, basin size, rock type
drainage are is ____ with area of a fan
proportional
how do terraces form?
river is meandering and creating a flood plain but then an incision event occurs and the river deepens and forms a new floodplain, old floodplain left as a terrace
methods for estimating paleo flood discharge
sediment deposits, ripple marks, largest rock size
what is geomorphic work?
sediment transferred during a given flow * frequency of that given flow
characteristics of a flash flood
small area, thin soil, high relief, equidistant basin, sparse vegetation
why does the slope of rivers adjust?
so that discharge is sufficient to transport the sediment discharge
what is the equation for shear stress?
tau = p*g*depth*slope p = density g = gravity
causes of segmentation
tectonic movements, climate (increase Q), change in Qs
what is competence?
the largest particle a stream can entrain under a hydraulic condition
what is entrainment?
the process under which particles start moving
stream power rule supposition
the thing that controls bedrock river incision rate is the rate of energy expenditure by the river on the river bed
what is drainage density?
total channel length / basin area
re > 1000
turbulent flow
what affects n?
vegetation, particle size, change in bed forms
thickness of viscous sublayer
vs = 11.5 * mew // sqrt(t//p) mew = viscosity t = shear stress p = density of fluid
how does hydraulic geometry change downstream due to a change in discharge?
water discharge increases downstream as drainage area (and the runoff from it) increases
variables that affect fan slope
water discharge, source area lithology, vegetation,
