Fluvial Processes III
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Transcript of Fluvial Processes III
PTYS 554
Evolution of Planetary Surfaces
Fluvial Processes IIIFluvial Processes III
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Fluvial Processes I Rainfall and runoff Channelization and erosion Drainage networks Sediment transport – Shields curve Velocity and discharge, Manning vs Darcy Weisback
Fluvial Processes II Stream power and stable bedforms from ripples to antidunes Floodplains, Levees, Meanders and braided streams Alluvial fans and Deltas Wave action and shoreline Processes
Fluvial Processes III Groundwater tables Subterranean flow rates Springs and eruption of pressurized groundwater Sapping as an erosional mechanism
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Fluid mostly infiltrates surface Infiltration rate fast at first until near-surface pores are filled, constant rate thereafter set by
permeability
Fluid that doesn’t infiltrate the subsurface can runoff Causes erosion
Surface with high infiltration rates are
very resistant to erosion
Melosh 2011
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Nomenclature
Unsaturated (Vadose) zone
Saturated zone
Capillary zoneGroundwater table
Phreatic Surface
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Ponded liquids (Precipitation – evaporation) vs. transport into the groundwater table
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Flow rate per unit area (not the same as flow velocity!)
η is the viscosity dp/dx is the applied pressure gradient k is the permeability Permeability generally increases with porosity Permeability has units of area
1 Darcy is 10-12 m-2 or (1 μm2)
Discharge = flow velocity x area
Where Φ is porosity i.e. fraction of area covered by pores on a rock face is
porosity
Groundwater flow – Darcy’s Law
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Models for permeability Permeability is usually very directional Not always directly related to pore space
Carman-Kozeny model relates flow through a packed bed to porosity
Where C’ is ~1/180 (for spherical particles) and depends on particle shape and tortuosity
Bigger particles or higher porosity means larger permeability
Medium Sand
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Within the saturated zone Porosity decreases with depth Salt precipitation increases with depth as water
migration speeds slow
In a regolith, porosity scales exponentially with depth
Based on Apollo seismic data
On Earth permeability scales as a power law with depth
Not-applicable to surface permeabilities
Scaling to other planets then assume it’s the overburden pressure that matters
Replace z with z(g1/g2) Where g1 is the gravity where the relationship was
established… …and g2 is the gravity on the planet that you’re
interested in.
Clifford & Parker 2001
Scaled to Mars
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Hydrologists usually work with hydraulic head instead of permeability H: the height a column of water would rise to if unconfined Height relative to what? Doesn’t matter, only relative heights drive flow.
Darcy’s law becomes:
Define a hydraulic conductivity:
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Flow in a confined aquifer:
Turcotte & Schubert, 2002
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Flow in an unconfined aquifer Discharge per meter of width
(breaks down near h=0)
Applied to a dam w meters thick Dupuit-Fuchheimer discharge
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Changes with time Liquid in u(x)h(x) Liquid out u(x+dx)h(x+dx)
Examine small changes i.e.
Diffusion equation If ε varies periodically then waves propagate out through the groundwater table Wave amplitude decreases exponentially with x with e-folding distance
P = Period
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Mix of permeable and impermeable layers can lead to perched aquifers and spring discharge
Especially true on the Colorado Plateau where permeable sandstone overlies impermeable slitstones
Seeps weaken rock by transporting cementing agents to the surface
Discharge transports sediment away
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Sapping Seeps weaken rock by transporting cementing agents to the surface Discharge or runoff transports sediment away
Backwasting here undermines rock above
Collapse produces alcove that lengthens into channel
Floor is set by the impermeable layer
Brown Canyon, Utah
Aharonson et al., 2002
e.g. Najavo Sandstone
e.g. Kayenta formation
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Characteristics of sapping channels Usually one main channel Theatre-shaped alcove at head Short stubby tributaries Not a dendritic network – low stream order
Sapping channels vs. runoff Sapping: Propagate backward via head-ward erosion Runoff: down-cutting of pre-existing terrain
Idaho and Utah
Pelletier and Baker 2011Mars, msss.com
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Longitudinal profiles Logarithmic for runoff Piecewise linear for sapping channels
Knick points are common and migrate ‘upstream’
Aharonson et al., 2002
Ma’adim Vallis
Al-Qahira Vallis Brown’s Canyon
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Runoff dominates over sub-surface flow
Sub-surface flow dominates over runoff
Pelletier and Baker 2011
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More Mars Examples
Pelletier and Baker 2011
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Canyon de Chelly, Earth
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Sapping vs runoff
Runoff Sapping
Downcutting through terrain Headward erosion of alcove
Dendritic network – high order Few tributaries – low order
Channels narrow to points Channel head is theatre-shaped
Logarithmic longitudinal profile Flat piecewise segments for floors
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Sapping on Titan?
Huygens descent probe Dendritic channels leading into dark areas River-like features – up to forth order channels Sapping like features in other areas
Sodeblom et al., 2007
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Penetrometer data and methane detection indicate Titan’s surface is wet
Rounded cobbles indicate runoff has occured
Zarnecki et al., 2005
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Huge flood carved channels Contains streamlined Islands Likely that a large underground reservoir emptied
catastrophically Source region collapses to chaos terrain Flood empties into northern lowlands
Up to 400km across and 2.5km deep
Discharge estimates up to 104-109 m3/sec
Outflow Channels
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Terrestrial analogue End of the last ice-age Glacial lake Missoula- Ice-dam breaks
Channeled scablands, Washington
Outflow channel, Mars
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Fluvial Processes I Rainfall and runoff Channelization and erosion Drainage networks Sediment transport – Shields curve Velocity and discharge, Manning vs Darcy Weisback
Fluvial Processes II Stream power and stable bedforms from ripples to antidunes Floodplains, Levees, Meanders and braided streams Alluvial fans and Deltas Wave action and shoreline Processes
Fluvial Processes III Groundwater tables Subterranean flow rates Springs and eruption of pressurized groundwater Sapping as an erosional mechanism