PTYS 411 Geology and Geophysics of the Solar System Fluvial Processes PYTS 411 – Fluvial Processes l Earth n n l Liquid = water Sediment = mostly quartz Mars n n l Liquid =
Download ReportTranscript PTYS 411 Geology and Geophysics of the Solar System Fluvial Processes PYTS 411 – Fluvial Processes l Earth n n l Liquid = water Sediment = mostly quartz Mars n n l Liquid =
PTYS 411 Geology and Geophysics of the Solar System
Fluvial Processes
PYTS 411 – Fluvial Processes
l
Earth
n n
Liquid = water Sediment = mostly quartz
2 l
Mars
n n
Liquid = water Sediment = mostly basalt
l
Titan
n n
Liquid = Methane (mostly) Sediment = organic stuff and H 2 O ice
PYTS 411 – Fluvial Processes
3 l l
Lower gravity
n n
Slower flow …but easier to transport sediment Fluid viscosity and density
n n
Affects particle buoyancy and settling velocity Water can carry bigger particles in suspension
l
PYTS 411 – Fluvial Processes Growth of Drainage Features Fluvial erosion starts with rainfall
n n n
Rainsplash is similar to micrometeorite bombardment
‘
Ejecta
’
is preferentially transported downslope Diffusive smoothing where dominant
n
Channel formation suppressed
4
PYTS 411 – Fluvial Processes
l
Fluid mostly infiltrates surface
n
Infiltration rate fast at first until near-surface pores are filled, constant rate thereafter set by permeability
n
Fluid that doesn ’t infiltrate the subsurface can run off
w
Causes erosion
5 n
Surface with high infiltration rates are very resistant to erosion Melosh 2011
l
Permeability effects on erosion PYTS 411 – Fluvial Processes
6
High permeability cinders 50 Kyr after eruption Low permeability ash 5 months after eruption
PYTS 411 – Fluvial Processes
l
Flow thickness increases with distance from the divide
n
Shear stress depends on flow thickness
t = r
gh
sin a 7 n
Transport of debris is a threshold process
n n
No channelization where flow is thin Rainsplash smoothing suppresses rille formation
n
At some point transition from sheet flow to rille formation
PYTS 411 – Fluvial Processes
l
If surface is eroded until stress falls below the threshold…
n
x is distance from drainage divide
n
Solving this gives a logarithmic profile
z
-
z o
= -
C
ln
ç
x L
ö ø n
Result is a characteristic terrestrial hillshape z = z o
8
l
Rilles combine to form networks
n
E.g. Pinatubo ash deposit
w
Low permeability, high runoff
w
A wet environment PYTS 411 – Fluvial Processes
9
5 months after the eruption
PYTS 411 – Fluvial Processes
l
Drainage basins
n n n
Range from a few m across to continental in scale Bounded by divides Area, length relation
L
» 3
A
Montgomery and Dietrich 1992
10
PYTS 411 – Fluvial Processes
l
Networks characterized by Strahler number
n
Stream order
w w w
1 – initial rilles 2 – combining 2 order 1 streams 3 – combining 2 order 2 streams etc…
w
Combining streams of different order gives a stream with the higher order – side branches
n n
Some rivers can by up to 10 th order (Mississippi) The nearby Gila river is 8 th order e.g. fifth order network Turcotte 1997
11
PYTS 411 – Fluvial Processes
l
Two types of channel
n n
Bedrock channel – detachment limited Alluvial channel – transport limited
12
l
PYTS 411 – Fluvial Processes Sediment Transport Moving material
n
Bedload – saltating and rolling material
n n
Suspended load Washload
w
Very fine particles (essentially part of the fluid)
13
Washload
PYTS 411 – Fluvial Processes
l
Transport threshold - The Shields curve
n
Define the shear velocity
u
* = t r
f
n
Define the boundary Reynolds number
Re * = r
f u
* h
d
n n p 4
d
2 t
t so
: q
t
= = p
d
3 ( r
s
r
f
)
g
q
t
6 ( r
s
t
t
r
f
)
gd
= ( r
f u
* 2 r
s
r
f
)
gd
This factor is the Shields criterion
w
Function of Rayleigh number
w
Empirically determined Burr et al. 2006
14
PYTS 411 – Fluvial Processes
l l l
All Re * and θ t
n
values along that line can be converted to u * U * is proportional to u ave (constant depends on bed roughness) and d Yields different threshold curves for different material parameters and gravity Frictional velocities on Earth, Titan and Mars differ by only a factor of ~3
15
Burr et al. 2006
PYTS 411 – Fluvial Processes
l
Sediment flux
n
Similar to aeolian transport
n
With a threshold
µ ( t t
t
) µ
u
* 3 µ t 3 2 3 2
or
µ ( q q
t
) 3 2
where
q = ( r
s
t r
f
)
gd
n
where Β depends on transport stage (T s ):
T s
= t t
t
= q q
q
*
s
= q
t
) 3 2 b =
1.6ln
T s
-
4.65
l
θ t
l
Re-dimentionalize q *s
q s
=
q
*
s
( ( r
s
r
f
) r
f
)
gd
3
q s is sediment flux per meter of stream width
16
Burr et al. 2006
PYTS 411 – Fluvial Processes
l
Hjulstrom diagram often used instead
n n n
Uses dimensional velocity instead of shear velocity Curves are different for every depth Not as flexible as the Shields curve – not easily transferred to other planets
17
PYTS 411 – Fluvial Processes
l
Stream power is a measure of how much sediment a river can carry
n
P s /w is the unit stream power
P s P s P s w
= = @ r
f g
r
f gSQ
(
hw
1
m
) r
f gSQ w SV
or
P s w
= t
V P s
= r
f u
* 2
u
*
w P s w
= ( 8
f c
r
f
) 8
u
* 3
f c
Reduction in unit stream power can switch erosion to deposition Increase in w Decrease in Q
18
PYTS 411 – Fluvial Processes
l l
Turbulent flow entraining particles is complex Adding a deformable bed is even more complex
l
Antidunes
n
High-velocity shallow flows
n
Shallow water wavespeed
c
=
gh
n
Froude number
Fr
=
V c
n n n
Surface wave setup λ~2πh that that causes variable bed erosion
n
Upstream propagating bedforms oversteepen and break
19
PYTS 411 – Fluvial Processes
l
MER discoveries of festoon cross-bedding indicates beds deposited by flowing water
n
Estimated flow speeds of ~0.5 m/s
20
Grotzinger et al. 2006
PYTS 411 – Fluvial Processes Bedrock Erosion
l
Proceeds by:
n n n
Abrasion from suspended sediment Plucking Cavitation
l
Bedrock abrasion on Titan
n n n
Roughly as easy to do as on Earth Various properties of the two bodies and materials involved cancel
w
Abrasion of water ice is easier
w
Lower kinetic energy on Titan Big question is how much run-off there is and the nature of the debris
l
Bedrock stream erosion on Mars
n
Harder than Earth – lower kinetic energy
n
Equally hard rocks
21
l
PYTS 411 – Fluvial Processes Flow velocities and discharges Open channel flow described by the manning equation:
n
R h is the hydraulic radius Flow-cross-section / Wet-perimeter
V
= 1
n R h
2 3
S
1 2
w
i.e. for a rectangular trough When w>>h then R h ~ h
R
h
= ( + 2
h
)
h
n n
n is the Manning coefficient of roughness Varies from: ~0.02 – smooth beds and straight plans to ~0.08 – rough beds and sinuous plans
n
n is determined empirically
l l l
Empirically
‘
discovered
’
in 19 th century by averaging a bunch of pre-existing flow laws Problem is that
‘
n
’
has dimensions – can
’
t be generalized to other planets Chezy
’
s law has the same problem:
V
=
c R h
1 2
S
1 2 22
PYTS 411 – Fluvial Processes
l
Darcy-Weisbach law
n
Balance shear stress with friction
n
Downhill force per unit length
r
f ghw
sin a n n
Friction with walls in terms of mean velocity: i.e.
u
* = (
f c
8 )
V
(
f c
n n n
Flow velocity is: Discharge is:
V
=
Q
= 8
A R h f c gR h
sin a 8
f c
sin a
Q
µ
wh
3 2 n 8
Empirical relations that relate f c
w w
Grain-size of bed material
f c
= 5.75 log æ è 2
h D
50 ö ø
or
to
V
=
u
* 1 0.4
ln æ ç
h D
50 2 4 ö ø 8 ) r
f V
2 ( 2
h
+
w
)
h
Compare to eolian flow, law of the wall Julien et al. 2006
w
23
PYTS 411 – Fluvial Processes
l
Mature rivers slow down and deposit material
n
Levees and floodplains
n
Terraced floodplains
w
Tectonic change in landscape height
w
Decreases in sediment load
w
Increasing discharge
24
PYTS 411 – Fluvial Processes
l
Inverted relief
n n
Channels contain coarser grained material than floodplains Coarser material is more resistant to erosion and remains as a ridge
25
Burr et al. 2009
PYTS 411 – Fluvial Processes
l
Meanders
n
Water channel migrates by erosion of one bank and deposition on the other
n n n
Faster water on the outside bank Scroll-bars from sedimentation on inner bank Friction with wall leads to helical flow
w
Fastest velocity not on outer edge
26
Press and Siever 1998
PYTS 411 – Fluvial Processes
l
Meander wavelength and radius of curvature
n
Width of river related to meander radius of curvature Titan Earth Malaska et al. 2011
27
l
Martian meanders
n
Including scroll bars PYTS 411 – Fluvial Processes
28
Burr et al. 2009
PYTS 411 – Fluvial Processes
l l
Underfit rivers have two meander wavelengths superposed
n
Shorter one is present-day Likely due to change in river discharge (width) with climate
29
Posamentier 2001
l
Meander with Scrollbars PYTS 411 – Fluvial Processes
30
msss.com
l
Meanders of lava channels on Venus ?
PYTS 411 – Fluvial Processes
31
Melosh 2011