Eroding landscapes fluvial processes Sediments and bedrock erosion

Eroding landscapes: fluvial processes Sediments and bedrock erosion Mikaël ATTAL Acknowledgements: Jérôme Lavé, Peter van der Beek and other scientists from LGCA (Grenoble) and CRPG (Nancy) Marsyandi valley, Himalayas, Nepal

Lecture overview I. Field testing of fluvial erosion laws: do sediments matter? II. How do sediments modulate bedrock erosion rates? III. What does control sediment characteristics in bedrock rivers?

General form: fluvial incision laws m n E = KA S. f(qs) Ø Stream Power Law(s) (laws 1, 2, 3): f(qs) = 1 Ø Laws including the role of the sediments: f(qs) ≠ 1 Threshold for erosion (law 4), slope set by necessity for river to transport sediment downstream (law 5), cover effect (law 6), tools + cover effects (law 7). Similar predictions at SS: concave up profile with power relationship between S and A. Different predictions in terms of transient response of the landscape to perturbation.

I. Field testing of fluvial incision laws (1) Excess shear stress model (law 4): Lavé & Avouac, 2001 V D Basal shear stress: τ = ρ g R S, where R = WD / (W+2 D) τ = ρ g D S, if W >10 D. Fluvial erosion law: E = K (τ - τc)

Fluvial incision along Himalayan rivers MFT

Fluvial incision measured using terraces

Comparison between fluvial incision (terraces) and excess shear stress (channel geometry) Shields stress (non-dimensional): τ* = τ / (ρs – ρ)g. D 50 [Lavé and Avouac, 2001]

E = K (τ* - τc*) Independent measurements: E from terraces and τ from channel geometry. τc* value used = 0. 03 See Buffington and Montgomery, 1997, for extensive description of the critical shear stress concept. Important role of lithology

S LH HHC TSS Modified from Lavé & Avouac, 2001: maximum fluvial erosion rate in the HHC zone for 6 main Himalayan rivers

S LH HHC TSS Lavé & Avouac, 2001: maximum fluvial erosion rate in the HHC zone for 6 main Himalayan rivers Modified from Lavé & Avouac, 2001

I. Field testing of fluvial incision laws (2) Using the transient response of the landscape Ø All laws predict similar steady-state topographies (concave-up profile, etc. ), Ø Predicted transient response to a disturbance depends on the fluvial incision law chosen.

Transient response of fluvial systems Detachment-limited law (SPL, laws 1, 2, 3) (2002) Transport limited law (law 5) (2002)

Transient response to tectonic disturbance (Whittaker et al. , 2007 a, b, 2008; Cowie et al. , 2008, Attal et al. , 2008) Fiamignano, Italy Xerias, Greece 0 600 km http: //www. phys. uu. nl/~gdevries/maps. cgi

Transient response to tectonic disturbance (Whittaker et al. , 2007 a, b, 2008; Cowie et al. , 2008, Attal et al. , 2008) Fiamignano, Italy Xerias, Greece

Transient response to tectonic disturbance Erosion efficiency f(Qs) Italy closer to DL end-member, Greece closer to TL end-member (Cowie et al. , 2008) Qs/Qc 0 Fiamignano, Italy Xerias, Greece SEDIMENTS DO MATTER! 1

II. How do sediments modulate bedrock erosion rates? Role of sediment: the “tools and cover” effects (Gilbert, 1877) Cover Tools Experimental study of bedrock abrasion by saltating particles Sklar & Dietrich, 2001

Role of sediment: the “tools and cover” effects Sklar & Dietrich, 1998, 2004 1998: theoretical E = V i Ir F e 2004: mechanistic Vi = volume of rock detached / particle impact, Ir = rate of particle impacts per unit area per unit time, Fe = fraction of the river bed made up of exposed bedrock.

Role of sediment: the “tools and cover” effects Turowski et al. , 2007 Erosion efficiency f(Qs) Maximum bedrock erosion for sediment supply = Qc (“dynamic cover effect”) Sklar & Dietrich 0 Qs/Qc Turowski et al. 1 Sediment SUPPLY / Qc Sediment SUPPLY ≤ Qc Qs/Qc = Sediment SUPPLY / Qc Sediment SUPPLY > Qc Qs/Qc = 1

Role of sediment: the “tools and cover” effects Effect of grain size? (Sklar & Dietrich, 2004) But very simplistic model: bedload is made of only 1 grain size!

Role of sediment: the “tools and cover” effects Effect of grain size? Bedload is made of a wide range of grain sizes At low flow: bedload is motionless and protects bedrock from erosion. During floods, the smallest particles will be put in motion ( tools) while the largest might remain motionless ( cover): difference in sediment MOBILITY will affect bedrock erosion Not only size will affect sediment mobility: interactions between particle will do it as well (e. g. patches, gravel-bars)

Sediment mobility in bedrock rivers Cellular Automata model Movement probability 0. 05 0. 2 0. 05 Courtesy Rebecca Hodge, University of Glasgow

Role of sediment: the “tools and cover” effects Erosion efficiency f(Qs) Hodge et al. , work in progress Lower erosion rates for higher sediment supply because of increasing likelihood of jams Sklar & Dietrich Turowski et al. 0 Qs/Qc 1 Sediment SUPPLY / Qc Sediment SUPPLY ≤ Qc Qs/Qc = Sediment SUPPLY / Qc Sediment SUPPLY > Qc Qs/Qc = 1

Characterizing sediment mobility in the field Calder River, Renfrewshire

Characterizing sediment mobility in the lab Schmeeckle et al. Modelling sediment motion… http: //www. markschmeeckle. com/ Ideally, we would include pebble and bedrock abrasion in such models. But the computer that can do that efficiently doesn’t exist yet…

III. What does control sediment characteristics in bedrock rivers? Bedrock erosion in (1) will depend on sediment characteristics in (1): - what proportion of sediment is bedload? ( tools and cover) - what is the grain size distribution of the bedload? ( for a given flood, what will be tools, what will be cover, and how efficient the tools will be) - what is the lithologic content of the bedload? ( how efficient the tools will be) http: //projects. crustal. ucsb. edu/nepal/ What does control the characteristics of sediments in (1)? (2) Characteristics of the source of sediment (location, amount, grain size distribution, Sediment lithology) mobility (1) (3) Transport and abrasion processes along the channel

III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport Sediments entering the channel are usually angular Marsyandi River, Nepal

III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport Angular pebbles in the river Pebbles are abraded during fluvial transport Each pebble is reduced in size and gets more rounded Marsyandi River, Nepal

III. What does control sediment characteristics in bedrock rivers? Common pebble abrasion processes: Chipping Crushing Cracking Grinding Splitting These processes reduce the size of pebbles and tend to make them more rounded Marsyandi River, Nepal

III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport Not necessarily, because fresh material is added along the river course in mountain rivers (≠ alluvial rivers) Downstream fining?

III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport If the 2 rock types are eroded at the same rate A Rock-type content in bedload (coarse fraction, > ~1 mm) 100% B Change in rock type proportion? 50% 0% A B Distance downstream (km)

III. What does control sediment characteristics in bedrock rivers? Pebble abrasion during fluvial transport If the pink rock type is more resistant than the orange one A Rock-type content in bedload (coarse fraction, > ~1 mm) 100% B Change in rock type proportion? 50% 0% A B Distance downstream (km)

Experimental study of pebble abrasion during fluvial transport Scale 1/5 model

The « machine a laver » … on its frame The circular flume Piping suspended 1. 35 m above the ground

Non-abrasive floor condition physical laws of pebble abrasion Abrasion = f (pebble size, velocity, lithology, amount of sediment) Videos: the flume in action…

Experimental study of pebble abrasion during fluvial transport Differences in pebble abrasion rates can be up to a factor 200! Pebble abrasion rate (% / km) Attal and Lavé, 2006

Colchen et al. , 1986, modified III. What does control sediment characteristics in bedrock rivers? Field study: the sediments of the Marsyandi river (Himalayas) Size distributions Rock type proportions DOMINANT LITHOLOGIES: Limestone Gneiss Schist Sandstone & Schist Measurement sites: Gravel bar Source of sediment

Shear stress (N/m²) Gravel bar D 50 (cm) Lavé and Avouac, 2001 Increase in grain size due to change from moraine-type source (a) to landslide-type source (b) Increase in grain size due to drop in shear stress the river is less likely to move large particles supplied from hillslopes and upstream (a) (b) Distance from source (km) “Source” and “transport” effects Attal and Lavé, 2006

% Area Lithologies exposed Gneiss and granite Schist Distance from source (km) Sandstone Quartzite Limestone % Weight Resistant rock types (Quartzite, Gneiss + Granite) are overrepresented with respect to poorly resistant rock types (Schist, Sandstones) – “Abrasion” effect Gravel bar content Distance from source (km) Attal and Lavé, 2006

III. What does control sediment characteristics in bedrock rivers? Red Deer River, Alberta, Canada (Parker, 1991) After a few hundreds of km of transport, Quartz becomes the dominant rock type in bedload Kali Gandaki - Narayani, Nepal (Mezaki and Yabiku, 1984) % (km) Downstream

III. What does control sediment characteristics in bedrock rivers? Angular pebbles, varied rock types TO SUMMARIZE Pebbles are abraded during fluvial transport What happens to the other rock types? They are turned into sand, silt transported to sedimentary basins, mostly in suspension Rounded pebbles, resistant rock types dominant Marsyandi River, Nepal

III. What does control sediment characteristics in bedrock rivers? Perfectly rounded quartz pebbles on the Isle of Arran

The ideal model of fluvial erosion and landscape evolution? Sediment characteristics strongly influence bedrock erosion rates. To better understand predict how these characteristics evolve along rivers, the ideal model would need to include: - characteristics of the sources of sediment (2), http: //projects. crustal. ucsb. edu/nepal/ - fluvial transport law (3), (2) - law of pebble abrasion during fluvial transport (3), - law of bedrock abrasion due to impacts by moving particles, (1) (3) - particles tracking, from hillslopes to rivers, from mountain range to basins.