Autogenic terraces and non-linear river incision rates under steady external forcing
Luca C Malatesta1, Jeffrey P Prancevic1, Jean-Philippe Avouac1,2 California Institute of Technology, University of Cambridge
ics Obser ton vato c ry Te
California Institute of Technology Common in incising alluvial rivers: Abandonment of wide terraces followed by (accelerated) entrenchment
Commonly interpreted as an acceleration of tectonic or climatic forcing during incision discussed in Bull (1991); Molnar et al. (1994); Poisson & Avouac (2004); Pazzaglia (2012); Gong et al. (2014); Finnegan et al. (2014) among others Kuitun River, Tian Shan, China. Luca Malatesta Common in incising alluvial rivers: Abandonment of wide terraces followed by (accelerated) entrenchment
Can accelerated incision rates result from autogenic processes?
Commonly interpreted as an acceleration of tectonic or climatic forcing during incision discussed in Bull (1991); Molnar et al. (1994); Poisson & Avouac (2004); Pazzaglia (2012); Gong et al. (2014); Finnegan et al. (2014) among others Kuitun River, Tian Shan, China. Luca Malatesta Our understanding of incision in transport-limited rivers is rooted in 1D
1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 DEGRADATION TO A NEW MOBILE-BED EQUILIBRIUM
Bed evolution Bed evolution
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80 0 yr 80 0 yr 0.2 yr 2 yr 70 70 0.4 yr 4 yr 60 0.6 yr 60 6 yr 0.8 yr 8 yr 50 50 1 yr 10 yr 40 Ultimate 40 Ultimate
30 30 Elevation in m in Elevation m in Elevation 20 20
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0 0 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 Distance in m Distance in m
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Gary Parker’s morphodynamics e-book available online We propose to test two autogenic processes that enhance vertical incision as entrenchment proceeds.
1. high banks reduce the channel 2. large cliff collapse cannot be lateral migration rate, vertical immediately removed by the river, creating incision happens over the same a talus that shields the bank from erosion area and is enhanced while vertical incision continues.
mentioned in Hancock and Anderson, 2002; Nicholas and
Anji Hai River tributary, Tian Shan, China. Quine, 2007a,b; Clarke et al., 2010; Nicholas et al., 2009 Model for cross-section evolution of entrenchment
Erosivity of a food Bankfull lateral migration
Strength of bank COVARY
Anji Hai River tributary, Tian Shan, China. The geometric model is made of a series of independent cross-sections
Sediment fux in each cross-section depends on channel slope, width and water discharge Erosion is defned by the divergence of sediment fux between the profles (Exner) In each cross-section erosion is partitioned between bed erosion and lateral erosion Lateral migration is independently picked in each cross-section following a random walk 1
foodplain river elevation
0 0
distance along river 1 1 0 distance across valley the geometric model reflects the main processes: starting with a random migration to erode a set area
f(S,W) erosion capacity
lateral migration channel foodplain
total erosion capacity the geometric model reflects the main processes: migration and erosion within the floodplain
f(S,W) =
lateral migration channel pre-incision foodplain pre-incision channel post-incision foodplain post-incision total erosion capacity within-foodplain incision the geometric model reflects the main processes: migration against a short bank
lateral migration channel foodplain
total erosion capacity the geometric model reflects the main processes: results in floodplain and bank erosion
= + +
lateral migration channel pre-incision foodplain pre-incision channel post-incision foodplain post-incision total erosion capacity bank-abutting incision within-foodplain incision bank erosion the geometric model reflects the main processes: against a tall bank (cliff), excess material is
=
+
lateral migration channel pre-incision foodplain pre-incision channel post-incision foodplain post-incision total erosion capacity bank-abutting incision within-foodplain incision bank erosion the geometric model reflects the main processes: deposited as a talus and constrains the channel
channel pre-talus foodplain pre-talus channel post-talus foodplain post-talus talus area 180 runs to test parameters Erosion capacity captures variability of entrenchment dynamics best
High erosion capacity result in Low erosion capacity result in deep rectangular canyons funnel-shaped canyons
foodplain foodplain channel channel
1 1
0 0 0 0
1 1
1 1 0 0
Limited terrace record Extensive terrace record Moderate channel narrowing Important channel narrowing 180 runs to test parameters Erosion capacity captures variability of entrenchment dynamics best
High erosion capacity result in Low erosion capacity result in deep rectangular canyons funnel-shaped canyons
foodplain foodplain channel channel
1 1
0 0 0 0
1 1
1 1 0 0
Limited terrace record Extensive terrace record Moderate channel narrowing Important channel narrowing High erosion capacity steady incision rate Low erosion capacity acceleration of incision rate River incision at high erosion capacity River incision at low erosion capacity
500 500 2D incision 1D incision
400 400 Elevation Elevation 2D incision 300 1D incision 300
200 200 0 2000 4000 6000 0 2000 4000 6000 Time Time Evolution of cross-section geometry Evolution of cross-section geometry
500 500
400 400 elevation elevation timesteps timesteps 0 0 375 1250 300 750 300 2500 1125 3750 1500 5000 4000 6000 8000 4000 6000 8000 distance across stream distance across stream Channel narrowing mitigates the Channel narrows while the channel remains slowdown of incision. steep, forcing acceleration of the incision Conclusions
Terrace record Autogenic effects have to be factored in to quantify the relationship between forcing and incision Accelerated incision does not require a change in external forcing. A combination of topography and ages is generally required to identify external forcing
Sediment routing system Lateral feedbacks cause channel narrowing, and rivers can erode (and remobilize) deeper and older strata.