Geomorphology 320 - UW Madison

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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,


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